FRAGMENTS OF SCIENCE:

A Series of
Detached

ESSAYS, ADDRESSES, AND REVIEWS.

BY

JOHN
TYNDALL, F.R.S.

Printed
By Spottiswoode AND CO.

NEW-STREET SQUARE

PARLIAMENT STREET

SIXTH EDITION,

VOL. 1.

LONDON:
LONGMANS, GREEN, AND CO.

1879.

All rights reserved.

VOL. I. INORGANIC NATURE *

VOL. II. *

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PREFACE TO THE SIXTH EDITION.

TO AVOID unwieldiness of bulk this edition of the ‘Fragments’
is published in two volumes, instead of, as heretofore, in
one.

The first volume deals almost exclusively with the laws and
phenomena of matter. The second trenches upon questions in which
the phenomena of matter interlace more or less with those of
mind.

New Essays have been added, while old ones have been revised,
and in part recast. To be clear, without being superficial, has
been my aim throughout.

In neither volume have I aspired to sit in the seat of the
scornful, but rather to treat the questions touched upon with a
tolerance, if not a reverence, befitting their difficulty and
weight.

Holding, as I do, the nebular hypothesis, I am logically bound
to deduce the life of the world from forces inherent in the
nebula. With this view, which is set forth in the second volume,
it seemed but fair to associate the reasons which cause me to
conclude that every attempt made in our day to generate life
independently of antecedent life has utterly broken down.

A discourse on the Electric Light winds up the Second volume.
The incongruity of its position is to be referred to the lateness
of its delivery.

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VOL. I. INORGANIC NATURE

I. THE CONSTITUTION OF NATURE.

[Footnote:
‘Fortnightly Review,’ 1865, vol. iii. p.
129.]

WE cannot think of space as finite, for wherever in
imagination we erect a boundary, we are compelled to think of
space as existing beyond it. Thus by the incessant dissolution of
limits we arrive at a more or less adequate idea of the infinity
of space. But, though compelled to think of space as unbounded,
there is no mental necessity compelling us to think of it either
as filled or empty; whether it is so or not must be decided by
experiment and observation. That it is not entirely void, the
starry heavens declare; but the question still remains, Are the
stars themselves hung in vacuo? Are the vast regions which
surround them, and across which their light is propagated,
absolutely empty? A century ago the answer to this question,
founded on the Newtonian theory, would have been, ‘No, for
particles of light are incessantly shot through space.’ The reply
of modern science is also negative, but on different grounds. It
has the best possible reasons for rejecting the idea of
luminiferous particles; but, in support of the conclusion that
the celestial spaces are occupied by matter, it is able to offer
proofs almost as cogent as those which can be adduced of the
existence of an atmosphere round the earth. Men’s minds, indeed,
rose to a conception of the celestial and universal atmosphere
through the study of the terrestrial and local one. From the
phenomena of sound, as displayed in the air, they ascended to the
phenomena of light, as displayed in the aether; which is
the name given to the interstellar medium.

The notion of this medium must not be considered as a vague or
fanciful conception on the part of scientific men. Of its reality
most of them are as convinced as they are of the existence of the
sun and moon. The luminiferous aether has definite mechanical
properties. It is almost infinitely more attenuated than any
known gas, but its properties are those of a solid rather than of
a gas. It resembles jelly rather than air. This was not the first
conception of the aether, but it is that forced upon us by a more
complete knowledge of its phenomena. A body thus constituted may
have its boundaries; but, although the aether may not be
co-extensive with space, it must at all events extend as far as
the most distant visible stars. In fact it is the vehicle of
their light, and without it they could not be seen. This
all-pervading substance takes up their molecular tremors, and
conveys them with inconceivable rapidity to our organs of vision.
It is the transported shiver of bodies countless millions of
miles distant, which translates itself in human consciousness
into the splendour of the firmament at night.

If the aether have a boundary, masses of ponderable matter
might be conceived to exist beyond it, but they could emit no
light. Beyond the aether dark suns might burn; there, under
proper conditions, combustion might be carried on; fuel might
consume unseen, and metals be fused in invisible fires. A body,
moreover, once heated there, would continue for ever heated; a
sun or planet once molten, would continue for ever molten. For,
the loss of heat being simply the abstraction of molecular motion
by the aether, where this medium is absent no cooling could
occur. A sentient being on approaching a heated body in this
region, would be conscious of no augmentation of temperature. The
gradations of warmth dependent on the laws of radiation would not
exist, and actual contact would first reveal the heat of an extra
ethereal sun.

Imagine a paddle-wheel placed in water and caused to rotate.
From it, as a centre, waves would issue in all directions, and a
wader as he approached the place of disturbance would be met by
stronger and stronger waves. This gradual augmentation of the
impression made upon the wader is exactly analogous to the
augmentation of light when we approach a luminous source. In the
one case, however, the coarse common nerves of the body suffice;
for the other we must have the finer optic nerve. But suppose the
water withdrawn; the action at a distance would then cease, and,
as far as the sense of touch is concerned, the wader would be
first rendered conscious of the motion of the wheel by the blow
of the paddles. The transference of motion from the paddles to
the water is mechanically similar to the transference of
molecular motion from the heated body to the aether; and the
propagation of waves through the liquid is mechanically similar
to the propagation of light and radiant heat.

As far as our knowledge of space extends, we are to conceive
it as the holder of the luminiferous aether, through which are
interspersed, at enormous distances apart, the ponderous nuclei
of the stars. Associated with the star that most concerns us we
have a group of dark planetary masses revolving at various
distances round it, each again rotating on its own axis; and,
finally, associated with some of these planets we have dark
bodies of minor note — the moons. Whether the other fixed
stars have similar planetary companions or not is to us a matter
of pure conjecture, which may or may not enter into our
conception of the universe. But probably every thoughtful person
believes, with regard to those distant suns, that there is, in
space, something besides our system on which they shine.

From this general view of the present condition of space, and
of the bodies contained in it, we pass to the enquiry whether
things were so created at the beginning. Was space furnished at
once, by the fiat of Omnipotence, with these burning orbs? In
presence of the revelations of science this view is fading more
and more. Behind the orbs, we now discern the nebulae from which
they have been condensed. And without going so far back as the
nebulae, the man of science can prove that out of common
non-luminous matter this whole pomp of stars might have been
evolved.

The law of gravitation enunciated by Newton is, that every
particle of matter in the universe attracts every other particle
with a force which diminishes as the square of the distance
increases. Thus the sun and the earth mutually pull each other;
thus the earth and the moon are kept in company, the force which
holds every respective pair of masses together being the
integrated force of their component parts. Under the operation of
this force a stone falls to the ground and is warmed by the
shock; under its operation meteors plunge into our atmosphere and
rise to incandescence. Showers of such meteors doubtless fall
incessantly upon the sun. Acted on by this force, the earth, were
it stopped in its orbit to-morrow, would rush towards, and
finally combine with, the sun. Heat would also be developed by
this collision. Mayer first, and Helmholtz and Thomson
afterwards, have calculated its amount. It would equal that
produced by the combustion of more than 5,000 worlds of solid
coal, all this heat being generated at the instant of collision.
In the attraction of gravity, therefore, acting upon non-luminous
matter, we have a source of heat more powerful than could be
derived from any terrestrial combustion. And were the matter of
the universe thrown in cold detached fragments into space, and
there abandoned to the mutual gravitation of its own parts, the
collision of the fragments would in the end produce the fires of
the stars.

The action of gravity upon matter originally cold may, in
fact, be the origin of all light and heat, and also the proximate
source of such other powers as are generated by light and heat.
But we have now to enquire what is the light and what is the heat
thus produced? This question has already been answered in a
general way. Both light and heat are modes of motion. Two planets
clash and come to rest; their motion, considered as that of
masses, is destroyed, but it is in great part continued as a
motion of their ultimate particles. It is this latter motion,
taken up by the aether, and propagated through it with a velocity
of 186,000 miles a second, that comes to us as the light and
heat of suns and stars. The atoms of a hot body swing with
inconceivable rapidity — billions of times in a second
— but this power of vibration necessarily implies the
operation of forces between the atoms themselves. It reveals to
us that while they are held together by one force, they are kept
asunder by another, their position at any moment depending on the
equilibrium of attraction and repulsion. The atoms behave as if
connected by elastic springs, which oppose at the same time their
approach and their retreat, but which tolerate the vibration
called heat. The molecular vibration once set up is instantly
shared with the aether, and diffused by it throughout space.

We on the earth’s surface live night and day in the midst of
aethereal commotion. The medium is never still. The cloud canopy
above us may be thick enough to shut out the light of the stars;
but this canopy is itself a warm body, which radiates its thermal
motion through the aether. The earth also is warm, and sends its
heat-pulses incessantly forth. It is the waste of its molecular
motion in space that chills the earth upon a clear night; it is
the return of thermal motion from the clouds which prevents the
earth’s temperature, on a cloudy night, from falling so low. To
the conception of space being filled, we must therefore add the
conception of its being in a state of incessant tremor.

The sources of this vibration are the ponderable masses of the
universe. Let us take a sample of these and examine it in detail.
When we look to our planet, we find it to be an aggregate of
solids, liquids, and gases. Subjected to a sufficiently low
temperature, the two last, would also assume the solid form. When
we look at any one of these, we generally find it composed of
still more elementary parts. We learn, for example, that the
water of our rivers is formed by the union, in definite
proportions, of two gases, oxygen and hydrogen. We know how to
bring these constituents together, so as to form water: we also
know how to analyse the water, and recover from it its two
constituents. So, likewise, as regards the solid portions of the
earth. Our chalk hills, for example, are formed by a combination
of carbon, oxygen, and calcium. These are the so-called
elements the union of which, in definite proportions, has
resulted in the formation of chalk. The flints within the chalk
we know to be a compound of oxygen and silicium, called silica;
and our ordinary clay is, for the most part, formed by the union
of silicium, oxygen, and the well-known light metal, aluminium.
By far the greater portion of the earth’s crust is compounded of
the elementary substances mentioned in these few lines.

The principle of gravitation has been already described as an
attraction which every particle of matter, however small, exerts
on every other particle. With gravity there is no selection; no
particular atoms choose, by preference, other particular atoms as
objects of attraction; the attraction of gravitation is
proportional simply to the quantity of the attracting matter,
regardless of its quality. But in the molecular world which we
have now entered matters are otherwise arranged. Here we have
atoms between which a strong attraction is exercised, and also
atoms between which a weak attraction is exercised. One atom can
jostle another out of its place, in virtue of a superior force of
attraction. But, though the amount of force exerted varies thus
from atom to atom, it is still an attraction of the same
mechanical quality, if I may use the term, as that of gravity
itself. Its intensity might be measured in the same way, namely
by the amount of motion which it can generate in a certain
time. Thus the attraction of gravity at the earth’s surface is
expressed by the number 32; because, when acting freely on a body
for a second of time, gravity imparts to the body a velocity of
thirty-two feet a second. In like manner the mutual attraction of
oxygen and hydrogen might be measured by the velocity imparted to
the atoms in their rushing together. Of course such a unit
of time as a second is not here to be thought of, the whole
interval required by the atoms to cross the minute spaces which
separate them amounting only to an inconceivably small fraction
of a second.

It has been stated that when a body falls to the earth it is
warmed by the shock. Here, to use the terminology of Mayer, we
have a mechanical combination of the earth and the body.
Let us suffer the falling body and the earth to dwindle in
imagination to the size of atoms, and for the attraction of
gravity let us substitute that of chemical affinity; we have then
what is called a chemical combination. The effect of the
union in this case also is the development of heat, and from the
amount of heat generated we can infer the intensity of the atomic
pull. Measured by ordinary mechanical standards, this is
enormous. Mix eight pounds of oxygen with one of hydrogen, and
pass a spark through the mixture; the gases instantly combine,
their atoms rushing over the little distances which separate
them. Take a weight of 47,000 pounds to an elevation of 1,000
feet above the earth’s surface, and let it fall; the energy with
which it will strike the earth will not exceed that of the eight
pounds of oxygen atoms, as they dash against one pound of
hydrogen atoms to form water.

It is sometimes stated that gravity is distinguished from all
other forces by the fact of its resisting conversion into other
forms of force. Chemical affinity, it is said, can be converted
into heat and light, and these again into magnetism and
electricity: but gravity refuses to be so converted; being a
force maintaining itself under all circumstances, and not capable
of disappearing to give place to another. The statement arises
from vagueness of thought. If by it be meant that a particle of
matter can never be deprived of its weight, the assertion is
correct; but the law which affirms the convertibility of natural
forces was never intended, in the minds of those who understood
it, to affirm that such a conversion as that here implied occurs
in any case whatever. As regards convertibility into heat,
gravity and chemical affinity stand on precisely the same
footing. The attraction in the one case is as indestructible as
in the other. Nobody affirms that when a stone rests upon the
surface of the earth, the mutual attraction of the earth and
stone is abolished; nobody means to affirm that the mutual
attraction of oxygen for hydrogen ceases, after the atoms have
combined to form water. What is meant, in the case of chemical
affinity, is, that the pull of that affinity, acting through a
certain space, imparts a motion of translation of the one atom
towards the other. This motion is not heat, nor is the force that
produces it heat. But when the atoms strike and recoil, the
motion of translation is converted into a motion of vibration,
which is heat. The vibration, however, so far from causing the
extinction of the original attraction, is in part carried on by
that attraction. The atoms recoil, in virtue of the elastic force
which opposes actual contact, and in the recoil they are driven
too far back. The original attraction then triumphs over the
force of recoil, and urges the atoms once more together. Thus,
like a pendulum, they oscillate, until their motion is imparted
to the surrounding aether; or, in other words, until their heat
becomes radiant heat.

In this sense, and in this sense only, is chemical affinity
converted into heat. There is, first of all, the attraction
between the atoms; there is, secondly, space between them.
Across this space the attraction urges them. They collide, they
recoil, they oscillate. There is here a change in the form of the
motion, but there is no real loss. It is so with the attraction
of gravity. To produce motion by gravity space must also
intervene between the attracting bodies. When they strike
together motion is apparently destroyed, but in reality there is
no destruction. Their atoms are suddenly urged together by the
shock; by their own perfect elasticity these atoms recoil; and
thus is set up the molecular oscillation which, when communicated
to the proper nerves, announces itself as heat.

It was formerly universally supposed that by the collision of
unelastic bodies force was destroyed. Men saw, for example, that
when two spheres of clay, painter’s putty, or lead for example,
were urged together, the motion possessed by the masses, prior to
impact, was more or less annihilated. They believed in an
absolute destruction of the force of impact. Until recent times,
indeed, no difficulty was experienced in believing this, whereas,
at present, the ideas of force and its destruction refuse to be
united in most philosophic minds. In the collision of elastic
bodies, on the contrary, it was observed that the motion with
which they clashed together was in great part restored by the
resiliency of the masses, the more perfect the elasticity the
more complete being the restitution. This led to the idea of
perfectly elastic bodies — bodies competent to restore by
their recoil the whole of the motion which they possessed before
impact — and this again to the idea of the
conservation of force, as opposed to that destruction of
force which was supposed to occur when unelastic bodies met in
collision.

We now know that the principle of conservation holds equally
good with elastic and unelastic bodies. Perfectly elastic bodies
would develop no heat on collision. They would retain their
motion afterwards, though its direction might be changed; and it
is only when sensible motion is wholly or partly destroyed, that
heat is generated. This always occurs in unelastic collision, the
heat developed being the exact equivalent of the sensible motion
extinguished. This heat virtually declares that the property of
elasticity, denied to the masses, exists among their atoms; by
the recoil and oscillation of which the principle of conservation
is vindicated.

But ambiguity in the use of the term ‘force’ makes itself more
and more felt as we proceed. We have called the attraction of
gravity a force, without any reference to motion. A body resting
on a shelf is as much pulled by gravity as when, after having
been pushed off the shelf, it falls towards the earth. We applied
the term force also to that molecular attraction which we called
chemical affinity. When, however, we spoke of the conservation of
force, in the case of elastic collision, we meant neither a pull
nor a push, which, as just indicated, might be exerted upon inert
matter, but we meant force invested in motion — the vis
viva, as it is called, of the colliding masses.

Force in this form has a definite mechanical measure, in the
amount of work that it can perform. The simplest form of work is
the raising of a weight. A man walking up-hill, or up-stairs,
with a pound weight in his hand, to an elevation say of sixteen
feet, performs a certain amount of work, over and above the
lifting of his own body. If he carries the pound to a height of
thirty-two feet, he does twice the work; if to a height of
forty-eight feet, he does three times the work; if to sixty-four
feet, he does four times the work, and so on. If, moreover, he
carries up two pounds instead of one, other things being equal,
he does twice the work; if three, four, or five pounds, he does
three, four, or five times the work. In fact, it is plain that
the work performed depends on two factors, the weight raised and
the height to which it is raised. It is expressed by the product
of these two factors.

But a body may be caused to reach a certain elevation in
opposition to the force of gravity, without being actually
carried up. If a hodman, for example, wished to land a brick at
an elevation of sixteen feet above the place where he stood, he
would probably pitch it up to the bricklayer. He would thus
impart, by a sudden effort, a velocity to the brick sufficient to
raise it to the required height; the work accomplished by that
effort being precisely the same as if he had slowly carried up
the brick. The initial velocity to be imparted, in this case, is
well known. To reach a height of sixteen feet, the brick must
quit the man’s hand with a velocity of thirty-two feet a second.
It is needless to say, that a body starting with any velocity,
would, if wholly unopposed or unaided, continue to move for ever
with the same velocity. But when, as in the case before us, the
body is thrown upwards, it moves in opposition to gravity, which
incessantly retards its motion, and finally brings it to rest at
an elevation of sixteen feet. If not here caught by the
bricklayer, it would return to the hodman with an accelerated
motion, and reach his hand with the precise velocity it possessed
on quitting it.

An important relation between velocity and work is here to be
pointed out. Supposing the hodman competent to impart to the
brick, at starting, a velocity of sixty-four feet a second, or
twice its former velocity, would the amount of work performed be
twice what it was in the first instance? No; it would be four
times that quantity; for a body starting with twice the velocity
of another, will rise to four times the height. In like manner, a
three-fold velocity will give a nine-fold elevation, a four-fold
velocity will give a sixteen-fold elevation, and so on. The
height attained, then, is not proportional to the initial
velocity, but to the square of the velocity. As before, the work
is also proportional to the weight elevated. Hence the work which
any moving mass whatever is competent to perform, in virtue of
the motion which it at any moment possesses, is jointly
proportional to its weight and the square of its velocity.
Here, then, we have a second measure of work, in which we simply
translate the idea of height into its equivalent idea of
motion.

In mechanics, the product of the mass of a moving body into
the square of its velocity, expresses what is called the
vis viva, or living force. It is also sometimes
called the ‘mechanical effect.’ If, for example, a cannon pointed
to the zenith urge a ball upwards with twice the velocity
imparted to a second ball, the former will rise to four times the
height attained by the latter. If directed against a target, it
will also do four times the execution. Hence the importance of
imparting a high velocity to projectiles in war. Having thus
cleared our way to a perfectly definite conception of the
vis viva of moving masses, we are prepared for the
announcement that the heat generated by the shock of a falling
body against the earth is proportional to the vis
viva annihilated. The heat is proportional to the square
of the velocity. In the case, therefore, of two cannon-balls of
equal weight, if one strike a target with twice the velocity of
the other, it will generate four times the heat, if with three
times the velocity, it will generate nine times the heat, and so
on.

Mr. Joule has shown that a pound weight falling from a height
of 772 feet, or 772 pounds falling through one foot, will
generate by its collision with the earth an amount of heat
sufficient to raise a pound of water one degree Fahrenheit in
temperature. 772 “foot-pounds” constitute the mechanical
equivalent of heat. Now, a body falling from a height of 772
feet, has, upon striking the earth, a velocity of 223 feet a
second; and if this velocity were imparted to the body, by any
other means, the quantity of heat generated by the stoppage of
its motion would be that stated above. Six times that velocity,
or 1,338 feet, would not be an inordinate one for a cannon-ball
as it quits the gun. Hence, a cannon-ball moving with a velocity
of 1,338 feet a second, would, by collision, generate an amount
of heat competent to raise its own weight of water 36 degrees
Fahrenheit in temperature. If composed of iron, and if all the
heat generated were concentrated in the ball itself, its
temperature would be raised about 360 degrees Fahrenheit; because
one degree in the case of water is equivalent to about ten
degrees in the case of iron. In artillery practice, the heat
generated is usually concentrated upon the front of the bolt, and
on the portion of the target first struck. By this concentration
the heat developed becomes sufficiently intense to raise the dust
of the metal to incandescence, a flash of light often
accompanying collision with the target.

Let us now fix our attention for a moment on the gunpowder
which urges the cannon-ball. This is composed of combustible
matter, which if burnt in the open air would yield a certain
amount of heat. It will not yield this amount if it perform the
work of urging a ball. The heat then generated by the gunpowder
will fall short of that produced in the open air, by an amount
equivalent to the vis viva of the ball; and this
exact amount is restored by the ball on its collision with the
target. In this perfect way are heat and mechanical motion
connected.

Broadly enunciated, the principle of the conservation of force
asserts, that the quantity of force in the universe is as
unalterable as the quantity of matter; that it is alike
impossible to create force and to annihilate it. But in what
sense are we to understand this assertion? It would be manifestly
inapplicable to the force of gravity as defined by Newton; for
this is a force varying inversely as the square of the distance;
and to affirm the constancy of a varying force would be
self-contradictory. Yet, when the question is properly
understood, gravity forms no exception to the law of
conservation. Following the method pursued by Helmholtz, I will
here attempt an elementary exposition of this law. Though
destined in its applications to produce momentous changes in
human thought, it is not difficult of comprehension.

For the sake of simplicity we will consider a particle of
matter, which we may call F, to be perfectly fixed, and a second
movable particle, D, placed at a distance from F. We will assume
that these two particles attract each other according to the
Newtonian law. At a certain distance, the attraction is of a
certain definite amount, which might be determined by means of a
spring balance. At half this distance the attraction would be
augmented four times; at a third of the distance, nine times; at
one-fourth of the distance, sixteen times, and so on. In every
case, the attraction might be measured by determining, with the
spring balance, the amount of tension just sufficient to prevent
D from moving towards F. Thus far we have nothing whatever to do
with motion; we deal with statics, not with dynamics. We simply
take into account the distance of D from F, and the pull exerted
by gravity at that distance.

It is customary in mechanics to represent the magnitude of a
force by a line of a certain length, a force of double magnitude
being represented by a line of double length, and so on. Placing
then the particle D at a distance from F, we can, in imagination,
draw a straight line from D to F, and at D erect a perpendicular
to this line, which shall represent the amount of the attraction
exerted on D. If D be at a very great distance from F, the
attraction will be very small, and the perpendicular consequently
very short. If the distance be practically infinite, the
attraction is practically nil. Let us now suppose at every point
in the line joining F and D a perpendicular to be erected,
proportional in length to the attraction exerted at that point;
we thus obtain an infinite number of perpendiculars, of gradually
increasing length, as D approaches F. Uniting the ends of all
these perpendiculars, we obtain a curve, and between this curve
and the straight line joining F and D we have an area containing
all the perpendiculars placed side by side. Each one of this
infinite series of perpendiculars representing an attraction, or
tension, as it is sometimes called, the area just referred to
represents the sum of the tensions exerted upon the particle D,
during its passage from its first position to F.

Up to the present point we have been dealing with tensions,
not with motion. Thus far vis viva has been
entirely foreign to our contemplation of D and F. Let us now
suppose D placed at a practically infinite distance from F; here,
as stated, the pull of gravity would be infinitely small, and the
perpendicular representing it would dwindle almost to a point. In
this position the sum of the tensions capable of being exerted on
D would be a maximum. Let D now begin to move in obedience to the
infinitesimal attraction exerted upon it. Motion being once set
up, the idea of vis viva arises. In moving towards
F the particle D consumes, as it were, the tensions. Let us fix
our attention on D, at any point of the path over which it is
moving. Between that point and F there is a quantity of unused
tensions; beyond that point the tensions have been all consumed,
but we have in their place an equivalent quantity of vis
viva. After D has passed any point, the tension previously
in store at that point disappears, but not without having added,
during the infinitely small duration of its action, a due amount
of motion to that previously possessed by D. The nearer D
approaches to F, the smaller is the sum of the tensions
remaining, but the greater is the vis viva; the
farther D is from F, the greater is the sum of the unconsumed
tensions, and the less is the living force. Now the principle of
conservation affirms not the constancy of the value of the
tensions of gravity, nor yet the constancy of the vis
viva, taken separately, but the absolute constancy of the
value of both taken together. At the beginning the vis
viva was zero, and the tension area was a maximum; close
to F the vis viva is a maximum, while the tension
area is zero. At every other point, the work-producing power of
the particle D consists in part of vis viva, and in
part of tensions.

If gravity, instead of being attraction, were repulsion, then,
with the particles in contact, the sum of the tensions between D
and F would be a maximum, and the vis viva zero.
If, in obedience to the repulsion, D moved away from F,
vis viva would be generated; and the farther D
retreated from F the greater would be its vis viva,
and the less the amount of tension still available for producing
motion. Taking repulsion as well as attraction into account, the
principle of the conservation of force affirms that the
mechanical value of the tensions and vires vivae of
the material universe, so far as we know it, is a constant
quantity. The universe, in short, possesses two kinds of property
which are mutually convertible. The diminution of either carries
with it the enhancement of the other, the total value of the
property remaining unchanged.

The considerations here applied to gravity apply equally to
chemical affinity. Ina mixture of oxygen and hydrogen the atoms
exist apart, but by the application of proper means they may be
caused to rush together across that space that separates them.
While this space exists, and as long as the atoms have not begun
to move towards each other, we have tensions and nothing else.
During their motion towards each other the tensions, as in the
case of gravity, are converted into vis viva. After
they clash we have still vis viva, but in another
form. It was translation, it is vibration. It was molecular
transfer, it is heat.

It is possible to reverse these processes, to unlock the
combined atoms and replace them in their first positions. But, to
accomplish this, as much heat would be required as was generated
by their union. Such reversals occur daily and hourly in nature.
By the solar waves, the oxygen of water is divorced from its
hydrogen in the leaves of plants. As molecular vis
viva the waves disappear, but in so doing they re-endow
the atoms of oxygen and hydrogen with tension. The atoms are thus
enabled to recombine, and when they do so they restore the
precise amount of heat consumed in their separation. The same
remarks apply to the compound of carbon and oxygen, called
carbonic acid, which is exhaled from our lungs, produced by our
fires, and found sparingly diffused everywhere throughout the
air. In the leaves of plants the sunbeams also wrench the atoms
of carbonic acid asunder, and sacrifice themselves in the act;
but when the plants are burnt, the amount of heat consumed in
their production is restored.

This, then, is the rhythmic play of Nature as regards her
forces. Throughout all her regions she oscillates from tension to
vis viva, from vis viva to tension.
We have the same play in the planetary system. The earth’s orbit
is an ellipse, one of the foci of which is occupied by the sun.
Imagine the earth at the most distant part of the orbit. Her
motion, and consequently her vis viva, is then a
minimum. The planet rounds the curve, and begins its approach to
the sun. In front it has a store of tensions, which are gradually
consumed, an equivalent amount of vis viva being
generated. When nearest to the sun the motion, and consequently
the vis viva, reach a maximum. But here the
available tensions have been used up. The earth rounds this
portion of the curve and retreats from the sun. Tensions are now
stored up, but vis viva is lost, to be again
restored at the expense of the complementary force on the
opposite side of the curve. Thus beats the heart of the universe,
but without increase or diminution of its total stock of
force.

I have thus far tried to steer clear amid confusion, by fixing
the mind of the reader upon things rather than upon names. But
good names are essential; and here, as yet, we are not provided
with such. We have had the force of gravity and living force
— two utterly distinct things. We have had pulls and
tensions; and we might have had the force of heat, the force of
light, the force of magnetism, or the force of electricity
— all of which terms have been employed more or less
loosely by writers on physics. This confusion is happily avoided
by the introduction of the term ‘energy,’ which embraces both
tension and vis viva. Energy is possessed by
bodies already in motion; it is then actual, and we agree to call
it actual or dynamic energy. It is our old
vis viva. On the other hand, energy is possible to
bodies not in motion, but which, in virtue of attraction or
repulsion, possess a power of motion which would realise itself
if all hindrances were removed. Looking, for example, at gravity;
a body on the earth’s surface in a position from which it cannot
fall to a lower one possesses no energy. It has neither motion
nor power of motion. But the same body suspended at a height
above the earth has a power of motion, though it may not have
exercised it. Energy is possible to such a body, and we agree to
call this potential energy. It consists of our old
tensions. We, moreover, speak of the conservation of energy,
instead of the conservation of force; and say that the sum of the
potential and dynamic energies of the material universe is a
constant quantity.

A body cast upwards consumes the actual energy of projection,
and lays up potential energy. When it reaches its utmost height
all its actual energy is consumed, its potential energy being
then a maximum. When it returns, there is a reconversion of the
potential into the actual. A pendulum at the limit of its swing
possesses potential energy; at the lowest point of its arc its
energy is all actual. A patch of snow resting on a mountain slope
has potential energy; loosened, and shooting down as an
avalanche, it possesses dynamic energy. The pine-trees growing on
the Alps have potential energy; but rushing down the Holzrinne of
the woodcutters they possess actual energy. The same is true of
the mountains themselves. As long as the rocks which compose them
can fall to a lower level, they possess potential energy, which
is converted into actual when the frost ruptures their cohesion
and hands them over to the action of gravity. The stone
avalanches of the Matterhorn and Weisshorn are illustrations in
point. The hammer of the great bell of Westminster, when raised
before striking, possesses potential energy; when it falls, the
energy becomes dynamic; and after the stroke, we have the
rhythmic play of potential and dynamic in the vibrations of the
bell. The same holds good for the molecular oscillations of a
heated body. An atom is driven against its neighbour, and
recoils. The ultimate amplitude of the recoil being attained, the
motion of the atom in that direction is checked, and for an
instant its energy is all potential. It is then drawn towards its
neighbour with accelerated speed; thus, by attraction, converting
its potential into dynamic energy. Its motion in this direction
is also finally checked, and again, for an instant, its energy is
all potential. It once more retreats, converting, by repulsion,
its potential into dynamic energy, till the latter attains a
maximum, after which it is again changed into potential energy.
Thus, what is true of the earth, as she swings to and fro in her
yearly journey round the sun, is also true of her minutest atom.
We have wheels within wheels, and rhythm within rhythm.

When a body is heated, a change of molecular arrangement
always occurs, and to produce this change heat is consumed.
Hence, a portion only of the heat communicated to the body
remains as dynamic energy. Looking back on some of the statements
made at the beginning of this article, now that our knowledge is
more extensive, we see the necessity of qualifying them. When,
for example, two bodies clash, heat is generated; but the heat,
or molecular dynamic energy, developed at the moment of
collision, is not the exact equivalent of the sensible dynamic
energy destroyed. The true equivalent is this heat, plus the
potential energy conferred upon the molecules by the placing of
greater distances between them. This molecular potential energy
is afterwards, on the cooling of the body, converted into
heat.

Wherever two atoms capable of uniting together by their mutual
attractions exist separately, they form a store of potential
energy. Thus our woods, forests, and coal-fields on the one hand,
and our atmospheric oxygen on the other, constitute a vast store
of energy of this kind — vast, but far from infinite. We
have, besides our coal-fields, metallic bodies more or less
sparsely distributed through the earth’s crust. These bodies can
be oxydised; and hence they are, so far as they go, stores of
energy. But the attractions of the great mass of the earth’s
crust are already satisfied, and from them no further energy can
possibly be obtained. Ages ago the elementary constituents of our
rocks clashed together and produced the motion of heat, which was
taken up by the aether and carried away through stellar space. It
is lost for ever as far as we are concerned. In those ages the
hot conflict of carbon, oxygen, and calcium produced the chalk
and limestone hills which are now cold; and from this carbon,
oxygen, and calcium no further energy can be derived. So it is
with almost all the other constituents of the earth’s crust. They
took their present form in obedience to molecular force; they
turned their potential energy into dynamic, and yielded it as
radiant heat to the universe, ages before man appeared upon this
planet. For him a residue of potential energy remains, vast,
truly, in relation to the life and wants of an individual, but
exceedingly minute in comparison with the earth’s primitive
store.

To sum up. The whole stock of energy or working-power in the
world consists of attractions, repulsions, and motions. If the
attractions and repulsions be so circumstanced as to be able to
produce motion, they are sources of working-power, but not
otherwise. As stated a moment ago, the attraction exerted between
the earth and a body at a distance from the earth’s surface, is a
source of working-power; because the body can be moved by the
attraction, and in falling can perform work. When it rests at its
lowest level it is not a source of power or energy, because it
can fall no farther. But though it has ceased to be a source of
energy, the attraction of gravity still acts as a force, which
holds the earth and weight together.

The same remarks apply to attracting atoms and molecules. As
long as distance separates them, they can move across it in
obedience to the attraction; and the motion thus produced may, by
proper appliances, be caused to perform mechanical work. When,
for example, two atoms of hydrogen unite with one of oxygen, to
form water, the atoms are first drawn towards each other —
they move, they clash, and then by virtue of their resiliency,
they recoil and quiver. To this quivering motion we give the name
of heat. This atomic vibration is merely the redistribution of
the motion produced by the chemical affinity; and this is the
only sense in which chemical affinity can be said to be converted
into heat. We must not imagine the chemical attraction destroyed,
or converted into anything else. For the atoms, when mutually
clasped to form a molecule of water, are held together by the
very attraction which first drew them towards each other. That
which has really been expended is the pull exerted through the
space by which the distance between the atoms has been
diminished.

If this be understood, it will be at once seen that gravity,
as before insisted on, may, in this sense, be said to be
convertible into heat; that it is in reality no more an
outstanding and inconvertible agent, as it is sometimes stated to
be, than is chemical affinity. By the exertion of a certain pull
through a certain space, a body is caused to clash with a certain
definite velocity against the earth. Heat is thereby developed,
and this is the only sense in which gravity can be said to be
converted into heat. In no case is the force, which
produces the motion annihilated or changed into anything else.
The mutual attraction of the earth and weight exists when they
are in contact, as when they were separate but the ability of
that attraction to employ itself in the production of motion does
not exist.

The transformation, in this case, is easily followed by the
mind’s eye. First, the weight as a whole is set in motion by the
attraction of gravity. This motion of the mass is arrested by
collision with the earth, being broken up into molecular tremors,
to which we give the name of heat.

And when we reverse the process, and employ those tremors of
heat to raise a weight — which is done through the
intermediation of an elastic fluid in the steam-engine — a
certain definite portion of the molecular motion is consumed. In
this sense, and in this sense only, can the heat be said to be
converted into gravity; or, more correctly, into potential energy
of gravity. Here the destruction of the heat has created no new
attraction; but the old attraction has conferred upon it a power
of exerting a certain definite pull, between the starting-point
of the falling weight and the earth.

When, therefore, writers on the conservation of energy speak
of tensions being ‘consumed’ and ‘generated,’ they do not mean
thereby that old attractions have been annihilated, and new ones
brought into existence, but that, in the one case, the power of
the attraction to produce motion has been diminished by the
shortening of the distance between the attracting bodies, while,
in the other case, the power of producing motion has been
augmented by the increase of the distance. These remarks apply to
all bodies, whether they be sensible masses or molecules.

Of the inner quality that enables matter to attract matter we
know nothing; and the law of conservation makes no statement
regarding that quality. It takes the facts of attraction as they
stand, and affirms only the constancy of working-power. That
power may exist in the form of MOTION; or it may exist in the
form of FORCE, with distance to act through. The former is
dynamic energy, the latter is potential energy, the constancy of
the sum of both being affirmed by the law of conservation. The
convertibility of natural forces consists solely in
transformations of dynamic into potential, and of potential into
dynamic energy. In no other sense has the convertibility of force
any scientific meaning.

.

Grave errors have been entertained as to what is really
intended to be conserved by the doctrine of conservation. This
exposition I hope will tend to remove them.

.

.

.

.

.

.

II. RADIATION.

[Footnote: The Rede Lecture
delivered in the Senate House before the University of Cambridge,
May 16, 1865.]

1. Visible and Invisible
Radiation.

BETWEEN the mind of man and the
outer world are interposed the nerves of the human body, which
translate, or enable the mind to translate, the impressions of
that world into facts of consciousness and thought.

Different nerves are suited to the perception of different
impressions. We do not see with the ear, nor hear with the eye,
nor are we rendered sensible of sound by the nerves of the
tongue. Out of the general assemblage of physical actions, each
nerve, or group of nerves, selects and responds to those for the
perception of which it is specially organised.

The optic nerve passes from the brain to the back of the
eyeball and there spreads out, to form the retina, a web of nerve
filaments, on which the images of external objects are projected
by the optical portion of the eye. This nerve is limited to the
apprehension of the phenomena of radiation, and, notwithstanding
its marvellous sensibility to certain impressions of this class,
it is singularly obtuse to other impressions.

