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FEAGMENTS OP SCIENCE

VOL. I

FEAGMENTS OF

SCIENCE,

A SERIES OF DETACHED

ESSAYS, ADDRESSES, AND REVIEWS

BY

JOHN TYNDALL, F.R.S.

AUTHOR OF NEW FRAGMENTS, HEAT AS A MODE OF MOTION,

ON SOUND, RADIANT HEAT, ON FORMS OF WATER,

HOURS OF EXERCISE IN THE ALPS, ETC.

VOL. I

NEW YORK
D. APPLETON AND COMPANY

1896

Authorized Editiorio

PUBLISHERS' NOTE.

The first edition of Prof. TyndaU's " Fragments of
Science " was published some twenty years ago as a sin-
gle volume, which was made up of a score or more of
his detached essays, addresses, and reviews. The book
was afterward revised, some of the papers recast, and
from time to time new ones added, until, the size of the
work becoming somewhat unwieldy, the present two-
volume edition was decided upon. This contains fifteen
additional papers, and represents the author's latest
changes and revisions. The volumes are uniform with
"New Fragments," recently issued, and the three to-
gether include all the occasional writings which their
author has decided to preserve in permanent form.

PEBFAOB

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 in-
terlace 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 logi-
cally bound to deduce the life of the world from forces

viii PEEFACK

inherent in the nebula. With this view, which is set
forth in the second volume, it seemed but fair to asso-
ciate 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.

CONTENTS

OF

THE FIRST VOLUME

CHAFTBB PAGB

I. THE CONSTITUTION OP NATUKB » . . 3

n. EAOIATION . . . , . . , 28

ni. ON RADIANT HEAT IN RELATION TO THE COLOUR

AND CHEMICAL CONSTITUTION OP BODIES . 74

rV. NEW CHEMICAL REACTIONS PRODUCED BY LIGHT . 96

V. THE SKY , , . . . , .131

VI. VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSB • 142

VII. NIAGARA . . " . . . • .175

VIII. THE PARALLEL ROADS OP GLEN ROY , , . 205

IX. ALPINE SCULPTURE . . , • . 229

X. RECENT EXPERIMENTS ON POG-SIGNALS • , , 253

XI. ON THE STUDY OP PHYSICS . . » .281

XII. ON CRYSTALLINE AND SLATY CLEAVAGE . . . 304

XIII. ON PARAMAGNETIC AND DIAMAGNETIC FORCES . 321

XIV. PHYSICAL BASIS OF SOLAR CHEMISTRY . , , 329
XV. ELEMENTARY MAGNETISM • • • • 343

XVI. ON FORCE . . ,• . . • • 369

XVIL CONTRIBUTIONS TO MOLECULAR PHYSICS . .386

X CONTENTS.

CHAPTEB PAOK

XVIII. LIFE AND LETTERS OP FARADAY. , , . 399

XIX. THE COPLEY MEDALLIST OP 1870 • • • 422

XX. THE COPLEY MEDALLIST OP 1871 . • • . 429

XXI. DEATH BY LIGHTNING . • • • . 439

XXII. SCIENCE AND THE SPIRITS . • , , 444

MAP

SHOWING THE PARALLEL ROADS OF GLEN ROY . tofocep, 227

VOL. t

mORGAHIC NATUKE

X CONTENTS.

CHAPTBR PAOJJ

XVIII. LIFE AND LETTERS OP FARADAY . , , . 399

XIX. THE COPLEY MEDALLIST OP 1870 , . • 422

XX. THE COPLEY MEDALLIST OP 1871 . • • . 429

XXI. DEATH BY LIGHTNING . • , « .439

2LXII. SCIENCE AND THE SPIRITS . • , • 444

MAP

SHOWING THE PARALLEL ROADS OP GLEN ROY , tofocep, 227

VOL. L

mOKGANIO NATUKE

THE CONSTITUTION OF NATURE}

WE cannot think of space as finite, for wherever
in imagination we erect a boundary, we are com-
pelled 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 difierent grounds. It has the best possible
reasons for rejecting the idea of luminiferous particles ;
but, in support of th^ conclusion that the celestial
spaces are occupied by matter, it is able to offer proofs

» 'Fortnightly Beview,' 1865, voL iii. p. 129.

4 FEAGMENTS OF SCIENCE.

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 terres-
trial and local one. From the phenomena of sound,
as displayed in the air, they ascended to the phenomena
of light, as displayed in the ether ; 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 ether 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 ether, 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 ether 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 ether have a boundary, masses of ponderable
matter might be conceived to exist beyond it, but they
could emit no light. Beyond the ether 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.

THE CONSTITUTION OF NATURE. 6

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 ab-
straction of molecular motion by the ether, 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 aug-
mentation 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 ether; and the propagation of waves
through the liquid is mechanically similar to the pro-
pagation of light and radiant heat.

As far as our knowledge of space extends, we are
to conceive it as the holder of the luminiferous ether,
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

6 FRAGMENTS OF SCIENCE.

round it, each again rotating on its o-wti 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 be-
ginning. 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 nebulas
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 in-
tegrated 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

THE CONSTITUTION OF NATUEE. 7

developed by this collision. Mayer first, and Helm-
holtz and Thomson afterwards, have calculated its
amount. It would equal that produced by the com-
bustion 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 pro-
duce 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 ether, and propagated through it with a velo-
city 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 neces-
sarily 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, tlieir position at any moment depending
on the equilibrium of attraction and repulsion. The
atoms behave as if connected by elastic springs, which

8 FRAGMENTS OF SCIENCE.

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
>vith the ether, and diffused by it throughout space.

We on the earth's surface live night and day in the
midst of ethereal 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 ether. 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 falliag so low. To the con-
ception 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 car-
bon, oxygen, and calcium. These are the so-called
elements the union of which, in definite proportions, has

THE CONSTITUTION OF NATUEE. 9

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
gi eater portion of the earth's crust is compounded
of the elementary substances mentioned in these few
lines.

The principle of gravitation has been already de-
scribed 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 attrac-
tion ; 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 exer-
cised, 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, gra-
vity 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 im-
parted 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

10 FRAGMENTS OF SCIENCE.

minute spaces whicli 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 ter-
minology 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 conver-
sion 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 re-
fuses to be so converted ; being a force maintaining
itself under all circumstances, and not capable of dis-
appearing 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

THE CONSTITUTION OF NATUEB. H

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 in-
destructible 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 con-
verted 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, s'pace
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 to-

12 FRAGMENTPJ OF SCIENCE.

gether 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 moleculaif
oscillation which, when communicated to the proper
nerves, announces itself as heat.

It was formerly universally supposed that by the col-
lision 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 destruc-
tion 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 to-
gether 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 con-
servation 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. Per-
fectly 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 gene-
rated. This always occurs in unelastic collision, the
heat developed being the exact equivalent of the sensible

THE CONSTITUTION OF NATUEE. 13

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 mea-
sure, 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 per-
formed 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 elevqr

14 FRAGMENTS OF SCIENCE.

tion 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 un-
opposed 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 gra-
vity, 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 com-
petent 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 eleva-
tion, a four-fold velocity will give a sixteen-fold elevation,
and so on. The height attained, then, is not propor-
tional 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

THE CONSTITUTION OF NATURE. 15

mass whatever is competent to perform, in virtue of the
motion which it at any momentpossesses, is jointly pro'
portional 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 annihil-
ated. 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 velo-
city 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 failing 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

16 FKAGMENTS OF SCIENCE.

i

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 concen-
trated 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 com-
posed 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 uni-
verse 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^

THE CONSTITUTION OF NATURE. 17

as defined by Newton ; for this is a force varying inversely
as the square of the distance ; and to affirm the con-
stancy of a varying force would be self-contradictory.
Yet, when the question is properly understood, gravity
forms no exception to the law of conservation. Follow-
ing the method pursued by Helmholtz, I will here at-
tempt an elementary exposition of this law. Though
destined in its applications to produce momentous
changes in human thought, it is not difficult of compre-
hension.

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, n, 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 deter-
mining, 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 gi-avity at that distance.

It is customary in mechanics to represent the magni-
tude 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 dis-
tance from F, we can, in imagination, draw a straight
line from n 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

18 FRAGMENTS OF SCIENCE.

attraction will be very small, and the perpendicular conse*
quently 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 ap-
proaches 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 attrac-
tion, 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 pre-
viously in store at that point disappears, but not with-

THE CONSTITUTION OF NATUKE. 19

out 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 un consumed tensions, and
tne less is the living force. Now the principle of con-
servation afiBrms not the constancy of the value of the
tensions of gravity, nor yet the constancy of the via
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 maxi-
mum ; 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 stiU 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 virea
vivcB 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 enhance-
ment of the other, the total value of the property
remaining unchanged.

The considerations here applied to gravity apply
equally to chemical affinity. In a mixture of oxygen and
hydrogen the atoms exist apart, but by the application

20 FRAGMENTS OF SCIENCE.

of proper means they may be caused to rusli together I
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 con-
verted into vis viva. After they clash we have still
via 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 re-
versals 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 re combine, 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 ia
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 par<^

THE CONSTITUTION OF NATURE. 21

of the orbit. Her motion, and consequently her via
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 e(iuivalent amount of vis viva being generated.
When nearest to the sun the motion, and consequently
the vis mva, 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 pos-
sessed 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
pc»ssible 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 re-
moved. Looking, for example, at gravity ; a body on
the earth's surface in a position from which it cannot
fell to a lower one possesses no energy. It has neithei

22 FRAGMENTS OF SCIENCE.

motion nor power of motion. But the same body sus-
pended 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, more-
over, 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 con-
sumed, its potential energy being then a maximum.
When it returns, there is a reconversion of the poten-
tial 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 pos-
sesses 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 Weisshomj
are illustrations in point. The hammer of the great]
bell of Westminster, when raised before striking, pos-J
sesses potential energy ; when it falls, the energys
becomes dynamic ; and after the stroke, we have tnej
rhythmic play of potential and dynamic in the vibra-j
tions of the bell. The same holds good for the molecular^
oscillations of a heated body. An atom is drivcDJ

THE CONSTITUTION OF NATURE. 23

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 re-
pulsion, 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 con-
ferred upon the molecules by the placing of greater
distances between them. This molecular potential
energy is afterwards, on the cooling of the body, con-
verted 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

3

d4 FEAGMENTS OF SCIENCE.

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 dis-
tributed 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
fin^her 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 ether 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 ap-
peared 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 com-
parison 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

THE CONSTITUTION OF NATURE. 25

thougli it has ceased to be a source of energy^ the at-
traction of gravity still acts as 2i 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 ex-
ample, two atoms of hydrogen unite with one of oxygen,
to form water, the atoms are first drawn towards each
otlier — they move, they clash, and then by virtue of
their resiliency, they recoil and quiver. To this quiver-
ing 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 con-
verted into heat. We must not imagine the chemical
attraction destroyed, or converted into anything else.
For the atoms, when mutually clasped to form a mole-
cule of water, are held together by the very attraction
which first drew them towards each other. That which
has really been expended is the ^ull 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 some-
times stated to be, than is chemical affinity. By the
exertion of a certain pull through a cert.ain space, a
body is caused to clash with a certain definite velocity
againr-t the earth. Heat is thereby developed, and this
is the only sense in which gravity can be said to be con-
verted into heat. In no case is the force which produces
the motion annihilated or changed into anything else.

26 FEAGMENTS OF SCIENCE.

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 ' gene-
rated,' they do not mean thereby that old attractions
have been annihilated, and new ones brought into exist-
ence, but that, in the one case, the power of the attrac-
tion 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 sen-
sible 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 CONSTITUTION OF NATURE. 27

the facts of attraction as they stand, and afl&rms 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 con-
stancy of the sum of both being affirmed by the law of
conservation. The convertibility of natural forces con-
sists 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 con-
servation. This exposition I hope will tend to remove
them.

1

28 FOAGMENXa OF SCIENCE.

n.

RABIATIOm

1. Visible arid 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 im-
pressions 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 assem-
blage 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 ex-
ternal 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.

' The Rede Lecture delivered in the Senate House before th©
University of Cambridge, May 16, 1865.

RADIATION. 29

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 aflfect the optic nerve a
certain temperature is necessary. A cool poker thrust
into a fire remains dark for a time, but when its tem-
perature 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 hea.t augments, still however remaining obscure ;
at length we can no longer touch the metal with im-
punity ; 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 ()f the sun.

By means of a prism Sir Isaac Newton unravelled
the texture of solar light, and by the same simple instru-
ment 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 re-
fracted from their straight course; and, as different rays
are differently refracted by the prism, we are by it en-
abled to separate one class of rays fi:om another. By
such prismatic analysis Dr. Draper has shown, that when
the platinum wire first begins to glow, the light emitted
io sensibly red. As the glow augments the red becomes

30 FKAGMENTS OF SCIENCE.

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
wdre 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 kiud of obscure rays emitted by all the

RADIATION. 31

planets of our system are included in the present radia-
tion 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 suc-
cessive 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 sho\ved, 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. Eitter discovered tne 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 ad-
ihirable researches of Professor Stokes. The complete
spectrum of the sun consists, therefore, of three distinct
parts : — first, of ultra-red rays of high heating powCi-,
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, s^xe

32 FRAGMENTS OF SCIENCE.

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 Gha/racter of Radiation. The Ether,

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 origin-
ate, 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 vibra-
tion. 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 agita-
tion 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

mi

RADIATION. 33

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, aud when a certain efinite 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

i 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 insen-
sible 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

f the hot platinum, and hence the pertinence of the ques-
tion, 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 ether. Every vibration of every atom
of our platinum wire raises in this ether a wave, which
speeds through it at the rate of 186,000 miles asecond*

34 FEAGMENTS OF SCIENCE.

The ether 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.* 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 ethereal 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 ether 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 con-
scious impression of light.

Darkness might then be defined as ether at rest ;
light as ether in motion. But in reality the ether 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 ether,
that constitutes what we call the ' temperature of space.'
As the air of a room accommodates itself to the require-

* The action here described is analogous to the passage of sound*
wtkves through thick lelt whose interstices {g:e occupied by air.

RADIATION. 35

ments of an orchestra, transmitting each- vibration of
every pipe and string, so does the inter-stellar ether
accommodate itself to the requirements of light and
heat. Its waves mingle in space without disorder,
each being endowed with an individuality as inde-
structible 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 ether ; 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 im-
parting it to the medium in which they vibrate. On
the other hand, the waves of ether 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 ether to the molecules
consists the absorption of radiant heat. All the pheno-
mena 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 con-
scious of the action of heat and cold.

3. The Atomic Theory in reference to the Ether.

The word ' atoms ' has been more than once em-
ployed 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

36 FEAGMENTS OF SCIENCE.

merely rrtixecl,, but they may be made to comhvne 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 ele-
mentary bodies as little spheres, the molecules of com-
pound bodies must be pictured as gToups 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 ether 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 combina-
tion— whether as regards radiation and absorption, or,
in other words, whether as regards the communication
of motion to the ether, and the acceptance of motion
from it, the deportment of the uncombined atoms will:
be dififerent from that of the combined.

4. Absorption of Radiant Heat by Gases,

"We have now to submit these considerations to th^
only test by which they can be tried, namely, that
experiment. An experiment is well defined as a quee
tion put to Nature ; but, to avoid the risk of askii
amiss, we ought to purify the question from all adjunct
which do not necessarily belong to it. Matter hasi
been shown to be composed of elementary constituents,
by the compounding of which all its varieties are pi

RADIATION. 37

duced. 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 cohe-
sion, 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 absorp-
tion, 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 inter-
molecular 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

38 FRAGMENTS OF SCIENCE.

upon the anterior face of a thermo-electric pile,* which
instantly converts the heat into an electric current.
This current conducted round a magnetic needle de-
flects it, and the magriitude 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 cun-ent 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 ad-
justing 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,

> In the Appendix to the first chapter of * Heat as a Mode o£
Motion,* the construction of the thermo-electric pile is fully ex-
plained.

V

RABAITION.

39

* 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 follow-
ing table were examined, a small portion only of each
being admitted into the glass tube. The quantity ad-
mitfifed 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
atmospheric air being taken as unity.

Radiation through Gases,

Name of gas
Air
Oxygen . . •

Nitrogen •
Hydrogen . •
Carbonic oxide •
Carbonic acid
Hydrochloric acid .
Nitric oxide .
Nitrous oxide
Sulphide of hydrogen
Ammonia
defiant gas .
Sulphurous acid .

BelatlTo

absorption

1

1

1

1

750

972

1,005

1,590

1,860

2,100

5,460

6,030

6,480

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 absorp-
tive power are manifested. These differences illustrate
4

40 FKAGMENTS OF SCIENCE.

in the most unexpected manner the influence of chemi-
cal 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 inter-
cepts, perfectly transparent ammonia intercepts 5,460
units, defiant 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 ether pass with-
out 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.

6. Formation of Invisible FocL

This extraordinary deportment of the elementary
gases naturally directed attention to elementary bodies
in other states of aggregation. Some of Melloni's re-
sults 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 per-

RADIATION. 41

fectly diathermic. The dense and deeply-coloured
element bromine was examined, and found competent
to cut oflf 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 imcombined
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.^

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,

* 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.