Nor does the optic nerve embrace the entire range even of
radiation. Some rays, when they reach it, are incompetent to
evoke its power, while others never reach it at all, being
absorbed by the humours of the eye. To all rays which, whether
they reach the retina or not, fail to excite vision, we give the
name of invisible or obscure rays. All non-luminous bodies emit
such rays. There is no body in nature absolutely cold, and every
body not absolutely cold emits rays of heat. But to render
radiant heat fit to affect the optic nerve a certain temperature
is necessary. A cool poker thrust into a fire remains dark for a
time, but when its temperature has become equal to that of the
surrounding coals, it glows like them. In like manner, if a
current of electricity, of gradually increasing strength, be sent
through a wire of the refractory metal platinum, the wire first
becomes sensibly warm to the touch; for a time its heat augments,
still however remaining obscure; at length we can no longer touch
the metal with impunity; and at a certain definite temperature it
emits a feeble red light. As the current augments in power the
light augments in brilliancy, until finally the wire appears of a
dazzling white. The light which it now emits is similar to that
of the sun.

By means of a prism Sir Isaac Newton unravelled the texture of
solar light, and by the same simple instrument we can investigate
the luminous changes of our platinum wire. In passing through the
prism all its rays (and they are infinite in variety) are bent or
refracted from their straight course; and, as different rays are
differently refracted by the prism, we are by it enabled to
separate one class of rays from another. By such prismatic
analysis Dr. Draper has shown, that when the platinum wire first
begins to glow, the light emitted is sensibly red. As the glow
augments the red becomes more brilliant, but at the same time
orange rays are added to the emission. Augmenting the temperature
still further, yellow rays appear beside the orange; after the
yellow, green rays are emitted; and after the green come, in
succession, blue, indigo, and violet rays. To display all these
colours at the same time the platinum wire must be
white-hot: the impression of whiteness being in fact
produced by the simultaneous action of all these colours on the
optic nerve.

In the experiment just described we began with a platinum wire
at an ordinary temperature, and gradually raised it to a white
heat. At the beginning, and even before the electric current had
acted at all upon the wire, it emitted invisible rays. For some
time after the action of the current had commenced, and even for
a time after the wire had become intolerable to the touch, its
radiation was still invisible. The question now arises, What
becomes of these invisible rays when the visible ones make their
appearance? It will be proved in the sequel that they maintain
themselves in the radiation; that a ray once emitted continues to
be emitted when the temperature is increased, and hence the
emission from our platinum wire, even when it has attained its
maximum brilliancy, consists of a mixture of visible and
invisible rays. If, instead of the platinum wire, the earth
itself were raised to incandescence, the obscure radiation which
it now emits would continue to be emitted. To reach incandescence
the planet would have to pass through all the stages of
non-luminous radiation, and the final emission would embrace the
rays of all these stages. There can hardly be a doubt that from
the sun itself, rays proceed similar in kind to those which the
dark earth pours nightly into space. In fact, the various kind of
obscure rays emitted by all the planets of our system are
included in the present radiation of the sun.

The great pioneer in this domain of science was Sir William
Herschel. Causing a beam of solar light to pass through a prism,
he resolved it into its coloured constituents; he formed what is
technically called the solar spectrum. Exposing thermometers to
the successive colours he determined their heating power, and
found it to augment from the violet or most refracted end, to the
red or least refracted end of the spectrum. But he did not stop
here. Pushing his thermometers into the dark space beyond the red
he found that, though the light had disappeared, the radiant heat
falling on the instruments was more intense than that at any
visible part of the spectrum. In fact, Sir William Herschel
showed, and his results have been verified by various
philosophers since his time, that, besides its luminous rays, the
sun pours forth a multitude of other rays, more powerfully
calorific than the luminous ones, but entirely unsuited to the
purposes of vision.

At the less refrangible end of the solar spectrum, then, the
range of the sun’s radiation is not limited by that of the eye.
The same statement applies to the more refrangible end. Ritter
discovered the extension of the spectrum into the invisible
region beyond the violet; and, in recent times, this ultra-violet
emission has had peculiar interest conferred upon it by the
admirable researches of Professor Stokes. The complete spectrum
of the sun consists, therefore, of three distinct parts :—
first, of ultra-red rays of high heating power, but unsuited to
the purposes of vision; secondly, of luminous rays which display
the succession of colours, red, orange, yellow, green, blue,
indigo, violet; thirdly, of ultra-violet rays which, like the
ultra-red ones, are incompetent to excite vision, but which,
unlike the ultra-red rays, possess a very feeble heating power.
In consequence, however, of their chemical energy these
ultra-violet rays are of the utmost importance to the organic
world.

.

.

2. Origin and Character of
Radiation. The Aether.

When we see a platinum wire
raised gradually to a white heat, and emitting in succession all
the colours of the spectrum, we are simply conscious of a series
of changes in the condition of our own eyes. We do not see the
actions in which these successive colours originate, but the mind
irresistibly infers that the appearance of the colours
corresponds to certain contemporaneous changes in the wire. What
is the nature of these changes? In virtue of what condition does
the wire radiate at all? We must now look from the wire, as a
whole, to its constituent atoms. Could we see those atoms, even
before the electric current has begun to act upon them, we should
find them in a state of vibration. In this vibration, indeed,
consists such warmth as the wire then possesses. Locke enunciated
this idea with great precision, and it has been placed beyond the
pale of doubt by the excellent quantitative researches of Mr.
Joule. ‘ Heat,’ says Locke, ‘is a very brisk agitation of
the insensible parts of the object, which produce in us that
sensation from which we denominate the object hot: so what in our
sensations is heat in the object is nothing but
motion.’ When the electric current, still feeble, begins
to pass through the wire, its first act is to intensify the
vibrations already existing, by causing the atoms to swing
through wider ranges. Technically speaking, the amplitudes
of the oscillations are increased. The current does this,
however, without altering the periods of the old vibrations, or
the times in which they were executed. But besides intensifying
the old vibrations the current generates new and more rapid ones,
and when a certain definite rapidity has been attained, the wire
begins to glow. The colour first exhibited is red, which
corresponds to the lowest rate of vibration of which the eye is
able to take cognisance. By augmenting the strength of the
electric current more rapid vibrations are introduced, and orange
rays appear. A quicker rate of vibration produces yellow, a still
quicker green; and by further augmenting the rapidity, we pass
through blue, indigo, and violet, to the extreme ultra-violet
rays.

Such are the changes recognised by the mind in the wire
itself, as concurrent with the visual changes taking place in
the eye. But what connects the wire with this organ? By what means
does it send such intelligence of its varying condition to the
optic nerve? Heat being as defined by Locke, ‘a very brisk
agitation of the insensible parts of an object,’ it is readily
conceivable that on touching a heated body the agitation may
communicate itself to the adjacent nerves, and announce itself to
them as light or heat. But the optic nerve does not touch the hot
platinum, and hence the pertinence of the question, By what
agency are the vibrations of the wire transmitted to the eye?

The answer to this question involves one of the most important
physical conceptions that the mind of man has yet achieved: the
conception of a medium filling space and fitted mechanically for
the transmission of the vibrations of light and heat, as air is
fitted for the transmission of sound. This medium is called the
luminiferous aether. Every vibration of every atom of our
platinum wire raises in this aether a wave, which speeds through
it at the rate of 186,000 miles a second.

The aether suffers no rupture of continuity at the surface of
the eye, the inter-molecular spaces of the various humours are
filled with it; hence the waves generated by the glowing platinum
can cross these humours and impinge on the optic nerve at the
back of the eye. [Footnote: The action here described is
analogous to the passage of sound-waves through thick felt whose
interstices are occupied by air.] Thus the sensation of light
reduces itself to the acceptance of motion. Up to this point we
deal with pure mechanics; but the subsequent translation of the
shock of the aethereal waves into consciousness eludes mechanical
science. As an oar dipping into the Cam generates systems of
waves, which, speeding from the centre of disturbance, finally
stir the sedges on the river’s bank, so do the vibrating atoms
generate in the surrounding aether undulations, which finally
stir the filaments of the retina. The motion thus imparted is
transmitted with measurable, and not very great velocity to the
brain, where, by a process which the science of mechanics does
not even tend to unravel, the tremor of the nervous matter is
converted into the conscious impression of light.

Darkness might then be defined as aether at rest; light as
aether in motion. But in reality the aether is never at rest, for
in the absence of light-waves we have heat-waves always speeding
through it. In the spaces of the universe both classes of
undulations incessantly commingle. Here the waves issuing from
uncounted centres cross, coincide, oppose, and pass through each
other, without confusion or ultimate extinction. Every star is
seen across the entanglement of wave-motions produced by all
other stars. It is the ceaseless thrill caused by those distant
orbs collectively in the aether, that constitutes what we call
the ‘temperature of space.’ As the air of a room accommodates
itself to the requirements of an orchestra, transmitting each
vibration of every pipe and string, so does the inter-stellar
aether accommodate itself to the requirements of light and heat.
Its waves mingle in space without disorder, each being endowed
with an individuality as indestructible as if it alone had
disturbed the universal repose.

All vagueness with regard to the use of the terms ‘radiation’
and ‘absorption’ will now disappear. Radiation is the
communication of vibratory motion to the aether; and when a body
is said to be chilled by radiation, as for example the grass of a
meadow on a starlight night, the meaning is, that the molecules
of the grass have lost a portion of their motion, by imparting it
to the medium in which they vibrate. On the other hand, the waves
of aether may so strike against the molecules of a body exposed
to their action as to yield up their motion to the latter; and in
this transfer of the motion from the aether to the molecules
consists the absorption of radiant heat. All the phenomena of
heat are in this way reducible to interchanges of motion; and it
is purely as the recipients or the donors of this motion, that we
ourselves become conscious of the action of heat and cold.

.

.

3. The Atomic Theory in reference to
the Aether.

The word ‘atoms’ has been more
than once employed in this discourse. Chemists have taught us
that all matter is reducible to certain elementary forms to which
they give this name. These atoms are endowed with powers of
mutual attraction, and under suitable circumstances they coalesce
to form compounds. Thus oxygen and hydrogen are elements when
separate, or merely mixed, but they may be made to
combine so as to form molecules, each consisting of two
atoms of hydrogen and one of oxygen. In this condition they
constitute water. So also chlorine and sodium are elements, the
former a pungent gas, the latter a soft metal; and they unite
together to form chloride of sodium or common salt. In the same
way the element nitrogen combines with hydrogen, in the
proportion of one atom of the former to three of the latter, to
form ammonia. Picturing in imagination the atoms of elementary
bodies as little spheres, the molecules of compound bodies must
be pictured as groups of such spheres. This is the atomic theory
as Dalton conceived it. Now if this theory have any foundation in
fact, and if the theory of an aether pervading space, and
constituting the vehicle of atomic motion, be founded in fact, it
is surely of interest to examine whether the vibrations of
elementary bodies are modified by the act of combination —
whether as regards radiation and absorption, or, in other words,
whether as regards the communication of motion to the aether, and
the acceptance of motion from it, the deportment of the
uncombined atoms will be different from that of the
combined.

.

.

.

4. Absorption of Radiant Heat by
Gases.

We have now to submit these
considerations to the only test by which they can be tried,
namely, that of experiment. An experiment is well defined as a
question put to Nature; but, to avoid the risk of asking amiss,
we ought to purify the question from all adjuncts which do not
necessarily belong to it. Matter has been shown to be composed of
elementary constituents, by the compounding of which all its
varieties are produced. But, besides the chemical unions which
they form, both elementary and compound bodies can unite in
another and less intimate way. Gases and vapours aggregate to
liquids and solids, without any change of their chemical nature.
We do not yet know how the transmission of radiant heat may be
affected by the entanglement due to cohesion; and, as our object
now is to examine the influence of chemical union alone, we shall
render our experiments more pure by liberating the atoms and
molecules entirely from the bonds of cohesion, and employing them
in the gaseous or vaporous form.

Let us endeavour to obtain a perfectly clear mental image of
the problem now before us. Limiting in the first place our
enquiries to the phenomena of absorption, we have to picture a
succession of waves issuing from a radiant source and passing
through a gas; some of them striking against the gaseous
molecules and yielding up their motion to the latter; others
gliding round the molecules, or passing through the
intermolecular spaces without apparent hindrance. The problem
before us is to determine whether such free molecules have any
power whatever to stop the waves of heat; and if so, whether
different molecules possess this power in different degrees.

In examining the problem let us fall back upon an actual piece
of work, choosing as the source of our heat-waves a plate of
copper, against the back of which a steady sheet of flame is
permitted to play. On emerging from the copper, the waves, in the
first instance, pass through a space devoid of air, and then
enter a hollow glass cylinder, three feet long and three inches
wide. The two ends of this cylinder are stopped by two plates of
rock-salt, a solid substance which offers a scarcely sensible
obstacle to the passage of the calorific waves. After passing
through the tube, the radiant heat falls upon the anterior face
of a thermo-electric pile, [Footnote: In the Appendix to
the first chapter of ‘Heat as a Mode of Motion,’ the
construction of the thermo-electric pile is fully explained.]
which instantly converts the heat into an electric current. This
current conducted round a magnetic needle deflects it, and the
magnitude of the deflection is a measure of the heat falling upon
the pile. This famous instrument, and not an ordinary
thermometer, is what we shall use in these enquiries, but we
shall use it in a somewhat novel way. As long as the two opposite
faces of the thermo-electric pile are kept at the same
temperature, no matter how high that may be, there is no current
generated. The current is a consequence of a difference of
temperature between the two opposite faces of the pile. Hence, if
after the anterior face has received the heat from our radiating
source, a second source, which we may call the compensating
source, be permitted to radiate against the posterior face, this
latter radiation will tend to neutralise the former. When the
neutralisation is perfect, the magnetic needle connected with the
pile is no longer deflected, but points to the zero of the
graduated circle over which it hangs.

And now let us suppose the glass tube, through which the waves
from the heated plate of copper are passing, to be exhausted by
an air-pump, the two sources of heat acting at the same time on
the two opposite faces of the pile. When by means of an adjusting
screen, perfectly equal quantities of heat are imparted to the
two faces, the needle points to zero. Let any gas be now
permitted to enter the exhausted tube; if its molecules possess
any power of intercepting the calorific waves, the equilibrium
previously existing will be destroyed, the compensating source
will triumph, and a deflection of the magnetic needle will be the
immediate consequence. From the deflections thus produced by
different gases, we can readily deduce the relative amounts of
wave-motion which their molecules intercept.

In this way the substances mentioned in the following table
were examined, a small portion only of each being admitted into
the glass tube. The quantity admitted in each case was just
sufficient to depress a column of mercury associated with the
tube one inch: in other words, the gases were examined at a
pressure of one-thirtieth of an atmosphere. The numbers in the
table express the relative amounts of wave-motion absorbed by the
respective gases, the quantity intercepted by air being taken as
unity.

.

Every gas in this table is perfectly transparent to light,
that is to say, all waves within the limits of the visible
spectrum pass through it without obstruction; but for the waves
of slower period, emanating from our heated plate of copper,
enormous differences of absorptive power are manifested. These
differences illustrate in the most unexpected manner the
influence of chemical combination. Thus the elementary gases,
oxygen, hydrogen, and nitrogen, and the mixture atmospheric air,
prove to be practical vacua to the rays of heat; for every ray,
or, more strictly speaking, for every unit of wave-motion, which
any one of them intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while
sulphurous acid gas absorbs 6,480 units. What, becomes of the
wave-motion thus intercepted? It is applied to the heating of the
absorbing gas. Through air, oxygen, hydrogen, and nitrogen, the
waves of aether pass without absorption, and these gases are not
sensibly changed in temperature by the most powerful calorific
rays. The position of nitrous oxide in the foregoing table is
worthy of particular notice. In this gas we have the same atoms
in a state of chemical union, that exist uncombined in the
atmosphere; but the absorption of the compound is 1,800 times
that of air.

.

.

.

5. Formation of Invisible
Foci.

This extraordinary deportment
of the elementary gases naturally directed attention to
elementary bodies in other states of aggregation. Some of
Melloni’s results now attained a new significance. This
celebrated experimenter had found crystals of sulphur to be
highly pervious to radiant heat; he had also proved that
lamp-black, and black glass, (which owes its blackness to the
element carbon) were to a considerable extent transparent to
calorific rays of low refrangibility. These facts, harmonising so
strikingly with the deportment of the simple gases, suggested
further enquiry. Sulphur dissolved in bisulphide of carbon was
found almost perfectly diathermic. The dense and deeply-coloured
element bromine was examined, and found competent to cut off the
light of our most brilliant flames, while it transmitted the
invisible calorific rays with extreme freedom. Iodine, the
companion element of bromine, was next thought of, but it was
found impracticable to examine the substance in its usual solid
condition. It however dissolves freely in bisulphide of carbon.
There is no chemical union between the liquid and the iodine; it
is simply a case of solution, in which the uncombined atoms of
the element can act upon the radiant heat. When permitted to do
so, it was found that a layer of dissolved iodine, sufficiently
opaque to cut off the light of the midday sun, was almost
absolutely transparent to the invisible calorific rays.
[Footnote: Professor Dewar has recently succeeded in
producing a medium highly opaque to light, and highly transparent
to obscure heat, by fusing together sulphur and
iodine.]

By prismatic analysis Sir William Herschel separated the
luminous from the non-luminous rays of the sun, and he also
sought to render the obscure rays visible by concentration.
Intercepting the luminous portion of his spectrum he brought, by
a converging lens, the ultra-red rays to a focus, but by this
condensation he obtained no light. The solution of iodine offers
a means of filtering the solar beam, or failing it, the beam of
the electric lamp, which renders attainable far more powerful
foci of invisible rays than could possibly be obtained by the
method of Sir William Herschel. For to form his spectrum he was
obliged to operate upon solar light which had passed through a
narrow slit or through a small aperture, the amount of the
obscure heat being limited by this circumstance. But with our
opaque solution we may employ the entire surface of the largest
lens, and having thus converged the rays, luminous and
non-luminous, we can intercept the former by the iodine, and do
what we please with the latter. Experiments of this character,
not only with the iodine solution, but also with black glass and
layers of lampblack, were publicly performed at the Royal
Institution in the early part of 1862, and the effects at the
foci of invisible rays, then obtained, were such as had never
been witnessed previously.

In the experiments here referred to, glass lenses were
employed to concentrate the rays. But glass, though highly
transparent to the luminous, is in a high degree opaque to the
invisible, heat-rays of the electric lamp, and hence a large
portion of those rays was intercepted by the glass. The obvious
remedy here is to employ rock-salt lenses instead of glass ones,
or to abandon the use of lenses wholly, and to concentrate the
rays by a metallic mirror. Both of these improvements have been
introduced, and, as anticipated, the invisible foci have been
thereby rendered more intense. The mode of operating remains
however the same, in principle, as that made known in 1862. It
was then found that an instant’s exposure of the face of the
thermoelectric pile to the focus of invisible rays, dashed the
needles of a coarse galvanometer violently aside. It is now found
that on substituting for the face of the thermo-electric pile a
combustible body, the invisible rays are competent to set that
body on fire.

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6. Visible and Invisible Rays of the
Electric Light.

We have next to examine what
proportion the non-luminous rays of the electric light bear to
the luminous ones. This the opaque solution of iodine enables us
to do with an extremely close approximation to the
truth.

The pure bisulphide of carbon, which is the solvent of the
iodine, is perfectly transparent to the luminous, and almost
perfectly transparent to the dark, rays of the electric lamp.
Supposing the total radiation of the lamp to pass through the
transparent bisulphide, while through the solution of iodine only
the dark rays are transmitted. If we determine, by means of a
thermoelectric pile, the total radiation, and deduct from it the
purely obscure, we obtain the value of the purely luminous
emission. Experiments, performed in this way, prove that if all
the visible rays of the electric light were converged to a focus
of dazzling brilliancy, its heat would only be one-eighth of that
produced at the unseen focus of the invisible rays.

Exposing his thermometers to the successive colours of the
solar spectrum, Sir William Herschel determined the heating power
of each, and also that of the region beyond the extreme red. Then
drawing a straight line to represent the length of the spectrum,
he erected, at various points, perpendiculars to represent the
calorific intensity existing at those points. Uniting the ends of
all his perpendiculars, he obtained a curve which showed at a
glance the manner in which the heat was distributed in the solar
spectrum. Professor Müller of Freiburg, with improved
instruments, afterwards made similar experiments, and constructed
a more accurate diagram of the same kind. We have now to examine
the distribution of heat in the spectrum of the electric light;
and for this purpose we shall employ a particular form of the
thermo-electric pile, devised by Melloni. Its face is a
rectangle, which by means of movable side-pieces can be rendered
as narrow as desired. We can, for example, have the face of the
pile the tenth, the hundredth, or even the thousandth of an inch
in breadth. By means of an endless screw, this linear
thermo-electric pile may be moved through the entire spectrum,
from the violet to the red, the amount of heat falling upon the
pile at every point of its march, being declared by a magnetic
needle associated with the pile.

When this instrument is brought up to the violet end of the
spectrum of the electric light, the heat is found to be
insensible. As the pile is gradually moved from the violet end
towards the red, heat soon manifests itself, augmenting as we
approach the red. Of all the colours of the visible spectrum the
red possesses the highest heating power. On pushing the pile into
the dark region beyond the red, the heat, instead of vanishing,
rises suddenly and enormously in intensity, until at some
distance beyond the red it attains a maximum. Moving the pile
still forward, the thermal power falls, somewhat more rapidly
than it rose. It then gradually shades away, but, for a distance
beyond the red greater than the length of the whole visible
spectrum, signs of heat may be detected.

Drawing a datum line, and erecting along it perpendiculars,
proportional in length to the thermal intensity at the respective
points, we obtain the extraordinary curve, shown on the opposite
page, which exhibits the distribution of heat in the spectrum of
the electric light. In the region of dark rays, beyond the red,
the curve shoots up to B, in a steep and massive peak — a
kind of Matterhorn of heat, which dwarfs the portion of the
diagram C D E, representing the luminous radiation. Indeed the
idea forced upon the mind by this diagram is that the light rays
are a mere insignificant appendage to the heat-rays represented
by the area A B C D, thrown in as it were by nature for the
purpose of vision.

.

.

Figure 1. Spectrum of
Electric Light.

The diagram drawn by Professor Müller to represent the
distribution of heat in the solar spectrum is not by any means so
striking as that just described, and the reason, doubtless, is
that prior to reaching the earth the solar rays have to traverse
our atmosphere. By the aqueous vapour there diffused, the summit
of the peak representing the sun’s invisible radiation is cut
off. A similar lowering of the mountain of invisible heat is
observed when the rays from the electric light are permitted to
pass through a film of water, which acts upon them as the
atmospheric vapour acts upon the rays of the sun.

.

.

7. Combustion by Invisible
Rays.

The sun’s invisible rays far
transcend the visible ones in heating power, so that if the
alleged performances of Archimedes during the siege of Syracuse
had any foundation in fact, the dark solar rays would have been
the philosopher’s chief agents of combustion. On a small scale we
can readily produce, with the purely invisible rays of the
electric light, all that Archimedes is said to have performed
with the sun’s total radiation. Placing behind the electric light
a small concave mirror, the rays are converged, the cone of
reflected rays and their point of convergence being rendered
clearly visible by the dust always floating in the air. Placing
between the luminous focus and the source of rays our solution of
iodine, the light of the cone is entirely cut away; but the
intolerable heat experienced when the band is placed, even for a
moment, at the dark focus, shows that the calorific rays pass
unimpeded through the opaque solution.

Almost anything that ordinary fire can effect may be
accomplished at the focus of invisible rays; the air at
the focus remaining at the same time perfectly cold, on account
of its transparency to the heat-rays. An air thermometer, with a
hollow rack-salt bulb, would be unaffected by the heat of the
focus: there would be no expansion, and in the open air there is
no convection. The aether at the focus, and not the air, is the
substance in which the heat is embodied. A block of wood, placed
at the focus, absorbs the heat, and dense volumes of smoke rise
swiftly upwards, showing the manner in which the air itself would
rise, if the invisible rays were competent to heat it. At the
perfectly dark focus dry paper is instantly inflamed: chips of
wood are speedily burnt up: lead, tin, and zinc are fused: and
disks of charred paper are raised to vivid incandescence. It
might be supposed that the obscure rays would show no preference
for black over white; but they do show a preference, and to
obtain rapid combustion, the body, if not already black, ought to
be blackened. When metals are to be burned, it is necessary to
blacken or otherwise tarnish them, so as to diminish their
reflective power. Blackened zinc foil, when brought into the
focus of invisible rays, is instantly caused to blaze, and burns
with its peculiar purple light. Magnesium wire flattened, or
tarnished magnesium ribbon, also bursts into flame. Pieces of
charcoal suspended in a receiver full of oxygen are also set on
fire when the invisible focus falls upon them; the dark rays
after having passed through the receiver, still possessing
sufficient power to ignite the charcoal, and thus initiate the
attack of the oxygen. If, instead of being plunged in oxygen, the
charcoal be suspended in vacuo, it immediately glows at the place
where the focus falls.

.

.

8. Transmutation of Rays:
Calorescence.

[Footnote: I borrow
this term from Professor Challis, ‘Philosophical Magazine,’ vol.
xii. p. 521.]

Eminent experimenters were long occupied in demonstrating the
substantial identity of light and radiant heat, and we have now
the means of offering a new and striking proof of this identity.
A concave mirror produces, beyond the object which it reflects,
an inverted and magnified image of the object. Withdrawing, for
example, our iodine solution, an intensely luminous inverted
image of the carbon points of the electric light is formed at the
focus of the mirror employed in the foregoing experiments. When
the solution is interposed, and the light is cut away, what
becomes of this image? It disappears from sight; but an invisible
thermograph remains, and it is only the peculiar constitution of
our eyes that disqualifies us from seeing the picture formed by
the calorific rays. Falling on white paper, the image chars
itself out: falling on black paper, two holes are pierced in it,
corresponding to the images of the two coke points: but falling
on a thin plate of carbon in vacuo, or upon a thin sheet of
platinised platinum, either in vacuo or in air, radiant heat is
converted into light, and the image stamps itself in vivid
incandescence upon both the carbon and the metal. Results similar
to those obtained with the electric light have also been obtained
with the invisible rays of the lime-light and of the sun.

Before a Cambridge audience it is hardly necessary to refer to
the excellent researches of Professor Stokes at the opposite end
of the spectrum. The above results constitute a kind of
complement to his discoveries. Professor Stokes named the
phenomena which he has discovered and investigated
Fluorescence; for the new phenomena here described I have
proposed the term Calorescence. He, by the interposition
of a proper medium, so lowered the refrangibility of the
ultraviolet rays of the spectrum as to render them visible. Here,
by the interposition of the platinum foil, the refrangibility of
the ultra-red rays is so exalted as to render them visible.
Looking through a prism at the incandescent image of the carbon
points, the light of the image is decomposed, and a complete
spectrum is obtained. The invisible rays of the electric light,
remoulded by the atoms of the platinum, shine thus visibly forth;
ultra-red rays being converted into red, orange, yellow, green,
blue, indigo, violet, and ultraviolet ones. Could we, moreover,
raise the original source of rays to a sufficiently high
temperature, we might not only obtain from the dark rays of such
a source a single incandescent image, but from the dark rays of
this image we might obtain a second one, from the dark rays of
the second a third, and so on — a series of complete images
and spectra being thus extracted from the invisible emission of
the primitive source.[Footnote: On investigating the
calorescence produced by rays transmitted through glasses of
various colours, it was found that in the case of certain
specimens of blue glass, the platinum foil glowed with a pink or
purplish light. The effect was not subjective, and considerations
of obvious interest are suggested by it. Different kinds of black
glass differ notably as to their power of transmitting radiant
heat. When thin, some descriptions tint the sun with a greenish
hue: others make it appear a glowing red without any trace of
green. The latter are far more diathermic than the former. In
fact, carbon when perfectly dissolved and incorporated with a
good white glass, is highly transparent to the calorific rays,
and by employing it as an absorbent the phenomena of
‘calorescence’ may be obtained, though in a less striking form
than with the iodine. The black glass chosen for thermometers,
and intended to absorb completely the solar heat, may entirely
fail in this object, if the glass in which the carbon is
incorporated be colourless. To render the bulb of a thermometer a
perfect absorbent, the glass ought in the first instance to be
green. Soon after the discovery of fluorescence the late Dr.
William Allen Miller pointed to the lime-light as an illustration
of exalted refrangibility. Direct experiments have since entirely
confirmed the view expressed at page 210 of his work on
‘Chemistry,’ published in 1855.]

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9. Deadness of the Optic Nerve to
the Calorific Rays.

The layer of iodine used in the
foregoing experiments intercepted the rays of the noonday sun. No
trace of light from the electric lamp was visible in the darkest
room, even when a white screen was placed at the focus of the
mirror employed to concentrate the light. It was thought,
however, that if the retina itself were brought into the focus
the sensation of light might be experienced. The danger of this
experiment was twofold. If the dark rays were absorbed in a high
degree by the humours of the eye the albumen of the humours might
coagulate along the line of the rays. If, on the contrary, no
such high absorption took place, the rays might reach the retina
with a force sufficient to destroy it. To test the likelihood of
these results, experiments were made on water and on a solution
of alum, and they showed it to be very improbable that in the
brief time requisite for an experiment any serious damage could
be done. The eye was therefore caused to approach the dark focus,
no defence, in the first instance, being provided; but the heat,
acting upon the parts surrounding the pupil, could not be borne.
An aperture was therefore pierced in a plate of metal, and the
eye, placed behind the aperture, was caused to approach the point
of convergence of invisible rays. The focus was attained, first
by the pupil and afterwards by the retina. Removing the eye, but
permitting the plate of metal to remain, a sheet of platinum foil
was placed in the position occupied by the retina a moment
before. The platinum became red-hot. No sensible damage was done
to the eye by this experiment; no impression of light was
produced; the optic nerve was not even conscious of
heat.

But the humours of the eye are known to be highly impervious
to the invisible calorific rays, and the question therefore
arises, ‘Did the radiation in the foregoing experiment reach the
retina at all?’ The answer is, that the rays were in part
transmitted to the retina, and in part absorbed by the humours.
Experiments on the eye of an ox showed that the proportion of
obscure rays which reached the retina amounted to 18 per cent. of
the total radiation; while the luminous emission from the
electric light amounts to no more than 10 per cent. of the same
total. Were the purely luminous rays of the electric lamp
converged by our mirror to a focus, there can be no doubt as to
the fate of a retina placed there. Its ruin would be inevitable;
and yet this would be accomplished by an amount of wave-motion
but little more than half of that which the retina, without
exciting consciousness, bears at the focus of invisible rays.

This subject will repay a moment’s further attention. At a
common distance of a foot the visible radiation of the electric
light employed in these experiments is 800 times the light of a
candle. At the same distance, the portion of the radiation of the
electric light which reaches the retina, but fails to excite
vision, is about 1,500 times the luminous radiation of the
candle. [Footnote: It will be borne in mind that the heat
which any ray, luminous or non-luminous, is competent to generate
is the true measure of the energy of the ray.] But a
candle on a clear night can readily be seen at a distance of a
mile, its light at this distance being less than 1/20,000,000 of
its light at the distance of a foot.

Hence, to make the candle-light a mile off equal in power to
the non-luminous radiation received from the electric light at a
foot distance, its intensity would have to be multiplied by 1,500
x 20,000,000, or by thirty thousand millions. Thus the thirty
thousand millionth part of the invisible radiation from the
electric light, received by the retina at the distance of a foot,
would, if slightly changed in character, be amply sufficient to
provoke vision. Nothing could more forcibly illustrate that
special relationship supposed by Melloni and others to subsist
between the optic nerve and the oscillating periods of luminous
bodies. The optic nerve responds, as it were, to the waves with
which it is in consonance, while it refuses to be excited by
others of almost infinitely greater energy, whose periods of
recurrence are not in unison with its own.

.

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.

10. Persistence of
Rays.

At an early part of this
lecture it was affirmed, that when a platinum wire was, gradually
raised to a state of high incandescence, new rays were constantly
added, while the intensity of the old ones was increased. Thus,
in Dr. Draper’s experiments, the rise of temperature that
generated the orange, yellow, green, and blue augmented the
intensity of the red. What is true of the red is true of every
other ray of the spectrum, visible and invisible. We cannot
indeed see the augmentation of intensity in the region beyond the
red, but we can measure it and express it numerically. With this
view the following experiment was performed: A spiral of platinum
wire was surrounded by a small glass globe to protect it from
currents of air; through an orifice in the globe the rays could
pass from the spiral and fall afterwards upon a thermo-electric
pile. Placing in front of the orifice an opaque solution of
iodine, the platinum was gradually raised from a low dark heat to
the fullest incandescence, with the following results
:—

Thus the augmentation of the electric current, which raises
the wire from its primitive dark condition to an intense white
heat, exalts at the same time the energy of the obscure
radiation, until at the end it is fully 440 times what it was at
the beginning.

What has been here proved true of the totality of the
ultra-red rays is true for each of them singly. Placing our
linear thermo-electric pile in any part of the ultra-red
spectrum, it may be proved that a ray once emitted continues to
be emitted with increased energy as the temperature is augmented.
The platinum spiral, so often referred to, being raised to
whiteness by an electric current, a brilliant spectrum was formed
from its light. A linear thermo-electric pile was placed in the
region of obscure rays beyond the red, and by diminishing the
current the spiral was reduced to a low temperature. It was then
caused to pass through various degrees of darkness and
incandescence, with the following results :—

Here, as in the former case, the dark and bright radiations
reached their maximum together; as the one augmented, the other
augmented, until at last the energy of the obscure rays of the
particular refrangibility here chosen, became 122 times what it
was at first. To reach a white heat the wire has to pass through
all the stages of invisible radiation, but in its most brilliant
condition it embraces, in an intensified form, the rays of all
those stages.

And thus it is with all other kinds of matter, as far as they
have hitherto been examined. Coke, whether brought to a white
heat by the electric current, or by the oxyhydrogen jet, pours
out invisible rays with augmented energy, as its light is
increased. The same is true of lime, bricks, and other
substances. It is true of all metals which are capable of being
heated to incandescence. It also holds good for phosphorus
burning in oxygen. Every gush of dazzling light has associated
with it a gush of invisible radiant heat, which far transcends
the light in energy. This condition of things applies to all
bodies capable of being raised to a white heat, either in the
solid or the molten condition. It would doubtless also apply to
the luminous fogs formed by the condensation of incandescent
vapours. In such cases when the curve representing the radiant
energy of the body is constructed, the obscure radiation towers
upwards like a mountain, the luminous radiation resembling a mere
‘spur’ at its base. From the very brightness of the light
of some of the fixed stars we may infer the intensity of that
dark radiation, which is the precursor and inseparable associate
of their luminous rays.

We thus find the luminous radiation appearing when the radiant
body has attained a certain temperature; or, in other words, when
the vibrating atoms of the body have attained a certain width of
swing. In solid and molten bodies a certain amplitude cannot be
surpassed without the introduction of periods of vibration, which
provoke the sense of vision. How are we to figure this? If
permitted to speculate, we might ask, are not these more rapid
vibrations the progeny of the slower? Is it not really the mutual
action of the atoms, when they swing through very wide spaces,
and thus encroach upon each other, that causes them to tremble in
quicker periods? If so, whatever be the agency by which the large
swinging space is obtained, we shall have light-giving vibrations
associated with it. It matters not whether the large amplitudes
be produced by the strokes of a hammer, or by the blows of the
molecules of a non-luminous gas, like air at some height above a
gas-flame; or by the shock of the aether particles when
transmitting radiant heat. The result in all cases will be
incandescence. Thus, the invisible waves of our filtered electric
beam may be regarded as generating synchronous vibrations among
the atoms of the platinum on which they impinge; but, once these
vibrations have attained a certain amplitude, the mutual jostling
of the atoms produces quicker tremors, and the light-giving waves follow as the necessary
product of the heat-giving ones.

.

.

11. Absorption of Radiant Heat by
Vapours and Odours.

We commenced the demonstrations
brought forward in this lecture by experiments on permanent
gases, and we have now to turn our attention to the vapours of
volatile liquids. Here, as in the case of the gases, vast
differences have been proved to exist between various kinds of
molecules, as regards their power of intercepting the calorific
waves. While some vapours allow the waves a comparatively free
passage, the faintest mixture of other vapours causes a
deflection of the magnetic needle. Assuming the absorption
effected by air, at a pressure of one atmosphere, to be unity,
the following are the absorptions effected by a series of vapours
at a pressure of 1/60th of an atmosphere :—

Bisulphide of carbon is the
most transparent vapour in this list; and acetic ether the most
opaque; 1/60th of an atmosphere of the former, however, produces
47 times the effect of a whole atmosphere of air, while 1/60th of
an atmosphere of the latter produces 612 times the effect of a
whole atmosphere of air. Reducing dry air to the pressure of the
acetic ether here employed, and comparing them then together, the
quantity of wave-motion intercepted by the ether would be many
thousand times that intercepted by the air.

Any one of these vapours discharged into the free atmosphere,
in front of a body emitting obscure rays, intercepts more or less
of the radiation. A similar effect is. produced by perfumes
diffused in the air, though their attenuation is known to be
almost infinite. Carrying, for example, a current of dry air over
bibulous paper, moistened by patchouli, the scent taken up by the
current absorbs 30 times the quantity of heat intercepted by the
air which carries it; and yet patchouli acts more feebly on
radiant heat than any other perfume yet examined.

Here follow the results obtained with various essential oils,
the odour, in each case, being carried by a current of dry air
into the tube already employed for gases and vapours:—

Thus the absorption by a tube
full of dry air being 1, that of the odour of patchouli diffused
in it is 30, at of lavender 60, that of rosemary 74, whilst that
of aniseed amounts to 372. It would be idle to speculate the
quantities of matter concerned in these actions.