42 FRAaMENTS OF SCIENCE.

luminous and non-luminous, we can intercept the formei
by the iodine, and do what we please with the latter.
Experiments of this character, not only with the iodine
solution, buL also with black glass and layers of lamp-
black, were publicly performed at the Koyal 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 inter-
cepted by the glass. The obvious remedy here is to
employ rock-salt lenses instead of glass ones, or to aban-
don 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 prin-
ciple, as that made known in 1862. It was then found
that an instant's exposure of the face of the thermo-
electric 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.

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.

KADIATION. 43

The pure bisulphide of carbon, which is the solvent
of the iodine, is perfectly transparent to the luminouSj
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 thermo-
electric 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 tliat 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 Miiller 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 par-
ticular 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 naiTOW as de-
sired. 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

44 FRAGMENTS OF SCIENCE. ,

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 mani-
fests 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 vanish-
ing, 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. ^^Ml

Drawing a datum line, and erecting along it per-
pendiculars, proportional in length to the thermal
intensity at the respective points, we obtain the extra-
ordinary 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 insigni-
ficant appendage to the heat-rays represented by the
area a B c d, thrown in as it were by nature for
purpose of vision.

The diagram drawn by Professor Miiller to represeiit

11

sedt I

mu

KADIATION.

45

(4
H

to
o

;^

(4
H

04

46 FRAaMENTS OF SCIENCE.

the distribution of heat in the solar spectrum is not bj
any means so striking as that just described, and th<
reason, doubtless, is that prior to reaching the eartl
the solar rays have to traverse our atmosphere. By the
aqueous vapour there diffused, the summit of the pe
representing the sun's invisible radiation is cut off.
similar lowering of the mountain of invisible heat h
observed when the rays from the electric light are per-
mitted to pass through a film of water, which acts upoi
them as the atmospheric vapour acts upon the rays
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 perfor-
mances of Archimedes during the siege of Syracuse had
any foundation in fact, the dark solar rays would hav
been the philosopher's chief agents of combustion. 0:
a small scale we can readily produce, with the pure!
invisible rays of the electric light, all that Archi-
medes 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
entirely cut away ; but th^ intolerable heat experience(
when the hand is placed, even for a moment, at the
dark focus, shows that the calorific rays pass unimpedec
through the opaque solution.

Almost anything that ordinary fire can effect maj
be accomplished at the focus of invisible rays ; the ail

d

I

RADIATION. 47

at the focus remaining at the same time perfectly cold,
on account of its transparency to the heat-rays. An air
tliermometer, with a hollow rock-salt bulb, would be
unafifected by the heat of the focus : there would be no
expansion, and in the open air there is no convection.
The ether at the focus, and not the air, is the sub-
stance 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
fuU 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 immedi*
ately glows at the place where the focus falls.

48 FEAGMENTS OF SCIENCE.

8. Transmutation of Rays : ' GaZorescence,

Eminent experimenters were long occupied in
monstrating 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 conca"\'(;
mirror produces, beyond the object which it reflectsj
an inverted and magnified image of the objecbl
Withdrawing, for example, our iodine solution, an i]ij
tensely luminous inverted image of the carbon points o'
the electric light is formed at the focus of the miii >
employed in the foregoing experiments. When ti,
solution is interposed, and the light is cut away, whaj
becomes of this image? It disappears from sight |
but an invisible thermograph remains, and it is onlj
the peculiar constitution of our eyes that disqualifiei
us from seeing the picture formed by the calorific ray
Falling on white paper, the image chars itself out]
falling on black paper, two holes are pierced in i
corresponding to the images of the two coke poin.e
but falling on a thin plate of carbon in vacuo, or upc
a thin sheet of platinised platinum, either in vacuo or i'
air, radiant heat is converted into light, and the imaf
stamps itself in vivid incandescence upon both the ca
bon and the metal. Eesults similar to those obtai:i<
with the electric light have also been obtained ^ i
the invisible rays of the lime-light and of the sun.

Before a Cambridge audience it is hardly necessa
to refer to the excellent researches of Professor Stcl^i
at the opposite end of the spectrum. The above ]•
suits constitute a kind of complement to his disco ve: i .
Professor Stokes named the phenomena which hi

» I borrow this term from Professor Challis, *Phil(
Magazine/ vol, zii. p. 621.

RADIATION. 49

liscovered and investigated Fluorescence ; for the new
jhenomena here described I have proposed the term
Ocdorescence. He, by the interposition of a proper
medium, so lowered the refrangibility of the ultra-
violet rays of the spectrum as to render them visible.
;Here, by the interposition of the platinum foil, the
irefrangibility of the ultra-red rays is so exalted as to
(render them visible. Looking through a prism at tlie
fmcandescent 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,
)range, yellow, green, blue, indigo, violet, and ultra-
violet ones. Could we, moreover, raise the original
>urce of rays to a sufficiently high temperature, we
light 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.*

* 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 oxpii/rplish light. The effect was not subjective, and consider-
ations 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 em-
ploying 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 com-
, pletely the solar heat, may entirely fail in this object, if the glass
: in which the carbon is incorporated be colourless. To render tJ>f
i

50 FRAGMENTS OF SCIENCE.

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

The layer of iodine used in the foregoing experi-
ments 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 de-
stroy it. To test the likelihood of these results, experi-
ments 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 daik focus, no defence, in the first in-
stance, 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 after-
wards by the retina. Kemoving the eye, but per-
mitting the plate of metal to remain, a sheet of
platinum foil was placed in the position occupied by 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.

EADIATION. 51

retina a moment before. The platinum became red-hot.
No sensible damage was done to the eye by this experi-
ment ; 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. Experi-
ments on the eye of an ox showed that the proportion
of obscure rays which reached the retina amounted to
1 8 per cent, of the total radiation ; while the luminous
emission from the electric light amounts to no more
than 1 0 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.'
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 ^^^ of its light at the distance of a foot.

' 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.

5i FRAGMENTS OF SCIENCE.

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 1,500x20,000,000, 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.

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 aug-
mentation of intensity in the region beyond the red,
but we can measure it and express it numerically.
With this \'iew 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

HADIATIOK.

5d

spiral and fall afterwards upon a thermo-electric pile.
Placing in front of the orifice an opaque solution of
iodine, the platinum s^as gradually raised from a low.
dark heat to the fullest incandescence, with the follow-
ing results : —

Appearance

Energy of

of spiial obscure radiation

Dark 1

Dark, but hotter .

3

Dark, but still hotter

5

Dark, but still hotter ,

10

Feeble red .

19

Dull red

25

Bed . . .

37

Full red.

62

Orange .

89

Bright orange

. 144

Yellow .

. 202

White .

. 276

Intense white . ,

440

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 obscm-e 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

54 FRAGMENTS OP SCIENCfi.

various degrees of darkness and incandescence, with
the following results : — -

Appearance Energy of

of spiral obscure rays

Dark 1

Dark 6

Faint red 10

DuUred 13

Red 18

FuUred 27

Orange 60

Yellow 93

White 122

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 refrangi-
bility 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 con-
dition 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 incan-
descent vapours. In such cases when the curve repre-
senting the radiant energy of the bo'dy 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 asso-
ciate of their luminous rays.

We thus find the luminous radiation appearing
when the radiant body has attained a certain tem-
perature ; 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 pro-
duced 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 ether
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,
5

56 FRAGMENTS OF SCIENCE.

and the light-giving waives 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 -^-Qth of an atmospliere : —

Name of vapour

Absorption

Bisnlphide of carbon ... 47

Iodide of methyl . ,

115

Benzol .

136

Amylene . .

. 321

Sulphuric ether .

. 440

Formic ether.

. 648

Acetic ether .

. 612

Bisulphide of carbon is the most transparent vapour
in this list ; and acetic ether the most opaque ; -g^th of
an atmosphere of the former, however, produces 47
times the effect of a whole atmosphere of air, while -g^th
of an atmosphere of the latter produces 612 times the
effect of a whole atmosphere of air. Eeducing dry air
to the pressure of the acetic ether here employed, and
comparing them then together, the quantity of wave-

IL

RADIATION. 57

motion intercepted by the ether would be many thousand
times that intercepted by the air.