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12. Aqueous Vapour in relation to
the Terrestrial Temperatures.

We are now fully prepared for a
result which, without such preparation, might appear. incredible.
Water is, to some extent, a volatile body, and our atmosphere,
resting as it does upon the surface of the ocean, receives from
it a continual supply of aqueous vapour. It would be an error to
confound clouds or fog or any visible mist with the vapour of
water, which is a perfectly impalpable gas, diffused, even on the
clearest days, throughout the atmosphere. Compared with the great
body of the air, the aqueous vapour it contains is of almost
infinitesimal amount, 99.5 out of every 100 parts of the
atmosphere being composed of oxygen and nitrogen. In the absence
of experiment, we should never think of ascribing to this scant
and varying constituent any important influence on terrestrial
radiation; and yet its influence is far more potent than that of
the great body of the air. To say that on a day of average
humidity in England, the atmospheric vapour exerts 100 times the
action of the air itself, would certainly be an understatement of
the fact. Comparing a single molecule of aqueous vapour with an
atom of either of the main constituents of our atmosphere, I am
not prepared to say how many thousand times the action of the
former exceeds that of the latter.

But it must be borne in mind that these large numbers depend,
in part, on the extreme feebleness of the air; the power of
aqueous vapour seems vast, because that of the air with which it
is compared is infinitesimal. Absolutely considered, however,
this substance, notwithstanding its small specific gravity,
exercises a very potent action. Probably from 10 to 15 per cent.
of the heat radiated from the earth is absorbed within 10 or 20
feet of the earth’s surface. This must evidently be of the utmost
consequence to the life of the world. Imagine the superficial
molecules of the earth agitated with the motion of heat, and
imparting it to the surrounding aether; this motion would be
carried rapidly away, and lost for ever to our planet, if the
waves of aether had nothing but the air to contend with in their
outward course. But the aqueous vapour takes up the motion, and
becomes hereby heated, thus wrapping the earth like a warm
garment, and protecting its surface from the deadly chill which
it would otherwise sustain. Various philosophers have speculated
on the influence of an atmospheric envelope. De Saussure,
Fourier, M. Pouillet, and Mr. Hopkins have, one and all, enriched
scientific literature with contributions on this subject, but the
considerations which these eminent men have applied to
atmospheric air, have, if my experiments be correct, to be
transferred to the aqueous vapour.

The observations of meteorologists
furnish important, though hitherto unconscious evidence of the
influence of this agent. Wherever the air is dry we are liable to
daily extremes of temperature. By day, such places, the sun’s
heat reaches the earth unimpeded, and renders the maximum high;
by night, on the other hand, the earth’s heat escapes unhindered
to space, and renders the minimum low. Hence the difference
between the maximum and minimum is greatest where the air is
driest. In the plains of India, the heights of the Himalaya, in
central Asia, in Australia — wherever drought reigns, we
have the heat of day forcibly contrasted with the chill of night.
In the Sahara itself, when the sun’s rays cease to impinge on the
burning soil, the temperature runs rapidly down to freezing,
because there is no vapour overhead to check the calorific drain.
And here another instance might be added to the numbers already
known, in which nature tends as it were to check her own excess.
By nocturnal refrigeration, the aqueous vapour of the air is
condensed to water on the surface of the earth; and, as only the
superficial portions radiate, the act of condensation makes water
the radiating body. Now experiment proves that to the rays
emitted by water, aqueous vapour is especially opaque. Hence the
very act of condensation, consequent on terrestrial cooling,
becomes a safeguard to the earth, imparting to its radiation that
particular character which renders it most liable to be prevented
from escaping into space.

It might however be urged that, inasmuch as we derive all our
heat from the sun, the selfsame covering which protects the earth
from chill must also shut out the solar radiation. This is
partially true, but only partially; the sun’s rays are different
in quality from the earth’s rays, and it does not at all follow
that the substance which absorbs the one must necessarily absorb
the other. Through a layer of water, for example, one tenth of an
inch in thickness, the sun’s rays are transmitted with
comparative freedom; but through a layer half this thickness, as
Melloni has proved, no single ray from the warmed earth could
pass. In like manner, the sun’s rays pass with comparative
freedom through the aqueous vapour of the air: the absorbing
power of this substance being mainly exerted upon the invisible
heat that endeavours to escape from the earth. In consequence of
this differential action upon solar and terrestrial heat, the
mean temperature of our planet is higher than is due to its
distance from the sun.

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13. Liquids and their Vapours in
relation to Radiant Heat.

The deportment here assigned to
atmospheric vapour has been established by direct experiments on
it taken from the streets and parks of London, from the downs of
Epsom, from the hills and sea-beach of the Isle of Wight, and
also by experiments on air in the first instance dried, and
afterwards rendered artificially humid by pure distilled water.
It has also been established in the following way: Ten volatile
liquids were taken at random and the power of these ;liquids, at a
common thickness, to intercept the waves of heat, was carefully
determined. The vapours of the liquids were next taken, in
quantities proportional to the quantities of liquid, and the power
of the vapours intercept the waves of heat was also
determined.

Commencing with the substance which exerted the least
absorptive power, and proceeding onwards to the most energetic,
the following order of absorption was observed :—

We here find the order of
absorption in both cases be the same. We have liberated the
molecules from the bonds which trammel them more or less in a
liquid condition; but this change in their state of aggregation
does not change their relative powers of absorption. Nothing
could more clearly prove that the act of absorption depends upon
the individual molecule, which equally asserts its power in the
liquid and the gaseous state. We may safely conclude from the
above table that the position of a vapour is determined by that
of its liquid. Now at the very foot of the list of liquids stands
water, signalising itself above all others by its enormous power
of absorption. And from this fact, even if no direct experiment
on the vapour of water had ever been made, we should be entitled
to rank that vapour as our most powerful absorber of radiant
heat. Its attenuation, however, diminishes its action. I have
proved that a shell of air two inches in thickness surrounding
our planet, and saturated with the vapour of sulphuric aether,
would intercept 35 per cent. of the earth’s radiation. And though
the quantity of aqueous vapour necessary to saturate air is much
less than the amount of sulphuric aether vapour which it can
sustain, it is still extremely probable that the estimate already
made of the action of atmospheric vapour within 10 feet of the
earth’s surface, is under the mark; and that we are indebted to
this wonderful substance, to an extent not accurately determined,
but certainly far beyond what has hitherto been imagined, for the
temperature now existing at the surface of the globe.

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14. Reciprocity of Radiation and
Absorption.

Throughout the reflections
which have hitherto occupied us, the image before the mind has
been that of a radiant source sending forth calorific waves,
which on passing among the molecules of a gas or vapour were
intercepted by those molecules in various degrees. In all cases
it was the transference of motion from the aether to the
comparatively quiescent molecules of the gas or vapour that
occupied our thoughts. We have now to change the form of our
conception, and to figure these molecules not as absorbers but as
radiators, not as the recipients but as the originators of
wave-motion. That is to say, we must figure them vibrating, and
generating in the surrounding aether undulations which speed
through it with the velocity of light. Our object now is to
enquire whether the act of chemical combination, which proves so
potent as regards the phenomena of absorption, does not also
manifest its power in the phenomena of radiation. For the
examination of this question it is necessary, in the first place,
to heat our gases and vapours to the same temperature, and then
examine their power of discharging the motion thus imparted to
them upon the aether in which they swing.

A heated copper ball was placed above a ring gas-burner
possessing a great number of small apertures, the burner being
connected by a tube with vessels containing the various gases to
be examined. By gentle pressure the gases were forced through the
orifices of the burner against the copper ball, where each of
them, being heated, rose in an ascending column. A thermoelectric
pile, entirely screened from the hot ball, was exposed to the
radiation of the warm gas, while the deflection of a magnetic
needle connected with the pile declared the energy of the
radiation.

By this mode of experiment it was proved that the selfsame
molecular arrangement which renders a gas a powerful absorber,
renders it a powerful radiator — that the atom or molecule
which is competent to intercept the calorific waves is, in the
same degree, competent to send them forth. Thus, while the atoms
of elementary gases proved themselves unable to emit any sensible
amount of radiant heat, the molecules of compound gases were
shown to be capable of powerfully disturbing the surrounding
aether. By special modes of experiment the same was proved to
hold good for the vapours of volatile liquids, the radiative
power of every vapour being found proportional to its absorptive
power.

The method of experiment here pursued, though not of the
simplest character, is still easy to grasp. When air is permitted
to rush into an exhausted tube, the temperature of the air is
raised to a degree equivalent to the vis viva
extinguished. [Footnote: See above for a definition of
vis viva.] Such air is said to be
dynamically heated, and, if pure, it shows itself incompetent to
radiate, even when a rock-salt window is provided for the passage
of its rays. But if instead of being empty the tube contain a
small quantity of vapour, the warmed air communicates its heat by
contact to the vapour, the molecules of which convert into the
radiant form the heat imparted to them by the atoms of the air.
By this process also, which I have called Dynamic Radiation, the
reciprocity of radiation and absorption has been conclusively
proved.[Footnote: When heated air imparts its motion to
another gas or vapour, the transference of heat is accompanied by
a change of vibrating period. The Dynamic Radiation of vapours is
rendered possible by this transmutation of
vibrations.]

In the excellent researches of Leslie, De la Provostaye and
Detains, and Balfour Stewart, the same reciprocity, as regards
solid bodies, has been variously illustrated; while the labours,
theoretical and experimental, of Kirchhoff have given this
subject a wonderful expansion, and enriched it by applications of
the highest kind. To their results are now to be added the
foregoing, whereby gases and vapours, which have been hitherto
thought inaccessible to experiments with the thermo-electric
pile, are proved by it to exhibit the indissoluble duality of
radiation and absorption, the influence of chemical combination
on both being exhibited in the most decisive and extraordinary
way.

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15. Influence of Vibrating Period
and Molecular Form. Physical Analysis of the Human
Breath.

In the foregoing experiments
with gases and vapours have employed throughout invisible rays,
and found some of these bodies so impervious to radiant heat, that
lengths of a few feet they intercept every ray as actually as a
layer of pitch. The substances, however, which show themselves thus
opaque to radiant heat perfectly transparent to light. Now the
rays of light differ from those of invisible heat merely in point of
period, the former failing to affect the retina because their
periods of recurrence are too slow. Hence, in one way or other,
the transparency of our gases and vapours depends upon the
periods of the waves which impinge upon them. What is the nature
of this dependence? The admirable researches of Kirchhoff help us
an answer. The atoms and molecules of every gas have certain
definite rates of oscillation, and those waves aether are most
copiously absorbed whose periods recurrence synchronise with
those of the atomic groups amongst which they pass. Thus, when we
find invisible rays absorbed and the visible ones transmitted by
a layer of gas, we conclude that the oscillating periods of the
atoms constituting the gaseous molecules coincide with those of
the invisible, and not with those of the visible
spectrum.

It requires some discipline of the imagination to form a clear
picture of this process. Such a picture is, however, possible,
and ought to be obtained. When the waves of aether impinge upon
molecules whose periods of vibration coincide with the recurrence
of the undulations, the timed strokes of the waves augment the
vibration of the molecules, as a heavy pendulum is set in motion
by well-timed puffs of breath. Millions of millions of shocks are
received every second from the calorific waves; and it is not
difficult to see that as every wave arrives just in time to
repeat the action of its predecessor, the molecules must finally
be caused to swing through wider spaces than if the arrivals were
not so timed. In fact, it is not difficult to see that an
assemblage of molecules, operated upon by contending waves, might
remain practically quiescent. This is actually the case when the
waves of the visible spectrum pass through a transparent gas or
vapour. There is here no sensible transference of motion from the
aether to the molecules; in other words, there is no sensible
absorption of heat.

One striking example of the influence of period may be here
recorded. Carbonic acid gas is one of the feeblest absorbers of
the radiant heat emitted by solid bodies. It is, for example, to
a great extent transparent to the rays emitted by the heated
copper plate already referred to. There are, however, certain
rays, comparatively few in number, emitted by the copper, to
which the carbonic acid is impervious; and could we obtain a
source of heat emitting such rays only, we should find carbonic
acid more opaque to the radiation from that source, than any
other gas. Such a source is actually found in the flame of
carbonic oxide, where hot carbonic acid constitutes the main
radiating body. Of the rays emitted by our heated plate of
copper, olefiant gas absorbs ten times the quantity absorbed by
carbonic acid. Of the rays emitted by a carbonic oxide flame,
carbonic acid absorbs twice as much as olefiant gas. This
wonderful change in the power of the former, as an absorber, is
simply due to the fact, that the periods of the hot and cold
carbonic acid are identical, and that the waves from the flame
freely transfer their motion to the molecules which synchronise
with them. Thus it is that the tenth an atmosphere of carbonic
acid, enclosed in a tube four feet long, absorbs 60 per cent. of
the radiation from carbonic oxide flame, while one-thirtieth of
an atmosphere absorbs 48 per cent. of the heat from the same
source.

In fact, the presence of the minutest quantity of carbonic
acid may be detected by its action on the rays from the carbonic
oxide flame. Carrying, for example, the dried human breath into a
tube four feet long, the absorption there effected by the
carbonic acid of the breath amounts to 50 per cent. of the entire
radiation. Radiant heat may indeed be employed as a means of
determining practically the amount of carbonic acid expired from
the lungs. My late assistant, Mr. Barrett, while under my
direction, made this determination. The absorption produced by
the breath freed from its moisture, but retaining its carbonic
acid, was first determined. Carbonic acid, artificially prepared,
was then mixed with dry air in such proportions that the action
of the mixture upon the rays of heat was the same as that of the
dried breath. The percentage of the former being known,
immediately gave that of the latter. The same breath, analysed
chemically by Dr. Frankland, and physically by Mr. Barrett, gave
the following results :—

Percentage of Carbonic Acid in the Human Breath.

It is thus proved that in the quantity of aethereal motion
which it is competent to take up, we have a practical measure of
the carbonic acid of the breath, and hence of the combustion
going on in the human lungs.

Still this question of period, though of the utmost
importance, is not competent to account for the whole of the
observed facts. The aether, as far as we know, accepts vibrations
of all periods with the same readiness. To it the oscillations of
an atom of free oxygen are just as acceptable as those of the
atoms in a molecule of olefiant gas; that the vibrating oxygen
then stands so far below the olefiant gas in radiant power must
be referred not to period, but to some other peculiarity. The
atomic group which constitutes the molecule of olefiant gas,
produces many thousand times the disturbance caused by the
oxygen, it may be because the group is able to lay a vastly more
powerful hold upon the aether than the single atoms can.
Another, and probably very potent cause of the difference may be,
that the vibrations, being those of the constituent atoms of the
molecule, [Footnote: See ‘Physical Considerations,’ Art.
iv.] are generated in highly condensed aether,
which acts like condensed air upon sound. But whatever may be
the fate of these attempts to visualise the physics of the
process, it will still remain true, that to account for the
phenomena of radiation and absorption we must take into
consideration the shape, size, and condition of the aether within
the molecules, by which the external aether is disturbed.

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16. Summary and
Conclusion.

Let us now cast a momentary
glance over the ground that we have left behind. The general
nature of light and heat was first briefly described: the
compounding of matter from elementary atoms, and the influence of
the act of combination on radiation and absorption, were
considered and experimentally illustrated. Through the
transparent elementary gases radiant heat was found to pass as
through a vacuum, while many of the compound gases presented
almost impassable obstacles to the calorific-waves. This
deportment of the simple gases directed our attention to other
elementary bodies, the examination of which led to the discovery
that the element iodine, dissolved in bisulphide of carbon,
possesses the power of detaching, with extraordinary sharpness, the
light of the spectrum from its heat, intercepting all luminous
rays up to the extreme red, and permitting the calorific rays
beyond the red to pass freely through it. This substance was then
employed to filter the beams of the electric light, and to form
foci of invisible rays so intense as to produce almost all the
effects obtainable in ordinary fire. Combustible bodies were
burnt, and refractory ones were raised to a white heat, by the
concentrated invisible rays. Thus, by exalting their
refrangibility, the invisible rays of the electric light were
rendered visible, and all the colours of the solar spectrum were
extracted from utter darkness. The extreme richness of the
electric light in invisible rays of low refrangibility was
demonstrated, one-eighth only of its radiation consisting of
luminous rays. The deadness of the optic nerve to those invisible
rays was proved, and experiments were then added to show that the
bright and the dark rays of a solid body, raised gradually to
incandescence, are strengthened together; intense dark heat being
an invariable accompaniment of intense white heat. A sun could
not be formed, or a meteorite rendered luminous, on any other
condition. The light-giving rays constituting only a small
fraction of the total radiation, their unspeakable importance to
us is due to the fact, that their periods are attuned to the
special requirements of the eye.

Among the vapours of volatile liquids vast differences were
also found to exist, as regards their powers of absorption. We
followed various molecules from a state of liquid to a state of
gas, and found, in both states of aggregation, the power of the
individual molecules equally asserted. The position of a vapour
as an absorber of radiant heat was shown to be determined by that
of the liquid from which it is derived. Reversing our
conceptions, and regarding the molecules of gases and vapours not
as the recipients but as the originators of wave-motion; not as
absorbers but as radiators; it was proved that the powers of
absorption and radiation went hand in hand, the self-same
chemical act which rendered a body competent to intercept the
waves of aether, rendering it competent, in the same degree, to
generate them. Perfumes were next subjected to examination, and,
notwithstanding their extraordinary tenuity, they were found
vastly superior, in point of absorptive power, to the body of the
air in which they were diffused. We were led thus slowly up to
the examination of the most widely diffused and most important of
all vapours — the aqueous vapour of our atmosphere, and we
found in it a potent absorber of the purely calorific rays. The
power of this substance to influence climate, and its general
influence on the temperature of the earth, were then briefly
dwelt upon. A cobweb spread above a blossom is sufficient to
protect it from nightly chill; and thus the aqueous vapour of our
air, attenuated as it is, checks the drain of terrestrial heat,
and saves the surface of our planet from the refrigeration which
would assuredly accrue, were no such substance interposed between
it and the voids of space. We considered the influence of
vibrating period, and molecular form, on absorption and
radiation, and finally deduced, from its action upon radiant
heat, the exact amount of carbonic acid expired by the human
lungs.

Thus, in brief outline, were placed before you some of the
results of recent enquiries in the domain of Radiation, and my
aim throughout has been to raise in your minds distinct physical
images of the various processes involved in our researches. It is
thought by some that natural science has a deadening influence on
the imagination, and a doubt might fairly be raised as to the
value of any study which would necessarily have this effect. But
the experience of the last hour must, I think, have convinced
you, that the study of natural science goes hand in hand with the
culture of the imagination. Throughout the greater part of this
discourse we have been sustained by this faculty. We have been
picturing atoms, and molecules, and vibrations, and waves, which
eye has never seen nor ear heard, and which can only be discerned
by the exercise of imagination. This, in fact, is the faculty
which enables us transcend the boundaries of sense, and connect
the phenomena of our visible world with those of an invisible
one. Without imagination we never could have risen to the
conceptions which have occupied us here today; and in proportion
to your power of exercising this faculty aright, and of
associating definite mental images with the terms employed, will
be the pleasure and the profit which you will derive from this
lecture.

The outward facts of nature are insufficient to satisfy the
mind. We cannot be content with knowing that the light and heat
of the sun illuminate and warm the world. We are led irresistibly
to enquire, ‘What is light, and what is heat?’ and this
question leads us at once out of the region of sense into that of
imagination. [Footnote: This line of thought was pursued
further five years subsequently. See ‘Scientific Use of the
Imagination’ in Vol. II.]

Thus pondering, and questioning, and striving to supplement
that which is felt and seen, but which is incomplete, by
something unfelt and unseen which is necessary to its
completeness, men of genius have in part discerned, not only the
nature of light and heat, but also, through them, the general
relationship of natural phenomena. The working power of Nature
consists of actual or potential motion, of which all its
phenomena are but special forms. This motion manifests itself in
tangible and in intangible matter, being incessantly transferred
from the one to the other, and incessantly transformed by the
change. It is as real in the waves of the aether as in the waves
of the sea; the latter — derived as they are from winds,
which in their turn are derived from the sun — are, indeed,
nothing more than the heaped-up motion of the aether waves. It is
the calorific waves emitted by the sun which heat our air,
produce our winds, and hence agitate our ocean. And whether they
break in foam upon the shore, or rub silently against the ocean’s
bed, or subside by the mutual friction of their own parts, the
sea waves, which cannot subside without producing heat, finally
resolve themselves into waves of aether, thus regenerating the
motion from which their temporary existence was derived. This
connection is typical. Nature is not an aggregate of independent
parts, but an organic whole. If you open a piano and sing into
it, a certain string will respond. Change the pitch of our voice;
the first string ceases to vibrate, but another replies. Change
again the pitch; the first two strings are silent, while another
resounds. Thus is sentient man acted on by Nature, the optic, the
auditory, and other nerves of the human body being so many
strings differently tuned, and responsive to different forms of
the universal power.

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III ON RADIANT HEAT IN RELATION TO
THE COLOUR AND CHEMICAL CONSTITUTION OF
BODIES.

[Footnote: A
discourse delivered in the Royal Institution of Great Britain,
Jan. 19, 1866.]

ONE of the most important functions of physical science,
considered as a discipline of the mind, is to enable us by means
of the sensible processes of Nature to apprehend the insensible.
The sensible processes give direction to the line of thought; but
this once given, the length of the line is not limited by the
boundaries of the senses. Indeed, the domain of the senses, in
Nature, is almost infinitely small in comparison with the vast
region accessible to thought which lies beyond them. From a few
observations of a comet, when it comes within the range of his
telescope, an astronomer can calculate its path in regions which
no telescope can reach: and in like manner, by means of data
furnished in the narrow world of the senses, we make ourselves at
home in other and wider worlds, which are traversed by the
intellect alone.

From the earliest ages the questions, ‘What is light?’
and ‘What is heat?’ have occurred to the minds of men; but these
questions never would have been answered had they not been
preceded by the question, ‘What is sound?’ Amid the grosser
phenomena of acoustics the mind was first disciplined,
conceptions being thus obtained from direct observation, which
were afterwards applied to phenomena of a character far too
subtle to be observed directly. Sound we know to be due to
vibratory motion. A vibrating tuning-fork, for example, moulds
the air around it into undulations or waves, which speed away on
all sides with a certain measured velocity, impinge upon the drum
of the ear, shake the auditory nerve, and awake in the brain the
sensation of sound. When sufficiently near a sounding body we can
feel the vibrations of the air. A deaf man, for example, plunging
his hand into a bell when it is sounded, feels through the common
nerves of his body those tremors which, when imparted to the
nerves of healthy ears, are translated into sound. There are
various ways of rendering those sonorous vibrations not only
tangible but visible; and it was not until numberless experiments
of this kind had been executed, that the scientific investigator
abandoned himself wholly, and without a shadow of misgiving, to
the conviction that what is sound within us is, outside of us, a
motion of the air.

But once having established this fact — once having
proved beyond all doubt that the sensation of sound is produced
by an agitation of the auditory nerve — the thought soon
suggested itself that light might be due to an agitation of the
optic nerve. This was a great step in advance of that ancient
notion which regarded light as something emitted by the eye, and
not as anything imparted to it. But if light be produced by an
agitation of the retina, what is it that produces the agitation?
Newton, you know, supposed minute particles to be shot through
the humours of the eye against the retina, which he supposed to
hang like a target at the back of the eye. The impact of these
particles against the target, Newton believed to be
the cause of light. But Newton’s notion has not held its
ground, being entirely driven from the field by the more
wonderful and far more philosophical notion that light, like
sound, is a product of wave-motion.

The domain in which this motion of light is carried on lies
entirely beyond the reach of our senses. The waves of light
require a medium for their formation and propagation; but we
cannot see, or feel, or taste, or smell this medium. How, then,
has its existence been established? By showing, that by the
assumption of this wonderful intangible aether, all the phenomena
of optics are accounted for, with a fulness, and clearness, and conclusiveness, which leave no
desire of the intellect unsatisfied. When the law of gravitation
first suggested itself to the mind of Newton, what did he do? He
set himself to examine whether it accounted for all the facts. He
determined the courses of the planets; he calculated the rapidity
of the moon’s fall towards the earth; he considered the
precession of the equinoxes, the ebb and flow of the tides, and
found all explained by the law of gravitation. He therefore
regarded this law as established, and the verdict of science
subsequently confirmed his conclusion. On similar, and, if
possible, on stronger grounds, we found our belief in the
existence of the universal aether. It explains facts far more
various and complicated than those on which Newton based his law.
If a single phenomenon could be pointed out which the aether is
proved incompetent to explain, we should have to give it up; but
no such phenomenon has ever been pointed out. It is, therefore,
at least as certain that space is filled with a medium, by means
of which suns and stars diffuse their radiant power, as that it
is traversed by that force which holds in its grasp, not only our
planetary system, but the immeasurable heavens themselves.

There is no more wonderful instance than this of the
production of a line of thought, from the world of the senses
into the region of pure imagination. I mean by imagination here,
not that play of fancy which can give to airy nothings a local
habitation and a name, but that power which enables the mind to
conceive realities which lie beyond the range of the senses
— to present to itself distinct images of processes which,
though mighty in the aggregate beyond all conception, are so
minute individually as to elude all observation. It is the waves
of air excited by a tuning-fork which render its vibrations
audible. It is the waves of aether sent forth from those lamps
overhead which render them luminous to us; but so minute are
these waves, that it would take from 30,000 to 60,000 of them
placed end to end to cover a single inch. Their number, however,
compensates for their minuteness. Trillions of them have entered
your eyes, and hit the retina at the backs of your eyes, in the
time consumed in the utterance of the shortest sentence of this
discourse. This is the steadfast result of modern research; but
we never could have reached it without previous discipline. We
never could have measured the waves of light, nor even imagined
them to exist, had we not previously exercised ourselves among
the waves of sound. Sound and light are now mutually helpful, the
conceptions of each being expanded, strengthened, and defined by
the conceptions of the other.

The aether which conveys the pulses of light and heat not only
fills celestial space, swathing suns, and planets, and moons, but
it also encircles the atoms of which these bodies are composed.
It is the motion of these atoms, and not that of any sensible
parts of bodies, that the aether conveys. This motion is the
objective cause of what, in our sensations, are light and heat.
An atom, then, sending its pulses through the aether, resembles a
tuning-fork sending its pulses through the air. Let us look for a
moment at this thrilling medium, and briefly consider its
relation to the bodies whose vibrations it conveys. Different
bodies, when heated to the same temperature, possess very
different powers of agitating the aether: some are good
radiators, others are bad radiators; which means that some are so
constituted as to communicate their atomic motion freely to the
aether, producing therein powerful undulations; while the atoms
of others are unable thus to communicate their motions, but glide
through the medium without materially disturbing its repose.
Recent experiments have proved that elementary bodies, except
under certain anomalous conditions, belong to the class of bad
radiators. An atom, vibrating in the aether, resembles a naked
tuning-fork vibrating in the air. The amount of motion
communicated to the air by the thin prongs is too small to evoke
at any distance the sensation of sound. But if we permit the
atoms to combine chemically and form molecules, the result, in
many cases, is an enormous change in the power of radiation. The
amount of aethereal disturbance, produced by the combined atoms
of a body, may be many thousand times that produced by the same
atoms when uncombined.

The pitch of a musical note depends upon the rapidity of its
vibrations, or, in other words, on the length of its waves. Now,
the pitch of a note answers to the colour of light. Taking a
slice of white light from the sun, or from an electric lamp, and
causing the light to pass through an arrangement of prisms, it is
decomposed. We have the effect obtained by Newton, who first
unrolled the solar beam into the splendours of the solar
spectrum. At one end of this spectrum we have red light, at the
other, violet; and between those extremes lie the other prismatic
colours. As we advance along the spectrum from the red to the
violet, the pitch of the light — if I may use the
expression — heightens, the sensation of violet being
produced by a more rapid succession of impulses than that which
produces the impression of red. The vibrations of the violet are
about twice as rapid as those of the red; in other words, the
range of the visible spectrum is about an octave.

There is no solution of continuity in this spectrum one colour
changes into another by insensible gradations. It is as if an
infinite number of tuning-forks, of gradually augmenting pitch,
were vibrating at the same time. But turning to another spectrum
— that, namely, obtained from the incandescent vapour of
silver — you observe that it consists of two narrow and
intensely luminous green bands. Here it is as if two forks only,
of slightly different pitch, were vibrating. The length of the
waves which produce this first band is such that 47,460 of them,
placed end to end, would fill an inch. The waves which produce
the second band are a little shorter; it would take of these
47,920 to fill an inch. In the case of the first band, the number
of impulses imparted, in one second, to every eye which sees it,
is 677 millions of millions; while the number of impulses
imparted, in the same time, by the second band is 600 millions of
millions. We may project upon a white screen the beautiful stream
of green light from which these bands were derived. This luminous
stream is the incandescent vapour of silver. The rates of
vibration of the atoms of that vapour are as rigidly fixed as
those of two tuning-forks; and to whatever height the temperature
of the vapour may be raised, the rapidity of its vibrations, and
consequently its colour, which wholly depends upon that rapidity,
remain unchanged.

The vapour of water, as well as the vapour of silver, has its
definite periods of vibration, and these are such as to
disqualify the vapour, when acting freely as such, from being
raised to a white heat. The oxyhydrogen flame, for example,
consists of hot aqueous vapour. It is scarcely visible in the air
of this room, and it would be still less visible if we could burn
the gas in a clean atmosphere. But the atmosphere, even at the
summit of Mont Blanc, is dirty; in London it is more than dirty;
and the burning dirt gives to this flame the greater portion of
its present light. But the heat of the flame is enormous. Cast
iron fuses at a temperature of 2,000° Fahr.; while the
temperature of the oxyhydrogen flame is 6,000° Fahr. A piece
of platinum is heated to vivid redness, at a distance of two
inches beyond the visible termination of the flame. The vapour
which produces incandescence is here absolutely dark. In the
flame itself the platinum is raised to dazzling whiteness, and is
even pierced by the flame. When this flame impinges on a piece of
lime, we have the dazzling Drummond light. But the light is here
due to the fact that when it impinges upon the solid body, the
vibrations excited in that body by the flame are of periods
different from its own.

Thus far we have fixed our attention on atoms and molecules in
a state of vibration, and surrounded by a medium which accepts
their vibrations, and transmits them through space. But suppose
the waves generated by one system of molecules to impinge upon
another system, how will the waves be affected? Will they be
stopped, or will they be permitted to pass? Will they transfer
their motion to the molecules on which they impinge, or will they
glide round the molecules, through the intermolecular spaces, and
thus escape?

The answer to this question depends upon a condition which may
be beautifully exemplified by an experiment on sound. These two
tuning-forks are tuned absolutely alike. They vibrate with the
same rapidity, and, mounted thus upon their resonant cases, you
hear them loudly sounding the same musical note. Stopping one of
the forks, I throw the other into strong vibration, and bring
that other near the silent fork, but not into contact with it.
Allowing them to continue in this position for four or five
seconds, and then stopping the vibrating fork, the sound does not
cease. The second fork has taken up the vibrations of its
neighbour, and is now sounding in its turn. Dismounting one of
the forks, and permitting the other to remain upon its stand, I
throw the dismounted fork into strong vibration. You cannot hear
it sound. Detached from its case, the amount of motion which it
can communicate to the air is too small to be sensible at any
distance. When the dismounted fork is brought close to the
mounted one, but not into actual contact with it, out of the
silence rises a mellow sound. Whence comes it? From the
vibrations which have been transferred from the dismounted fork
to the mounted one.

That the motion should thus transfer itself through the air it
is necessary that the two forks should be in perfect unison. If a
morsel of wax not larger than a pea be placed on one of the
forks, it is rendered thereby powerless to affect, or to be
affected by, the other. It is easy to understand this experiment.
The pulses of the one fork can affect the other, because they are
perfectly timed. A single pulse causes the prong of the silent
fork to vibrate through an infinitesimal space. But just as it
has completed this small vibration another pulse is ready to
strike it. Thus, the impulses add themselves together. In the
five seconds during which the forks were held near each other,
the vibrating fork sent 1,280 waves against its neighbour and
those 1,280 shocks, all delivered at the proper moment, all, as I
have said, perfectly timed, have given such strength to the
vibrations of the mounted fork as to render them audible to
all.

Another curious illustration of the influence of synchronism
on musical vibrations, is this: Three small gas-flames are
inserted into three glass tubes of different lengths. Each of
these flames can be caused to emit a musical note, the pitch of
which is determined by the length of the tube surrounding the
flame. The shorter the tube the higher is the pitch. The flames
are now silent within their respective tubes, but each of them
can be caused to respond to a proper note sounded anywhere in
this room. With an instrument called a syren, a powerful musical
note, of gradually increasing pitch, can be produced. Beginning
with a low note, and ascending gradually to a higher one, we
finally attain the pitch of the flame in the longest tube. The
moment it is reached, the flame bursts into song. The other
flames are still silent within their tubes. But by urging the
instrument on to higher notes, the second flame is started, and
the third alone remains. A still higher note starts it also.
Thus, as the sound of the syren rises gradually in pitch, it
awakens every flame in passing, by striking it with a series of
waves whose periods of recurrence are similar to its own.

Now the wave-motion from the syren is in part taken up by the
flame which synchronises with the waves; and were these waves to
impinge upon a multitude of flames, instead of upon one flame
only, the transference might be so great as to absorb the whole
of the original wave motion. Let us apply these facts to radiant
heat. This blue flame is the flame of carbonic oxide; this
transparent gas is carbonic acid gas. In the blue flame we have
carbonic acid intensely heated, or, in other words, in a state of
intense vibration. It thus resembles the sounding fork, while
this cold carbonic acid resembles the silent one. What is the
consequence? Through the synchronism of the hot and cold gas, the
waves emitted by the former are intercepted by the latter, the
transmission of the radiant heat being thus prevented. The cold
gas is intensely opaque to the radiation from this particular
flame, though highly transparent to heat of every other kind. We
are here manifestly dealing with that great principle which lies
at the basis of spectrum analysis, and which has enabled
scientific men to determine the substances of which the sun, the
stars, and even the nebulae are composed; the principle, namely,
that a body which is competent to emit any ray, whether of heat
or light, is competent in the same degree to absorb that ray. The
absorption depends on the synchronism existing between the
vibrations of the atoms from which the rays, or more correctly the
waves, issue, and those of the atoms on which they impinge.

To its almost total incompetence to emit white light, aqueous
vapour adds a similar incompetence to absorb white light. It
cannot, for example, absorb the luminous rays of the sun, though
it can absorb the non-luminous rays of the earth. This
incompetence of the vapour to absorb luminous rays is shared by
water and ice — in fact, by all really transparent
substances. Their transparency is due to their inability to
absorb luminous rays. The molecules of such substances are in
dissonance with luminous waves; and hence such waves pass through
transparent bodies without disturbing the molecular rest. A
purely luminous beam, however intense may be its heat, is
sensibly incompetent to melt ice. We can, for example, converge a
powerful luminous beam upon a surface covered with hoar frost,
without melting a single spicula of the crystals. How then, it
may be asked, are the snows of the Alps swept away by the
sunshine of summer? I answer, they are not swept away by sunshine
at all, but by rays which have no sunshine whatever in them. The
luminous rays of the sun fall upon the snow-fields and are
flashed in echoes from crystal to crystal, but they find next to
no lodgment within the crystals. They are hardly at all absorbed,
and hence they cannot produce fusion. But a body of powerful dark
rays is emitted by the sun; and it is these that cause the
glaciers to shrink and the snows to disappear; it is they that
fill the banks of the Arve and Arveyron, and liberate from their
frozen captivity the Rhone and the Rhine.

Placing a concave silvered mirror behind the electric light
its rays are converged to a focus of dazzling brilliancy. Placing
in the path of the rays, between the light and the focus, a
vessel of water, and introducing at the focus a piece of ice, the
ice is not melted by the concentrated beam. Matches, at the same
place, are ignited, and wood is set on fire. The powerful heat,
then, of this luminous beam is incompetent to melt the ice. On
withdrawing the cell of water, the ice immediately liquefies, and
the water trickles from it in drops. Reintroducing the cell of
water, the fusion is arrested, and the drops cease to fall. The
transparent water of the cell exerts no sensible absorption on
the luminous rays, still it withdraws something from the beam,
which, when permitted to act, is competent to melt the ice. This
something is the dark radiation of the electric light. Again, I
place a slab of pure ice in front of the electric lamp; send a
luminous beam first through our cell of water and then through
the ice. By means of a lens an image of the slab is cast upon a
white screen. The beam, sifted by the water, has little power
upon the ice. But observe what occurs when the water is removed;
we have here a star and there a star, each star resembling a
flower of six petals, and growing visibly larger before our eyes.
As the leaves enlarge, their edges become serrated, but there is
no deviation from the six-rayed type. We have here, in fact, the
crystallisation of the ice reversed by the invisible rays of the
electric beam. They take the molecules down in this wonderful
way, and reveal to us the exquisite atomic structure of the
substance with which Nature every winter roofs our ponds and
lakes.