Any one of these vapours discharged into the free
tmosphere, 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 bibu-
lous 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 pat-
chouli acts more feebly on radiant heat than any other
perfume yet examined. Here follow the results ob-
tained 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 : —

Name of perfnme Absorption

Patchouli . . . . • 30

Sandal wood H2

Geranium 33

Oil of cloves 34

Otto of roses 37

Bergamot 44

Neroli 47

Lavender 60

Lemon 65

Portugal 67

Thyme 68

Kosemary 74

Oil of laurel 80

Camomile flowers .... 87

Cassia . . ... 109

Spikenard 355

Aniseed ' 372

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

68 FRAGMENTS OF SCIENCR

12. Aqueous Vapour i/n, 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, diflfused, 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^ 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 terres-
trial 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,

i

RADIATION. 59

WL5 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 tc
the life of the world. Imagine the superficial molecules
of the earth agitated with the motion of heat, and
imparting it to the surrounding ether; this motion
would be carried rapidly away, and lost for ever to our
planet, if the waves of ether had nothing but the air
to contend with in their outward course. But the
aqueous vapour takes up the motion, and becomes
thereby heated, thus wrapping the earth like a warm
garment, and protecting its surface from the deadly
chill which it would otherwise sustain. Various philo-
sophers have speculated on the influence of an atmo-
spheric 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 impor-
tant, 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,
in such places, the sun's heat reaches the earth unim-
peded, and renders tlie maximum high ; by night, on
the other hand, the earth's heat escapes unhindered
into space, and renders the minimum low. Hence the
difi'erence between the maximum and minimum is
greatest where the air is driest. In the plains of India,
on 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

60 FRAGMENTS OF SCIENCE.

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 trans-
mitted 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 suu.

RADIATION. 61

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

The deportment here assigned to atmospheric
vapour has been established by direct experiments on
air 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 arti-
ficially 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
to 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 : —

Liquids Vapours

Bisulphide of carbon. Bisulphide of carbon.

Chloroform. Chloroform.

Iodide of methyl. Iodide of methyl.

Iodide of ethyl. Iodide of ethyl.

Benzol. Benzol.

Amylene. Amylene.

Sulphuric ether. Sulphuric ether.

Acetic ether. Acetic ether.

Formic ether. Formic ether.

Alcohol. Alcohol.
Water.

We here find the order of absorption in both cases
to 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

62 FRAGMENTS OF SCIENCE.

does not change their relative powers of absorption.
Nothing could more clearly prove that the act of ab-
sorption 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 sur-
rounding our planet, and saturated with the vapour of
sulphuric ether, 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 ether 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 tem-
perature now existing at the surface of the globe.

14. Reciprocity of Radiation and Absorption,

Throughout the reflections which have hitherto oc-
cupied 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 v£irious degrees. In

RADIATION. 63

all cases it was the transference of motion from the
ether 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 re-
cipients but as the originators of wave-motion. That is
to say, we must figure them vibrating, and generating in
the surrounding ether 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 oiu: gases and
vapours to the same temperature, and then examine
their power of discharging the motion thus imparted to
them upon the ether 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 con-
taining 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 thermo-
electric 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

64 FKAGMENTS OF SCIENCE.

amount of radiant heat, the molecules of compound
gases were shown to be capable of powerfully disturbing
the surrounding ether. 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 equi-
valent to the vis viva extinguished.' 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 con-
tact 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 Kadiation, the reciprocity of radiation
and absorption has been conclusively proved.^

In the excellent researches of Leslie, De la Pro-
vostaye and Desains, and Balfour Stewart, the same
reciprocity, as regards solid bodies, has been variously
illustrated; while the labours, theoretical and ex-
perimental, of Kirchhoff have given this subject a
wonderful expansion, and enriclied 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 ex-

' See page 15 for a definition of vis viva.

* 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
tiiib transmutation of vibrations.

KADIATION. 65

periments 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 moat decisive and extra-
ordinary way.

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

In the foregoing experiments with gases and vapours
we have employed throughout invisible rays, and fouad
some of these bodies so impervious to radiant heat, that
in lengths of a few feet they intercept every ray as
effectually as a layer of pitch. The substances, how-
ever, which show themselves thus opaque to radiant heat
are 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
some 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 depend-
ence ? The admirable researches of Kirchhoff help us
to an answer. The atoms and molecules of every gas
have certain definite rates of oscillation, and those waves
of ether are most copiously absorbed whose periods
of recurrence synchronise with those of the atomic
groups amongst which they pass. Thus, when we find
the invisible rays absorbed and the visible ones trans-
mitted 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.

66 FEAGMENTS OF SCIENCE.

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 ether 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 molecides, 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 aa
every wave arrives just in time to repeat the action oi
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 qiuescent. 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 ether
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

IK):

RADIATION. 67

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 won-
derful 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
of an atmosphere of carbonic acid, enclosed in a tube
four feet long, absorbs 60 per cent, of the radiation from
a carbonic oxide flame, while one-thirtieth of an atmo-
sphere absorbs 48 per cent, of the heat from the same
source.

In fact, the presence of the minutest quantity of car-
bonic 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.
Eadiant 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 deter-
mined. 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

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 : —

68 FEAGMENTS OF SCIENCE.

Percentage of Carbonic Add in the Human Breath,

Chemical analysis Physical analysh

4-66 4-56

6-33 6-22.

It is thus proved that in the quantity of ethereal
motion which it is competent to take up, we have a prac-
tical 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 ether, 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 re-
ferred 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 ether
than the single atoms can. Another, and probably very
potent cause of the difference may be, that the vibra-
tions, being those of the constituent atoms of the mole-
cule,^ are generated in highly condensed ether, which
acts like condensed air upon sound. But whatever may
be the fata 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 etlier within the molecules, by which the external
ether is disturbed.

* See * Physical, Considerations,* Art. iv. p. 102,

RADIATION. 60

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 calor-
ific waves. This deportment of the simple gases directed
our attention to other elementary bodies, the examina-
tion 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 sub-
stance was then employed to filter the beams of the
electric light, and to form foci of invisible rays so in-
tense as to produce almost all the efiects obtainable in
an ordinary fire. Combustible bodies were burnt, and
refractory ones were raised to a white heat, by the con-
centrated invisible rays. Thus, by exalting their re-
frangibility, 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 rich-
ness of the electric light in invisible rays of low re-
frangibility 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

70 FRAGMENTS OF SCIENCE.

incandescence, are strengthened together; intense
dark heat being an invariable accompaniment of in
tense 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 differ-
ences 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 mole-
cules 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. Re-
versing 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 ether, rendering it competent, in the same
degree, to generate them. Perfumes were next sub-
jected 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 sub-
stance 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

RADIATION. 71

sufiScient 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 pro-
cesses 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 ima-
gination. 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 ima-
gination. This, in fact, is the faculty which enables us
to transcend the boundaries of sense, and connect the
phenomena of our visible world with those of an in-
visible one. Without imagination we never could have
risen to the conceptions which have occupied us here
to-day ; 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.

72 FRAaMENTS OF SCIENCE.

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 ilhiminate 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.*
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 ether 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 ether
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 ether,
thus regenerating the motion from which their tempo-
rary 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

• This line of thought was pursued further five years subsa*
queutly. See ' Scientific Use of the Imagination ' in Vol. IL

RADIATIOIT. 73

it, a certain string will respond. Change the pitch of
your 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 tlie human body being so many strings
differently tuned, and responsive to different forms of
the universal power.

m.

ON RADIANT HEAT IN RELATION TO THE
COLOUR AND CHEMICAL CONSTITUTION OP
BODIES."^

ONE of the most important functioDS 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 com-
parison 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 ques-
tion, ' What is sound ? ' Amid the grosser phenomena
of acoustics the mind was first disciplined, conceptions

> A discourse delivered in tlie Kojal Institution of Great;
Britain, Jan. 19, 1866.

RADIANT HEAT AND ITS RELATIONS. 75

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 un-
dulations 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 seientific 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 any-
thing 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 th^ eye. The impact of thesft
particles against the target, Newton believed to be

76 FRAGMENTS OF SCIENCE.

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 assump-
tion of this wonderful intangible ethGV, all the pheno-
mena 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 ether. 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 ether 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

i

RADIANT HEAT AND ITS KELATIONS. 77

planetary system, but the immeasurable heavens them-
selves.

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 he beyond tlie range of the senses — to
present to itself distinct images of processes which,
though miglity 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 ether
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, how-
ever, 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 ether 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 oi

78

FEAGMENTS OF SCIENCE.

bodies, that the ether conveys. This motion is the
objective cause of what, in our sensations, are light and
heat. An atom, then, sending its pulses through the
ether, 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. Dififerent bodies,
when heated to the same temperature, possess very dif-
ferent powers of agitating the ether : some are good
radiators, others are bad radiators ; which means that
some are so constituted as to communicate their atomic
motion freely to the ether, 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. Eecent
experiments have proved that elementary bodies, except
under certain anomalous conditions, belong to the class
of bad radiators. An atom, vibrating in the ether, re-
sembles 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 sensa-
tion of sound. But if we permit the atoms to com-
bine chemically and form molecules, the result, in
many cases, is an enormous change in the power of
radiation. The amount of ethereal disturbance, pro-
duced 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,

RADIANT HEAT AND ITS RELATIONS. 79

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 gradu-
ally augmenting pitch, were vibrating at the same time.
But turning to another spectrum — that, namely, ob-
tained 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 difierent 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
577 millions of millions ; wldle 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 tern-

80 FEAGMENTS OF SCIENCE.

perature 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 tempera-
ture 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

"he

RADIANT HEAT AND ITS RELATIONS. 81

pinge, or will they glide round the molecules, through
;lie intermolecular spaces, and thus escape ?

The answer to this question depends upon a condi-
tion which may be beautifully exemplified by an experi-
ment on sound. These two tuning-forks are tuned
absolutely alike. They vibrate with the same rapidity,
and, mounted thus upon their resonant cases, yuu hear
them loudly sounding the same musical note. Si-opping
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 vibra-
tion. 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 t]ie 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 per-
fectly 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.

82 FRAGMENTS OF SCIENCE.

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 : Tliree 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 wavefi^
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

RADIANT HEAT AND ITS RELATIONS. 83

motion. Let us apply these facts to radiant heat. This
blue flame is the flame of carbooic oxide ; this trans-
parent 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 lumi-
nous rays of the sun, though it can absorb the non-lumi-
nous 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. Tlieir trans-
parency is due to their inability to absorb luminous rays.
The molecules of such substances are in dissonance with
the luminous waves ; and hence such waves pass through
transparent bodies without disturbing the molecular
rest. A purely luminous beam, however intense may

84 FEAGMENTS OF SCIENCE.

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 sun-
shine 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 power-
ful dark rays is emitted \ y 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 Ehone and
the Rhine.

Placing a concave silvered mirror behind the electric
light its rays are converged to a focus of dazzling bril-
liancy. 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, tlie ice imme-
diately 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

RADIANT HEAT AND ITS RELATIONS. 85

cell of water and then through the ice. By mean? 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 re-
moved ; 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 crystal-
lisation 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 pf 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 dis-
covered, by which these dark rays may be detached from

86 FRAOMENTS OF SCIENCE.

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 mer-
cilessly 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 trans-
formed 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 im-
pression.

The dark rays being thus collected, you see nothing
at their place of convergence. With a proper thermo-
meter 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 ether at
the focus to be practically detached from the air, — that
the most violent ethereal 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 fusd
gold, silver, copper, and aluminium. The moment,
moreover, that wood is placed at the focus it bursts intd
a blaze.

It has been already affirmed that, whether as re^
gards radiation or absorption, the elementary atoms

KADIANT HEAT AND ITS RELATIONS. 87

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 ex-
posed to the intense radiation of an ordinary fire with-
out bursting into flame. It may also be exposed for
twenty or thirty seconds at an obscure focus, of suffi-
cient power to raise platinum to a red heat, without
ignition. Notwithstanding the energy of the ethereal
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 powerfid waves of low refrangi-
bility, 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 difierent 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
7

88 FKAGMENTS OF SCIENCE.

requires no thermometer to answer this qiiestioiu
Simply pressing the back of the card, on which the
white powder is strewn, against the cheek or fore-
head, 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 experiment of Franklin. Again, here are sii
pended two delicate mercurial thermometers at the-
same distance from a gas- flame. The bulb of one o:
them is covered by a dark substance, the bulb of th(
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

II

RADIANT HEAT AND ITS RELATIONS. 89

to the visible, rays. In the case of the radiation from
our fire, about 98 per cent, of the whole emission con
sists 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 pro-
cesses 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 inter-
dependence," 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 accom-
pany 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 incom-

90 FBAGMENTS OF SCIENCE. ^

petent 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
amount 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
experiment.

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

J

I

RADIANT HEAT ANT) ITS RELATIONS. 91

With regard to gases and vapours, and to the liquids
om 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. Melloni, for example,
had found the same radiant and absorbent power for
chalk and lamp-black. MM. Masson and Courtepee
had performed a most ela.borate 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 opera-
tions of experimental philosophy ; while an experiment
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-lumi-
nous ones. It is a powerful emitter of dark rays ; it

92 FRAGMENTS OF SCIENCE.

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 Courtepee expeii
mented thus : they mixed their powders and precipi
tates 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 amount 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 refer jed 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

RADIANT HEAT AND ITS RELATIONS. 93

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 com-
pound 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, how-
ever, 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
becaiise 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
tthe white radiates far more than the black. Deter-
mining, 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 absoi-p-
tion 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 alto-
gether delusive. This is the strict scientific upshot of

94 FKAGMENTS OF SCIENCE.

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 experiment of Franklin-
black dresses are more potent than white ones as ab-
sorbers of solar heat.

Tims, ill 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 ethereal 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 applica-
tion 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, lie knows full well that the
greatest practical triumphs have been episodes in the
search after pure natural truth. The electric telegraph

RADIANT HEAT AND ITS RELATIONS. 95

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 ? Tliis, I am bound to say, was done
by two men, the one a dweller in Italy,^ the other a
dweller in England,^ 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.

» Volta, • Faraday.

I

IV.

JfBW CHEMICAL BEACTI0N8 PRODUCED BY
LIGHT,

1868-69.

Measured by their power, not to excite vision, but to
produce heat — in other words, measured by their ab-
solute 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 thi3_
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 od
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

DECOMPOSITION BY LIGHT.

97

FlO. 2.

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 purifica-
tion 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
(6) 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 in-
variably 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 piirified 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 6,
and escaped by the small orifice at tlie lower end of
h into the liquid. Through this it bubbled, loading

98

FRAGMENTS OF SCIENCE.

DECOMPOSITION BY LIGHT. 99

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,
i- pp' the pipe leading to the air-pump, and f the test-
tube containing the volatile liquid. The tube t V is
plugged with cotton-wool intended to intercept the float-
ing 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 extraordinaiy.
When the experiment is made in a darkened room, a
tube which in ordinary daylight appears absolutely
clean, is often shown by the present mode of examina-
tion 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

100 FEAGMENTS OF SCIENCE.

particles was precipitated on the beam. The cloud
thus generated became denser as the light continued
to act, showing 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 dark-
ness. When the light was sent through it, the pre-
cipitation 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 witli
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 ob-
serve, 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.

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 vehiclf, 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 deportments
of the vapour itself. When it was permitted to enter

DECOMPOSITION BY LIGHT. 101

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 experi-
ments 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 experiment 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 asimder 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
tlie 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 sift-
ing of the beam by the vapour occurs, which diminishes
the chemical action in the posterior portions. In some
experiments the precipitated cloud only extended half-
way 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.

102 FRAGMENTS OF SCIENCE.

Solar light also effects the decomposition of th<
nitrite-of-amyl vapour. On October 10, 1868, I
tially darkened a small room in the Koyal Institutionj
into which the sun shone, permitting the light to entei
through an open portion of the window-shutter. In the
track of the beam was placed a large plano-convex lensj
which formed a fine convergent cone in tlie dust of the
room behind it. The experimental tube was filled ii
the laboratory, covered with a black cloth, and carri(
into the partially darkened room. On thrusting one en(
of the tube into the cone of rays behind the lens, pre
cipitation within the cone was copious and immediate.
The vapour at the distant end of the tube was in pai
shielded by that in front, and was also more feeblj
acted on through the divergence of the rays. On re-
versing the tube, a second and similar cone was precipi
tated.

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 introfluced before the removal of the yellow,
or the red, on taking the latter away prompt precipit
tion occurred along the track of the blue beam. Henc
in this case, the more refrangible rays are the mc
chemically active. The colour of the liquid nitrite o{
amyl indicates that this must be the case ; it is a feebh
but distinct yellow : in other words, the yellow portioi
of the beam is most freely transmitted. It is no<
however, the transmitted portion of any beam whicl

DECOMPOSITION BY LIGHT. 103

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 thick-
ness, 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.^ 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
absoi-ption, and where is its seat ? ^ 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

* ' Phil. Trans/ 1864 ; ♦ Heat, a Mode of Motion,' chap. xii. ; and
p. 61 of this volume.

* My attention was very forcibly directed to this subject some
years ago by a conversation with my excellent friend Professoi
Claosius.

8

104

FRAGMENTS OF SCIENCE.

be regarded as an assemblage of such molecules,
question now before us is this : In the act of absorptioi
is it the molecules that are effective, or is it their coi
stituent atoms? Is the vis viva of the intercept
light-waves transferred to the molecule as a whole, or
its constituent parts ?