Numberless effects, apparently anomalous, might be adduced in
illustration of the action of these lightless rays. These two
powders, for example, are both white, and undistinguishable from
each other by the eye. The luminous rays of the sun are
unabsorbed by both — from such rays these powders acquire
no heat; still one of them, sugar, is heated so highly by the
concentrated beam of the electric lamp, that it first smokes and
then violently inflames, while the other substance, salt, is
barely warmed at the focus. Placing two perfectly transparent
liquids in test-tubes at the focus, one of them boils in a couple
of seconds, while the other, in a similar position, is hardly
warmed. The boiling-point of the first liquid is 78°C., which
is speedily reached; that of the second liquid is only 48°C.,
which is never reached at all. These anomalies are entirely due
to the unseen element which mingles with the luminous rays of the
electric beam, and indeed constitutes 90 per cent. of its
calorific power.

A substance, as many of you know, has been discovered, by
which these dark rays may be detached from the total emission of
the electric lamp. This ray-filter is a liquid, black as pitch to
the luminous, but bright as a diamond to the non-luminous,
radiation. It mercilessly cuts off the former, but allows the
latter free transmission. When these invisible rays are brought
to a focus, at a distance of several feet from the electric lamp,
the dark rays form an invisible image of their source. By proper
means, this image may be transformed into a visible one of
dazzling brightness. It might, moreover, be shown, if time
permitted, how, out of those perfectly dark rays, could be
extracted, by a process of transmutation, all the colours of the
solar spectrum. It might also be proved that those rays, powerful
as they are, and sufficient to fuse many metals, can be permitted
to enter the eye, and to break upon the retina, without producing
the least luminous impression.

The dark rays being thus collected, you see nothing at their
place of convergence. With a proper thermometer it could be
proved that even the air at the focus is just as cold as the
surrounding air. And mark the conclusion to which this leads. It
proves the aether at the focus to be practically detached from
the air, — that the most violent aethereal motion may there
exist, without the least aerial motion. But, though you see it
not, there is sufficient heat at that focus to set London on
fire. The heat there is competent to raise iron to a temperature
at which it throws off brilliant scintillations. It can heat
platinum to whiteness, and almost fuse that refractory metal. It
actually can fuse gold, silver, copper, and aluminium. The
moment, moreover, that wood is placed at the focus it bursts into
a blaze.

It has been already affirmed that, whether as regards
radiation or absorption, the elementary atoms possess but little
power. This might be illustrated by a long array of facts; and
one of the most singular of these is furnished by the deportment
of that extremely combustible substance, phosphorus, when placed
at the dark focus. It is impossible to ignite there a fragment of
amorphous phosphorus. But ordinary phosphorus is a far quicker
combustible, and its deportment towards radiant heat is still
more impressive. It may be exposed to the intense radiation of an
ordinary fire without bursting into flame. It may also be exposed
for twenty or thirty seconds at an obscure focus, of sufficient
power to raise platinum to a red heat, without ignition.
Notwithstanding the energy of the aethereal waves here
concentrated, notwithstanding the extremely inflammable character
of the elementary body exposed to their action, the atoms of that
body refuse to partake of the motion of the powerful waves of low
refrangibility, and consequently cannot be affected by their
heat.

The knowledge we now possess will enable us to analyse with
profit a practical question. White dresses are worn in summer,
because they are found to be cooler than dark ones. The
celebrated Benjamin Franklin placed bits of cloth of various
colours upon snow, exposed them to direct sunshine, and found
that they sank to different depths in the snow. The black cloth
sank deepest, the white did not sink at all. Franklin inferred
from this experiment that black bodies are the best absorbers,
and white ones the worst absorbers, of radiant heat. Let us test
the generality of this conclusion. One of these two cards is
coated with a very dark powder, and the other with a perfectly
white one. I place the powdered surfaces before a fire, and leave
them there until they have acquired as high a temperature as they
can attain in this position. Which of the cards is then most
highly heated? It requires no thermometer to answer this
question. Simply pressing the back of the card, on which the
white powder is strewn, against the cheek or forehead, it is
found intolerably hot. Placing the dark card in the same
position, it is found cool. The white powder has absorbed far
more heat than the dark one. This simple result abolishes a
hundred conclusions which have been hastily drawn from the
experiments of Franklin. Again, here are suspended two delicate
mercurial thermometers at the same distance from a gas-flame. The
bulb of one of them is covered by a dark substance, the bulb of
the other by a white one. Both bulbs have received the radiation
from the flame, but the white bulb has absorbed most, and its
mercury stands much higher than that of the other thermometer.
This experiment might be varied in a hundred ways: it proves that
from the darkness of a body you can draw no certain conclusion
regarding its power of absorption.

The reason of this simply is, that colour gives us
intelligence of only one portion, and that the smallest one, of
the rays impinging on the coloured body. Were the rays all
luminous, we might with certainty infer from the colour of a body
its power of absorption; but the great mass of the radiation from
our fire, our gas-flame, and even from the sun itself, consists
of invisible calorific rays, regarding which colour teaches us
nothing. A body may be highly transparent to the one class of
rays, and highly opaque to the other. Thus the white powder,
which has shown itself so powerful an absorber, has been
specially selected on account of its extreme perviousness to the
visible rays, and its extreme imperviousness to the invisible
ones; while the dark powder was chosen on account of its extreme
transparency to the invisible, and its extreme opacity to the
visible, rays. In the case of the radiation from our fire, about
98 per cent of the whole emission consists of invisible rays; the
body, therefore, which was most opaque to these triumphed as an
absorber, though that body was a white one.

And here it is worth while to consider the manner in which we
obtain from natural facts what may be called their intellectual
value. Throughout the processes of Nature we have interdependence
and harmony; and the main value of physics, considered as a
mental discipline, consists in the tracing out of this
interdependence, and the demonstration of this harmony. The
outward and visible phenomena are the counters of the intellect;
and our science would not be worthy of its name and fame if it
halted at facts, however practically useful, and neglected the
laws which accompany and rule the phenomena. Let us endeavour,
then, to extract from the experiment of Franklin all that it can
yield, calling to our aid the knowledge which our predecessors
have already stored. Let us imagine two pieces of cloth of the
same texture, the one black and the other white, placed upon
sunned snow. Fixing our attention on the white piece, let us
enquire whether there is any reason to expect that it will sink
in the snow at all. There is knowledge at hand which enables us
to reply at once in the negative. There is, on the contrary,
reason to expect that, after a sufficient exposure, the bit of
cloth will be found on an eminence instead of in a hollow; that
instead of a depression, we shall have a relative elevation of
the bit of cloth. For, as regards the luminous rays of the sun,
the cloth and the snow are alike powerless; the one cannot be
warmed, nor the other melted, by such rays. The cloth is white
and the snow is white, because their confusedly mingled fibres
and particles are incompetent to absorb the luminous rays.
Whether, then, the cloth will sink or not depends entirely upon
the dark rays of the sun. Now the substance which absorbs these
dark rays with the greatest avidity is ice, — or snow,
which is merely ice in powder. Hence, a less amounts of heat will
be lodged in the cloth than in the surrounding snow. The cloth
must therefore act as a shield to the snow on which it rests;
and, in consequence of the more rapid fusion of the exposed snow,
its shield must, in due time, be left behind, perched upon an
eminence like a glacier-table.

But though the snow transcends the cloth, both as a radiator
and absorber, it does not much transcend it. Cloth is very
powerful in both these respects. Let us now turn our attention to
the piece of black cloth, the texture and fabric of which I
assume to be the same as that of the white. For our object being
to compare the effects of colour, we must, in order to study this
effect in its purity, preserve all the other conditions constant.
Let us then suppose the black cloth to be obtained from the
dyeing of the white. The cloth itself, without reference to the
dye, is nearly as good an absorber of heat as the snow around it.
But to the absorption of the dark solar rays by the undyed cloth,
is now added the absorption of the whole of the luminous rays,
and this great additional influx of heat is far more than
sufficient to turn the balance in favour of the black cloth. The
sum of its actions on the dark and luminous rays, exceeds the
action of the snow on the dark rays alone. Hence the cloth will
sink in the snow, and this is the complete analysis of Franklin’s
experiments.

Throughout this discourse the main stress has been laid on
chemical constitution, as influencing most powerfully the
phenomena of radiation and absorption.

With regard to gases and vapours, and to the liquids from
which these vapours are derived, it has been proved by the most
varied and conclusive experiments that the acts of radiation and
absorption are molecular — that they depend upon chemical,
and not upon mechanical, condition. In attempting to extend this
principle to solids I was met by a multitude of facts, obtained
by celebrated experimenters, which seemed flatly to forbid such
an extension. Mellon, for example, had found the same radiant and
absorbent power for chalk and lamp-black. MM. Masson and
Courtépée had performed a most elaborate series of
experiments on chemical precipitates of various kinds, and found
that they one and all manifested the same power of radiation.
They concluded from their researches, that when bodies are
reduced to an extremely fine state of division, the influence of
this state is so powerful as entirely to mask and override
whatever influence may be due to chemical constitution.

But it appears to me that through the whole of these
researches an oversight has run, the mere mention of which will
show what caution is essential in the operations of experimental
philosophy; while an experiments or two will make clear wherein
the oversight consists. Filling a brightly polished metal cube
with boiling water, I determine the quantity of heat emitted by
two of the bright surfaces. As a radiator of heat one of them far
transcends the other. Both surfaces appear to be metallic; what,
then, is the cause of the observed difference in their radiative
power? Simply this: one of the surfaces is coated with
transparent gum, through which, of course, is seen the metallic
lustre behind; and this varnish, though so perfectly transparent
to luminous rays, is as opaque as pitch, or lamp-black, to
non-luminous ones. It is a powerful emitter of dark rays; it is
also a powerful absorber. While, therefore, at the present
moment, it is copiously pouring forth radiant heat itself, it
does not allow a single ray from the metal behind to pass through
it. The varnish then, and not the metal, is the real
radiator.

Now Melloni, and Masson, and Courtépée
experimented thus: they mixed their powders and precipitates with
gum-water, and laid them, by means of a brush, upon the surfaces
of a cube like this. True, they saw their red powders red, their
white ones white, and their black ones black, but they saw these
colours through the coat of varnish which surrounded every
particle. When, therefore, it was concluded that colour had no
influence on radiation, no chance had been given to it of
asserting its influence; when it was found that all chemical
precipitates radiated alike, it was the radiation from a varnish,
common to them all, which showed the observed constancy.
Hundreds, perhaps thousands, of experiments on radiant heat have
been performed in this way, by various enquirers, but the work
will, I fear, have to be done over again. I am not, indeed,
acquainted with an instance in which an oversight of so trivial a
character has been committed by so many able men in succession,
vitiating so large an amounts of otherwise excellent work. Basing
our reasonings thus on demonstrated facts, we arrive at the
extremely probable conclusion that the envelope of the particles,
and not the particles themselves, was the real radiator in the
experiments just referred to. To reason thus, and deduce their
more or less probable consequences from experimental facts, is an
incessant exercise of the student of physical science. But having
thus followed, for a time, the light of reason alone through a
series of phenomena, and emerged from them with a purely
intellectual conclusion, our duty is to bring that conclusion to
an experimental test. In this way we fortify our science.

For the purpose of testing our conclusion regarding the
influence of the gum, I take two powders presenting the same
physical appearance; one of them is a compound of mercury, and
the other a compound of lead. On two surfaces of a cube are
spread these bright red powders, without varnish of any kind.
Filling the cube with boiling water, and determining the
radiation from the two surfaces, one of them is found to emit
thirty-nine units of heat, while the other emits seventy-four.
This, surely, is a great difference. Here, however, is a second
cube, having two of its surfaces coated with the same powders,
the only difference being that the powders are laid on by means
of a transparent gum. Both surfaces are now absolutely alike in
radiative power. Both of them emit somewhat more than was emitted
by either of the unvarnished powders, simply because the gum
employed is a better radiator than either of them. Excluding all
varnish, and comparing white with white, vast differences are
found; comparing black with black, they are also different; and
when black and white are compared, in some cases the black
radiates far more than the white, while in other cases the white
radiates far more than the black. Determining, moreover, the
absorptive power of those powders, it is found to go hand-in-hand
with their radiative power. The good radiator is a good absorber,
and the bad radiator is a bad absorber. From all this it is
evident that as regards the radiation and absorption of
non-luminous heat, colour teaches us nothing; and that even as
regards the radiation of the sun, consisting as it does mainly of
non-luminous rays, conclusions as to the influence of colour may
be altogether delusive. This is the strict scientific upshot of
our researches. But it is not the less
true that in the case of wearing apparel — and this for
reasons which I have given in analysing the experiments of
Franklin — black dresses are more potent than white ones as
absorbers of solar heat.

Thus, in brief outline, have been brought before you a few of
the results of recent enquiry. If you ask me what is the use of
them, I can hardly answer you, unless you define the term use. If
you meant to ask whether those dark rays which clear away the
Alpine snows, will ever be applied to the roasting of turkeys, or
the driving of steam-engines — while affirming their power
to do both, I would frankly confess that they are not at present
capable of competing profitably with coal in these particulars.
Still they may have great uses unknown to me; and when our
coal-fields are exhausted, it is possible that a more aethereal
race than we are may cook their victuals, and perform their work,
in this transcendental way. But is it necessary that the student
of science should have his labours tested by their possible
practical applications? What is the practical value of Homer’s
Iliad? You smile, and possibly think that Homer’s Iliad is good
as a means of culture. There’s the rub. The people who demand of
science practical uses, forget, or do not know, that it also is
great as a means of culture — that the knowledge of this
wonderful universe is a thing profitable in itself, and requiring
no practical application to justify its pursuit.

But while the student of Nature distinctly refuses to have his
labours judged by their practical issues, unless the term
practical be made to include mental as well as material good, he
knows full well that the greatest practical triumphs have been
episodes in the search after pure natural truth. The electric
telegraph is the standing wonder of this age, and the men whose
scientific knowledge, and mechanical skill, have made the
telegraph what it is, are deserving of all honour. In fact, they
have had their reward, both in reputation and in those more
substantial benefits which the direct service of the public
always carries in its train. But who, I would ask, put the soul
into this telegraphic body? Who snatched from heaven the fire
that flashes along the line? This, I am bound to say, was done by
two men, the one a dweller in Italy, [Footnote:
Volta] the other a dweller in England, [Footnote:
Faraday] who never in their enquiries consciously set a
practical object before them — whose only stimulus was the
fascination which draws the climber to a never-trodden peak, and
would have made Caesar quit his victories for the sources of the
Nile. That the knowledge brought to us by those prophets,
priests, and kings of science is what the world calls
‘useful knowledge,’ the triumphant application of their
discoveries proves. But science has another function to fulfil,
in the storing and the training of the human mind; and I would
base my appeal to you on the specimen which has this evening been
brought before you, whether any system of education at the
present day can be deemed even approximately complete, in which
the knowledge of Nature is neglected or ignored.

.

.

.

.

——————–

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.

IV. NEW CHEMICAL REACTIONS PRODUCED
BY LIGHT.

1868-69.

1. DECOMPOSITION BY
LIGHT.

MEASURED by their power, not to
excite vision, but to produce heat — in other words,
measured by their absolute energy — the ultra-red waves of
the sun and of the electric light, as shown in the preceding
articles, far transcend the visible. In the domain of chemistry,
however, there are numerous cases in which the more powerful
waves are ineffectual, while the more minute waves, through what
may be called their timeliness of application, are able to
produce great effects. A series of these, of a novel and
beautiful character, discovered in 1868, and further illustrated
in subsequent years, may be exhibited by subjecting the vapours of
volatile liquids to the action of concentrated sunlight, or to
the concentrated beam of the electric light. Their investigation
led up to the discourse on ‘Dust and Disease’ which follows
in this volume; and for this reason some account of them is
introduced here.

—–

A glass tube 3 feet long and 3 inches wide, which had been
frequently employed in my researches on radiant heat, was
supported horizontally on two stands. At one end of the tube was
placed an electric lamp, the height and position of both being so
arranged, that the axis of the tube, and that of the beam issuing
from the lamp, were coincident. In the first experiments the two
ends of the tube were closed by plates of rock-salt, and
subsequently by plates of glass. For the sake of distinction, I
call this tube the experimental tube. It was connected with an
air-pump, and also with a series of drying and other tubes used
for the purification of the air.

A number of test-tubes, like F, fig. 2 (I have used at least
fifty of them), were converted into Woulf’s flasks. Each of them
was stopped by a cork, through which passed two glass tubes: one
of these tubes (a) ended immediately below the cork, while the
other (b) descended to the bottom of the flask, being drawn out
at its lower end to an orifice about 0.03 of an inch in diameter.
It was found necessary to coat the cork carefully with cement. In
the later experiments corks of vulcanised India-rubber were
invariably employed.

The little flask, thus formed, being partially filled with the
liquid whose vapour was to be examined, was introduced into the
path of the purified current of air. The experimental tube being
exhausted, and the cock which cut off the supply of purified air
being cautiously turned on, the air entered the flask through the
tube b, and escaped by the small orifice at the lower end of b into
the liquid. Through this it bubbled, loading itself with vapour,
after which the mixed air and vapour, passing from the flask by
the tube a, entered the experimental tube, where they were
subjected to the action of light.

The whole arrangement is shown in fig. 3, where L represents
the electric lamp, s s’ the experimental tube, pp’ the pipe
leading to the air-pump, and F the test-tube containing the
volatile liquid. The tube t t’ is plugged with cotton-wool
intended to intercept the floating matter of the air; the bent
tube T’ contains caustic potash, the tube T sulphuric acid, the
one intended to remove the carbonic acid and the other the
aqueous vapour of the air.

The power of the electric beam to reveal the existence of
anything within the experimental tube, or the impurities of the
tube itself, is extraordinary. When the experiments is made in a
darkened room, a tube which in ordinary daylight appears
absolutely clean, is often shown by the present mode of
examination to be exceedingly filthy.

The following are some of the results obtained with this
arrangement :—

Nitrite of amyl. — The vapour of this
liquid was in the first instance permitted to enter the
experimental tube, while the beam from the electric lamp was
passing through it. Curious clouds, the cause of which was then
unknown, were observed to form near the place of entry, being
afterwards whirled through the tube.

The tube being again exhausted, the mixed air and vapour were
allowed to enter it in the dark. The slightly convergent beam of
the electric light was then sent through the mixture. For a
moment the tube was optically empty, nothing whatever being seen
within it; but before a second had elapsed a shower of particles
was precipitated on the beam. The cloud thus generated became
denser as the light continued to act, slowing at some places
vivid iridescence.

The lens of the electric lamp was now placed so as to form
within the tube a strongly convergent cone of rays. the tube was
cleansed and again filled in darkness. When the light was sent
through it, the precipitation upon the beam was so rapid and
intense that the cone, which a moment before was invisible,
flashed suddenly forth like a solid luminous spear. The effect
was the same when the air and vapour were allowed to enter the
tube in diffuse daylight. The cloud, however, which shone with
such extraordinary radiance under the electric beam, was
invisible in the ordinary light of the laboratory.

The quantity of mixed air and vapour within the experimental
tube could of course be regulated at pleasure. The rapidity of
the action diminished with the attenuation of the vapour. When,
for example, the mercurial column associated with the
experimental tube was depressed only five inches, the action was
not nearly so rapid as when the tube was full. In such cases,
however, it was exceedingly interesting to observe, after some
seconds of waiting, a thin streamer of delicate bluish-white
cloud slowly forming along the axis of the tube, and finally
swelling so as to fill it.

.

Fig. 2.

Fig. 3.

When dry oxygen was employed to carry in the vapour the effect
was the same as that obtained with air.

When dry hydrogen was used as a vehicle, the effect was also
the same.

The effect, therefore, is not due to any interaction between
the vapour of the nitrite and its vehicle.

This was further demonstrated by the deportment of the vapour
itself. When it was permitted to enter the experimental tube
unmixed with air or any other gas, the effect was substantially
the same. Hence the seat of the observed action is the
vapour.

This action is not to be ascribed to heat. As regards the
glass of the experimental tube, and the air within the tube, the
beam employed in these experiments was perfectly cold. It had
been sifted by passing it through a solution of alum, and through
the thick double-convex lens of the lamp. When the unsifted beam
of the lamp was employed, the effect was still the same; the
obscure calorific rays did not appear to interfere with the
result.

My object here being simply to point out to chemists a method
of experiments which reveals a new and beautiful series of
reactions, I left to them the examination of the products of
decomposition. The group of atoms forming the molecule of nitrite
of amyl is obviously shaken asunder by certain specific waves of
the electric beam, nitric oxide and other products, of which the
nitrate of amyl is probably one, being the result of the
decomposition. The brown fumes of nitrous acid were seen mingling
with the cloud within the experimental tube. The nitrate of amyl,
being less volatile than the nitrite, and not being able to
maintain itself in the condition of vapour, would be precipitated
as a visible cloud along the track of the beam.

In the anterior portions of the tube a powerful sifting of the
beam by the vapour occurs, which diminishes the chemical action
in the posterior portions. In some experiments the precipitated
cloud only extended halfway down the tube. When, under these
circumstances, the lamp was shifted so as to send the beam
through the other end of the tube, copious precipitation occurred
there also.

Solar light also effects the decomposition of the
nitrite-of-amyl vapour. On October 10, 1868, I partially darkened
a small room in the Royal Institution, into which the sun shone,
permitting the light to enter through an open portion of the
window-shutter. In the track of the beam was placed a large
plano-convex lens, which formed a fine convergent cone in the
dust of the room behind it. The experimental tube was filled in
the laboratory, covered with a black cloth, and carried into the
partially darkened room. On thrusting one end of the tube into
the cone of rays behind the lens, precipitation within the cone
was copious and immediate. The vapour at the distant end of the
tube was in part shielded by that in front, and was also more
feebly acted on through the divergence of the rays. On reversing
the tube, a second and similar cone was precipitated.

Physical
Considerations.

I sought to determine the
particular portion of the light which produced the foregoing
effects. When, previous to entering the experimental tube, the
beam was caused to pass through a red glass, the effect was
greatly weakened, but not extinguished. This was also the case
with various samples of yellow glass. A blue glass being
introduced before the removal of the yellow or the red, on taking
the latter away prompt precipitation occurred along the track of
the blue beam. Hence, in this case, the more refrangible rays are
the most chemically active. The colour of the liquid nitrite of
amyl indicates that this must be the case; it is a feeble but
distinct yellow: in other words, the yellow portion of the beam
is most freely transmitted. It is not, however, the transmitted
portion of any beam which
produces chemical action, but the absorbed portion. Blue, as
the complementary colour to yellow, is here absorbed, and hence
the more energetic action of the blue rays.

This reasoning, however, assumes that the same rays are
absorbed by the liquid and its vapour. The assumption is worth
testing. A solution of the yellow chromate of potash, the colour
of which may be made almost, if not altogether, identical with
that of the liquid nitrite of amyl, was found far more effective
in stopping the chemical rays than either the red or the yellow
glass. But of all substances the liquid nitrite itself is most
potent in arresting the rays which act upon its vapour. A layer
one-eighth of an inch in thickness, which scarcely perceptibly
affected the luminous intensity, absorbed the entire chemical
energy of the concentrated beam of the electric light.

The close relation subsisting between a liquid and its vapour,
as regards their action upon radiant heat, has been already amply
demonstrated. [Footnote: ‘Phil. Trans.’ 1864; ‘Heat,
a Mode of Motion,’ chap, xii.; and P. 61 of this volume.]
As regards the nitrite of amyl, this relation is more specific
than in the cases hitherto adduced; for here the special
constituent of the beam, which provokes the decomposition of the
vapour, is shown to be arrested by the liquid.

A question of extreme importance in molecular physics here
arises: What is the real mechanism of this absorption, and where
is its seat? [Footnote: My attention was very forcibly
directed to this subject some years ago by a conversation with my
excellent friend Professor Clausius.]

I figure, as others do, a molecule as a group of atoms, held
together by their mutual forces, but still capable of motion
among themselves. The vapour of the nitrite of amyl is to be
regarded as an assemblage of such molecules. The question now
before us is this: In the act of absorption, is it the molecules
that are effective, or is it their constituent atoms? Is the
vis viva of the intercepted light-waves transferred
to the molecule as a whole, or to its constituent parts?

The molecule, as a whole, can only vibrate in virtue of the
forces exerted between it and its neighbour molecules. The
intensity of these forces, and consequently the rate of
vibration, would, in this case, be a function of the distance
between the molecules. Now the identical absorption of the liquid
and of the vaporous nitrite of amyl indicates an identical
vibrating period on the part of liquid and vapour, and this, to
my mind, amounts to an experimental proof that the absorption
occurs in the main within the molecule. For it can hardly be
supposed, if the absorption were the act of the molecule as a
whole, that it could continue to affect waves of the same period
after the substance had passed from the vaporous to the liquid
state.

In point of fact, the decomposition of the nitrite of amyl is
itself to some extent an illustration of this internal molecular
absorption; for were the absorption the act of the molecule as a
whole, the relative motions of its constituent atoms would remain
unchanged, and there would be no mechanical cause for their
separation. It is probably the synchronism of the vibrations of
one portion of the molecule with the incident waves, that enables
the amplitude of those vibrations to augment, until the chain
which binds the parts of the molecule together is snapped
asunder.

I anticipate wide, if not entire, generality for the fact that
a liquid and its vapour absorb the same rays. A cell of liquid
chlorine would, I imagine, deprive light more effectually of its
power of causing chlorine and hydrogen to combine than any other
filter of the luminous rays. The rays which give chlorine its
colour have nothing to do with this combination, those that are
absorbed by the chlorine being the really effective rays. A
highly sensitive bulb, containing chlorine and hydrogen, in the
exact proportions necessary for the formation of hydrochloric
acid, was placed at one end of an experimental tube, the beam of
the electric lamp being sent through it from the other. The bulb
did not explode when the tube was filled with chlorine, while the
explosion was violent and immediate when the tube was filled with
air. I anticipate for the liquid chlorine an action similar to,
but still more energetic than, that exhibited by the gas. If this
should prove to be the case, it will favour the view that
chlorine itself is molecular and not monatomic.

Production of Sky-blue by the
Decomposition of Nitrite of Amyl.

When the quantity of nitrite
vapour is considerable, and the light intense, the chemical
action is exceedingly rapid, the particles precipitated being so
large as to whiten the luminous beam. Not so, however, when a
well-mixed and highly attenuated vapour fills the experimental
tube. The effect now to be described was first obtained when the
vapour of the nitrite was derived from a portion of its liquid
which had been accidentally introduced into the passage through
which the dry air flowed into the experimental tube.

In this case, the electric beam traversed the tube for several
seconds before any action was visible. Decomposition then visibly
commenced, and advanced slowly. _When the light was very strong,
the cloud appeared of a milky blue. When, on the contrary, the
intensity was moderate, the blue was pure and deep. In
Brücke’s important experiments on the blue of the sky and
the morning and evening red, pure mastic is dissolved in alcohol,
and then dropped into water well stirred. When the proportion of
mastic to alcohol is correct, the resin is precipitated so finely
as to elude the highest microscopic power. By reflected light,
such a medium appears bluish, by transmitted light yellowish,
which latter colour, by augmenting the quantity of the
precipitate, can be caused to pass into orange or red.

But the development of colour in the attenuated
nitrite-of-amyl vapour is doubtless more similar to what takes
place in our atmosphere. The blue, moreover, is far purer and
more sky-like than that obtained from Bruecke’s turbid medium.
Never, even in the skies of the Alps, have I seen a richer or a
purer blue than that attainable by a suitable disposition of the
light falling upon the precipitated vapour.

Iodide of Allyl. — Among the liquids
hitherto subjected to the concentrated electric light, iodide of
allyl, in point of rapidity and intensity of action, comes next
to the nitrite of amyl. With the iodide I have employed both
oxygen and hydrogen, as well as air, as a vehicle, and found the
effect in all cases substantially the same. The cloud-column here
was exquisitely beautiful. It revolved round the axis of the
decomposing beam; it was nipped at certain places like an
hour-glass, and round the two bells of the glass delicate
cloud-filaments twisted themselves in spirals. It also folded
itself into convolutions resembling those of shells. In certain
conditions of the atmosphere in the Alps I have often observed
clouds of a special pearly lustre; when hydrogen was made the
vehicle of the iodide-of allyl vapour a similar lustre was most
exquisitely shown. With a suitable disposition of the light, the
purple hue of iodine-vapour came out very strongly in the
tube.

The remark already made, as to the bearing of the
decomposition of nitrite of amyl by light on the question of
molecular absorption, applies here also; for were the absorption
the work of the molecule as a whole, the iodine would not be
dislodged from the allyl with which it is combined. The
non-synchronism of iodine with the waves of obscure heat is
illustrated by its marvellous transparency to such heat. May not
its synchronism with the waves of light in the present instance
be the cause of its divorce from the allyl?

Iodide of Isopropyl. — The action of light
upon the vapour of this liquid is, at first, more languid than
upon iodide of allyl; indeed many beautiful reactions may be
overlooked, in consequence of this languor at the commencement.
After some minutes’ exposure, however, clouds begin to form,
which grow in density and in beauty as the light continues to
act. In every experiment hitherto made with this substance the
column of cloud filling the experimental tube, was divided into
two distinct parts near the middle of the tube. In one
experiments a globe of cloud formed at the centre, from which,
right and left, issued an axis uniting the globe with two
adjacent cylinders. Both globe and cylinders were animated by a
common motion of rotation. As the action continued, paroxysms of
motion were manifested; the various parts of the cloud would rush
through each other with sudden violence. During these motions
beautiful and grotesque cloud-forms were developed. At some
places the nebulous mass would become ribbed so as to resemble
the graining of wood; a longitudinal motion would at times
generate in it a series of curved, transverse bands, the
retarding influence of the sides the tube causing an appearance
resembling, on a small scale, the dirt-bands of the Mer de Glace.
In the anterior portion of the tube those sudden commotion were
most intense; here buds of cloud would sprout forth, and grow in
a few seconds into perfect flower-like forms. The cloud of iodide
of isopropyl had a character Of its own, and differed materially
from all others that I had seen. A gorgeous mauve colour was
observed in the last twelve inches of the tube; the vapour of
iodine was present, and it may have been the sky-blue scattered
by the precipitated particles which, mingling with the purple of
the iodine, produced the mauve. As in all other cases here
adduced, the effects were proved to be due to the light; they
never occurred in darkness.

The forms assumed by some of those actinic clouds, as I
propose to call them, in consequence of rotations and other
motions, due to differences of temperature, are perfectly
astounding. I content myself here with a meagre description of
one more of them.

The tube being filled with the sensitive mixture, the beam was
sent through it, the lens at the same time being so placed as to
produce a cone of very intense light. Two minutes elapsed before
anything was visible; but at the end of this time a faint bluish
cloud appeared to hang itself on the most concentrated portion of
the beam.

Soon afterwards a second cloud was formed five inches farther
down the experimental tube. Both clouds were united by a slender
cord of the same bluish tint as themselves.

As the action of the light continued, the first cloud
gradually resolved itself into a series of parallel disks of
exquisite delicacy, which rotated round an axis perpendicular to
their surfaces, and finally blended to a screw surface with an
inclined generatrix. This gradually changed into a filmy funnel,
from the narrow end of which the ‘cord’ extended to the
cloud in advance.

The latter also underwent slow but incessant modification. It
first resolved itself into a series of strata resembling those of
the electric discharge. After a little time, and through changes
which it was difficult to follow, both clouds presented the
appearance of a series of concentric funnels set one within the
other, the interior ones being seen through the outer ones. Those
of the distant cloud resembled claret-glasses in shape. As many
as six funnels were thus concentrically set together, the two
series being united by the delicate cord of cloud already
referred to. Other cords and Blender tubes were afterwards
formed, which coiled themselves in delicate spirals around the
funnels.

Rendering the light along the connecting-cord more intense, it
diminished in thickness and became whiter; this was a consequence
of the enlargement of its particles. The cord finally
disappeared, while the funnels melted into two ghost-like films,
shaped like parasols. They were barely visible, being of an
exceedingly delicate blue tint. They seemed woven of blue air. To
compare them with cobweb or with gauze would be to liken them to
something infinitely grosser than themselves.

In all cases a distant candle-flame, when looked at through
the cloud, was sensibly undimmed.

.

.

.

.

§ 2. ON THE BLUE COLOUR OF THE
SKY, AND THE POLARISATION OF SKYLIGHT.

[Footnote: In my
‘Lectures on Light’ (Longman), the polarisation of light will be
found briefly, but, I trust, clearly explained.]

1869.

After the communication to the Royal Society of the foregoing
brief account of a new Series of Chemical Reactions produced by
Light, the experiments upon this subject were continued, the
number of substances thus acted on being considerably
increased.

I now, however, beg to direct attention to two questions
glanced at incidentally in the preceding pages — the blue
colour of the sky, and the polarisation of skylight. Reserving
the historic treatment of the subject for a more fitting
occasion, I would merely mention now that these questions
constitute, in the opinion of our most eminent authorities, the
two great standing enigmas of meteorology. Indeed it was the
interest manifested in them by Sir John Herschel, in a letter of
singular speculative power, addressed to myself, that caused me
to enter upon the consideration of these questions so soon.

The apparatus with which I work consists, as already stated,
of a glass tube about a yard in length, and from 2.5 to 3 inches
internal diameter. The vapour to be examined is introduced into
this tube in the manner already described, and upon it the
condensed beam of the electric lamp is permitted to act, until
the neutrality or the activity of the substance has been
declared.

It has hitherto been my aim to render the chemical action of
light upon vapours visible. For this purpose substances have been
chosen, one at least of whose products of decomposition under
light shall have a boiling-point so high, that as soon as the
substance is formed it shall be precipitated. By graduating the
quantity of the vapour, this precipitation may be rendered of any
degree of fineness, forming particles distinguishable by the
naked eye, or far beyond the reach of our highest microscopic
powers. I have no reason to doubt that particles may be thus
obtained, whose diameters constitute but a small fraction of the
length of a wave of violet light.

In all cases when the vapours of the liquids employed are
sufficiently attenuated, no matter what the liquid may be, the
visible action commences with the formation of a blue cloud. But
here I must guard myself against all misconception as to the use
of this term. The ‘cloud’ here referred to is totally invisible
in ordinary daylight. To be seen, it requires to be surrounded by
darkness, it only being illuminated by a powerful beam of light.
This blue cloud differs in many important particulars from the
finest ordinary clouds, and might justly have assigned to it an
intermediate position between such clouds and true vapour. With
this explanation, the term ‘cloud,’ or ‘incipient cloud,’ or
‘actinic cloud,’ as I propose to employ it, cannot, I
think, be misunderstood.

I had been endeavouring to decompose carbonic acid gas by
light. A faint bluish cloud, due it may be, or it may not be, to
the residue of some vapour previously employed, was formed in the
experimental tube. On looking across this cloud through a Nicol’s
prism, the line of vision being horizontal, it was found that
when the short diagonal of the prism was vertical, the quantity
of light reaching the eye was greater than when the long diagonal
was vertical. When a plate of tourmaline was held between the eye
and the bluish cloud, the quantity of light reaching the eye when
the axis of the prism was perpendicular to the axis of the
illuminating beam, was greater than when the axes of the crystal
and of the beam were parallel to each other.

This was the result all round the experimental tube. Causing
the crystal of tourmaline to revolve round the tube, with its
axis perpendicular to the illuminating beam, the quantity of
light that reached the eye was in all its positions a maximum.
When the crystallographic axis was parallel to the axis of the
beam, the quantity of light transmitted by the crystal was a
minimum.

From the illuminated bluish
cloud, therefore, polarised light was discharged, the direction
of maximum polarisation being at right angles to the illuminating
beam; the plane of vibration of the polarised light was
perpendicular to the beam. [Footnote: This is still an
undecided point; but the probabilities are so much in its favour,
and it is in my opinion so much preferable to have a physical
image on which the mind can rest, that I do not hesitate to
employ the phraseology in the text.]

Thin plates of selenite or of quartz, placed between the Nicol
and the actinic cloud, displayed the colours of polarised light,
these colours being most vivid when the line of vision was at
right angles to the experimental tube. The plate of selenite
usually employed was a circle, thinnest at the centre, and
augmenting uniformly in thickness from the centre outwards. When
placed in its proper position between the Nicol and the cloud, it
exhibited a system of splendidly-coloured rings.

The cloud here referred to was the first operated upon in the
manner described. It may, however, be greatly improved upon by
the choice of proper substances, and by the application, in
proper quantities, of the substances chosen. Benzol, bisulphide
of carbon, nitrite of amyl, nitrite of butyl, iodide of allyl,
iodide of isopropyl, and many other substances may be employed. I
will take the nitrite of butyl as illustrative of the means
adopted to secure the best result, with reference to the present
question.

And here it may be mentioned that a vapour, which when alone,
or mixed with air in the experimental tube, resists the action of
light, or shows but a feeble result of this action, may, when
placed in proximity with another gas or vapour, exhibit vigorous,
if not violent action. The case is similar to that of carbonic
acid gas, which, diffused in the atmosphere, resists the
decomposing action of solar light, but when placed in contiguity
with chlorophyl in the leaves of plants, has its molecules shaken
asunder.

Dry air was permitted to bubble through the liquid nitrite of
butyl, until the experimental tube, which had been previously
exhausted, was filled with the mixed air and vapour. The visible
action of light upon the mixture after fifteen minutes’ exposure
was slight. The tube was afterwards filled with half an
atmosphere of the mixed air and vapour, and a second
half-atmosphere of air which had been permitted to bubble through
fresh commercial hydrochloric acid. On sending the beam through
this mixture, the tube, for a moment, was optically empty. But
the pause amounted only to a small fraction of a second, a dense
cloud being immediately precipitated upon the beam.