The molecule, as a whole, can only vibrate in virtu^
of the forces exerted between it and its neighbour mole
cules. 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 idei
tical absorption of the liquid and of the vaporous nitril
of amyl indicates an identical vibrating period on thi
part of liquid and vapour, and this, to my mine
amounts to an experimental proof that the absorptioi
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
amyl is itself to some extent an illustration of this ii
ternal molecular absorption ; for were the absorptio^
the act of the molecule as a whole, the relative motioi
of its constituent atoms would remain unchanged, ai
there would be no mechanical cause. for their separatioi
It is probably the synchronism of the vibrations of on(
portion of the molecule with the incident waves, thj
enables the amplitude of those vibrations to augment
until the chain which binds the parts of the moleci
together is snapped asunder.

I anticipate wide, if not entire, generality for tl
fact that a liquid and its vapour absorb the same ra]
A cell of liquid chlorine would, I imagine, deprive Ugl
more effectually of its power of causing chlorine

DECOMPOSITION BY LIGHT. 105

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-Uue by the Decomposition of
Nitrite of Am/yl,

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 ex-
perimental 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. Decom-
position then visibly commenced, and advanced slowly.
When the light was very strong, the cloud appeared of

106 FRAGMENTS OF SCIENCE.

a milky blue. When, on the contrary, the intensity
was moderate, the blue was pure and deep. In Briicke'a
important experiments on the blue of the sky and tb
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 highe
microscopic power. By reflected light, such a medium
appears bluish, by transmitted light yellowish, which
latter colour, by augmenting the quantity of the pre-
cipitate, 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, more-
over, is far purer and more sky-like than that obtained
from Briicke'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 sub-
jected 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 em-
ployed 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 decom-i
posing 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.

DECOMPOSITION BY LIGHT. 107

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 mar-
vellous transparency to such heat. May not its syn-
chronism 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, how-
ever, clouds begin to form, which grow in density and in
beauty as the light continues to act. In every experi-
ment 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
experiment 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 mani-
fested ; the various parts of tbe 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
80 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
of the tube causing an appearance resembling, on a

108 FRAGMENTS OF SCIENCE.

small scale, the dirt-bands of the Mer de Glace. In the
anterior portion of the tube those sudden commotions
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 scat-
tered 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 astoimding. 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 perpen-
dicular 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,

ARTIFICIAL SKY. 109

The latter also underwent slow but incessant modifica-
tion. It first resolved itself into a series of strata re-
sembling 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. ThosB
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
slender tubes were afterwards formed, which coiled
themselves in delicate spirals around the funnels.

Kenderihg the light along the connecting-cord more
intense, it diminished in thickness and became whiter ;
this was a consequence of the enlargement of its par-
ticles. The cord finally disappeared, while the funnels
melted into two ghost-like films, shaped like parasols.
They were barely visible, being of an exceedingly deli-
cate blue tint. They seemed woven of blue air. To com-
pare 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 Blub Colour of the Sky, and thb
Polarisation of Skylight.*

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

' In my * Lectures on Light * (Longmans), the polarisation of
light will be found briefly, but, I trust, clearly explained.

110 FKAGMENTS OF SCIENCE.

this subject were continued, the number of substances'
thus acted on being considerably increased,

I now, however, beg to direct attention to two ques- 1
tions glanced at incidentally in the preceding pages — thei
blue colour of the sky, and the polarisation of skylight, i
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 j
meteorology. Indeed it was the interest manifested inj
them by Sir John Herschel, in a letter of singular]
speculative power, addressed to myself, that caused m©^
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^ 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 pro-
ducts 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 con-
stitute but a small fraction of the length of a wave of
violet light.

In all cases when the vapours of the liquids em-

ARTIFICIAL SKY. Ill

ployed 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 inter-
mediate 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 pre-
viously 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 foimd 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 revolye 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.

112 FRAGMENTS OF SCIENCE.

From the illuminated bluish cloud, therefore, polarised
light was discharged, the direction of maximum polari-
sation being at right angles to the illuminating beam ;
the plane of vibration of the polarised light was per-
pendicular to the beam.*

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 sub-
stances 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 de-

• 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.

ABTIFICIAL SKY. 113

composing 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 experimentaV 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-atmo-
sphere of air which had been permitted to bubble
through fresh commercial hydrochloric acid. On send-
ing 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 per-
pendicularly 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 -^ of an inch. In other words,
the air and vapour, united, exercised a pj-essure not
exceeding -^^ of an atmosphere. Air, passed through
a solution of hydrochloric acid, was then added, till the
mercury column was depressed three inches. The con-
densed 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

114 FRAGMENTS OF SCIENCE.

lon^ 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 ex-
ceedingly brilliant. The direction of maximum polari
sation was distinctly at right angles to the illuminating
beam. This continued to be the case as long as th
cloud maintained a decided blue colour, and even fo;
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 re-
presentative. In all cases, and with all substances, the
cloud formed at the commencement, when the preci-
pitated particles are sufficiently fine, is blue, and it can
he 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

AETIFICIAL SKY. 115

or fifteen minutes after its first appearance the light
jfrom 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
I 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 cessa-
tion 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.^ 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 dis-
charged normally.

The perfection of the polarisation, in a direction
perpendicular to the illuminating beam, is also illus-
trated by the following experiment : A Nicol's prism,
large enough to embrace the entire beam of the electric
lamp, was placed between the lamp and the experi-
mental 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

* This shows that particles too large to polarise the blue, polar*
iaa perfectly light of lower refrangibility.

116 FRAGMENTS OF SCIENCE.

beam being therefore also vertical. As the light con
tinned to act, a superb blue cloud, visible to both myj
assistant and myself, was slowly formed. But thi
cloud, so deep and rich when looked at from the posi
tions mentioned, utterly disappeared when looked
vertically downwards^ or vertically upwards, Kefl
tion from the cloud was not possible in these directio
When the large Nicol was slowly turned round its axi
the eye of the observer being on the level of the bea:
and the line of vision perpendicular to it, entire extinc-
tion of the light emitted horizontally occurred who
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 experiment,
which I contemplated making on my own account, was
first definitely suggested by a remark in a letter ad-
dressed 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 re-
flection which produces perfect polarisation would
require to be made in air wpon air ; and indeed this
led many of our most eminent men, Brewster himself
among the number, to entertain the idea of aerial
molecular reflection.* I have, however, operated upon

' * The cause of the polarisation is evidently a reflection of the
sun's light upon sometliing. The question is on what 1 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 there-
abouts, is the correct angle, and that therefore whatever be the
body on which the light has been reflected, if jjolarised hy a singU
refleotiony the polarising angle must be 46°, and the index of refrac-
tion, which is the tangent of that angle, unity ; in other words, the

ARTIFICIAL SKY. 117

substances of widely different refractive indices, ami
therefore of very different polarising angles as ordi-
narily defined, but the polarisation of the beam, by the
incipient cloud, has thus far proved itself to be abso-
lutely independent of the polarising angle. The law
of Brewster does not apply to matter in this condition,
and it rests with tlie 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 sun-
ward 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 througli an incipient cloud. The
sunbeam would be blue, and it would discharge later-
ally light in precisely the same condition as that dis-
charged 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.' Con-
versely our ' blue cloud ' is, to all intents and purposes,
an artificial shy}

^reflection would require to be made in air upon air I * (Sir John
terschel, * 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-
Fparticles on a hot summer's day in the higher regions of our atmo-
\ iphere inconceivable ? It is to be remembered that the oxygen and
[nitrogen of the air behave as a vacuum to radiant heat, the exceed-
kingly attenuated vapour of the higher atmosphere being therefore
fin practical contact with the cold of space.

' The opinion of Sir John Herschel, connecting the polarisation
imnd the blue colour of the sky, i& verified by the foregoing result*

118 FRAGMENTS OF SCIENCE.

But, as regards the polarisation of the sky, we kno\i
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, ' reutral 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 com-
plete 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 polar-
isation 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

* 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.'

ABTIFICIAL SKY. 119

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 ob-
liquely. 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 Eoyal 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 aii w :s 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 ammo-
nium, the smoke of brown paper, and tobacco-smoke, I
had varied and confirmed in many ways those experi-
ments on neutral points, when my attention was drawn
by Sir Charles Wheatstone to an important observation
communicated to the Paris Academy in 1860 by Pro-

120 FRAGMEr^TS OF SCIENCK

fessor Govi, of Turin.' M. Oovi bad been led to examine
a beam of light sent through a room in which were ajuc-
cessively diffused the smoke of incense, and tobacco-
smoke. His first brief communication stated the fact
of polarisation by such smoke ; but in his second com-
munication 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 eac
other.