This cloud began blue, but the advance to whiteness was so
rapid as almost to justify the application of the term
instantaneous. The dense cloud, looked at perpendicularly to its
axis, showed scarcely any signs of polarisation. Looked at
obliquely the polarisation was strong.

The experimental tube being again cleansed and exhausted, the
mixed air and nitrite-of-butyl vapour was permitted to enter it
until the associated mercury column was depressed 1/10 of an
inch. In other words, the air and vapour, united, exercised a
pressure not exceeding 1/300^th of an atmosphere. Air,
passed through a solution of hydrochloric acid, was then added,
till the mercury column was depressed three inches. The condensed
beam of the electric light was passed for some time through this
mixture without revealing anything within the tube competent to
scatter the light. Soon, however, a superbly blue cloud was
formed along, the track of the beam, and it continued blue
sufficiently long to permit of its thorough examination. The
light discharged from the cloud, at right angles to its own
length, was at first perfectly polarised. It could be totally
quenched by the Nicol. By degrees the cloud became of whitish
blue, and for a time the selenite colours, obtained by looking at
it normally, were exceedingly brilliant. The direction of maximum
polarisation was distinctly at right angles to the illuminating
beam. This continued to be the case as long as the cloud
maintained a decided blue colour, and even for some time after
the blue had changed to whitish blue. But, as the light continued
to act, the cloud became coarser and whiter, particularly at its
centre, where it at length ceased to discharge polarised light in
the direction of the perpendicular, while it continued to do so
at both ends.

But the cloud which had thus ceased to polarise the light
emitted normally, showed vivid selenite colours when looked at
obliquely, proving that the direction of maximum polarisation
changed with the texture of the cloud. This point shall receive
further illustration subsequently.

A blue, equally rich and more durable, was obtained by
employing the nitrite-of-butyl vapour in a still more attenuated
condition. The instance here cited is representative. In all
cases, and with all substances, the cloud formed at the
commencement, when the precipitated particles are sufficiently
fine, is blue, and it can be made to display a colour rivalling
that of the purest Italian sky. In all cases, moreover, this fine
blue cloud polarises perfectly the beam which illuminates it, the
direction of polarisation enclosing an angle of 90° with the
axis of the illuminating beam.

It is exceedingly interesting to observe both the perfection
and the decay of this polarisation. For ten or fifteen minutes
after its first appearance the light from a vividly illuminated
actinic cloud, looked at perpendicularly, is absolutely quenched
by a Nicol’s prism with its longer diagonal vertical. But as the
sky-blue is gradually rendered impure by the growth of the
particles — in other words, as real clouds begin to be
formed — the polarisation begins to decay, a portion of the
light passing through the prism in all its positions. It is
worthy of note, that for some time after the cessation of perfect
polarisation, the residual light which passes, when the Nicol is
in its position of minimum transmission, is of a gorgeous blue,
the whiter light of the cloud being extinguished.
[Footnote: This shows that particles too large to polarise
the blue, polarise perfectly light of lower
refrangibility.] When the cloud texture has become
sufficiently coarse to approximate to that of ordinary clouds,
the rotation of the Nicol ceases to have any sensible effect on
the quantity of light discharged normally.

The perfection of the polarisation, in a direction
perpendicular to the illuminating beam, is also illustrated by
the following experiments: A Nicol’s prism, large enough to
embrace the entire beam of the electric lamp, was placed between
the lamp and the experimental tube. A few bubbles of air, carried
through the liquid nitrite of butyl, were introduced into the
tube, and they were followed by about three inches (measured by
the mercurial gauge) of air which had passed through aqueous
hydrochloric acid. Sending the polarised beam through the tube, I
placed myself in front of it, my eye being on a level with its
axis, my assistant occupying a similar position behind the tube.
The short diagonal of the large Nicol was in the first instance
vertical, the plane of vibration of the emergent beam being
therefore also vertical. As the light continued to act, a superb
blue cloud, visible to both my assistant and myself, was slowly
formed. But this cloud, so deep and rich when looked at from the
positions mentioned, utterly disappeared when looked at
vertically downwards, or vertically upwards. Reflection from the
cloud was not possible in these directions. When the large Nicol
was slowly turned round its axis, the eye of the observer being
on the level of the beam, and the line of vision perpendicular to
it, entire extinction of the light emitted horizontally occurred
when the longer diagonal of the large Nicol was vertical. But now
a vivid blue cloud was seen when looked at downwards or upwards.
This truly fine experiments, which I contemplated making on my
own account, was first definitely suggested by a remark in a
letter addressed to me by Professor Stokes.

As regards the polarisation of skylight, the greatest
stumbling-block has hitherto been, that, in accordance with the
law of Brewster, which makes the index of refraction the tangent
of the polarising angle, the reflection which produces perfect
polarisation would require to be made in air upon air; and indeed
this led many of our most eminent men, Brewster himself among the
number, to entertain the idea of aerial molecular reflection.
[Footnote: ‘The cause of the polarisation is evidently a
reflection of the sun’s light upon something. The question
is on what? Were the angle of maximum polarisation 76°, we
should look to water or ice as the reflecting body, however
inconceivable the existence in a cloudless atmosphere and a hot
summer’s day of unevaporated molecules (particles?) of water. But
though we were once of this opinion, careful observation has
satisfied us that 90°, or thereabouts, is the correct angle,
and that therefore whatever be the body on which the light has
been reflected, if polarised by a single reflection, the
polarising angle must be 45°, and the index of refraction,
which is the tangent of that angle, unity; in other words, the
reflection would require to be made in air upon
air!’ (Sir John Herschel, ‘Meteorology,’ par. 233.)

Any particles, if small enough, will produce both the colour
and the polarisation of the sky. But is the existence of small
water-particles on a hot summer’s day in the higher regions of
our atmosphere inconceivable? It is to be remembered that the
oxygen and nitrogen of the air behave as a vacuum to radiant
heat, the exceedingly attenuated vapour of the higher atmosphere
being therefore in practical contact with the cold of
space.]

I have, however, operated upon
substances of widely different refractive indices, and therefore
of very different polarising angles as ordinarily defined, but
the polarisation of the beam, by the incipient cloud, has thus
far proved itself to be absolutely independent of the polarising
angle. The law of Brewster does not apply to matter in this
condition, and it rests with the undulatory theory to explain
why. Whenever the precipitated particles are sufficiently fine,
no matter what the substance forming the particles may be, the
direction of maximum polarisation is at right angles to the
illuminating beam, the polarising angle for matter in this
condition being invariably 45°.

Suppose our atmosphere surrounded by an envelope impervious to
light, but with an aperture on the sunward side through which a
parallel beam of solar light could enter and traverse the
atmosphere. Surrounded by air not directly illuminated, the track
of such a beam would resemble that of the parallel beam of the
electric lamp through an incipient cloud. The sunbeam would be
blue, and it would discharge laterally light in precisely the
same condition as that discharged by the incipient cloud. In
fact, the azure revealed by such a beam would be to all intents
and purposes that which I have called a ‘blue cloud.’
Conversely our ‘blue cloud’ is, to all intents and purposes, an
artificial sky.’ [Footnote: The opinion of Sir John
Herschel, connecting the polarisation and the blue colour of the
sky, is verified by the foregoing results. ‘The more the
subject [the polarisation of skylight] is considered,’ writes
this eminent philosopher, ‘the more it will be found beset with
difficulties, and its explanation when arrived at will probably
be found to carry with it that of the blue colour of the sky
itself, and of the great quantity of light it actually does send
down to us.’ ‘We may observe, too,’ he adds, ‘that it is
only where the purity of the sky is most absolute that the
polarisation is developed in its highest degree, and that where
there is the slightest perceptible tendency to cirrus it is
materially impaired.’ This applies word for word to our
‘incipient clouds.’ ]

But, as regards the polarisation of the sky, we know that not
only is the direction of maximum polarisation at right angles to
the track of the solar beams, but that at certain angular
distances, probably variable ones, from the sun, ‘neutral
points,’ or points of no polarisation, exist, on both sides of
which the planes of atmospheric polarisation are at right angles
to each other. I have made various observations upon this subject
which are reserved for the present; but, pending the more
complete examination of the question, the following facts bearing
upon it may be submitted.

The parallel beam employed in these experiments tracked its
way through the laboratory air, exactly as sunbeams are seen to
do in the dusty air of London. I have reason to believe that a
great portion of the matter thus floating in the laboratory air
consists of organic particles, which are capable of imparting a
perceptibly bluish tint to the air. These also showed, though far
less vividly, all the effects of polarisation obtained with the
incipient clouds. The light discharged laterally from the track
of the illuminating beam was polarised, though not perfectly, the
direction of maximum polarisation being at right angles to the
beam. At all points of the beam, moreover, throughout its entire
length, the light emitted normally was in the same state of
polarisation. Keeping the positions of the Nicol and the selenite
constant, the same colours were observed throughout the entire
beam, when the line of vision was perpendicular to its
length.

The horizontal column of air, thus illuminated, was 18 feet
long, and could therefore be looked at very obliquely. I placed
myself near the end of the beam, as it issued from the electric
lamp, and, looking through the Nicol and selenite more and more
obliquely at the beam, observed the colours fading until they
disappeared. Augmenting the obliquity the colours appeared once
more, but they were now complementary to the former ones.

Hence this beam, like the sky, exhibited a neutral point, on
opposite sides of which the light was polarised in planes at
right angles to each other.

Thinking that the action observed in the laboratory might be
caused, in some way, by the vaporous fumes diffused in its air, I
had the light removed to a room at the top of the Royal
Institution. The track of the beam was seen very finely in the
air of this room, a length of 14 or 15 feet being attainable.
This beam exhibited all the effects observed with the beam in the
laboratory. Even the uncondensed electric light falling on the
floating matter showed, though faintly, the effects of
polarisation.

When the air was so sifted as to entirely remove the visible
floating matter, it no longer exerted any sensible action upon
the light, but behaved like a vacuum. The light is scattered and
polarised by particles, not by molecules or atoms.

By operating upon the fumes of chloride of ammonium, the smoke
of brown paper, and tobacco-smoke, I had varied and confirmed in
many ways those experiments on neutral points, when my attention
was drawn by Sir Charles Wheatstone to an important observation
communicated to the Paris Academy in 1860 by Professor Govi, of
Turin.[Footnote: Comptes Rendus,’ tome li, pp. 360 and
669.] M. Govi had been led to examine a beam of light sent
through a room in which were successively diffused the smoke of
incense, and tobacco-smoke. His first brief communication stated
the fact of polarisation by such smoke; but in his second
communication he announced the discovery of a neutral point in
the beam, at the opposite sides of which the light was polarised
in planes at right angles to each other.

But unlike my observations on the laboratory air, and unlike
the action of the sky, the direction of maximum polarisation in
M. Govi’s experiments enclosed a very small angle with the axis
of the illuminating beam. The question was left in this
condition, and I am not aware that M. Govi or any other
investigator has pursued it further.

I had noticed, as before stated, that as the clouds formed in
the experimental tube became denser, the polarisation of the
light discharged at right angles to the beam became weaker, the
direction of maximum polarisation becoming oblique to the beam.
Experiments on the fumes of chloride of ammonium gave me also
reason to suspect that the position of the neutral point was not
constant, but that it varied with the density of the illuminated
fumes.

The examination of these questions led to the following new
and remarkable results: The laboratory being well filled with the
fumes of incense, and sufficient time being allowed for their
uniform diffusion, the electric beam was sent through the smoke.
From the track of the beam polarised light was discharged; but
the direction of maximum polarisation, instead of being
perpendicular, now enclosed an angle of only 12° or 13°
with the axis of the beam.

A neutral point, with complementary effects at opposite sides
of it, was also exhibited by the beam. The angle enclosed by the
axis of the beam, and a line drawn from the neutral point to the
observer’s eye, measured in the first instance 66°.

The windows of the laboratory were now opened for some
minutes, a portion of the incense-smoke being permitted to
escape. On again darkening the room and turning on the light, the
line of vision to the neutral point was found to enclose, with
the axis of the beam, an angle of 63°.

The windows were again opened for a few minutes, more of the
smoke being permitted to escape. Measured as before, the angle
referred to was found to be 54°.

This process was repeated three additional times the neutral
point was found to recede lower and lower down the beam, the
angle between a line drawn from the eye to the neutral point and
the axis of the beam falling successively from 54° to
49°, 43° and 33°.

The distances, roughly measured, of the neutral point from the
lamp, corresponding to the foregoing series of observations, were
these :—

At the end of this series of
experiments the direction of maximum polarisation had again
become normal to the beam.

The laboratory was next filled with the fumes of gunpowder. In
five successive experiments, corresponding to five different
densities of the gunpowder-smoke, the angles enclosed between the
line of vision to the neutral point and the axis of the beam,
were 63 degrees, 50°, 47°, 42°, and 38°
respectively.

After the clouds of gunpowder had cleared away, the laboratory
was filled with the fumes of common resin, rendered so dense as
to be very irritating to the lungs. The direction of maximum
polarisation enclosed, in this case, an angle of 12°, or
thereabouts, with the axis of the beam. Looked at, as in the
former instances, from a position near the electric lamp, no
neutral point was observed throughout the entire extent of the
beam.

When this beam was looked at normally through the selenite and
Nicol, the ring-system, though not brilliant, was distinct.
Keeping the eye upon the plate of selenite, and the line of
vision perpendicular, the windows were opened, the blinds
remaining undrawn. The resinous fumes slowly diminished, and as
they did so the ring-system became paler. It finally disappeared.
Continuing to look in the same direction, the rings revived, but
now the colours were complementary to the former ones. The
neutral point had passed me in its motion down the beam,
consequent upon the attenuation of the fumes of resin.

With the fumes of chloride of ammonium substantially the same
results were obtained. Sufficient, however, has been here stated
to illustrate the variability of the position of the neutral
point.[Footnote: Brewster has proved the variability of
the position of the neutral point for skylight with the sun’s
altitude, a result obviously connected with the foregoing
experiments.]

By a puff of tobacco-smoke, or of condensed steam, blown into
the illuminated beam, the brilliancy of the selenite colours may
be greatly enhanced. But with different clouds two different
effects are produced. Let the ring-system observed in the common
air be brought to its maximum strength, and then let an
attenuated cloud of chloride of ammonium be thrown into the beam
at the point looked at; the ring system flashes out with
augmented brilliancy, but the character of the polarisation
remains unchanged. This is also the case when phosphorus, or
sulphur, is burned underneath the beam, so as to cause the fine
particles of phosphorus or of sulphur to rise into the light.
With the sulphur-fumes the brilliancy of the colours is
exceedingly intensified; but in none of these cases is there any
change in the character of the polarisation.

But when a puff of the fumes of hydrochloric acid, hydriodic
acid, or nitric acid is thrown into the beam, there is a complete
reversal of the selenite tints. Each of these clouds twists the
plane of polarisation 90°, causing the centre of the
ring-system to change from black to white, and the rings
themselves to emit their complementary colours. [Footnote:
Sir John Herschel suggested to me that this change of the
polarisation from positive to negative may indicate a change from
polarisation by reflection to polarisation by refraction. This
thought repeatedly occurred to me while looking at the effects;
but it will require much following up before it emerges into
clearness.]

Almost all liquids have motes in them sufficiently numerous to
polarise sensibly the light, and very beautiful effects may be
obtained by simple artificial devices. When, for example, a cell
of distilled water is placed in front of the electric lamp, and a
thin slice of the beam is permitted to pass through it, scarcely
any polarised light is discharged, and scarcely any colour
produced with a plate of selenite. But if a bit of soap be
agitated in the water above the beam, the moment the
infinitesimal particles reach the light the liquid sends forth
laterally almost perfectly polarised light; and if the selenite
be employed, vivid colours flash into existence. A still more
brilliant result is obtained with mastic dissolved in a great
excess of alcohol.

The selenite rings, in fact,
constitute an extremely delicate test as to the collective
quantity of individually invisible particles in a liquid.
Commencing with distilled water, for example, a thick slice of
light is necessary to make the polarisation of its suspended
particles sensible. A much thinner slice suffices for common
water; while, with Bruecke’s precipitated mastic, a slice too
thin to produce any sensible effect with most other liquids,
suffices to bring out vividly the selenite colours.

.

.

.

.

§ 3. THE SKY OF THE
ALPS.

The vision of an object always
implies a differential action on the retina of the observer. The
object is distinguished from surrounding space by its excess or
defect of light in relation to that space. By altering the
illumination, either of the object itself or of its environment,
we alter the appearance of the object. Take the case of clouds
floating in the atmosphere with patches of blue between them.
Anything that changes the illumination of either alters the
appearance of both, that appearance depending, as stated, upon
differential action. Now the light of the sky, being polarised,
may, as the reader of the foregoing pages knows, be in great part
quenched by a Nicol’s prism, while the light of a common cloud,
being unpolarised, cannot be thus extinguished. Hence the
possibility of very remarkable variations, not only in the aspect
of the firmament, which is really changed, but also in the aspect
of the clouds, which have that firmament as a background. It is
possible, for example, to choose clouds of such a depth of shade
that when the Nicol quenches the light behind them, they shall
vanish, being undistinguishable

from the residual dull tint which outlives the extinction of
the brilliancy of the sky. A cloud less deeply shaded, but still
deep enough, when viewed with the naked eye, to appear dark on a
bright ground, is suddenly changed to a white cloud on a dark
ground by the quenching of the light behind it. When a reddish
cloud at sunset chances to float in the region of maximum
polarisation, the quenching of the surrounding light causes it to
flash with a brighter crimson. Last Easter eve the Dartmoor sky,
which had just been cleansed by a snow-storm, wore a very wild
appearance. Round the horizon it was of steely brilliancy, while
reddish cumuli and cirri floated southwards. When the sky was
quenched behind them these floating masses seemed like dull
embers suddenly blown upon; they brightened like a fire.

In the Alps we have the most magnificent examples of crimson
clouds and snows, so that the effects just referred to may be
here studied under the best possible conditions. On August 23,
1869, the evening Alpenglow was very fine, though it did not
reach its maximum depth and splendour. The side of the Weisshorn
seen from the Bel Alp, being turned from the sun, was tinted
mauve; but I wished to observe one of the rose-coloured
buttresses of the mountain. Such a one was visible from a point a
few hundred feet above the hotel. The Matterhorn also, though for
the most part in shade, had a crimson projection, while a deep
ruddy red lingered along its western shoulder. Four distinct
peaks and buttresses of the Dom, in addition to its dominant head
— all covered with pure snow — were reddened by the
light of sunset. The shoulder of the Alphubel was similarly
coloured, while the great mass of the Fletschorn was all a-glow,
and so was the snowy spine of the Monte Leone.

Looking at the Weisshorn through the Nicol, the glow of its
protuberance was strong or weak according to the position of the
prism. The summit also underwent striking changes. In one
position of the prism it exhibited a pale white against a dark
background; in the rectangular position it was a dark mauve
against a light background. The red of the Matterhorn changed in
a similar manner; but the whole mountain also passed through
wonderful changes of definition. The air at the time was filled
with a silvery haze, in which the Matterhorn almost disappeared.
This could be wholly quenched by the Nicol, and then the mountain
sprang forth with astonishing solidity and detachment from the
surrounding air. The changes of the Dom were still more
wonderful. A vast amounts of light could be removed from the sky
behind it, for it occupied the position of maximum polarisation.
By a little practice with the Nicol it was easy to render the
extinction of the light, or its restoration, almost
instantaneous. When the sky was quenched, the four minor peaks
and buttresses, and the summit of the Dom, together with the
shoulder of the Alphubel, glowed as if set suddenly on fire. This
was immediately dimmed by turning the Nicol through an angle of
90°. It was not the stoppage of the light of the sky behind
the mountains alone which produced this startling effect; the air
between them and me was highly opalescent, and the quenching of
this intermediate glare augmented remarkably the distinctness of
the mountains.

On the morning of August 24 similar effects were finely shown.
At 10 A.M. all three mountains, the Dom, the Matterhorn, and the
Weisshorn, were powerfully affected by the Nicol. But in this
instance also, the line drawn to the Dom being very nearly
perpendicular to the solar beams, the effects on this mountain
were

most striking. The grey summit of the Matterhorn, at the same
time, could scarcely be distinguished from the opalescent haze
around it; but when the Nicol quenched the haze, the summit
became instantly isolated, and stood out in bold definition. It
is to be remembered that in the production of these effects the
only things changed are the sky behind, and the luminous haze in
front of the mountains; that these are changed because the light
emitted from the sky and from the haze is plane polarised light,
and that the light from the snows and from the mountains, being
sensibly unpolarised, is not directly affected by the Nicol. It
will also be understood that it is not the interposition of the
haze as an opaque body that renders the mountains
indistinct, but that it is the light of the haze which
dims and bewilders the eye, and thus weakens the definition of
objects seen through it.

These results have a direct bearing upon what artists call
‘aerial perspective.’ As we look from the summit of Mont Blanc,
or from a lower elevation, at the serried crowd of peaks,
especially if the mountains be darkly coloured — covered
with pines, for example — every peak and ridge is separated
from the mountains behind it by a thin blue haze which renders
the relations of the mountains as to distance unmistakable. When
this haze is regarded through the Nicol perpendicular to the
sun’s rays, it is in many cases wholly quenched, because the
light which it emits in this direction is wholly polarised. When
this happens, aerial perspective is abolished, and mountains very
differently distant appear to rise in the same vertical plane.
Close to the Bel Alp for instance, is the gorge of the Massa, and
beyond the gorge is a high ridge darkened by pines. This ridge
may be projected upon the dark slopes at the opposite side of the
Rhone valley, and between both we have the blue haze referred to,
throwing the distant mountains far away. But at certain hours of
the day the haze may be quenched, and then the Massa ridge and
the mountains beyond the Rhone seem almost equally distant from
the eye. The one appears, as it were, a vertical continuation of
the other. The haze varies with the temperature and humidity of
the atmosphere. At certain times and places it is almost as blue
as the sky itself; but to see its colour, the attention must be
withdrawn from the mountains and from the trees which cover them.
In point of fact, the haze is a piece of more or less perfect
sky; it is produced in the same manner, and is subject to the
same laws, as the firmament itself. We live in the sky, not under
it.

These points were further elucidated by the deportment of the
selenite, plate, with which the readers of the foregoing pages
are so well acquainted. On some of the sunny days of August the
haze in the valley of the Rhone, as looked at from the Bel Alp,
was very remarkable. Towards evening the sky above the mountains
opposite to my place of observation yielded a series of the most
splendidly-coloured iris-rings; but on lowering the selenite
until it had the darkness of the pines at the opposite side of
the Rhone ‘valley, instead of the darkness of space, as a
background, the colours were not much diminished in brilliancy. I
should estimate the distance across the valley, as the crow
flies, to the opposite mountain, at nine miles; so that a body of
air of this thickness can, under favourable circumstances,
produce chromatic effects of polarisation almost as vivid as
those produced by the sky itself.

Again: the light of a landscape, as of most other things,
consists of two parts; the one, coming purely from superficial
reflection, is always of the same colour

as the light which falls upon the landscape; the other Part
reaches us from a certain depth within the objects which compose
the landscape, and it is this portion of the total light which
gives these objects their distinctive colours. The white light of
the sun enters all substances to a certain depth, and is partly
ejected by internal reflection; each distinct substance absorbing
and reflecting the light, in accordance with the laws of its own
molecular constitution. Thus the solar light is sifted by the
landscape, which appears in such colours and variations of colour
as, after the sifting process, reach the observer’s eye. Thus the
bright green of grass, or the darker colour of the pine, never
comes to us alone, but is always mingled with an amounts of light
derived from superficial reflection. A certain hard brilliancy is
conferred upon the woods and meadows by this
superficially-reflected light. Under certain circumstances, it
may be quenched by a Nicol’s prism, and we then obtain the true
colour of the grass and foliage. Trees and meadows, thus
regarded, exhibit a richness and softness of tint which they
never show as long as the superficial light is permitted to
mingle with the true interior emission. The needles of the pines
show this effect very well, large-leaved trees still better;
while a glimmering field of maize exhibits the most extraordinary
variations when looked at through the rotating Nicol.

Thoughts and questions like those here referred to took me, in
August 1869, to the top of the Aletschhorn. The effects described
in the foregoing paragraphs were for the most part reproduced on
the summit of the mountain. I scanned the whole of the sky with
my Nicol. Both alone, and in conjunction with the selenite, it
pronounced the perpendicular to the solar beams to be the
direction of maximum polarisation.

But at no portion of the firmament was the polarisation
complete. The artificial sky produced in the experiments recorded
in the preceding pages could, in this respect, be rendered far
more perfect than the natural one; while the gorgeous ‘residual
blue’ which makes its appearance when the polarisation of the
artificial sky ceases to be perfect, was strongly contrasted with
the lack-lustre hue which, in the case of the firmament, outlived
the extinction of the brilliancy. With certain substances,
however, artificially treated, this dull residue may also be
obtained.

All along the arc from the Matterhorn to Mont Blanc the light
of the sky immediately above the mountains was powerfully acted
upon by the Nicol. In some cases the variations of intensity were
astonishing. I have already said that a little practice enables
the observer to shift the Nicol from one position to another so
rapidly as to render the alternative extinction and restoration
of the light immediate. When this was done along the arc to which
I have referred, the alternations of light and darkness resembled
the play of sheet lightning behind the mountains. There was an
element of awe connected with the suddenness with which the
mighty masses, ranged along the line referred to, changed their
aspect and definition under the operation of the prism.

—–

The physical reason of the blueness of both natural and
artificial skies is, I trust, correctly given in the essay on the
Scientific use of the Imagination published in the second volume
of these Fragments.

.

.

.

.

——————–

.

.

V. ON DUST AND DISEASE.

[Footnote: A
discourse delivered before the Royal Institution of Great
Britain, January 21, 1870.]

Experiments on Dusty
Air.

SOLAR light, in passing through
a dark room, reveals its track by illuminating the dust floating
in the air. ‘The sun,’ says Daniel Culverwell, ‘discovers atomes,
though they be invisible by candle-light, and makes them dance
naked in his beams.’

In my researches on the decomposition of vapours by light, I
was compelled to remove these ‘atoms’ and this dust. It was
essential that the space containing the vapours should embrace no
visible thing — that no substance capable of scattering
light in the slightest sensible degree should, at the outset of
an experiments, be found in the wide ‘experimental tube’ in which
the vapour was enclosed.

For a long time I was troubled by the appearance there of
floating matter, which, though invisible in diffuse daylight, was
at once revealed by a powerfully condensed beam. Two U-tubes were
placed in succession in the path of the air, before it entered
the liquid whose vapour was to be carried into the experimental
tube. One of the U-tubes contained fragments of marble wetted
with a strong solution of caustic potash; the other, fragments of
glass wetted with concentrated sulphuric acid which, while
yielding no vapour of its own, powerfully absorbs the aqueous
vapour of the air. [Footnote: The apparatus is figured at
p. 98.] To my astonishment, the air of the Royal
Institution, sent through these tubes at a rate sufficiently slow
to dry it, and to remove its carbonic acid, carried into the
experimental tube a considerable amounts of mechanically
suspended matter, which was illuminated when the beam passed
through the tube. The effect was substantially the same when the
air was permitted to bubble through the liquid acid, and through
the solution of potash.

I tried to intercept this floating matter in various ways; and
on October 5, 1868, prior to sending the air through the drying
apparatus, it was carefully permitted to pass over the tip of a
spirit-lamp flame. The floating matter no longer appeared, having
been burnt up by the flame. It was therefore organic matter. I
was by no means prepared for this result; having previously
thought that the dust of our air was, in great part, inorganic
and non-combustible. [Footnote: According to an analysis
kindly furnished to me by Dr. Percy, the dust collected from
the walls of the British Museum contains fully 50 per cent.
of inorganic matter. I have every confidence in the results of
this distinguished chemist; they show that the floating
dust of our rooms is, as it were, winnowed from the heavier
matter. As bearing directly upon this point I may quote the
following passage from Pasteur: ‘Mais ici se présente une
remarque: la poussière que Pon trouve à la surface
de tous les corps est soumise constamment à des courants
d’air, qui doivent soulever des particules les plus
légères, au nombre desquelles se trouvent, sans
doute, de préférence les corpuscules
organisés, oeufs ou spores, moins lourds
généralement que les particules
minérales.’]

I had constructed a small gas-furnace, now much employed by
chemists, containing a platinum tube, which could be heated to
vivid redness. [Footnote: Pasteur was, I believe, the
first to employ such a tube.] The tube contained a roll of
platinum gauze, which, while it permitted the air to pass through
it, ensured the practical contact of the dust with the
incandescent metal. The air of the laboratory was permitted to
enter the experimental tube, sometimes through the cold, and
sometimes through the heated, tube of platinum. In the first
column of the following fragment of a long table the quantity of
air operated on is expressed by the depression of the mercury
gauge of the air-pump. In the second column the condition of the
platinum tube is mentioned, and in the third the state of the air
in the experimental tube.

The phrase ‘optically
empty’ shows that when the conditions of perfect combustion were
present, the floating matter totally disappeared.

—–

In a cylindrical beam, which strongly illuminated the dust of
the laboratory, I placed an ignited spirit-lamp. Mingling with
the flame, and round its rim, were seen curious wreaths of
darkness resembling an intensely black smoke. On placing the
flame at some distance below the beam, the same dark masses
stormed upwards. They were blacker than the blackest smoke ever
seen issuing from the funnel of a steamer; and their resemblance
to smoke was so perfect as to lead the most practised observer to
conclude that the apparently pure flame of the alcohol lamp
required but a beam of sufficient intensity to reveal its clouds
of liberated carbon.

But is the blackness smoke? This question presented itself in
a moment and was thus answered: A red-hot poker was placed
underneath the beam: from it the black wreaths also ascended. A
large hydrogen flame was next employed, and it produced those
whirling masses of darkness, far more copiously than either the
spirit-flame or poker. Smoke was therefore out of the question.
[Footnote: In none of the public rooms of the United
States where I had the honour to lecture was this experiment
made. The organic dust was too scanty. Certain rooms in England
— the Brighton Pavilion, for example — also lack the
necessary conditions.]

What, then, was the blackness? It was simply that of stellar
space; that is to say, blackness resulting from the absence from
the track of the beam of all matter competent to scatter its
light. When the flame was placed below the beam the floating
matter was destroyed in situ; and the air, freed from this
matter, rose into the beam, jostled aside the illuminated
particles, and substituted for their light the darkness due to
its own perfect transparency. Nothing could more forcibly
illustrate the invisibility of the agent which renders all things
visible. The beam crossed, unseen, the black chasm formed by the
transparent air, while, at both sides of the gap, the
thick-strewn particles shone out like a luminous solid under the
powerful illumination.

It is not, however, necessary to burn the particles to produce
a stream of darkness. Without actual combustion, currents may be
generated which shall displace the floating matter, and appear
dark amid the surrounding brightness. I noticed this effect first
on placing a red-hot copper ball below the beam, and permitting
it to remain there until its temperature had fallen below that of
boiling water. The dark currents, though much enfeebled, were
still produced. They may also be produced by a flask filled with
hot water.

To study this effect a platinum wire was stretched across the
beam, the two ends of the wire being connected with the two poles
of a voltaic battery. To regulate the strength of the current a
rheostat was placed in the circuit. Beginning with a feeble
current the temperature of the wire was gradually augmented; but
long before it reached the heat of ignition, a flat stream of air
rose from it, which when looked at edgeways appeared darker and
sharper than one of the blackest lines of Fraunhofer in the
purified spectrum. Right and left of this dark vertical band the
floating matter rose upwards, bounding definitely the
non-luminous stream of air. What is the explanation? Simply this:
The hot wire rarefied the air in contact with it, but it did not
equally lighten the floating matter. The convection current of
pure air therefore passed upwards among the inert particles,
dragging them after it right and left, but forming between them
an impassable black partition. This elementary experiments
enables us to render an account of the dark currents produced by
bodies at a temperature below that of combustion.

But when the platinum wire is intensely heated, the floating
matter is not only displaced, but destroyed. I stretched a wire
about 4 inches long through the air of an ordinary glass shade
resting on cotton-wool, which also surrounded the rim. The wire
being raised to a white heat by an electric current, the air
expanded, and some of it was forced through the cotton-wool. When
the current was interrupted, and the air within the shade cooled,
the returning air did not carry motes along with it, being
filtered by the wool. At the beginning of this experiments the
shade was charged with floating matter; at the end of half an
hour it was optically empty.

On the wooden base of a cubical glass shade, a cubic foot in
volume, upright supports were fixed, and from one support to the
other 38 inches of platinum wire were stretched in four parallel
lines. The ends of the platinum wire were soldered to two stout
copper wires which passed through the base of the shade and could
be connected with a battery. As in the last experiments the shade
rested upon cotton-wool. A beam sent through the shade revealed
the suspended matter. The platinum wire was then raised to
whiteness. In five minutes there was a sensible diminution of the
matter, and in ten minutes it was totally consumed.

Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to
exclude all floating particles, produce, when poured or blown
into the beam, the darkness of stellar space. Coal-gas does the
same. An ordinary glass shade, placed in the air with its mouth
downwards, permits the track of the beam to be seen crossing it.
When coal-gas or hydrogen is allowed to enter the shade by a tube
reaching to its top, the gas gradually fills the shade from above
downwards. As soon as it occupies the space crossed by the beam,
the luminous track is abolished. Lifting the shade so as to bring
the common boundary of gas and air above the beam, the track
flashes forth. After the shade is full, if it be inverted, the
pure gas passes upwards like a black smoke among the illuminated
particles.

.

The Germ Theory of Contagious
Disease.

There is no respite to our
contact with the floating matter of the air; and the wonder is,
not that we should suffer occasionally from its presence, but
that so small a portion of it, and even that but rarely diffused
over large areas, should appear to be deadly to man. And what is
this portion? It was some time ago the current belief that
epidemic diseases generally were propagated by a kind of malaria,
which consisted of organic matter in a state of motor-decay; that
when such matter was taken into the body through the lungs, skin,
or stomach, it had the power of spreading there the destroying
process by which itself had been assailed. Such a power was
visibly exerted in the case of yeast. A little leaven was seen to
leaven the whole lump — a mere speck of matter, in this
supposed state of decomposition, being apparently competent to
propagate indefinitely its own decay. Why should not a bit of
rotten malaria act in a similar manner within the human frame? In
1836 a very wonderful reply was given to this question. In that
year Cagniard de la Tour discovered the yeast-plant
— a living organism, which when placed in a proper medium
feeds, grows, and reproduces itself, and in this way carries on
the process which we name fermentation. By this striking
discovery fermentation was connected with organic
growth.

Schwann, of Berlin, discovered the yeast-plant independently
about the same time; and in February, 1837, he also announced the
important result, that when a decoction of meat is effectually
screened from ordinary air, and supplied solely with calcined
air, putrefaction never sets in. Putrefaction, therefore, he
affirmed to be caused, not by the air, but by something which
could be destroyed by a sufficiently high temperature. The
results of Schwann were confirmed by the independent experiments
of Helmholtz, Ure, and Pasteur, while other methods, pursued by
Schultze, and by Schroeder and Dusch, led to the same result.

But as regards fermentation, the
minds of chemists, influenced probably by the great authority of
Gay-Lussac, fell back upon the old notion of matter in a state of
decay. It was not the living yeast-plant, but the dead or dying
parts of it, which, assailed by oxygen, produced the
fermentation. Pasteur, however, proved the real ‘ferments,’
mediate or immediate, to be organised beings which find in the
reputed ferments their necessary food.

Side by side with these researches and discoveries, and
fortified by them and others, has run the germ theory of
epidemic disease. The notion was expressed by Kircher, and
favoured by Linnaeus, that epidemic diseases may be due to germs
which float in the atmosphere, enter the body, and produce
disturbance by the development within the body of parasitic life.
The strength of this theory consists in the perfect parallelism
of the phenomena of contagious disease with those of life. As a
planted acorn gives birth to an oak, competent to produce a whole
crop of acorns, each gifted with the power of reproducing its
parent tree; and as thus from a single seedling a whole forest
may spring; so, it is contended, these epidemic diseases
literally plant their seeds, grow, and shake abroad new germs,
which, meeting in the human body their proper food and
temperature, finally take possession of whole populations. There
is nothing to my knowledge in pure chemistry which resembles the
power of propagation and self-multiplication possessed by the
matter which produces epidemic disease. If you sow wheat you do
not get barley; if you sow small-pox you do not get
scarlet-fever, but small-pox indefinitely multiplied, and nothing
else. The matter of each contagious disease reproduces itself as
rigidly as if it were (as Miss Nightingale puts it) dog or
cat.

.

Parasitic Diseases of Silkworms.
Pasteur’s Researches.

It is admitted on all hands
that some diseases are the product of parasitic growth. Both in
man and in lower creatures, the existence of such diseases has
been demonstrated. I am enabled to lay before you an account of
an epidemic of this kind, thoroughly investigated and
successfully combated by M. Pasteur. For fifteen years a plague
had raged among the silkworms of France. They had sickened and
died in multitudes, while those that succeeded in spinning their
cocoons furnished only a fraction of the normal quantity of silk.
In 1853 the silk culture of France produced a revenue of one
hundred and thirty millions of francs. During the twenty previous
years the revenue had doubled itself, and no doubt was
entertained as to its further augmentation. The weight of the
cocoons produced in 1853 was 26,000,000 kilogrammes; in 1865 it
had fallen to 4,000,000, the fall entailing, in a single year, a
loss of 100,000,000 francs.