But unlike my observations on the laboratory air;
and unlike the action of the sky, the direction of maxi-
mum polarisation in M. Grovi's experiment 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. Grovi or any other investigator has pur-
sued 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. Experi-
ments 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 follow-
ing new and remarkable results : The laboratory being
well filled with the fumes of incense, and sufiicient 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 direc-
tion of maximum polarisation, instead of being perpen-
dicular, now enclosed an angle of only 12° or 13° with
the axis of the beam.

* *Comptes Rendus,' tome li. pp. 360 and 669.

le

1

AETIFICIAL SKY.. 121

A neutral point, with complementary effects at oppo-
site 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 per-
mitted 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
Beries of observations, were these : —

1st observation •

. 2 feet 2 inches.

2nd „

. . 2 „ 6 „

3rd „

. . 2 „ 10 „

4th

. . 3 „ 2 „

5th M

. . 3 „ 7 „

6th

. . 4 „ 6 „

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, correspond-
ing to five different densities of the gunpowder-smoke,
the angles enclosed between the line of vision to the

122 FEAGMENTS OF SCIENCE.

neutral point and the axis of the beam, were 63°, 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 sele-
nite, and the line of vision perpendicular, the windows
were opened, the blinds remaining undrawn. The resi-
nous 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 re-
vived, but now the colours were complementary to the
former ones. The neutral point had passed me in its
Tnotion down the beam, consequent upon the attenua-
tion of the fumes of resin.

With the fumes of chloride of ammonium substan-
tially the same results were obtained. Sufficient, how-
ever, has been here stated to illustrate the variability
of the position of the neutral point.^

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. I>et
the ring-system observed in the common air be brought
to its maximum strength, and then let an attenuated^

* Brewster has proved the variability of the position of th<
neutral point for skylight with the sun's altitude, a result obviouslj
connected with the foregoing experimental

ARTIFICIAT. SKY. 123

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 sul-
phur-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 wliite, and the rings themselves to emit their
complementary colours.*

Almost all liijuids have motes in them sufficiently
numerous to polarise sensibly the light, and very beau-
tiful 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 moi'e brilliant result is

' Sir John Herscliel 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 clear-
Bess.

124 FRAGMENTS OF SCIENCE.

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 dis-
tilled water, for example, a thick slice of light is neces-
sary to make the polarisation of its suspended particles
sensible. A much thinner slice suffices for commor
water ; while, with Briicke's precipitated mastic, a
slice too thin to produce any sensible effect with mosl
other liquids, suffices to bring out vividly the selenit<=.
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 altei«hg
the illumination, either of the object itself or oi its
environment, we alter the appearance of the object.
Take the case of clouds floating in the atmop^^here with
patches of blue between them. Anything that changes
the illumination of either alters the appearance of both,
th9,t 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 un polarised, 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

ARTIFICIAL SKY. 125

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 sud-
denly 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 maxi-
mum polarisation, the quenching of the surroundiDg
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 wihi 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 Alpen-

glow 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 wislied to observe one of the rose-coloured

)uttresses of the mountain. Such a one was visible

rom a point a few hundred feet above the hotel. The

[atterhorn also, though for the most part in shade,

lad a crimson projection, while a deep ruddy red

lingered along its western shoulder. Four distinct

jaks 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.

126 FEAGMENTS OF SCIENCE.

Looking at the Weisshom through the Nicol, the'
glow of its protuberance was strong or weak according]
to the position of the prism. The summit also under-
<vent striking changes. In one position of the prism it]
exhibited a pale white against a dark background; inj
the rectangidar position it was a dark mauve against ai
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 amount 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 quench-
ing 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 thd solar beams, the effects on this mountain were

AETIFICIAL SKY. 127

most striking. The grey summit of the Matterhom,
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 iso-
lated, and stood out in bold definition. It is to be
remembered that in the production of these effects tlie
only things changed are the sky behind, and the lumi-
nous 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

128 FRAGMENTS OF SCIENCE.

we have the blue haze referred to, throwing the dis-
tant 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 atmo-
sphere. At certain times and places it is almost as
blue as the sky itself; but to see its colour, the atten-
tion 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 deport-
,ment 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 re-
markable. Towards evening the sky above the moun-
tains 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 th(
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 cir^
cumstances, produce chromatic effects of polarisation,
almost as vivid as those produced by the sky itself.

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

AETIFICIAL SKY. 129

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 distinc-
tive colours. The white light of the sun enters all
substances to a certain deptli, 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 amount of light
derived from superficial reflection. A certain hard
brilliancy is conferred upon the woods and meadows by
this superficially-reflected light. Under certain cir-
cumstances, 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 Aletsch-
[horn. The effects described in the foregoing para-
graphs 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.

130 FRAGMENTS OF SCIENCE.

But. at no portion of the firmament was the polarisation
complete. The artificial sky produced in the experi-
ments 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 tlie extinction of the brilliancy. With certain
substances, however, artificially treated, this dull residue
may also be obtained.

All along the arc from the Matterhom 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 astonish-
ing. 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 alternaltive ex-
tinction 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 sud-
denness with which the mighty masses, ranged along
the line referred to, changed their aspect and definition
under the operation of the prism.

[In t!iG last edition of the • Fragments of Science • an essay
on * Dust and Disease ' followed here ; but as almost all my
writings on the 'Germ Theory' are now collected in a single
volume entitled ' Essays on the Floating Matter of the Air,'
* Dust and Disease ' no longer appears in the * Fragments.* In
its place I venture to introduce a short article written early
last year for an important American magazine.]

V.

THE SKY}

Invited to write for the * Forum* an article that
would have brought me face to face with ' problems of
life and mind * for which I was at the moment unpre-
pared, and unwilling to decline a request so courteously
made, I offered, if the editor cared to accept it, to send
him a contribution on the subject here presented.

I mentioned this subject, thinking that, in addition
to its interest as a fragment of ' natural knowledge,' it
might permit of a giance at the workings of the
scientific mind when engaged on the deeper problems
which come before it. In the house of Science are
many mansions, occupied by tenants of diverse kinds.
Some of them execute with painstaking fidelity the
useful work of observation, recording from day to day
the aspects of Nature, or the indications of instruments
devised to reveal her ways. Others there are who add
to this capacity for observation a power over the
language of experiment, by means of which they put
questions to Nature, and receive from her intelligible
replies. There is, again, a third class of minds, that
cannot rest content with observation and experiment,
• From * The Forum,' February 1888.

132 FEAGMENTS OF SCIENCE.

whose love of causal unity tempts them perpetually
to break through the limitations of the senses, and to
seek beyond them the roots and reasons of the pheno-
mena which the observer and experimenter record.
To such spirits — adventurous and firm— we are indebted
for our deeper knowledge of the methods by which the
physical universe is ordered and ruled.

In his efforts to cross the common bourne of the
known and the unknown, the effective force of the man
of science must depend, to a great extent, upon his
acquired knowledge. But knowledge alone will not
do ; a stored memory will not suffice ; inspiration must
lend its aid. Scientific inspiration, however, is usually,
if not always, the fruit of long reflection — rof patiently
'intending the mind,' as Newton phrased it; and as
Copernicus, Newton, and Darwin practised it; until
outer darkness yields a glimmer, which in due time
opens out into perfect intellectual day. From some of
his expressions it might be inferred that Newton scorned
hypotheses ; but he allowed them, nevertheless, an open
avenue to his own mind. He propounded the famous
corpuscular theory of light, illustrating it and defending
it with a skill, power, and fascination which sub-
sequently won for it ardent supporters among the best
intellects of the world. This theory, moreover, was
weighted with a supplementary hypothesis, which as-
cribed to the luminiferous molecules ' fits of easy
reflection and transmission,' in virtue of which they were
sometimes repelled from the surfaces of bodies and
sometimes permitted to pass through. Newton may
have scorned the levity with which hypotheses are
sometimes framed ; but he lived in an atmosphere of
theory, which he, like all profound scientific thinkers,
found to be the very breath of his intellectual life.

The theorist takes his conceptions from the world

THE 8KY. 133

of fact, and refines and alters them to suit his needs.
The sensation of sound was known to be produced by
aerial waves impinging on the auditory nerve. Air being
a thing that could be felt, aud its vibrations, by suitable
treatment, made manifest to the eye, there was here a
physical basis for the ' scientific imagination ' to build
upon. Both Hooke and Huyghens built upon it with
effect. By the illustrious astronomer last named the
conception of waves was definitely transplanted from
its terrestrial birthplace to a universal medium whose
undulations could only be intellectually discerned.
Huyghens did not establi>h the undulatory theory, but
he took the first firm step towards establishing it.
Laying this theory at the root of the phenomena of
lij>ht, he went a good way towards showing that these
phenomena are the necessary outgrowth of the concep-
tion.