The country chiefly smitten by this calamity happened to be
that of the celebrated chemist Dumas, now perpetual secretary of
the French Academy of Sciences. He turned to his friend,
colleague, and pupil, Pasteur, and besought him, with an
earnestness which the circumstances rendered almost personal, to
undertake the investigation of the malady. Pasteur at this time
had never seen a silkworm, and he urged his inexperience in reply
to his friend. But Dumas knew too well the qualities needed for
such an enquiry to accept Pasteur’s reason for declining it.
‘Je mets,’ said he, ‘un prix extréme à
voir votre attention fixée sur la question qui
intéresse mon pauvre pays; la misére surpasse tout
ce que vous pouvez imaginer.’ Pamphlets about the plague had been
showered upon the public, the monotony of waste paper being
broken, at rare intervals, by a more or less useful publication.
‘The Pharmacopoeia of the Silkworm,’ wrote M. Cornalia in
1860, ‘is now as complicated as that of man. Gases,
liquids, and solids have been laid under contribution. From
chlorine to sulphurous acid, from nitric acid to rum, from sugar
to sulphate of quinine, — all has been invoked in behalf of
this unhappy insect.’ The helpless cultivators, moreover,
welcomed with ready trustfulness every new remedy, if only
pressed upon them with sufficient hardihood. It seemed impossible
to diminish their blind confidence in their blind guides. In 1863
the French Minister of Agriculture signed an agreement to pay
500,000 francs for the use of a remedy, which its promoter
declared to be infallible. It was tried in twelve different
departments of France, and found perfectly useless. In no single
instance was it successful. It was under these circumstances that
M. Pasteur, yielding to the entreaties of his friend, betook
himself to Alais in the beginning of June, 1865. As regards silk
husbandry, this was the most important department in France, and
it was the most sorely smitten by the plague.

The silkworm had been previously attacked by
muscardine, a disease proved by Bassi to be caused by a
vegetable parasite. This malady was propagated annually by the
parasitic spores. Wafted by winds they often sowed the disease in
places far removed from the centre of infection. Muscardine is
now said to be very rare, a deadlier malady having taken its
place. This new disease is characterised by the black spots which
cover the silkworms; hence the name pébrine, first
applied to the plague by M. de Quatrefages, and adopted by
Pasteur. Pébrine declares itself in the stunted and
unequal growth of the worms, in the languor of their movements,
in their fastidiousness as regards food, and in their premature
death. The course of discovery as regards the epidemic is this:
In 1849 Guérin Méneville noticed in the blood of
silkworms vibratory corpuscles, which he supposed from their
motions to be endowed with independent life. Filippi, however,
showed that the motion of the corpuscles was the well-known
Brownian motion; but he committed the error of supposing the
corpuscles to be normal to the life of the insect. Possessing the
power of indefinite self-multiplication, they are really the
cause of its mortality — the form and substance of its
disease. This was well described by Cornalia; while Lebert and
Frey subsequently found the corpuscles not only in the blood, but
in all the tissues of the insect. Osimo, in 1857, discovered them
in the eggs; and on this observation Vittadiani founded, in 1859,
a practical method of distinguishing healthy from diseased eggs.
The test often proved fallacious, and it was never extensively
applied.

These living corpuscles take possession of the intestinal
canal, and spread thence throughout the body of the worm. They
fill the silk cavities, the stricken insect often going
automatically through the motions of spinning, without any
material to work upon. Its organs, instead of being filled with
the clear viscous liquid of the silk, are packed to distension by
the corpuscles. On this feature of the plague Pasteur fixed his
entire attention. The cycle of the silkworm’s life is briefly
this: From the fertile egg comes the little worm, which grows,
and casts its skin. This process of moulting is repeated two or
three times at intervals during the life of the insect. After the
last moulting the worm climbs the brambles placed to receive it,
and spins among them its cocoon. It passes thus into a chrysalis;
the chrysalis becomes a moth, and the moth, when liberated, lays
the eggs which form the starting-point of a new cycle. Now
Pasteur proved that the plague-corpuscles might be incipient in
the egg, and escape detection; they might also be germinal in the
worm, and still baffle the microscope. But as the worm grows, the
corpuscles grow also, becoming larger and more defined. In the
aged chrysalis they are more pronounced than in the worm; while
in the moth, if either the egg or the worm from which it comes
should have been at all stricken, the corpuscles infallibly
appear, offering no difficulty of detection. This was the first
great point made out in 1865 by Pasteur. The Italian naturalists,
as aforesaid, recommended the examination of the eggs before
risking their incubation. Pasteur showed that both eggs and worms
might be smitten, and still pass muster, the culture of such eggs
or such worms being sure to entail disaster. He made the moth his
starting-point in seeking to regenerate the race.

Pasteur made his first communication on this subject to the
Academy of Sciences in September, 1865. It raised a cloud of
criticism. Here, forsooth, was a chemist rashly quitting his
proper métier and presuming to lay down the law for the
physician and biologist on a subject which was eminently theirs.
‘On trouva étrange que je fusse si peu au courant de
la question; on m’opposa des travaux qui avaient paru depuis
longtemps en Italie, dont les résultats montraient
l’inutilité de mes efforts, et l’impossibilité
d’arriver à un résultat pratique dans la direction
que je m’étais engagé. Que mon ignorance fut grande
au sujet des recherches sans nombre qui avaient paru depuis
quinze années.’ Pasteur heard the buzz, but he continued
his work. In choosing the eggs intended for incubation, the
cultivators selected those produced in the successful
‘educations’ of the year. But they could not understand the
frequent and often disastrous failures of their selected eggs;
for they did not know, and nobody prior to Pasteur was competent
to tell them, that the finest cocoons may envelope doomed
corpusculous moths. It was not, however, easy to make the
cultivators accept new guidance. To strike their imagination, and
if possible determine their practice, Pasteur hit upon the
expedient of prophecy. In 1866 he inspected, at St.
Hippolyte-du-Fort, fourteen different parcels of eggs intended
for incubation. Having examined a sufficient number of the moths
which produced these eggs, he wrote out the prediction of what
would occur in 1867, and placed the prophecy as a sealed letter
in the hands of the Mayor of St. Hippolyte.

In 1867 the cultivators communicated to the mayor their
results. The letter of Pasteur was then opened and read, and it
was found that in twelve out of fourteen cases there was absolute
conformity between his prediction and the observed facts. Many of
the groups had perished totally; the others had perished almost
totally; and this was the prediction of Pasteur. In two out of
the fourteen cases, instead of the prophesied destruction, half
an average crop was obtained. Now, the parcels of eggs here
referred to were considered healthy by their owners. They had
been hatched and tended in the firm hope that the labour expended
on them would prove remunerative. The application of the
moth-test for a few minutes in 1866, would have saved the labour
and averted the disappointment. Two additional parcels of eggs
were at the same time submitted to Pasteur. He pronounced them
healthy; and his words were verified by the production of an
excellent crop. Other cases of prophecy still more remarkable,
because more circumstantial, are recorded in Pasteur’s work.

Pasteur subjected the development of the corpuscles to a
searching investigation, and followed out with admirable skill
and completeness the various modes by which the plague was
propagated. From moths perfectly free from corpuscles he obtained
healthy worms, and selecting 10, 20, 30, 50, as the case might
be, he introduced into the worms the corpusculous matter. It was
first permitted to accompany the food. Let its take a single
example out of many. Rubbing up a small corpusculous worm in
water, he smeared the mixture over the mulberry-leaves. Assuring
himself that the leaves had been eaten, he watched the
consequences from day to day. Side by side with the infected
worms he reared their fellows, keeping them as much as possible
out of the way of infection. These constituted his ‘lot
témoin,’ — his standard of comparison. On April 16,
1868, he thus infected thirty worms. Up to the 23rd they remained
quite well. On the 25th they seemed well, but on that day
corpuscles were found in the intestines of two of them. On the
27th, or eleven days after the infected repast, two fresh worms
were examined, and not only was the intestinal canal found in
each case invaded, but the silk organ itself was charged with
corpuscles. On the 28th the twenty-six remaining worms were
covered by the black spots of pébrine. On the 30th
the difference of size between the infected and non-infected
worms was very striking, the sick worms being not more than
two-thirds of the bulk of the healthy ones. On May 2 a worm which
had just finished its fourth moulting was examined. Its whole
body was so filled with the parasite as to excite astonishment
that it could live.

The disease advanced, the worms died and were examined, and on
May 11 only six out of the thirty remained. They were the
strongest of the lot, but on being searched they also were found
charged with corpuscles. Not one of the thirty worms had escaped;
a single meal had poisoned them all. The standard lot, on the
contrary, spun their fine cocoons, two only of their moths being
proved to contain any trace of the parasite, which had doubtless
been introduced during the rearing of the worms.

As his acquaintance with the subject increased, Pasteur’s
desire for precision augmented, and he finally counted the
growing number of corpuscles seen in the field of his microscope
from day to day. After a contagious repast the number of worms
containing the parasite gradually augmented until finally it
became cent. per cent. The number of corpuscles would at the same
time rise from 0 to 1, to 10, to 100, and sometimes even to 1,000
or 1,500 in the field of his microscope. He then varied the mode
of infection. He inoculated healthy worms with the corpusculous
matter, and watched the consequent growth of the disease. He
proved that the worms inoculate each other by the infliction of
visible wounds with their claws. In various cases he washed the
claws, and found corpuscles in the water. He demonstrated the
spread of infection by the simple association of healthy and
diseased worms. By their claws and their dejections, the diseased
worms spread infection. It was no hypothetical infected medium
— no problematical pythogenic gas — that killed the
worms, but a definite organism. The question of infection at a
distance was also examined, and its existence demonstrated. As
might be expected from Pasteur’s antecedents, the investigation
was exhaustive, the skill and beauty of his manipulation finding
fitting correlatives in the strength and clearness of his
thought.

The following quotation from Pasteur’s work clearly shows the
relation in which his researches stand to the important question
on which he was engaged:

—–

Place (he says) the most skilful educator, even the most
expert microscopist, in presence of large educations which
present the symptoms described in our experiments; his judgment
will necessarily be erroneous if he confines himself to the
knowledge which preceded my researches. The worms will not
present to him the slightest spot of pébrine; the
microscope will not reveal the existence of corpuscles; the
mortality of the worms will be null or insignificant; and the
cocoons leave nothing to be desired. Our observer would,
therefore, conclude without hesitation that the eggs produced
will be good for incubation. The truth is, on the contrary, that
all the worms of these fine crops have been poisoned; that from
the beginning they carried in them the germ of the malady; ready
to multiply itself beyond measure in the chrysalides and the
moths, thence to pass into the eggs and smite with sterility the
next generation. And what is the first cause of the evil
concealed under so deceitful an exterior? In our experiments we
can, so to speak, touch it with our fingers. It is entirely the
effect of a single corpusculous repast; an effect more or less
prompt according to the epoch of life of the worm that has eaten
the poisoned food.

—–

Pasteur describes in detail his method of securing healthy
eggs. It is nothing less than a mode of restoring to France her
ancient silk husbandry. The justification of his work is to be
found in the reports which reached him of the application and the
unparalleled success of his method, while editing his researches
for final publication. In both France and Italy his method has
been pursued with the most surprising results. But it was an
up-hill fight which led to this triumph.

‘Ever,’ he says, ‘since the commencement of these
researches, I have been exposed to the most obstinate and unjust
contradictions; but I have made it a duty to leave no trace of
these conflicts in this book.’ And in reference to parasitic
diseases, generally, he uses the following weighty words: ‘Il est
au pouvoir de l’homme de faire disparaitre de la surface du globe
les maladies parasitaires, si, comme c’est ma conviction, la
doctrine des générations spontanées est une
chimère.’

Pasteur dwells upon the ease with which an island like Corsica
might be absolutely isolated from the silkworm epidemic. And with
regard to other epidemics, Mr. Simon describes an extraordinary
case of insular exemption, for the ten years extending from 1851
to 1860. Of the 627 registration districts of England, one only
had an entire escape from diseases which, in whole or in part,
were prevalent in all the others: ‘In all the ten years it had
not a single death by measles, nor a single death by small-pox,
nor a single death by scarlet-fever. And why? Not because of its
general sanitary merits, for it had an average amounts of other
evidence of unhealthiness. Doubtless, the reason of its escape
was that it was insular. It was the district of the Scilly Isles;
to which it was most improbable that any febrile contagion should
come from without. And its escape is an approximative proof that,
at least for those ten years, no contagium of measles, nor
any contagium of scarlet-fever, nor any contagium
of smallpox had arisen spontaneously within its limits.’ It may
be added that there were only seven districts in England in which
no death from diphtheria occurred, and that, of those seven
districts, the district of the Scilly Isles was one.

A second parasitic disease of silkworms, called in France la
flacherie, co-existent with pébrine, but quite
distinct from it, has also been investigated by Pasteur. Enough,
however, has been said to send the reader interested in these
questions to the original volumes for further information. To one
important practical point M. Pasteur, in a letter to myself,
directs attention:

—–

Permettez-moi de terminer ces quelques lignes que je dois
dicter, vaincu que je suis par la maladie, en vous faisant
observer que vous rendriez service aux Colonies de la
Grande-Bretagne en répandant la connaissance de ce livre,
et des principes que j’établis touchant la maladie des
vers à soie. Beaucoup de ces colonies pourraient cultiver
le mûrier avec succés, et, en jetant les yeux sur
mon ouvrage, vous vous convaincrez aisement qu’il est facile
aujourd’hui, nonseulement d’éloigner la maladie
régnante, mais en outre de donner aux récoltes de
la soie une prospérité qu’elles n’ont jamais
eue.

Origin and Propagation of Contagious
Matter.

Prior to Pasteur, the most
diverse and contradictory opinions were entertained as to the
contagious character of pébrine; some stoutly
affirmed it, others as stoutly denied it. But on one point all
were agreed. I They believed in the existence of a deleterious
medium, rendered epidemic by some occult and mysterious
influence, to which was attributed the cause of the disease.’
Those acquainted with our medical literature will not fail to
observe an instructive analogy here. We have on the one side
accomplished writers ascribing epidemic diseases to ‘deleterious
media’ which arise spontaneously in crowded hospitals and
ill-smelling drains. According to them, the contagia of
epidemic disease are formed de novo in a putrescent
atmosphere. On the other side we have writers, clear, vigorous,
with well-defined ideas and methods of research, contending that
the matter which produces epidemic disease comes always from a
parent stock. It behaves as germinal matter, and they do not
hesitate to regard it as such. They no more believe in the
spontaneous generation of such diseases, than they do in the
spontaneous generation of mice. Pasteur, for example, found that
pébrine had been known for an indefinite time as a
disease among silkworms. The development of it which he combated
was merely the expansion of an already existing power — the
bursting into open conflagration of a previously smouldering
fire. There is nothing surprising in this. For though epidemic
disease requires a special contagium to produce it,
surrounding conditions must have a potent influence on its
development. Common seeds may be duly sown, but the conditions of
temperature and moisture may be such as to restrict, or
altogether prevent, the subsequent growth. Looked at, therefore,
from the point of view of the germ theory, the exceptional energy
which epidemic disease from time to time exhibits, is in harmony
with the method of Nature. We sometimes hear diphtheria spoken of
as if it were a new disease of the last twenty years; but Mr.
Simon tells me that about three centuries ago tremendous
epidemics of it began to rage in Spain (where it was named
Garrotillo), and soon afterwards in Italy; and that since
that time the disease has been well known to all successive
generations of doctors. In or about 1758, for instance, Dr.
Starr, of Liskeard, in a communication to the Royal Society,
particularly described the disease, with all the characters which
have recently again become familiar, but under the name of
morbus strangulatorius, as then severely epidemic in
Cornwall. This fact is the more interesting, as diphtheria, in
its more modern reappearance, again showed predilection for that
remote county. Many also believe that the Black Death, of five
centuries ago, has disappeared as mysteriously as it came; but
Mr. Simon finds that it is believed to be prevalent at this hour
in some of the north-western parts of India.

Let me here state an item of my own experience. When I was at
the Bel Alp in 1869, the English chaplain received letters
informing him of the breaking out of scarlet-fever among his
children. He lived, if I remember rightly, on the healthful
eminence of Dartmoor, and it was difficult to imagine how
scarlet-fever could have been wafted to the place. A drain ran
close to his house, and on it his suspicions were manifestly
fixed. Some of our medical writers would fortify him in this
notion, and thus deflect him from the truth, while those of
another, and, in my opinion, a wiser school, would deny to a
drain, however foul, the power of generating de novo a specific
disease. After close enquiry he recollected that a hobby-horse
had been used both by his boy and another, who, a short time
previously, had passed through scarlet-fever.

Drains and cesspools, indeed, are by no means in such evil
odour as they used to be. A fetid Thames and a low death-rate
occur from time to time together in London. For, if the special
matter or germs of epidemic disorder be not present, a corrupt
atmosphere, however obnoxious otherwise, will not produce the
disorder. But, if the germs be present, defective drains and
cesspools become the potent distributors of disease and death.
Corrupted air may promote an epidemic, but cannot produce it. On
the other hand, through the transport of the special germ or
virus, disease may develop itself in regions where the drainage
is good and the atmosphere pure.

If you see a new thistle growing in your field, you feel sure
that its seed has been wafted thither. Just as sure does it seem
that the contagious matter of epidemic disease has been
transplanted to the place where it newly appears. With a
clearness and conclusiveness s not to be surpassed, Dr. William
Budd has traced such diseases from place to place; showing how
they plant themselves, at distinct foci, among populations
subjected to the same atmospheric influences, just as grains of
corn might be carried in the pocket and sown. Hildebrand, to
whose remarkable work, ‘Du Typhus contagieux,’ Dr. de Mussy has
directed my attention, gives the following striking case, both of
the durability and the transport of the virus of scarlatina: ‘Un
habit noir que j’avais en visitant une malade attaquée de
scarlatina, et que je portai de Vienne en Podolie, sans l’avoir
mis depuis plus d’un an et demi, me communiqua, dès que je
fus arrivé, cette maladie contagieuse, que je
répandis ensuite dans cette province, où elle
était jusqu’alors presque inconnue.’ Some years ago Dr. de
Mussy himself was summoned to a country house in Surrey, to see a
young lady who was suffering from a dropsy, evidently the
consequence of scarlatina. The original disease, being of a very
mild character, had been quite overlooked; but circumstances were
recorded which could leave no doubt upon the mind as to the
nature and cause of the complaint. But then the question arose,
How did the young lady catch the scarlatina? She had come there
on a visit two months previously, and it was only after she had
been a month in the house that she was taken ill. The housekeeper
at length cleared up the mystery. The young lady, on her arrival,
had expressed a wish to occupy a room in an isolated tower. Her
desire was granted; and in that room, six months previously, a
visitor had been confined with an attack of scarlatina. The room
had been swept and whitewashed, but the carpets had been
permitted to remain.

Thousands of cases could probably be cited in which the
disease has shown itself in this mysterious way, but where a
strict examination has revealed its true parentage and
extraction. Is it, then, philosophical to take refuge in the
fortuitous concourse of atoms as a cause of specific disease,
merely because in special cases the parentage may be indistinct?
Those best acquainted with atomic nature, and who are most ready
to admit, as regards even higher things than this, the
potentialities of matter, will be the last to accept these rash
hypotheses.

.

The Germ Theory applied to
Surgery.

Not only medical but still more
especially surgical science is now seeking light and guidance
from this germ theory. Upon it the antiseptic system of Professor
Lister of Edinburgh is founded. As already stated, the germ
theory of putrefaction was started by Schwann; but the
illustrations of this theory adduced by Professor Lister are of
such public moment as not only to justify, but to render
imperative, their introduction here.

Schwann’s observations (says Professor Lister) did not receive
the attention which they appeared to me to have deserved. The
fermentation of sugar was generally allowed to be occasioned by
the Torula cerevisiae; but it was not admitted that
putrefaction was due to an analogous agency. And yet the two
cases present a very striking parallel. In each a stable chemical
compound, sugar in the one case, albumen in the other, undergoes
extraordinary chemical changes under the influence of an
excessively minute quantity of a substance which, regarded
chemically, we should suppose inert. As an example of this in the
case of putrefaction, let us take a circumstance often witnessed
in the treatment of large chronic abscesses. In order to guard
against the access of atmospheric air, we used to draw off the
matter by means of a canula and trocar, such as you see here,
consisting of a silver tube with a sharp-pointed steel rod fitted
into it, and projecting beyond it. The instrument, dipped in oil,
was thrust into the cavity of the abscess, the trocar was
withdrawn, and the pus flowed out through the canula, care being
taken by gentle pressure over the part to prevent the possibility
of regurgitation. The canula was then drawn out with due
precaution against the reflux of air. This method was frequently
successful as to its immediate object, the patient being relieved
from the mass of the accumulated fluid, and experiencing no
inconvenience from the operation. But the pus was pretty certain
to reaccumulate in course of time, and it became necessary again
and again to repeat the process. And unhappily there was no
absolute security of immunity from bad consequences. However
carefully the procedure was conducted, it sometimes happened,
even though the puncture seemed healing by first intention, that
feverish symptoms declared themselves in the course of the first
or second day, and, on inspecting the seat of the abscess, the
skin was perhaps seen to be red, implying the presence of some
cause of irritation, while a rapid reaccumulation of the fluid
was found to have occurred. Under these circumstances, it became
necessary to open the abscess by free incision, when a quantity,
large in proportion to the size of the abscess, say, for example,
a quart, of pus escaped, fetid from putrefaction. Now, how had
this change been brought about? Without the germ theory, I
venture to say, no rational explanation of it could have been
given. It must have been caused by the introduction of something
from without. Inflammation of the punctured wound, even supposing
it to have occurred, would not explain the phenomenon. For mere
inflammation, whether acute or chronic, though it occasions the
formation of pus, does not induce Putrefaction. The pus
originally evacuated was perfectly sweet, and we know of nothing
to account for the alteration in its quality but the influence
of something derived from the external world. And what could that
something be? The dipping of the instrument in oil, and the
subsequent precautions, prevented the entrance of oxygen. Or even
if you allowed that a few atoms of the gas did enter, it would be
an extraordinary assumption to make that these could in so short
a time effect such changes in so large a mass of albuminous
material. Besides, the pyogenic membrane is abundantly supplied
with capillary vessels, through which arterial blood, rich in
oxygen, is perpetually flowing; and there can be little doubt
that the pus, before it was evacuated at all, was liable to any
action which the element might be disposed to exert upon it.

On the oxygen theory, then, the occurrence of putrefaction
under these circumstances is quite inexplicable. But if you admit
the germ theory, the difficulty vanishes at once. The canula and
trocar having been lying exposed to the air, dust will have been
deposited upon them, and will be present in the angle between the
trocar and the silver tube, and in that protected situation will
fail to be wiped off when the instrument is thrust through the
tissues. Then when the trocar is withdrawn, some portions of this
dust will naturally remain upon the margin of the canula, which
is left projecting into the abscess, and nothing is more likely
than that some particles may fail to be washed off by the stream
of out-flowing pus, but may be dislodged when the tube is taken
out, and left behind in the cavity. The germ theory tells us that
these particles of dust will be pretty sure to contain the germs
of putrefactive organisms, and if one such is left in the
albuminous liquid, it will rapidly develop at the high
temperature of the body, and account for all the phenomena.

But striking as is the parallel between putrefaction in this
instance and the vinous fermentation, as regards the greatness of
the effect produced, compared with the minuteness and the
inertness, chemically speaking, of the cause, you will naturally
desire further evidence of the similarity of the two processes.
You can see with the microscope the Torula of fermenting
must or beer. Is there, you may ask, any organism to be detected
in the putrefying pus? Yes, gentlemen, there is. If any drop of
the putrid matter is examined with a good glass, it is found to
be teeming with myriads of minute jointed bodies, called vibrios,
which indubitably proclaim their vitality by the energy of their
movements. It is not an affair of probability, but a fact, that
the entire mass of that quart of pus has become peopled with
living organisms as the result of the introduction of the canula
and trocar; for the matter first let out was as free from vibrios
as it was from putrefaction. If this be so, the greatness of the
chemical changes that have taken place in the pus ceases to be
surprising. We know that it is one of the chief peculiarities of
living structures that they possess extraordinary powers of
effecting chemical changes in materials in their vicinity, out of
all proportion to their energy as mere chemical compounds. And we
can hardly doubt that the animalcules which have been developed
in the albuminous liquid, and have grown at its expense, must
have altered its constitution, just as we ourselves alter that of
the materials on which we feed. [Footnote:
‘Introductory Lecture before the University of
Edinburgh.’]

In the operations of Professor Lister care is taken that every
portion of tissue laid bare by the knife shall be defended from
germs; that if they fall upon the wound they should be killed as
they fall. With this in view he showers upon his exposed surfaces
the spray of dilute carbolic acid, which is particularly deadly
to the germs, and he surrounds the wound in the most careful
manner with antiseptic bandages. To those accustomed to strict
experiment it is manifest that we have a strict experimenter here
— a man with a perfectly distinct object in view, which he
pursues with never-tiring patience and unwavering faith. And the
result, in his hospital practice, as described by himself, has
been, that even in the midst of abominations too shocking to be
mentioned here, and in the neighbourhood of wards where death was
rampant from pyaemia, erysipelas, and hospital gangrene, he was
able to keep his patients absolutely free from these terrible
scourges. Let me here recommend to your attention Professor
Lister’s ‘Introductory Lecture before the University of
Edinburgh,’ which I have already quoted; his paper on The Effect
of the Antiseptic System of Treatment on the Salubrity of a
Surgical Hospital;’ and the article in the ‘British Medical
Journal’ of January 14, 1871.

If, instead of using carbolic acid spray, he could surround
his wounds with properly filtered air, the result would, he
contends, be the same. In a room where the germs not only float
but cling to clothes and walls, this would be difficult, if not
impossible. But surgery is acquainted with a class of wounds in
which the blood is freely mixed with air that has passed through
the lungs, and it is a most remarkable fact that such air does
not produce putrefaction. Professor Lister, as far as I know, was
the first to give a philosophical interpretation of this fact,
which he describes and comments upon thus:

I have explained to my own mind the remarkable fact that in
simple fracture of the ribs, if the lung be punctured by a
fragment, the blood effused into the pleural cavity, though
freely mixed with air, undergoes no decomposition. The air is
sometimes pumped into the pleural cavity in such abundance that,
making its way through the wound in the pleura costalis, it
inflates the cellular tissue of the whole body. Yet this
occasions no alarm to the surgeon (although if the blood in the
pleura were to putrefy, it would infallibly occasion dangerous
suppurative pleurisy). Why air introduced into the pleural cavity
through a wounded lung, should have such wholly different effects
from that entering directly through a wound in the chest, was to
me a complete mystery until I heard of the germ theory of
putrefaction, when it at once occurred to me that it was only
natural that air should be filtered of germs by the air-passages,
one of whose offices is to arrest inhaled particles of dust, and
prevent them from entering the air-cells.

—–

I shall have occasion to refer to this remarkable hypothesis
farther on.

The advocates of the germ theory, both of putrefaction and
epidemic disease, hold that both arise, not from the air, but
from something contained in the air. They hold, moreover, that
this ‘something’ is not a vapour nor a gas, nor indeed a molecule
of any kind, but a particle. [Footnote: As regards
size, there is probably no sharp line of division between
molecules and particles; the one gradually shades into the other.
But the distinction that I would draw is this: the atom or the
molecule, if free, is always part of a gas, the particle is never
so. A particle is a bit of liquid or solid matter, formed by the
Aggregation of atoms or molecules.] The term
‘particulate ‘has been used in the Reports of the
Medical Department of the Privy Council to describe this supposed
constitution of contagious matter; and Dr. Sanderson’s
experiments render it in the highest degree probable, if they do
not actually demonstrate, that the virus of small-pox is
‘particulate.’ Definite knowledge upon this point is of
exceeding importance, because in the treatment of
particles methods are available which it would be futile
to apply to molecules.

The Luminous beam as a means of
Research.

My own interference with this
great question, while sanctioned by eminent names, has been also
an object of varied and ingenious attack. On this point I will
only say that when angry feeling escapes from behind the
intellect, where it may be useful as an urging force, and places
itself athwart the intellect, it is liable to produce all manner
of delusions. Thus my censors, for the most part, have levelled
their remarks against positions which were never assumed, and
against claims which were never made. The simple history of the
matter is this: During the autumn of 1868 I was much occupied
with the observations referred to at the beginning of this
discourse, and in part described in the preceding article. For
fifteen years it had been my habit to make use of floating dust
to reveal the paths of luminous beams through the air; but until
1868 I did not intentionally reverse the process, and employ a
luminous beam to reveal and examine the dust. In a paper
presented to the Royal Society in December, 1869, the
observations which induced me to give more special attention to
the question of spontaneous generation, and the germ theory of
epidemic disease, are thus described:

The Floating Matter of the
Air.

Prior to the discovery of the
foregoing action (the chemical action of light upon vapours,
Fragment IV.), and also during the experiments just referred to,
the nature of my work compelled me to aim at obtaining
experimental tubes absolutely clean upon the surface, and
absolutely free within from suspended matter. Neither condition
is, however, easily attained.

For however well the tubes might be washed and polished, and
however bright and pure they might appear in ordinary daylight,
the electric beam infallibly revealed signs and tokens of dirt.
The air was always present, and it was sure to deposit some
impurity. All chemical processes, not conducted in a vacuum, are
open to this disturbance. When the experimental tube was
exhausted, it exhibited no trace of floating matter, but on
admitting the air through the U-tubes (containing caustic potash
and sulphuric acid), a dust-cone
more or less distinct was always revealed by the powerfully
condensed electric beam.

The floating motes resembled minute particles of liquid which
had been carried mechanically from the U-tubes into the
experimental tube. Precautions were therefore taken to prevent
any such transfer. They produced little or no mitigation. I did
not imagine, at the time, that the dust of the external air could
find such free passage through the caustic potash and sulphuric
acid. This, however, was the case; the motes really came from
without. They also passed with freedom through a variety of
aethers and alcohols. In fact, it requires long-continued action
on the part of an acid first to wet the motes and afterwards to
destroy them. By carefully passing the air through the flame of a
spirit lamp, or through a platinum tube heated to bright redness,
the floating matter was sensibly destroyed. It was therefore
combustible, in other words, organic, matter. I tried to
intercept it by a large respirator of cotton-wool. Close pressure
was necessary to render the wool effective. A plug of the wool,
rammed pretty tightly into the tube through which the air passed,
was finally found competent to hold back the motes. They appeared
from time to time afterwards, and gave me much trouble; but they
were invariably traced in the end to some defect in the purifying
apparatus — to some crack or flaw in the sealing-wax
employed to render the tubes air-tight. Thus through proper care,
but not without a great deal of searching out of disturbances,
the experimental tube, even when filled with air or vapour,
contains nothing competent to scatter the light. The space within
it has the aspect of an absolute vacuum.

An experimental tube in this condition I call optically
empty.

The simple apparatus employed in these experiments will be at
once understood by reference to a figure printed in the last
article (Fig. 3.) s s’ is the glass experimental tube, which has
varied in length from 1 to 5 feet, and which may be from 2 to 3
inches in diameter. From the end s, the pipe p p’ passes to an
air-pump. Connected with the other end s’ we have the flask F,
containing the liquid whose vapour is to be examined; then
follows a U-tube, T, filled with fragments of clean glass, wetted
with sulphuric acid; then a second U-tube, T, containing
fragments of marble, wetted with caustic potash; and finally a
narrow straight tube t t’, containing a tolerably tightly fitting
plug of cotton-wool. To save the air-pump gauge from the attack
of such vapours as act on mercury, as also to facilitate
observation, a separate barometer tube was employed.

Through the cork which stops the flask F two glass tubes, a
and b, pass air-tight. The tube a ends immediately under the
cork; the tube b, on the contrary, descends to the bottom of the
flask and dips into the liquid. The end of the tube b is drawn
out so as to render very small the orifice through which the air
escapes into the liquid.

The experimental tube s s’ being exhausted, a cock at
the end s’ is turned carefully on. The air passes slowly through
the cotton-wool, the caustic potash, and the sulphuric acid in
succession. Thus purified, it enters the flask F and bubbles
through the liquid. Charged with vapour, it finally passes into
the experimental tube, where it is submitted to examination. The
electric lamp L placed at the end of the experimental tube
furnishes the necessary beam.

—–

The facts here forced upon my attention had a bearing too
evident to be overlooked. The inability of air which had been
filtered through cotton-wool to generate animalcular life, had
been demonstrated by Schroeder and Pasteur: here the cause of its
impotence was rendered evident to the eye. The experiment proved
that no sensible amount of light was scattered by the molecules
of the air; that the scattered light always arose from suspended
particles; and the fact that the removal of these abolished
simultaneously the power of scattering light and of originating
life, obviously detached the life-originating power from the air,
and fixed it on something suspended in the air. Gases of all
kinds passed with freedom through the plug of cotton-wool; hence
the thing whose removal by the cotton-wool rendered the gas
impotent, could not itself have been matter in the gaseous
condition. It at once occurred to me that the retina, protected
as it was, in these experiments, from all extraneous light, might
be converted into a new and powerful instrument of demonstration
in relation to the germ theory.

But the observations also revealed the danger incurred in
experiments of this nature; showing that without an amount of
care far beyond that hitherto bestowed upon them, such
experiments left the door open to errors of the gravest
description. It was especially manifest that the chemical method
employed by Schultze in his experiments, and so often resorted to
since, might lead to the most erroneous consequences; that
neither acids nor alkalies had the power of rapid destruction
hitherto ascribed to them. In short, the employment of the
luminous beam rendered evident the cause of success in
experiments rigidly conducted like those of Pasteur; while it
made equally evident the certainty of failure in experiments less
severely carried out.

Dr. Bennett’s
Experiments.

But I do not wish to leave an
assertion of this kind without illustration. Take, then, the
well-conceived experiments of Dr. Hughes Bennett, described
before the Royal Society of Surgeons in Edinburgh on January 17,
1868. [Footnote: ‘British Medical Journal,’ 13, pt.
ii. 1868.] Into flasks containing decoctions of
liquorice-root, hay, or tea, Dr. Bennett, by an ingenious method,
forced air. The air was driven through two U-tubes, the one
containing a solution of caustic potash, the other sulphuric
acid. ‘All the bent tubes were filled with fragments of
pumice-stone to break up the air, so as to prevent the
possibility of any germs passing through in the centre of
bubbles.’ The air also passed through a Liebig’s bulb containing
sulphuric acid, and also through a bulb containing
gun-cotton.

It was only natural for Dr. Bennett to believe that his
‘bent tubes’ entirely cut off the germs. Previous to the
observations just referred to, I also believed in their efficacy.
But these observations destroy any such notion. The gun-cotton,
moreover, will fail to arrest the whole of the floating matter,
unless it is tightly packed, and there is no indication in Dr.
Bennett’s memoir that it was so packed. On the whole, I should
infer, from the mere inspection of Dr. Bennett’s apparatus, the
very results which he has described — a retardation of the
development of life, a total absence of it in some cases, and its
presence in others.

In his first series of experiments, eight flasks were fed with
sifted air, and five with common air. In ten or twelve days all
the five had fungi in them; whilst it required from four to nine
months to develop fungi in the others. In one of the eight,
moreover, even after this interval no fungi appeared. In a second
series of experiments there was a similar exception. In a third
series the cork stoppers used in the first and second series were
abandoned, and glass stoppers employed. Flasks containing
decoctions of tea, beef, and hay were filled with common air, and
other flasks with sifted air. In every one of the former fungi
appeared and in not one of the latter. These experiments simply
ruin the doctrine that Dr. Bennett finally espouses.

In all these negative cases, the prepared air was forced into
the infusion when it was boiling hot. Dr. Bennett made a fourth
series of experiments, in which, previous to forcing in the air,
he permitted the flasks to cool. Into four bottles thus treated
he forced prepared air, and after a time found fungi in all of
them. What is his conclusion? Not that the boiling hot liquid,
employed in his first experiments, had destroyed such germs as
had run the gauntlet of his apparatus; but that air which,
previous to being sealed up, had been exposed to a temperature of
212°, is too rare to support life. This conclusion is so
remarkable that it ought to be stated in Dr. Bennett’s own words.
‘It may be easily conceived that air subjected to a boiling
temperature is so expanded as scarcely to merit the name of air,
and that it is more or less unfit for the purpose of sustaining
animal or vegetable life.’

Now numerical data are attainable here, and as a matter of
fact I live and flourish for a considerable portion of each year
in a medium of less density than that which Dr. Bennett describes
as scarcely meriting the name of air. The inhabitants of the
higher Alpine chalets, with their flocks and herds, and the
grasses which support these, do the same; while the chamois rears
its kids in air rarer still. Insect life, moreover, is sometimes
exhibited with monstrous prodigality at Alpine heights.

In a fifth series of experiments sixteen bottles were filled
with infusions. Into four of them, while cold, ordinary unheated
and unsifted air was pumped. In these four bottles fungi were
developed. Into four other bottles, containing a boiling
infusion, ordinary air was also pumped — no fungi were here
developed. Into four other bottles containing an infusion which
had been boiled and permitted to cool, sifted air was pumped
— no fungi were developed. Finally, into four bottles
containing a boiling infusion sifted air was pumped no fungi were
developed. Only, therefore, in the four cases where the infusions
were cold infusions, and the air ordinary air, did fungi
appear.