By analysis and synthesis Newton proved the white
light of the sun to be a skein of many colours. The
cause of colour was a question which immediately oc-
cupied his thoughts ; and here, as in other cates, he
freely resorted to hypothesis. He saw, with his mind's
eye, his luminiferous corpuscles crossing the bodily eye,
and imparting successive shocks to the retina behind.
To difierences of ' bigness ' in the light-awakening
molecules Newton ascribed the different colour-sensa-
tions. In the undulatory theory we are also confronted
with the question of colour ; and here again, to inform
and guide us, we have the analogy of sound. Aerial
waves of different lengths, or periods, produce notes of
different pitch ; and to differences of wave-length in
that mysterious medium, the all- pervading ether, dif-^
ferences of colour are ascribed. Hooke had already
discoursed of ' a very quick motion that causes light, as
well as a more robust that causes heat.' Newton had

IS4 FRAGMENTS OF SCIENCE,

ascribed the sensation of red to the shock of his grossest,
and that of violet to the shock of his finest luminiferous
projectiles. Defining the one, and displacing the other
of these notions, the wave-theory affirms red to be pro-
duced by the largest, and violet by the smallest waves
of the visible spectrum. The theory of undulation had
to encounter that fierce struggle for existence which
all great changes of doctrine, scientific or otherwise,
have had to endure. Mighty intellects, following the
mightiest of them all, were arrayed against it. But the
more it was discussed the more it grow in strength and
favour, until it finally supplanted its formidable rival.
No competent scientific man at the present day accepts
the theory of emission, or refuses to accept the theory
of undulation.

Boyle and Hooke had been fruitful experimenters on
those beautiful iridescences known as the 'colours of
thin plates.' The rich hues of the thin-blown soap-
bubble, of oil floating on water, and of the thin layer of
oxide on molten lead, are familiar illustrations of these
iris colours. Hooke showed that all transparent films, if
only thin enough, displayed such colours ; and he proved
that the particular colour displayed depended upon the
thickness of the film. Passing from solid and liquid
films to films of air, he says : ' Take two small pieces of
ground and polished looking-glass plate, each about the
bigness of a shilling ; take these two dry, and with your
forefingers and thumbs press them very hard and close
together, and you shall find that when they approach
each other very near, there will appear several irises or
coloured lines.' Newton, bent on knowing the exact
relation between the thickness of the film and the colour
it produced, varied Hooke's experiment. Taking two
pieces of glass, the one plane and the other very slightly
curved, and pressing both together, he obtained a film

I

1

THE SKY. 135

of air of gradually increasing thickness from the place
of contact outwards. As he expected, he found the place
of contact surrounded by a series of coloured circles,
still known all over the world as ' Newton's rings.' The
colours of his first circle, which immediately surrounded
a black central spot, Newton called ' colours of the first
order ; ' the colours of the second circle, ' colours of the
second order,' and so on. With unrivalled penetration
and apparent success, he applied his theory of ' fits'
to the explanation of the ' rings.' Here, however, the
only immortal parts of his labours are his facts and
measurements ; his theory has disappeared. It was re-
served for the illustrious Thomas Young, a man of in-
tellectual calibre resembling that of Newton himself, to
prove that the rings were produced by the mutual action
—in technical phrase, 'interference' — of the light-
waves reflected at the two surfaces of the film of air
inclosed between the plane and convex glasses. The
colours of thin plates were 'residual colours' — survivals
of the white light after the ravages of interference.
Young soon translated the theory of 'fits' into that of
'waves;' the measurements pertaining to the former
being so accurate as to render them immediately
available for the purposes of the latter.

It is here that Newton's researches and opinions
touch the subject of this article. The colour nearest to
the black spot, in the experiment above described, was
a faint blue — ' blue of the first order ' — corresponding
to the film of air when thinnest. If a solid or liquid
film, of the thickness requisite to produce this colour,
were broken into bits and scattered in the air, Newton
inferred that the tiny fragments would display the blue
colour. Tantamount to this, he considered, was the ac-
tion of minute water-particles in the incipient stage of
their condensation from aqueous vapour. Such particles

10

136 FKAGMENTS OF SCIENCB.

suspended in our atmosphere ought, he supposed, to
generate the serenest skies. Newton does not appear
to have bestowed much thought upon this subject ; for
to produce the particular blue which he regarded as
sky-blue, thin plates with parallel surfaces would be
required. The notion that cloud-particles are hollow
spheres, or vesicles, is prevalent on the Continent, but it
never made any way among the scientific men of Eng-
land. De Saussure thought that he had actually seen
the cloud- vesicles, and Faraday, as I learned from
himself, believed that he had once confirmed the observa-
tion of the illustrious Alpine traveller. During my long
acquaintance with the atmosphere of the Alps I have
often sought for these aqueous bladders, but have never
been able to find them. Clausius once published a
profound essay on the colours of the sky. The assump-
tion of small water drops, he proved, would lead to
optical consequences entirely at variance with facts.
For a time, therefore, he closed with the idea of vesicles,
and endeavoured to deduce from them the blue of the
firmament and the morning and evening red.

It is not, however, necessary to isvake the blue of
the first order to explain the colour of the sky ; nor i8
it necessary to impose upon condensing vapour the diffi-
cult, if not impossible, task of forming bladders, when
it passes into the liquid condition. Let us examine
the subject. Eau-de-Cologne is prepared by dissolving
aromatic gums or resins in alcohol. Dropped into
water, the scented liquid immediately produces a white
cloudiness, due to the precipitation of the substances
previously held in solution. The solid particles are,
however, comparatively gross; but by diminishing the
quantity of the dissolved gum, the precipitate may
be made to consist of extremely minute particles.
Briicke, for example, dissolved gum-mastic, in certain

THE SKY. 137

proportions, in alcohol, and carefully dropping hia
solution into a beaker of water, kept briskly stirred, he
was able to reduce the precipitate to an extremely fine
state of division. The particles of mastic can by no
means be imagined as forming bladders. Still, against
a dark ground — black velvet, for example — the water
that contains them shows a distinctly blue colour. The
bluish colour of many liquids is produced in a similar
manner. Thin milk is an example. Blue eyes are
also said to be simply turbid media. The rocks over
which glaciers pass are finely ground and pulverised by
the ice, or the stony emery imbedded in it ; and the
river which issues from the snout of every glacier is
laden with suspended matter. When such glacier water
is placed in a tall glass jar, and the heavier particles
are permitted to subside, the liquid column, when
viewed against a dark background, has a decidedly
bluish tinge. The exceptional blueness of the Lake of
Greneva, which is fed with glacier water, may be due,
in part, to particles small enough to remain suspended
long after their larger and heavier companions have
sunk to the bottom of the lake.

We need not, however, resort to water for the pro.
duction of the colour. We can liberate, in air, particles
of a size capable of producing a blue as deep and pure
as the azure of the firmament. In fact, artificial skies
may be thus generated, which prove their brotherhood
with the natm-al sky by exhibiting all its phenomena.
There are certain chemical compounds — aggregates of
molecules — the constituent atoms of which are readily
shaken asunder by the impact of special waves of light.
Probably, if not certainly, the atoms and the waves
are so related to each other, as regards vibrating period,
that the wave-motion can accumulate until it becomes
disruptive. A great number of substances might be

138 FRAGMENTS OF SCIENCE.

mentioned whose vapours, when mixed with air and sub-
jected to the action of a solar or an electric beam, are
thus decomposed, the products of decomposition hanging
as liquid or solid particles in the beam which genera
them. And here I must appeal to the inner vision already
spoken of. Remembering the different sizes of the waves
of light, it is not difficult to see that our minute par-
ticles are larger with respect to some waves than to
others. In the case of water, for example, a pebble will
intercept and reflect a larger fractional part of a ripple
than of a larger wave. We have now to imagine light-
undulations of diff'erent dimensions, but all exceedingly
minute, passing through air laden with extremely small
particles. It is plain that such particles, though scat-
tering portions of all the waves, will exert their most
conspicuous action upon the smallest ones ; and that
the colour-sensation answering to the smallest waves —
in other words, the colour blue — will be predominant
in the scattered light. This harmonises pei fectly with
what we observe in the firmament. The sky is blue,
but the blue is not pure. On looking at the sky
through a spectroscope, we observe all the colours of the
spectrum; blue is merely the predominant colour.
By means of our artificial skies we can take, as it were, J
the firmament in our hands and examine it at our
leisure. Like the natural sky, the artificial one show
all the colours of the spectrum, but blue in excess,
Mixing very small quantities of vapour with air, an
bringing the decomposing luminous beam into action
we produce particles too small to shed any sensibl
light, but which may, and doubtless do, exert an actio
on the ultra-violet waves of the spectrum. We cai
watch these particles, or rather the space they occupyj