Dr. Bennett does not draw from his experiments the conclusion
to which they so obviously point. On them, on the contrary, he
founds a defence of the doctrine of spontaneous generation, and
a general theory of spontaneous development. So strongly was he
impressed with the idea that the germs could not possibly pass
through his potash and sulphuric acid tubes, that the appearance
of fungi, even in a small minority of cases, where the air had
been sent through these tubes, was to him conclusive evidence of
the spontaneous origin of such fungi. And he accounts for the
absence of life in many of his experiments by an hypothesis which
will not bear a moment’s examination. But, knowing that organic
particles may pass unscathed through alkalies and acids, the
results of Dr. Bennett are precisely what ought wider the
circumstances to be expected. Indeed, their harmony with the
conditions now revealed is a proof of the honesty and accuracy
with which they were executed.

The caution exercised by Pasteur both in the execution of his
experiments, and in the reasoning based upon them, is perfectly
evident to those who, through the practice of severe experimental
enquiry, have rendered themselves competent to judge of good
experimental work. He found germs in the mercury used to isolate
his air. He was never sure that they did not cling to the
instruments he employed, or to his own person. Thus when he
opened his hermetically sealed flasks upon the Mer de Glace, he
had his eye upon the file used to detach the drawn-out necks of
his bottles; and he was careful to stand to leeward when each
flask was opened. Using these precautions, he found the glacier
air incompetent, in nineteen cases out of twenty, to generate
life; while similar flasks, opened amid the vegetation of the
lowlands, were soon crowded with living things. M. Pouchet
repeated Pasteur’s experiments in the Pyrenees, adopting the
precaution of holding his flasks above his head, and obtaining a
different result. Now great care would be needed to render this
procedure a real precaution. The luminous beam at once shows us
its possible effect. Let smoking brown paper be placed at the
open mouth of a glass shade, so that the smoke shall ascend and
fill the shade. A beam sent through the shade forms a bright
track through the smoke. When the closed fist is placed
underneath the shade, a vertical wind of surprising violence,
considering the small elevation of temperature, rises from the
band, displacing by comparatively dark air the illuminated smoke.
Unless special care were taken such a wind would rise from M.
Pouchet’s body as he held his flasks above his head, and thus the
precaution of Pasteur, of not coming between the wind and the
flask, would be annulled.

Let me now direct attention to another result of Pasteur, the
cause and significance of which are at once revealed by the
luminous beam. He prepared twenty one flasks, each containing a
decoction of yeast, filtered and clear. He boiled the decoction
so as to destroy whatever germs it might contain, and, while the
space above the liquid was filled with pure steam, he sealed his
flasks with a blow-pipe. He opened ten of them in the deep, damp
caves of the Paris Observatory, and eleven of them in the
courtyard of the establishment. Of the former, one only showed
signs of life subsequently. In nine out of the ten flasks no
organisms of any kind were developed. In all the others organisms
speedily appeared.

Now here is an experiment conducted in Paris, on which we can
throw obvious light in London. Causing our luminous beam to pass
through a large flask filled with the air of this room, and
charged with its germs and its dust, the beam is seen crossing
the flask from side to side. But here is another similar flask,
which cuts a clear gap out of the beam. It is filled with
unfiltered air, and still no trace of the beam is visible. Why?
By pure accident I stumbled on this flask in our apparatus room,
where it had remained quiet for some time. Acting upon this
obvious suggestion I set aside three other flasks, filled, in the
first instance, with mote-laden air. They are now optically
empty. Our former experiments proved that the life-producing
particles attach themselves to the fibres of cotton-wool. In the
present experiment the motes have been brought by gentle
air-currents, established by slight differences of temperature
within our closed vessels, into contact with the interior
surface, to which they adhere. The air of these flasks has
deposited its dust, germs and all, and is practically free from
suspended matter.

I had a chamber erected, the lower half of which is of wood,
its upper half being enclosed by four glazed window-frames. It
tapers to a truncated cone at the top. It measures in plan 3 ft.
by 2 ft. 6 in., and its height is 5 ft. 10 in. On February 6 it
was closed, every crevice that could admit dust, or cause
displacement of the air, being carefully pasted over with paper.
The electric beam at first revealed the dust within the chamber
as it did in the air of the laboratory. The chamber was examined
almost daily; a perceptible diminution of the floating matter
being noticed as time advanced. At the end of a week the chamber
was optically empty, exhibiting no trace of matter competent to
scatter the light. Such must have been the case in the stagnant
caves of the Paris Observatory. Were our electric beam sent
through the air of these caves its track would be invisible; thus
showing the indissoluble association of the scattering of light
by air and its power to generate life.

I will now turn to what seems to me a more interesting
application of the luminous beam than any hitherto described. My
reference to Professor Lister’s interpretation of the fact, that
air which has passed through the lungs cannot produce
putrefaction, is fresh in your memories. ‘Why air,’ said he,
‘introduced into the pleural cavity, through a wounded
lung, should have such wholly different effects from that
entering through a permanently open wound, penetrating from
without, was to me a complete mystery, till I heard of the germ
-theory of putrefaction, when it at once occurred to me that it
was only natural that the air should be filtered of germs by the
air passages, one of whose offices is to arrest inhaled particles
of. dust, and prevent them from entering the air-cells.’

Here is a surmise which bears the stamp of genius, but which
needs verification. If, for the words ‘it is only natural’
we were authorised to write ‘it is perfectly certain,’ the
demonstration would be complete. Such demonstration is furnished
by experiments with a beam of light. One evening, towards the
close of 1869, while pouring various pure gases across the dusty
track of a luminous beam, the thought occurred to me of using my
breath instead of the gases. I then noticed, for the first time,
the extraordinary darkness produced by the expired air, towards
the end of the expiration. Permit me to repeat the experiment in
your presence. I fill my lungs with ordinary air and breathe
through a glass tube across the beam. The condensation of the
aqueous vapour of the breath is shown by the formation of a
luminous white cloud of delicate texture. We abolish this cloud
by drying the breath previous to its entering the beam; or, still
more simply, by warming the glass tube. The luminous track of the
beam is for a time uninterrupted by the breath, because the dust
returning from the lungs makes good, in great part, the particles
displaced. After a time, however, an obscure disk appears in the
beam, the darkness of which increases, until finally, towards the
end of the expiration, the beam is, as it were, pierced by an
intensely black hole, in which no particles whatever can be
discerned. The deeper air of the lungs is thus proved to be
absolutely free from suspended matter. It is therefore in the
precise condition required by Professor Lister’s explanation.
This experiment may be repeated any number of times with the same
result. I think it must be regarded as a crowning piece of
evidence both of the correctness of Professor Lister’s views and
of the impotence, as regards vital development, of optically pure
air. [Footnote: Dr. Burden Sanderson draws attention to
the important observation of Brauell, which shows that the
contagium of a pregnant animal, suffering from splenic
fever, is not found in the blood of the foetus; the placental
apparatus acting as a filter, and holding back the infective
particles. ]

.

Application of Luminous beams to
Water.

The method of examination here
pursued is also applicable to water. It is in some sense
complementary to that of the microscope, and may, I think,
materially aid enquiries conducted with that instrument. In
microscopic examination attention is directed to a small portion
of the liquid, and the aim is to detect the individual particles.
By the present method a large portion of the liquid is
illuminated, the collective action of the particles being
revealed, by the scattered light. Care is taken to defend the eye
from the access of all other light, and, thus defended, it
becomes an organ of inconceivable delicacy. Indeed, an amount of
impurity so infinitesimal as to be scarcely expressible in
numbers, and the individual particles of which are so small as
wholly to elude the microscope, may, when examined by the method
alluded to, produce not only sensible, but striking, effects upon
the eye.

We will apply the method, in the first place, to an experiment
of M. Pouchet intended to prove conclusively that animalcular
life is developed in cases where no antecedent germs could
possibly exist. He produced water from the combustion of hydrogen
in air, justly arguing that no germ could survive the heat of a
hydrogen flame. But he overlooked the fact that his aqueous
vapour was condensed in the air, and was allowed as water to
trickle through the air. Indeed the experiment is one of a number
by which workers like M. Pouchet are differentiated from workers
like Pasteur. I will show you some water, produced by allowing a
hydrogen flame to play upon a polished silver condenser, formed
by the bottom of a silver basin, containing ice. The collected
liquid is pellucid in the common light; but in the condensed
electric beam it is seen to be laden with particles, so
thick-strewn and minute as to produce a continuous luminous cone.
In passing through the air the water loaded itself with this
matter; and the deportment of such water could obviously have no
influence in deciding this great question.

We are invaded with dirt not only in the air we breathe, but
in the water we drink. To prove this I take the bottle of water
intended to quench your lecturer’s thirst; which, in the track of
the beam, simply reveals itself as dirty water. And this water is
no worse than the other London waters. Thanks to the kindness of
Professor Frankland, I have been furnished with specimens of the
water of eight London companies. They are all laden with
impurities mechanically suspended. But you will ask whether
filtering will not remove the suspended matter? The grosser
matter, undoubtedly, but not the more finely divided matter.
Water may be passed any number of times through bibulous paper,
it will continue laden with fine matter. Water passed through
Lipscomb’s charcoal filter, or through the filters of the
Silicated Carbon Company, has its grosser matter removed, but it
is thick with fine matter. Nine-tenths of the light scattered by
these suspended particles is perfectly polarised in a direction
at right angles to the beam, and this release of the particles
from the ordinary law of polarisation is a demonstration of their
smallness. I should say by far the greater number of the
particles concerned in this scattering are wholly beyond the
range of the microscope, and no ordinary filter can intercept
such particles. It is next to impossible, by artificial means, to
produce a pure water. Mr. Hartley, for example, some time ago
distilled water while surrounded by hydrogen, but the water was
not free from floating matter. It is so hard to be clean in the
midst of dirt. In water from the Lake of Geneva, which has
remained long without being stirred, we have an approach to the
pure liquid. I have a bottle of it here, which was carefully
filled for me by my distinguished friend Soret. The track of the
beam through it is of a delicate sky-blue; there is scarcely a
trace of grosser matter.

The purest water that I have seen — probably the purest
which has been seen hitherto — has been obtained from the
fusion of selected specimens of ice. But extraordinary.
precautions are required to obtain this degree of purity. The
following apparatus has been constructed for this purpose:
Through the plate of an air-pump passes the shank of a large
funnel, attached to which below the plate is a clean glass bulb.
In the funnel is placed a block of the most transparent ice, and
over the funnel a glass receiver. This is first exhausted and
refilled several times with air, filtered by its passage through
cotton-wool, the ice being thus surrounded by pure moteless air.
But the ice has previously been in contact with mote-filled air;
it is therefore necessary to let it wash its own surface, and
also to wash the bulb which is to receive the water of
liquefaction. The ice is permitted to melt, the bulb is filled
and emptied several times, until finally the large block dwindles
to a small one. We may be sure that all impurity has been thus
removed from the surface of the ice. The water obtained in this
way is the purest hitherto obtained. Still I should hesitate to
call it absolutely pure. When condensed light is sent through it,
the track of the beam is not invisible, but of the most
exquisitely delicate blue. This blue is purer than that of the
sky, so that the matter which produces it must be finer than that
of the sky. It may be and indeed has been, contended that this
blue is scattered by the very molecules of the water, and not by
matter suspended in the water. But when we remember that this
perfection of blue is approached gradually through stages of less
perfect blue; and when we consider that a blue in all respects
similar is demonstrably obtainable from particles mechanically
suspended, we should hesitate, I think, to conclude that we have
arrived here at the last stage of purification. The evidence, I
think, points distinctly to the conclusion that, could we push
the process of purification still farther, even this last
delicate trace of blue would disappear.

Chalk-water. Clark’s Softening
Process.

But is it not possible to match
the water of the Lake of Geneva here in England? Undoubtedly it
is. We have in England a kind of rock which constitutes at once
an exceedingly clean recipient and a natural filter, and from
which we can obtain water extremely free from mechanical
impurities. I refer to the chalk formation, in which large
quantities of water are held in store. Our chalk hills are in
most cases covered with thin layers of soil, and with very scanty
vegetation. Neither opposes much obstacle to the entry of the
rain into the chalk, where any organic impurity which the water
may carry in is soon oxidised and rendered harmless. Those who
have scampered like myself over the downs of Hants and Wilts will
remember the scarcity of water in these regions. In fact, the
rainfall, instead of washing the surface and collecting in
streams, sinks into the fissured chalk and percolates through it.
When this formation is suitably tapped, we obtain water of
exceeding briskness and purity. A large glass globe, filled with
the water of a well near Tring, shows itself to be wonderfully
free from mechanical impurity. Indeed, it stands to reason that
water wholly withdrawn from surface contamination, and
percolating through so clean a substance, should be pure. It has
been a subject much debated, whether the supply of excellent
water which the chalk holds in store could not be rendered
available for London. Many of the most eminent engineers and
chemists have ardently recommended this source, and have sought
to show, not only that its purity is unrivalled, but that its
quantity is practically inexhaustible. Data sufficient to test
this are now, I believe, in existence; the number of wells sunk
in the chalk being so considerable, and the quantity of water
which they yield so well known.

But this water, so admirable as regards freedom from
mechanical impurity, labours under the disadvantage of being
rendered very hard by the carbonate of lime which it holds in
solution. The chalk-water in the neighbourhood of Watford
contains about seventeen grains of carbonate of lime per gallon.
This, in the old terminology, used to be called seventeen degrees
of hardness. This hard water is bad for tea, bad for washing, and
it furs our boilers, because the lime held in solution is
precipitated by boiling. If the water be used cold, its hardness
must be neutralised at the expense of soap, before it will give a
lather. These are serious objections to the use of chalk-water in
London. But they are successfully met by the fact that such water
can be softened inexpensively, and on a grand scale. I had long
known the method of softening water called Clark’s process, but
not until recently, under the guidance of Mr. Homersham, did I
see proof of its larger applications. The chalk-water is softened
for the supply of the city of Canterbury; and at the Chiltern
Hills it is softened for the supply of Tring and Aylesbury.
Caterham also enjoys the luxury.

I have visited all these places, and made myself acquainted
with the works. At Canterbury there are three reservoirs covered
in and protected, by a concrete roof and layers of pebbles, both
from the summer’s heat and the winter’s cold. Each reservoir
holds 120,000 gallons of water. Adjacent to these reservoirs are
others containing pure slaked lime — the so-called ‘cream
of lime.’ These being filled with water, the lime and water are
thoroughly mixed by air forced by an engine through apertures in
the bottom of the reservoir. The water soon dissolves all the
lime it is capable of dissolving. The mechanically suspended lime
is then allowed to subside to the bottom, leaving a perfectly
transparent lime-water behind.

The softening process is this: Into one of the empty
reservoirs is introduced a certain quantity of the clear
lime-water, and after this about nine times the quantity of the
chalk-water. The transparency immediately disappears — the
mixture of the two clear liquids becoming thickly turbid, through
the precipitation of carbonate of lime. The precipitate is
crystalline and heavy, and in about twelve hours a layer of pure
white carbonate of lime is formed at the bottom of the reservoir,
with a water of extraordinary beauty and purity overhead. A few
days ago I pitched some halfpence into a reservoir sixteen feet
deep at the Chiltern Hills. This depth hardly dimmed the coin.
Had I cast in a pin, it could have been seen at the bottom. By
this process of softening, the water is reduced from about
seventeen degrees of hardness, to three degrees of hardness. It
yields a lather immediately. Its temperature is constant
throughout the year. In the hottest summer it is cool, its
temperature being twenty degrees above the freezing point; and it
does not freeze in winter if conveyed in proper pipes. The
reservoirs are covered; a leaf cannot blow into them, and no
surface contamination can reach the water. It passes direct from
the main into the house tap; no cisterns are employed, and the
supply is always fresh and pure. This is the kind of water which
is supplied to the fortunate people of Tring, Caterham, and
Canterbury.

—–

The foregoing article, as far as it relates to the theory
which ascribes epidemic disease to the development of low
parasitic life within the human life, was embodied in a discourse
delivered before the Royal Institution in January 1870. In June
1871, after a brief reference to the polarisation of light by
cloudy matter, I ventured to recur to the subject in these terms:
What is the practical use of these curiosities? If we exclude the
interest attached to the observation of new facts, and the
enhancement of that interest through the knowledge that facts
often become the exponents of laws, these curiosities are in
themselves worth little. They will not enable us to add to our
stock of food, or drink, or clothes, or jewellery. But though
thus shorn of all usefulness in themselves, they may, by carrying
thought into places which it would not otherwise have entered,
become the antecedents of practical consequences. In looking, for
example, at our illuminated dust, we may ask ourselves what it
is. How does it act, not upon a beam of light, but upon our own
bodies? The question then assumes a practical character. We find
on examination that this dust is mainly organic matter — in
part living, in part dead. There are among it particles of ground
straw, torn rags, smoke, the pollen of flowers, the spores of
fungi, and the germs of other things. But what have they to do
with the animal economy? Let me give you an illustration to which
my attention has been lately drawn by Mr. George Henry Lewes, who
writes to me thus:

‘I wish to direct your attention to the experiments of von
Recklingshausen should you happen not to know them. They are
striking confirmations of what you say of dust and disease. Last
spring, when I was at his laboratory in Wuerzburg, I examined
with him blood that had been three weeks, a month, and five
weeks, out of the body, preserved in little porcelain cups under
glass shades. This blood was living and growing. Not only were
the Amoeba-like movements of the white corpuscles present, but
there were abundant evidences of the growth and development of
the corpuscles. (I also saw a frog’s heart still pulsating which
had been removed from the body I forget how many days, but
certainly more than a week). There were other examples of the
same persistent vitality, or absence of putrefaction. Von
Recklingshausen did not attribute this to the absence of germs
— germs were not mentioned by him; but when I asked him how
he represented the thing to himself, he said the whole mystery of
his operation consisted in keeping the blood free from dirt. The
instruments employed were raised to a red heat just before use;
the thread was silver thread and was similarly treated; and the
porcelain cups, though not kept free from air, were kept free
from currents. He said he often had failures, and these he
attributed to particles of dust having escaped his
precautions.’

Professor Lister, who has founded upon the removal or
destruction of this ‘dirt’ momentous improvements in
surgery, tells us the effect of its introduction into the blood
of wounds. The blood would putrefy and become fetid; and when you
examine more closely what putrefaction means, you find the
putrefying substance swarming with infusorial life, the germs of
which have been derived from the atmospheric dust.

We are now assuredly in the midst of practical matters; and
with your permission I will refer once more to a question which
has recently occupied a good deal of public attention. As regards
the lowest forms of life, the world is divided, and has for a
long time been divided, into two parties, the one affirming that
we have only to submit absolutely dead matter to certain physical
conditions, to evolve from it living things; the other (without
wishing to set bounds to the power of matter) affirming that, in
our day, life has never been found to arise independently of
pre-existing life. I belong to the party which claims life as a
derivative of life. The question has two factors — the
evidence, and the mind that judges of the evidence; and it may be
purely a mental set or bias on my part that causes me throughout
this long discussion, to see, on the one side, dubious facts and
defective logic, and on the other side firm reasoning and a
knowledge of what rigid experimental enquiry demands. But, judged
of practically, what, again, has the question of Spontaneous
Generation to do with us? Let us see. There are numerous diseases
of men and animals that are demonstrably the products of
parasitic life, and such diseases may take the most terrible
epidemic forms, as in the case of the silkworms of France,
referred to at an earlier part of this article. Now it is in the
highest degree important to know whether the parasites in
question are spontaneously developed, or whether they have been
wafted from without to those afflicted with the disease. The
means of prevention, if not of cure, would be widely different in
the two cases.

But this is not all. Besides these universally admitted cases,
there is the broad theory, now broached and daily growing in
strength and clearness — daily, indeed, gaining more and
more of assent from the most successful workers and profound
thinkers of the medical profession itself — the theory,
namely, that contagious disease, generally, is of this parasitic
character. Had I any cause to regret having introduced this
theory to your notice more than a year ago, that regret should
now be expressed. I would certainly renounce in your presence
whatever leaning towards the germ theory my words might then have
betrayed. But since the time referred to nothing has occurred to
shake my conviction of the truth of the theory. Let me briefly
state the grounds on which its supporters rely. From their
respective viruses you may plant typhoid fever, scarlatina, or
small-pox. What is the crop that arises from this husbandry? As
surely as a thistle rises from a thistle seed, as surely as the
fig comes from the fig, the grape from the grape, the thorn from
the thorn, so surely does the typhoid virus increase and multiply
into typhoid fever, the scarlatina virus into scarlatina, the
small-pox virus into small-pox. What is the conclusion that
suggests itself here? It is this: That the thing which we vaguely
call a virus is to all intents and purposes a seed. Excluding the
notion of vitality, in the whole range of chemical science you
cannot point to an action which illustrates this perfect
parallelism with the phenomena of life — this demonstrated
power of self-multiplication and reproduction. The germ theory
alone accounts for the phenomena.

In cases of epidemic disease, it is not on bad air or foul
drains that the attention of the physician of the future will
primarily be fixed, but upon disease germs, which no bad air or
foul drains can create, but which may be pushed by foul air into
virulent energy of reproduction. You may think I am treading on
dangerous ground, that I am putting forth views that may
interfere with salutary practice. No such thing. If you wish to
learn the impotence of medical practice in dealing with
contagious diseases, you have only to refer to the Harveian
oration for 1871, by Sir William Gull. Such diseases defy the
physician. They must run their course, and the utmost that can be
done for them is careful nursing. And this, though I do not
specially insist upon it, would favour the idea of their vital
origin. For if the seeds of contagious disease be themselves
living things, it may be difficult to destroy either them or
their progeny, without involving their living habitat in the same
destruction.

It has been said, and it is sure to be repeated, that I am
quitting my own métier, in speaking of these things. Not
so. I am dealing with a question on which minds accustomed to
weigh the value of experimental evidence are alone competent to
decide, and regarding which, in its present condition, minds so
trained are as capable of forming an opinion as regarding the
phenomena of magnetism or radiant heat. ‘The germ theory of
disease,’ it has been said, ‘appertains to the biologist
and the physician.’ Where, I would ask in reply, is the biologist
or physician, whose researches, in connection with this subject,
could for one instant be compared to those of the chemist
Pasteur? It is not the philosophic members of the medical
profession who are dull to the reception of truth not originated
within the pale of the profession itself. I cannot better
conclude this portion of my story than by reading to you an
extract from a letter addressed to me some time ago by Dr.
William Budd, of Clifton, to whose insight and energy the town of
Bristol owes so much in the way of sanitary improvement.

‘As to the germ theory itself,’ writes Dr. Budd, that is a
matter on which I have long since made up my mind. From the day
when I first began to think of these subjects I have never had a
doubt that the specific cause of contagious fevers must be living
organisms.

‘It is impossible, in fact, to make any statement bearing upon
the essence or distinctive characters of these fevers, without
using terms which are of all others the most distinctive of life.
Take up the writings of the most violent opponent of the germ
theory, and, ten to one, you will find them full of such terms as
“propagation,” “self-propagation,” “reproduction,” 61
self-multiplication,” and so on. Try as he may — if he has
anything to say of those diseases which is characteristic of
them — he cannot evade the use of these terms, or the exact
equivalents to them. While perfectly applicable to living things,
these terms express qualities which are not only inapplicable to
common chemical agents, but, as far as I can see, actually
inconceivable of them.’

.

.

Cotton-wool Respirator.

Once, then, established within
the body, this evil form of life, if you will allow me to call it
so, must run its course. Medicine as yet is powerless to arrest
its progress, and the great point to be aimed at is to prevent
its access to the body. It was with this thought in my mind that
I ventured to recommend, more than a year ago, the use of
cotton-wool respirators in infectious places. I would here repeat
my belief in their efficacy if properly constructed. But I do not
wish to prejudice the use of these respirators, by connecting
them indissolubly with the germ theory. There are too many trades
in England where life is shortened and rendered miserable by the
introduction of matters into the lungs which might be kept out of
them. Dr. Greenhow has shown the stony grit deposited in the
lungs of stonecutters. The black lungs of colliers is another
case in point. In fact, a hundred obvious cases might be cited,
and others that are not obvious might be added to them. We should
not, for example, think that printing implied labour where the
use of cotton-wool respirators might come into play; but the fact
is that the dust arising from the sorting of the type is very
destructive of health. I went some time ago into a manufactory in
one of our large towns, where iron
vessels are enamelled by coating them with a mineral powder, and
subjecting them to a heat sufficient to fuse the powder. The
organisation of the establishment was excellent, and one thing
only was needed to make it faultless. In a large room a number of
women were engaged covering the vessels. The air was laden with
the fine dust, and their faces appeared as white and bloodless as
the powder with which they worked. By the use of cotton-wool
respirators these women might be caused to breathe air as free
from suspended matter as that of the open street. Over a year ago
a Lancashire seedsman wrote to me, stating that during the seed
season his men suffered horribly from irritation and fever, so
that many of them left his service. He asked for help, and I gave
him my advice. At the conclusion of the season, this year, he
wrote to inform me that he had folded a little cotton-wool. in
muslin, and tied it in front of the mouth; and that with this
simple defence he had passed through the season in comfort, and
without a single complaint from his men.

Against the use of such a respirator the obvious objection
arises, that it becomes wet and heated by the breath. While
casting about for a remedy for this, a friend forwarded to me
from Newcastle a form of respirator invented by Mr. Carrick, a
hotel-keeper at Glasgow, which, by a slight modification, may be
caused to meet the case perfectly. The respirator, with its back
in part removed, is shown in fig. 4. Under the partition of
wire-gauze q r, is a space intended by Mr. Carrick for ‘medicated
substances,’ and which may be filled with cotton-wool. The mouth
is placed against the aperture o, which fits closely round the
lips, and the filtered air enters the mouth through a light valve
v, which is lifted by the act of inhalation.

During exhalation this valve closes; the breath escapes by a
second valve, v’, into the open. air. The wool is thus kept dry
and cool; the air in passing through it being filtered of
everything it holds in suspension. The respirator has since taken
other forms.

FIG. 4.

—–

Fireman’s Respirator.

We have thus been led by our
first unpractical experiments into a thicket of practical
considerations. But another step is possible. Admiring, as I do,
the bravery of our firemen, and hearing that smoke was a more
serious enemy than flame itself, I thought of devising a
fireman’s respirator.

Our fire-escapes are each in charge of a single man, and it
would be of obvious importance to place it in the power of each
of those men to penetrate through the densest smoke, into the
recesses of a house, and there to rescue those who would
otherwise be suffocated or burnt. Cotton-wool, which so
effectually arrested dust, was first tried; but, though found
soothing in certain gentle kinds of smoke, it was no match for
the pungent fumes of a resinous fire. For the purpose of catching
the atmospheric germs, M. Pouchet spread a film of glycerine on a
plate of glass, urged air against the film, and examined the dust
which stuck to it. The moistening of the cotton-wool with

glycerine was a decided improvement; still the respirator only
enabled us to remain in dense smoke for three or four minutes,
after which the irritation became unendurable. Reflection
suggested that, besides the smoke, there must be numerous
hydrocarbons produced, which, being in a state of vapour, would
be very imperfectly arrested by the cotton-wool. These, in all
probability, were the cause of the residual irritation; and if
these could be removed, a practically perfect respirator might
possibly be obtained.

I state the reasoning exactly as it occurred to my mind. Its
result will be anticipated by many present. All bodies possess
the power of condensing, in a greater or less degree, gases and
vapours upon their surfaces, and when the condensing body is very
porous, or in a fine state of division, the force of condensation
may produce very remarkable effects. Thus, a clean piece of
platinum-foil placed in a mixture of oxygen and hydrogen so
squeezes the gases together as to cause them to combine; and if
the experiment be made with care, the heat of combination may
raise the platinum to bright redness. The promptness of this
action is greatly augmented by reducing the platinum to a state
of fine division. A pellet of ‘spongy platinum,’ for
instance, plunged into a mixture of oxygen and hydrogen, causes
the gases to explode instantly. In virtue of its extreme
porosity, a similar power is possessed by charcoal. It is not
strong enough to cause the oxygen and hydrogen to combine like
the spongy platinum, but it so squeezes the more condensable
vapours, and acts with such condensing power upon the oxygen of
the air, as to bring both within the combining distance, thus
enabling the oxygen to attack and destroy the vapours in the
pores of the charcoal. In this way, effluvia of all kinds may be
virtually burnt up; and this is the principle of the excellent
charcoal respirators invented by Dr. Stenhouse. Armed with one of
these, you may go into the foulest-smelling places without having
your nose offended.

But, while powerful to arrest vapours, the charcoal respirator
is ineffectual as regards smoke. The smoke-particles get freely
through the respirator. With a number of such respirators, tested
in a proper room, from half a minute to a minute was the limit of
endurance. This might be exceeded by Faraday’s simple method of
emptying the lungs completely, and then filling them before going
into a smoky atmosphere. In fact, each solid smoke particle is
itself a bit of charcoal, and carries on it, and in it, its
little load of irritating vapour. It is this, far more than the
particles of carbon themselves, that produces the irritation.
Hence two causes of offence are to be removed: the carbon
particles which convey the irritant by adhesion and condensation,
and the free vapour which accompanies the particles. The
cotton-wool moistened with glycerine I knew would arrest the
first; fragments of charcoal I hoped would stop the second. In
the first fireman’s respirator, Mr. Carrick’s arrangement of two
valves, the one for inhalation, the other for exhalation, was
preserved. But the portion of the respirator which holds the
filtering and absorbent substances, was prolonged to a depth of
four or five inches (see fig. 5.) Under the partition of
wire-gauze q r at the bottom of the space which fronts the mouth
was placed a layer of cotton-wool, c, moistened with glycerine;
then a thin layer of dry wool, c’; then a layer of charcoal
fragments; and finally a second thin layer of dry cotton-wool.
The succession of the layers may be changed without prejudice to
the action. A wire-gauze cover, shown in plan under fig. 5, keeps
the substances from falling out of the respirator. A layer of
caustic lime may be added for the absorption of carbonic acid;
but in the densest smoke that we have hitherto employed, it has
not been found necessary, nor is it shown in the figure. In a
flaming building, indeed, the mixture of air with the smoke never
permits the carbonic acid to become so dense as to be
irrespirable; but in a place where the gas is present in undue
quantity, the fragments of lime would materially mitigate its
action.

In a small cellar-like chamber with a stone flooring and stone
walls, the first experiments were made. We Placed there furnaces
containing resinous pine-wood, lighted the wood, and, placing
over it a lid which prevented too brisk a circulation of the air,
generated dense volumes of smoke. With our eyes protected by
suitable glasses, my assistant and I have remained for half an
hour and more in smoke so dense and pungent that a single
inhalation, through the undefended mouth, would be perfectly
unendurable. We might have prolonged our stay for hours.

FIG. 5.

Having thus far perfected the instrument, I wrote to the chief
officer of the Metropolitan Fire Brigade, asking him whether such
a respirator would be of use to him. His reply was prompt; it
would be most valuable. He had, however, made himself acquainted
with every contrivance of the kind in this and other countries,
and had found none of them of any practical use. He offered to
come and test it here, or to place a room at my disposal in the
City. At my request he came here, accompanied by three of his
men. Our small room was filled with smoke to their entire
satisfaction. The three men went successively into it, and
remained there as long as Captain Shaw wished them. On coming out
they said that they had not suffered the slightest inconvenience;
that they could have remained all day in the smoke. Captain Shaw
then tested the respirator with the same result, and he
afterwards took great interest in the perfecting of the
instrument.

—–

Various ameliorations and improvements have recently been
introduced into the smoke respirator. The hood of Captain Shaw
has been improved upon by the simple and less expensive
mouthpiece of Mr. Sinclair; and this, in its turn, has been
simplified and improved by my assistant Mr. John Cottrell. The
respirator is now in considerable demand, and it has already done
good practical service. Care is, however, necessary, in
moistening the wool with glycerine. It must be carefully teazed,
so that the individual fibres may be moistened, and clots must be
avoided. I cannot recommend the layers of moistened flannel
which, in some cases, have been used instead of cotton-wool:
nothing equals the wool, when carefully treated.

An experiment made last year brought out very conspicuously
the necessity of careful packing, and the enormous comparative
power of resisting smoke irritation possessed by our firemen, and
the able officer who commands them. Having heard from Captain
Shaw that, in some recent very trying experiments, he had
obtained the best effects from dry cotton-wool, and thinking that
I could not have been mistaken in my first results, which proved
the dry so much inferior to the moistened wool and its associated
charcoal, I proposed to Captain Shaw to bring the matter to a
test at his workshops in the City. He was good enough to accept
my proposal, and thither I went on May 7, 1874. The smoke was
generated in a confined space from wet straw, and it was
certainly very diabolical.

At this season of the year I am usually somewhat shorn of
vigour, and therefore not in the best condition for severe
experiments; still I wished to test the matter in my own person.
With a respirator which had been in use some days previously, and
which was not carefully packed, I followed a fireman into the
smoke, he being provided with a dry-wool respirator. I was
compelled to quit the place in about three minutes, while the
fireman remained there for six or seven minutes.

I then tried his respirator upon myself, and found that with
it I could not remain more than a minute in the smoke; in fact
the first inhalation provoked coughing.

Thinking that Captain Shaw himself might have lungs more like
mine than those of his fireman, I proposed that we should try the
respirators together; but he informed me that his lungs were very
strong. He was, however, good enough to accede to my request.
Before entering the den a second time I repacked my respirator,
with due care, and entered the smoke in company with Captain
Shaw. I could hear him breathe long slow inhalations; his labour
was certainly greater than mine, and after the lapse of seven
minutes I heard him cough. In seven and a half minutes he had to
quit the place, thus proving that his lungs were able to endure
the irritation seven times as long as mine could bear it. I
continued in the smoke, with hardly any discomfort, for sixteen
minutes, and certainly could have remained in it much longer. The
advantage arising from the glycerine was thus placed beyond
question.

During this time I was in a condition to render very material
assistance to a person in danger of suffocation.

Helmholtz on Hay
Fever.

In my lecture on Dust and
Disease in 1870, I referred to an experiment made by Helmholtz
upon himself which strikingly connected hay fever with
animalcular life. About a year ago I received from Professor Binz
of Bonn a short, but important paper, embracing Helmholtz’s
account of his observation, to which Professor Binz has added
some remarks of his own. The paper, being mainly intended for
English medical men, was published in English, and though here
and there its style might be amended, I think it better to
publish it unaltered.

From what I have observed (says Professor Binz) of recent
English publications on the subject of hay fever, I am led to
suppose that English authorities are inaccurately acquainted with
the discovery of Professor Helmholtz, as far back as 1868, of the
existence of uncommon low organisms in the nasal secretions in
this complaint, and of the possibility of arresting their action
by the local employment of quinine. I therefore purpose to
republish the letter in which he originally announced these facts
to myself, and to add some further observations on this topic.
The letter is as follows :— [Footnote: Cf. Virchow’s
‘Archiv.’ vol. xlvi. p. 100]

‘I have suffered, as well as I can remember, since the year
1847, from the peculiar catarrh called by the English “hay
fever,” the speciality of which consists in its attacking its
victims regularly in the hay season (myself-between May 20 and
the end of June), that it ceases in the cooler weather, but on
the other hand quickly reaches a great intensity if the patients
expose themselves to heat and sunshine. An extraordinary violent
sneezing then sets in, and a strongly corrosive thin discharge,
with which much epithelium is thrown off. This increases, after a
few hours, to a painful inflammation of the mucous membrane and
of the outside of the nose, and excites fever with severe
headache and great depression, if the patient cannot withdraw
himself from the heat and the sunshine. In a cool room, however,
these symptoms vanish as quickly as they come on, and there then
only remains for a few days a lessened discharge and soreness, as
if caused by the loss of epithelium. I remark, by the way, that
in all my other years I had very little tendency to catarrh or
catching cold, while the hay fever has never failed during the
twenty-one years of which I have spoken, and has never attacked
me earlier or later in the year than the times named. The
condition is extremely troublesome, and increases, if one is
obliged to be much exposed to the sun, to an excessively severe
malady.

‘The curious dependence of the disease on the season of the
year suggested to me the thought that organisms might be the
origin of the mischief. In examining the secretion I regularly
found, in the last five years, certain vibrio-like bodies in it,
which at other times I could not observe in my nasal
secretion. . . . They are very small, and can only be recognised
with the immersion-lens of a very good Hartnack’s microscope. It
is characteristic of the common isolated single joints that they
contain four nuclei in a row, of which two pairs are more closely
united. The length of the joints is 0.004 millimetre. Upon the
warm objective-stage they move with moderate activity, partly in,
mere vibration, partly shooting backwards and forwards in the
direction of their long axis; in lower temperatures they are very
inactive. Occasionally one finds them arranged in rows upon each
other, or in branching series. Observed some days in the moist
chamber, they vegetated again, and appeared somewhat larger and
more conspicuous than immediately after their excretion. It is to
be noticed that only that kind of secretion contains them which
is expelled by violent sneezings; that which drops slowly does
not contain any. They stick tenaciously enough in the lower
cavities and recesses of the nose.

‘When I saw your first notice respecting the poisonous
action of quinine upon infusoria, I determined at once to make an
experiment with that substance, thinking that these vibrionic
bodies, even if they did not cause the whole illness, still could
render it much more unpleasant through their movements and the
decompositions caused by them. For that reason I made a neutral
solution of sulphate of quinine, which did not contain much of
the salt (1·800), but still was effective enough, and
caused moderate irritation on the mucus membrane of the nose. I
then lay flat on my back, keeping my head very low, and poured
with a pipette about four cubic centimetres into both nostrils.
Then I turned my head about in order to let the liquid flow in
all directions.

‘The desired effect was obtained immediately, and remained for
some hours; I could expose myself to the sun without fits of
sneezing and the other disagreeable symptoms coming on. It was
sufficient to repeat the treatment three times a day, even under
the most unfavourable circumstances, in order to keep myself
quite free. [Footnote: There is no foundation for the
objection that syringing the nose could not cure the asthma which
accompanies hay fever; for this asthma is only the reflex effect
arising from the irritation of the nose. — B.] There
were then no such vibrios in the secretion. If I only go out in
the evening, it suffices to inject the quinine once a day, just
before going. After continuing this treatment for some days the
symptoms disappear completely, but if I leave off they return
till towards the end of June.

‘My first experiments with quinine date from the summer
of 1867; this year (1868) I began at once as soon as the first
traces of the illness appeared, and I have thus been able to stop
its development completely.

‘I have hesitated as yet in publishing the matter,
because I have found no other patient [Footnote:
Helmholtz, now Professor of Physics at the University of Berlin,
is, although M.D., no medical practitioner. — B.] on
whom I could try the experiment. There is, it seems to me, no
doubt, considering the extraordinary regularity in the recurrence
and course of the illness, that quinine had here a most quick and
decided effect. And this again makes my hypothesis very probable,
that the vibrios, although of no specific form but a very
frequent one, are at least the cause of the rapid increase of the
symptoms in warm air, as heat excites them to lively action.

I should be very glad if the above lines would induce medical
men in England — the haunt of hay fever — to test the
observation of Helmholtz. To most patients the application with
the pipette may be too difficult or impossible; I have therefore
already suggested the use of Weber’s very simple but effective
nose-douche. Also it will be advisable to apply the solution of
quinine tepid. It can, further, not be repeated often enough that
quinine is frequently adulterated, especially with cinchona, the
action of which is much less to be depended upon.

Dr. Frickhoefer, of Schwalbach, has communicated to me a
second case in which hay fever was cured by local application of
quinine. [Footnote: Cf. Virchow’s ‘Archiv.’ (1870), vol.
li. p. 176.] Professor Busch, of Bonn, authorises me to
say that he succeeded in two cases of ‘catarrhus aestivus’ by the
same method: a third patient was obliged to abstain from the use
of quinine, as it produced an unbearable irritation of the
sensible nerves of the nose. In the autumn of 1872 Helmholtz told
me that his fever was quite cured, and that in the meantime two
other patients had, by his advice, tried this method, and with
the same success. [Footnote: Prof. Helmholtz, whom I had
the pleasure of meeting in Switzerland last year, then told me
that he was quite convinced that hay fever was produced by the
pollen afloat in early summer in the atmosphere.]

.

.

.

.

——————–

.

.

VI. VOYAGE TO ALGERIA TO OBSERVE THE
ECLIPSE.

1870.

THE opening of the Eclipse Expedition was not propitious.
Portsmouth, on Monday, December 5, 1870, was swathed by fog,
which was intensified by smoke, and traversed by a drizzle of
fine rain. At six P.M. I was on board the ‘Urgent.’ On
Tuesday morning the weather was too thick to permit of the ship’s
being swung and her compasses calibrated. The Admiral of the
port, a man of very noble presence, came on board. Under his
stimulus the energy which the weather had damped appeared to
become more active, and soon after his departure we steamed down
to Spithead. Here the fog had so far lightened as to enable the
officers to swing the ship.

At three P.M. on Tuesday, December 6, we got away, gliding
successively past Whitecliff Bay, Bembridge, Sandown, Shanklin,
Ventnor, and St. Catherine’s Lighthouse. On Wednesday morning we
sighted the Isle of Ushant, on the French side of the Channel.
The northern end of the island has been fretted by the waves into
detached tower-like masses of rock of very remarkable appearance.
In the Channel the sea was green, and opposite Ushant it was a
brighter green. On Wednesday evening we committed ourselves to
the Bay of Biscay. The roll of the Atlantic was full, but not
violent. There had been scarcely a gleam of sunshine throughout
the day, but the cloud-forms were fine, and their apparent
solidity impressive. On Thursday morning the green of the sea was
displaced by a deep indigo blue. The whole of Thursday we steamed
across the bay. We had little blue sky, but the clouds were again
grand and varied — cirrus, stratus, cumulus, and nimbus, we
had them all. Dusky hair-like trails were sometimes dropped from
the distant clouds to the sea.

These were falling showers, and they. sometimes occupied the
whole horizon, while we steamed across the rainless circle which
was thus surrounded. Sometimes we plunged into the rain, and once
or twice, by slightly changing our course, avoided a heavy
shower. From time to time perfect rainbows spanned the heavens
from side to side. At times a bow would appear in fragments,
showing the keystone of the arch midway in air, and its two
buttresses on the horizon. In all cases the light of the bow
could be quenched by a Nicol’s prism, with its long diagonal
tangent to the arc. Sometimes gleaming patches of the firmament
were seen amid the clouds. When viewed in the proper direction,
the gleam could be quenched by a Nicol’s prism, a dark aperture
being thus opened into stellar space.

At sunset on Thursday the denser clouds were fiercely fringed,
while through the lighter ones seemed to issue the glow of a
conflagration. On Friday morning we sighted Cape Finisterre
— the extreme end of the arc which sweeps from Ushant round
the Bay of Biscay. Calm spaces of blue, in which floated quietly
scraps of cumuli, were behind us, but in front of us was a
horizon of portentous darkness. It continued thus threatening
throughout the day. Towards evening the wind strengthened to a
gale, and at dinner it was difficult to preserve the plates and
dishes from destruction. Our thinned company hinted that the
rolling had other consequences. It was very wild when we went to
bed. I slumbered and slept, but after some time was rendered
anxiously conscious that my body had become a kind of projectile,
with the ship’s side for a target. I gripped the edge of my berth
to save myself from being thrown out. Outside, I could hear
somebody say that he had been thrown from his berth, and sent
spinning to the other side of the saloon. The screw laboured
violently amid the lurching; it incessantly quitted the water,
and, twirling in the air, rattled against its bearings, causing
the ship to shudder from stem to stern. At times the waves struck
us, not with the soft impact which might be expected from a
liquid, but with the sudden solid shock of battering-rams.
‘No man knows the force of water,’ said one of the
officers,’ until he has experienced a storm at sea.’ These blows
followed each other at quicker intervals, the screw rattling
after each of them, until, finally, the delivery of a heavier
stroke than ordinary seemed to reduce the saloon to chaos.
Furniture crashed, glasses rang, and alarmed enquiries
immediately followed. Amid the noises I heard one note of forced
laughter; it sounded very ghastly. Men tramped through the
saloon, and busy voices were heard aft, as if something there had
gone wrong.

I rose, and not without difficulty got into my clothes. In the
after-cabin, under the superintendence of the able and energetic
navigating lieutenant, Mr. Brown, a group of blue-jackets were
working at the tiller-ropes. These had become loose, and the helm
refused to answer the wheel. High moral lessons might be gained
on shipboard, by observing what steadfast adherence to an object
can accomplish, and what large effects are heaped up by the
addition of infinitesimals. The tiller-rope, as the blue-jackets
strained in concert, seemed hardly to move; still it did move a
little, until finally, by timing the pull to the lurching of the
ship, the mastery of the rudder was obtained. I had previously
gone on deck. Round the saloon-door were a few members of the
eclipse party, who seemed in no mood for scientific observation.
Nor did I; but I wished to see the storm. I climbed the steps to
the poop, exchanged a word with Captain Toynbee, the only member
of the party to be seen on the poop, and by his direction made
towards a cleat not far from the wheel. [Footnote: The
cleat is a T-shaped mass of metal employed for the fastening of
ropes.] Round it I coiled my arms. With the exception of
the men at the wheel, who stood as silent as corpses, I was
alone.

I had seen grandeur elsewhere, but this was a new form of
grandeur to me. The ‘Urgent’ is long and narrow, and during our
expedition she lacked the steadying influence of sufficient
ballast. She was for a time practically rudderless, and lay in
the trough of the sea. I could see the long ridges, with some
hundreds of feet between their crests, rolling upon the ship
perfectly parallel to her sides. As they approached, they so grew
upon the eye as to render the expression ‘mountains high’
intelligible. At all events, there was no mistaking their
mechanical might, as they took the ship upon their shoulders, and
swung her like a pendulum. The deck sloped sometimes at an angle
which I estimated at over forty-five degrees; wanting my previous
Alpine practice, I should have felt less confidence in my grip of
the cleat. Here and there the long rollers were tossed by
interference into heaps of greater height. The wind caught their
crests, and scattered them over the sea, the whole surface of
which was seething white. The aspect of the clouds was a fit
accompaniment to the fury of the ocean. The moon was almost full
— at times concealed, at times revealed, as the scud flew
wildly over it. These things appealed to the eye, while the ear
was filled by the groaning of the screw and the whistle and boom
of the storm.

Nor was the outward agitation the only object of interest to
me. I was at once subject and object to myself, and watched with
intense interest the workings of my own mind. The ‘Urgent’ is an
elderly ship. She had been built, I was told, by a contracting
firm for some foreign Government, and had been diverted from her
first purpose when converted into a troop-ship. She had been for
some time out of work, and I had heard that one of her boilers,
at least, needed repair. Our scanty but excellent crew, moreover,
did not belong to the ‘Urgent,’ but had been gathered from other
ships. Our three lieutenants were also volunteers. All this
passed swiftly through my mind as the steamer shook under the
blows of the waves, and I thought that probably no one on board
could say how much of this thumping and straining the
‘Urgent’ would be able to bear. This uncertainty caused me
to look steadily at the worst, and I tried to strengthen myself
in the face of it.

But at length the helm laid hold of the water, and the ship
was got gradually round to face the waves. The rolling
diminished, a certain amount of pitching taking its place. Our
speed had fallen from eleven knots to two. I went again to bed.
After a space of calm, when we seemed crossing the vortex of a
storm, heavy tossing recommenced. I was afraid to allow myself to
fall asleep, as my berth was high, and to be pitched out of it
might be attended with bruises, if not with fractures. From
Friday at noon to Saturday at noon we accomplished sixty-six
miles, or an average of less than three miles an hour. I
overheard the sailors talking about this storm. The ‘Urgent,’
according to those that knew her, had never previously
experienced anything like it. [Footnote: ‘There is,
it will be seen, a fair agreement between these impressions and
those so vigorously described by a scientific correspondent of
the ‘Times.’]

All through Saturday the wind, though somewhat sobered, blew
dead against us. The atmospheric effects were exceedingly fine.
The cumuli resembled mountains in shape, and their peaked summits
shone as white as Alpine snows. At one place this resemblance was
greatly strengthened by a vast area of cloud, uniformly
illuminated, and lying like a névé below the peaks.
From it fell a kind of cloud-river strikingly like a glacier. The
horizon at sunset was remarkable — spaces of brilliant
green between clouds of fiery red, Rainbows had been frequent
throughout the day, and at night a perfectly continuous lunar bow
spanned the heavens from side to side. Its colours were feeble;
but, contrasted with the black ground against which it rested,
its luminousness was extraordinary.

Sunday morning found us opposite to Lisbon, and at midnight we
rounded Cape St. Vincent, where the lurching seemed disposed to
recommence. Through the kindness of Lieutenant Walton, a cot had
been slung for me. It hung between a tiller-wheel and a flue, and
at one A.M. I was roused by the banging of the cot against its
boundaries. But the wind was now behind us, and we went along at
a speed of eleven knots. We felt certain of reaching Cadiz by
three. But a new lighthouse came in sight, which some affirmed to
be Cadiz Lighthouse, while the surrounding houses were declared
to be those of Cadiz itself. these statements, the navigating
lieutenant changed his course, and steered for the place. A pilot
came on board, and he informed us that we were before the mouth
of the Guadalquivir, and that the lighthouse was that of
Cipiòna. Cadiz was still some eighteen miles distant.

We steered towards the city, hoping to get into the harbour
before dark. But the pilot who would have guided us had been
snapped up by another vessel, and we did not get in. We beat
about during the night, and in the morning found ourselves about
fifteen miles from Cadiz. The sun rose behind the city, and we
steered straight into the light. The three-towered cathedral
stood in the midst, round which swarmed apparently a multitude of
chimney-stacks. A nearer approach showed the chimneys to be small
turrets. A pilot was taken on board; for there is a dangerous
shoal in the harbour. The appearance of the town as the sun shone
upon its white and lofty walls was singularly beautiful. We cast
anchor; some officials arrived and demanded a clean bill of
health. We had none. They would have nothing to do with us; so
the yellow quarantine flag was hoisted, and we waited for
permission to land the Cadiz party. After some hours’ delay the
English consul and vice-consul came on board, and with them a
Spanish officer ablaze with gold lace and decorations. Under
slight pressure the requisite permission had been granted. We
landed our party, and in the afternoon weighed anchor. Thanks to
the kindness of our excellent paymaster, I was here transferred
to a more roomy berth.

Cadiz soon sank beneath the sea, and we sighted in succession
Cape Trafalgar, Tarifa, and the revolving light of Ceuta. The
water was very calm, and the moon rose in a quiet heaven. She
swung with her convex surface downwards, the common boundary
between light and shadow being almost horizontal. A pillar of
reflected light shimmered up to us from the slightly rippled sea.
I had previously noticed the phosphorescence of the water, but
tonight it was stronger than usual, especially among the foam at
the bows. A bucket let down into the sea brought up a number of
the little sparkling organisms which caused the phosphorescence.
I caught some of them in my hand. And here an appearance was
observed which was new to most of us, and strikingly beautiful to
all. Standing at the bow and looking forwards, at a distance of
forty or fifty yards from the ship, a number of luminous
streamers were seen rushing towards us. On nearing the vessel
they rapidly turned, like a comet round its perihelion, placed
themselves side by side, and, in parallel trails of light, kept
up with the ship. One of them placed itself right in front of the
bow as a pioneer. These comets of the sea were joined at
intervals by others. Sometimes as many as six at a time would
rush at us, bend with extraordinary rapidity round a sharp curve,
and afterwards keep us company. I leaned over the bow, and
scanned the streamers closely. The frontal portion of each of
them revealed the outline of a porpoise. The rush of the
creatures through the water had started the phosphorescence,
every spark of which was converted by the motion of the retina
into a line of light. Each porpoise was thus wrapped in a
luminous sheath. The phosphorescence did not cease at the
creature’s tail, but was carried many porpoise-lengths behind
it.

To our right we had the African hills, illuminated by the
moon. Gibraltar Rock at length became visible, but the town
remained long hidden by a belt of haze, through which at length the brighter
lamps struggled. It was like the gradual resolution of a nebula
into stars. As the intervening depth became gradually less, the
mist vanished more and more, and finally all the lamps shone
through it They formed a bright foil to the sombre mass of rock
above them. The sea was so calm and the scene so lovely that Mr.
Huggins and myself stayed on deck till near midnight, when the
ship was moored. During our walking to and fro a striking
enlargement of the disk of Jupiter was observed, whenever the
heated air of the funnel came between us and the planet. On
passing away from the heated air, the flat dim disk would
immediately shrink to a luminous point. The effect was one of
visual persistence. The retinal image of the planet was set
quivering in all azimuths by the streams of heated air,
describing in quick succession minute lines of light, which
summed themselves to a disk of sensible area.

At six o’clock next morning, the gun at the Signal Station on
the summit of the rock, boomed. At eight the band on board the
‘Trafalgar’ training-ship, which was in the harbour, struck up
the national anthem; and immediately afterwards a crowd of
mite-like cadets swarmed up the rigging. After the removal of the
apparatus belonging to the Gibraltar party we went on shore.
Winter was in England when we left, but here we had the warmth of
summer. The vegetation was luxuriant — palm-trees,
cactuses, and aloes, all ablaze with scarlet flowers. A visit to
the Governor was proposed, as an act of necessary courtesy, and I
accompanied Admiral Ommaney and Mr. Huggins to ‘the Convent,’ or
Government House. We sent in our cards, waited for a time, and
were then conducted by an orderly to his Excellency. He is a fine
old man, over six feet high, and of frank military bearing. He
received us and conversed with us in a very genial manner. He
took us to see his garden, his palms, his shaded promenades, and
his orange-trees loaded with fruit, in all of which he took
manifest delight. Evidently ‘the hero of Kars’ had fallen upon
quarters after his own heart. He appeared full of good nature,
and engaged us on the spot to dine with him that day.

We sought the town-major for a pass to visit the lines. While
awaiting his arrival I purchased a stock of white glass bottles,
with a view to experiments on the colour of the sea. Mr. Huggins
and myself, who wished to see the rock, were taken by Captain
Salmond to the library, where a model of Gibraltar is kept, and
where we had a useful preliminary lesson. At the library we met
Colonel Maberly, a courteous and kindly man, who gave us good
advice regarding our excursion. He sent an orderly with us to the
entrance of the lines. The orderly handed us over to an
intelligent Irishman, who was directed to show us everything that
we desired to see, and to hide nothing from us. We took the
‘upper line,’ traversed the galleries hewn through the
limestone; looked through the embrasures, which opened like doors
in the precipice, towards the hills of Spain; reached St.
George’s hall, and went still higher, emerging on the summit of
one of the noblest cliffs I have ever seen.

Beyond were the Spanish lines, marked by a line of white
sentry-boxes; nearer were the English lines, less conspicuously
indicated; and between both was the neutral ground. Behind the
Spanish lines rose the conical hill called the Queen of Spain’s
Chair. The general aspect of the mainland from the rock is bold
and rugged. Doubling back from the galleries, we struck upwards
towards the crest, reached the Signal Station, where we indulged
in ‘shandy-gaff’ and bread and cheese. Thence to O’Hara’s Tower,
the highest point of the rock. It was built by a former Governor,
who, forgetful of the laws of terrestrial curvature, thought he
might look from the tower into-the port of Cadiz. The tower is
riven, and it may be climbed along the edges of the crack. We got
to the top of it; thence descended the curious Mediterranean
Stair — a zigzag, mostly of steps down a steeply falling
slope, amid palmetto brush, aloes, and prickly pear.

Passing over the Windmill Hill, we were joined at the
‘Governor’s Cottage’ by a car, and drove afterwards to the
lighthouse at Europa Point. The tower was built, I believe, by
Queen Adelaide, and it contains a fine dioptric apparatus of the
first order, constructed by Messrs. Chance, of Birmingham. At the
appointed hour we were at the Convent. During dinner the same
genial traits which appeared in the morning were still more
conspicuous. The freshness of the Governor’s nature showed itself
best when he spoke of his old antagonist in arms, Mouravieff.
Chivalry in war is consistent with its stern prosecution. These
two men were chivalrous, and after striking the last blow became
friends for ever. Our kind and courteous reception at Gibraltar
is a thing to be remembered with pleasure.

On December 15 we committed ourselves to the Mediterranean.
The views of Gibraltar with which we are most acquainted
represent it as a huge ridge; but its aspect, end on, both from
the Spanish lines and from the other side, is truly noble. There
is a sloping bank of sand at the back of the rock, which I was
disposed to regard simply as the débris of the limestone.
I wished to let myself down upon it, but had not the time. My
friend Mr. Busk, however, assures me that it is silica, and that
the same sand constitutes the adjacent neutral ground. There are
theories afloat as to its having been blown from Sahara. The
Mediterranean throughout this first day, and indeed throughout
the entire voyage to Oran, was of a less deep blue than the

Atlantic. Possibly the quantity of organisms may have modified
the colour. At night the phosphorescence was startling, breaking
suddenly out along the crests of the waves formed by the port and
starboard bows. Its strength was not uniform. Having flashed
brilliantly for a time, it would in part subside, and afterwards
regain its vigour. Several large phosphorescent masses of weird
appearance also floated past.

On the morning of the 16th we sighted the fort and lighthouse
of Marsa el Kibir, and beyond them the white walls of Oran lying
in the bight of a bay, sheltered by dominant hills. The sun was
shining brightly; during our whole voyage we had not had so fine
a day. The wisdom which had led us to choose Oran as our place of
observation seemed demonstrated. A rather excitable pilot came on
board, and he guided us in behind the Mole, which had suffered
much damage the previous year from an unexplained outburst of
waves from the Mediterranean. Both port and bow anchors were cast
in deep water. With three huge hawsers the ship’s stem was made
fast to three gun-pillars fixed in the Mole; and here for a time
the ‘Urgent’ rested from her labours.

M. Janssen, who had rendered his name celebrated by his
observations of the eclipse in India in 1868, when he showed the
solar flames to be eruptions of incandescent hydrogen, was
already encamped in the open country about eight miles from Oran.
On December 2 he had quitted Paris in a balloon, with a strong
young sailor as his assistant, had descended near the mouth of
the Loire, seen M. Gambetta, and received from him encouragement
and aid. On the day of our arrival his encampment was visited by
Mr. Huggins, and the kind and courteous Engineer of the Port
drove me subsequently, in his own phaeton, to the place. It bore
the best repute as regards freedom from haze and fog, and
commanded an open outlook; but it was inconvenient for us on
account of its distance from the ship. The place next in repute
was the railway station, between two and three miles distant from
the Mole. It was inspected, but, being enclosed, was abandoned
for an eminence in an adjacent garden, the property of Mr.
Hinshelwood, a Scotchman who had settled some years previously as
an Esparto merchant in Oran. [Footnote: Esparto is a kind
of grass now much used in the manufacture of paper.] He,
in the most liberal manner, placed his ground at the disposition
of the party. Here the tents were pitched, on the Saturday, by
Captain Salmond and his intelligent corps of sappers, the
instruments being erected on the Monday under cover of the
tents.

Close to the railway station runs a new loopholed wall of
defence, through which the highway passes into the open country.
Standing on the highway, and looking southwards, about twenty
yards to the right is a small bastionet, intended to carry a gun
or two. Its roof I thought would form an admirable basis for my
telescope, while the view of the surrounding country was
unimpeded in all directions. The authorities kindly allowed me
the use of this bastionet. Two men, one a blue-jacket named
Elliot, and the other a marine named Hill, were placed at my
disposal by Lieutenant Walton; and, thus aided, on Monday morning
I mounted my telescope. The instrument was new to me, and some
hours of discipline were spent in mastering all the details of
its manipulation.

Mr. Huggins joined me, and we visited together the Arab
quarter of Oran. The flat-roofed houses appeared very clean and
white. The street was filled with loiterers, and the thresholds
were occupied by picturesque groups. Some of the men were very
fine. We saw many straight, manly fellows who must have been six
feet four in height. They passed us with perfect indifference,
evincing no anger, suspicion, or curiosity, hardly caring in fact
to glance at us as we passed. In one instance only during my stay
at Oran was I spoken to by an Arab. He was a tall, good-humoured
fellow, who came smiling up to me, and muttered something about
‘les Anglais.’ The mixed population of Oran is picturesque in the
highest degree: the Jews, rich and poor, varying in their
costumes as their wealth varies; the Arabs more picturesque
still, and of all shades of complexion — the negroes, the
Spaniards, the French, all grouped together, each race preserving
its own individuality, formed a picture intensely interesting to
me.

On Tuesday, the 20th, I was early at the bastionet. The night
had been very squally. The sergeant of the sappers had taken
charge of our key, and on Tuesday morning Elliot went for it. He
brought back the intelligence that the tents had been blown down,
and the instruments overturned. Among these was a large and
valuable equatorial from the Royal Observatory, Greenwich. It
seemed hardly possible that this instrument, with its wheels and
verniers and delicate adjustments, could have escaped uninjured
from such a fall. This, however, was the case; and during the day
all the overturned instruments were restored to their places, and
found to be in practical working order. This and the following
day were devoted to incessant schooling. I had come out as a
general stargazer, and not with the intention of devoting myself
to the observation of any particular phenomenon. I wished to see
the whole — the first contact, the advance of the moon, the
successive swallowing up of the solar spots, the breaking of the
last line of crescent by the lunar mountains into Bailey’s beads,
the advance of the shadow through the air, the appearance of the
corona and prominences at the moment of totality, the radiant
streamer; of the corona, the internal structure of the flames, a
glance through a polariscope, a sweep round the landscape with
the naked eye, the reappearance of the soar limb through Bailey’s
beads, and, finally, the retreat of the lunar shadow through the
air.

I was provided with a telescope of admirable definition,
mounted, adjusted, packed, and most liberally placed at my
disposal by Mr. Warren De La Rue. The telescope grasped the whole
of the sun, and a considerable portion of the space surrounding
it. But it would not take in the extreme limits of the corona.
For this I had lashed on to the large telescope a light but
powerful instrument, constructed by Ross, and lent to me by Mr.
Huggins. I was also furnished with an excellent binocular by Mr.
Dallmeyer. In fact, no man could have been more efficiently
supported.

It required a strict parcelling out of the interval of
totality to embrace in it the entire series of observations.
These, while the sun remained visible, were to be made with an
unsilvered diagonal eye-piece, which reflected but a small
fraction of the sun’s light, this fraction, being still further
toned down by a dark glass. At the moment of totality the dark
glass was to be removed, and a silver reflector pushed in, so as
to get the maximum of light from the corona and prominences The
time of totality was distributed as follows:

In our rehearsals Elliot stood beside me, watch in hand, and
furnished with a lantern. He called out at the end of each
interval, while I moved from telescope to finder, from finder to
polariscope, from polariscope to naked eye, from naked eye back
to finder, from finder to telescope, abandoning the instrument
finally to observe the retreating shadow. All this we went over
twenty times, while looking at the actual sun, and keeping him
in the middle of the field. It was my object to render the
repetition of the lesson so mechanical as to leave no room for
flurry, forgetfulness, or excitement. Volition was not to be
called upon, nor judgment exercised, but a well-beaten path of
routine was to be followed. Had the opportunity occurred, I think
the programme would have been strictly carried out.

But the opportunity did not occur. For several days the
weather had been ill-natured. We had wind so strong As to render
the hawsers at the stern of the ‘Urgent’ as rigid as iron,
and to destroy the navigating lieutenant’s sleep. We had clouds,
a thunder-storm, and some rain. Still the hope was held out that
the atmosphere would cleanse itself, and if it did we were
promised air of extraordinary limpidity. Early on the 22nd we
were all at our posts. Spaces of blue in the early morning gave
us some encouragement, but all depended on the relation of these
spaces to the surrounding clouds. Which of them were to grow as
the day advanced? The wind was high, and to secure the steadiness
of my instrument I was forced to retreat behind a projection of
the bastionet, place stones upon its stand, and, further, to
avail myself of the shelter of a sail. My practised men fastened
the sail at the top, and loaded it with boulders at the bottom.
It was tried severely, but it stood firm.

The clouds and blue spaces fought for a time with varying
success. The sun was bidden and revealed at intervals, hope
oscillating in synchronism with the changes of the sky. At the
moment of first contact a dense cloud intervened; but a minute or
two afterwards the cloud had passed, and the encroachment of the
black body of the moon was evident upon the solar disk. The moon
marched onward, and I saw it at frequent intervals; a large group
of spots were approached and swallowed up. Subsequently I caught
sight of the lunar limb as it cut through the middle of a large
spot. The spot was not to be distinguished from the moon, but
rose like a mountain above it. The clouds, when thin, could be
seen as grey scud drifting across the black surface of the moon;
but they thickened more and more, and made the intervals of
clearness scantier. During these moments I watched with an
interest bordering upon fascination the march of the silver
sickle of the sun across the field of the telescope. It was so
sharp and so beautiful. No trace of the lunar limb could be
observed beyond the sun’s boundary. Here, indeed, it could only
be relieved by the corona, which was utterly cut off by the dark
glass. The blackness of the moon beyond the sun was, in fact,
confounded with the blackness of space.

Beside me was Elliot with the watch and lantern, while
Lieutenant Archer, of the Royal Engineers, had the kindness to
take charge of my note-book. I mentioned, and he wrote rapidly
down, such things as seemed worthy of remembrance. Thus my hands
and mind were entirely free; but it was all to no purpose. A
patch of sunlight fell and rested upon the landscape some miles
away. It was the only illuminated spot within view. But to the
north-west there was still a space of blue which might. reach us
in time. Within seven minutes of totality another space towards
the zenith became very dark. The atmosphere was, as it were, on
the brink of a precipice, being charged with humidity, which
required but a slight chill to bring it down in clouds. This was
furnished by the withdrawal of the solar beams: the clouds did
come down, covering up the space of blue on which our hopes had
so long rested. I abandoned the telescope and walked to and fro
in despair. As the moment of totality approached, the descent
towards darkness was as obvious as a falling stone. I looked
towards a distant ridge, where the darkness would first appear.
At the moment a fan of beams, issuing from the hidden sun, was
spread out over the southern heavens. These beams are bars of
alternate light and shade, produced in illuminated haze by the
shadows of floating cloudlets of varying density. The beams are
practically parallel, but by an effect of perspective they appear
divergent, having the sun, in fact, for their point of
convergence. The darkness took possession of the ridge referred
to, lowered upon M. Janssen’s observatory, passed over the
southern heavens, blotting out the beams as if a sponge had been
drawn across them. It then took successive possession of three
spaces of blue sky in the south-eastern atmosphere. I again
looked towards the ridge. A glimmer as of day-dawn was behind it,
and immediately afterwards the fan of beams, which had been for
more than two minutes absent, revived. The eclipse of 1870 had
ended, and, as far as the corona and flames were concerned, we
had been defeated.

Even in the heart of the eclipse the darkness was by no means
perfect. Small print could be read. In fact, the clouds which
rendered the day a dark one, by scattering light into the shadow,
rendered the darkness less intense than it would have been had
the atmosphere been without cloud. In the more open spaces I
sought for stars, but could find none. There was a lull in the
wind before and after totality, but during the totality the wind
was strong. I waited for some time on the bastionet, hoping to
get a glimpse of the moon on the opposite border of the sun, but
in vain. The clouds continued, and some rain fell. The day
brightened somewhat afterwards, and, having packed all up, in the
sober twilight Mr. Crookes and myself climbed the heights above
the fort of Vera Cruz. From this eminence we had a very noble
view over the Mediterranean and the flanking African hills. The
sunset was remarkable, and the whole outlook exceedingly
fine.

The able and well-instructed medical officer of the
‘Urgent,’ Mr. Goodman, observed the following temperatures
during the progress of the eclipse:

The minimum temperature
occurred some minutes after totality, when a slight rain
fell.

The wind was so strong on the 23rd that Captain Henderson
would not venture out. Guided by Mr. Goodman, I visited a cave in
a remarkable stratum of shell-breccia, and, thanks to my guide,
secured specimens. Mr. Busk informs me that a precisely similar
breccia, is found at Gibraltar, at approximately the same level.
During the afternoon, Admiral Ommaney and myself drove to the
fort of Marsa el Kibir. The fortification is of ancient origin,
the Moorish arches being still there in decay, but the fort is
now very strong. About four or five hundred fine-looking dragoons
were looking after their horses, waiting for a lull to enable
them to embark for France. One of their officers was wandering in
a very solitary fashion over the fort. We had some conversation
with him. He had been at Sedan, had been taken prisoner, but had
effected his escape. He shook his head when we spoke of the
termination of the war, and predicted its long continuance. There
was bitterness in his tone as he spoke of the charges of treason
so lightly levelled against French commanders.

The green waves raved round the promontory on which the fort
stands, smiting the rocks, breaking into foam, and jumping, after
impact, to a height of a hundred feet and more into the air. As
we returned our vehicle broke down through the loss of a wheel.
The Admiral went on board, while I remained long watching the
agitated sea. The little horses of Oran well merit a passing
word. Their speed and endurance, both of which are heavily drawn
upon by their drivers, are extraordinary.

The wind sinking, we lifted anchor on the 24th. For some hours
we went pleasantly along; but during the afternoon the storm
revived, and it blew heavily against us all the night. When we
came opposite the Bay of Almeria, on the 25th, the captain turned
the ship, and steered into the bay, where, under the shadow of
the Sierra Nevada, we passed Christmas night in peace. Next
morning ‘a rose of dawn’ rested on the snows of the adjacent
mountains, while a purple haze was spread over the lower hills. I
had no notion that Spain possessed so fine a range of mountains
as the Sierra Nevada. The height is considerable, but the form
also is such as to get the maximum of grandeur out of the height.
We weighed anchor at eight A.M., passing for a time through shoal
water, the bottom having been evidently stirred up. The adjacent
land seemed eroded in a remarkable manner. It has its floods,
which excavate these valleys and ravines, and leave those
singular ridges behind. Towards evening I climbed the mainmast,
and, standing on the cross-trees, saw the sun set amid a blaze of
fiery clouds. The wind was strong and bitterly cold, and I was
glad to slide back to the deck along a rope, which stretched from
the mast-head to the ship’s side. That night we cast anchor
beside the Mole of Gibraltar.

On the morning of the 27th, in company with two friends, I
drove to the Spanish lines, with the view of seeing the rock from
that side. It is an exceedingly noble mass. The Peninsular and
Oriental mail-boat had been signalled and had come. Heavy duties
called me homeward, and by transferring myself from the
‘Urgent’ to the mail-steamer I should gain three days. I
hired a boat, rowed to the steamer, learned that she was to start
at one, and returned with all speed to the ‘Urgent.’ Making
known to Captain Henderson my wish to get away, he expressed
doubts as to the possibility of reaching the mail-steamer in
time. With his accustomed kindness, he however placed a boat at
my disposal. Four hardy fellows and one of the ship’s officers
jumped into it; my luggage, hastily thrown together, was tumbled
in, and we were immediately on our way. We had nearly four miles
to row in about twenty minutes; but we hoped the mail-boat might
not be punctual. For a time we watched her anxiously; there was
no motion; we came nearer, but the flags were not yet hauled in.
The men put forth all their strength, animated by the
exhortations of the officer at the helm. The roughness of the sea
rendered their efforts to some extent nugatory: still we were
rapidly approaching the steamer. At length she moved, punctual
almost to the minute, at first slowly, but soon with quickened
pace.

We turned to the left, so as to cut across her bows. Five
minutes’ pull would have brought us up to her. The officer waved
his cap and I my hat. ‘If they could only see us, they
might back to us in a moment.’ But they did not see us, or if
they did, they paid us no attention. I returned to the ‘Urgent,’
discomfited, but grateful to the fine fellows who had wrought so
hard to carry out my wishes.

Glad of the quiet, in the sober afternoon I took a walk
towards Europa Point. The sky darkened and heavy squalls passed
at intervals. Private theatricals were at the Convent, and the
kind and courteous Governor had sent cards to the eclipse party.
I failed in my duty in not going. St. Michael’s Cave is said to
rival, if it does not outrival, the Mammoth Cave of Kentucky. On
the 28th Mr. Crookes, Mr. Carpenter, and myself, guided by a
military policeman who understood his work, explored the cavern.
The mouth is about 1,100 feet above the sea. We zigzagged up to
it, and first were led into an aperture in the rock, at some
height above the true entrance of the cave. In this upper cavern
we saw some tall and beautiful stalactite pillars.

The water drips from the roof charged with bicarbonate of
lime. Exposed to the air, the carbonic acid partially escapes,
and the simple carbonate of lime, which is hardly at all soluble
in water, deposits itself as a solid, forming stalactites and
stalagmites. Even the exposure of chalk or limestone water to the
open air partially softens it. A specimen of the Redbourne water
exposed by Professors Graham, Miller, and Hofmann, in a shallow
basin, fell from eighteen degrees to nine degrees of hardness.
The softening process of Clark is virtually a hastening of the
natural process. Here, however, instead of being permitted to
evaporate, half the carbonic acid is appropriated by lime, the
half thus taken up, as well as the remaining half, being
precipitated. The solid precipitate is permitted to sink, and the
clear supernatant liquid is limpid soft water.

We returned to the real mouth of St. Michael’s Cave, which is
entered by a wicket. The floor was somewhat muddy, and the roof
and walls were wet. We soon found ourselves in the midst of a
natural temple, where tall columns sprang complete from floor to
roof, while incipient columns were growing to meet each other,
upwards and downwards. The water which trickles from the
stalactite, after having in part yielded up its carbonate of
lime, falls upon the floor vertically underneath, and there
builds the stalagmite. Consequently, the pillars grow from above
and below simultaneously, along the same vertical. It is easy to
distinguish the stalagmitic from the stalactitic portion of the
pillars. The former is always divided into short segments by
protuberant rings, as if deposited periodically, while the latter
presents a uniform surface. In some cases the points of inverted
cones of stalactite rested on the centres of pillars of
stalagmite. The process of solidification and the consequent
architecture were alike beautiful.

We followed our guide through various branches and arms of the
cave, climbed and descended steps, halted at the edges of dark
shafts and apertures, and squeezed ourselves through narrow
passages. From time to time we halted, while Mr. Crookes
illuminated with ignited magnesium wire, the roof, columns,
dependent spears, and graceful drapery of the stalactites. Once,
coming to a magnificent cluster of icicle-like spears, we helped
ourselves to specimens. There was some difficulty in detaching
the more delicate ones, their fragility was so great. A
consciousness of vandalism, which smote me at the time, haunts me
still; for, though our requisitions were moderate, this beauty
ought not to be at all invaded. Pendent from the roof, in their
natural habitat, nothing can exceed their delicate beauty; they
live, as it were, surrounded by organic connections. In London
they are curious, but not beautiful. Of gathered shells Emerson
writes: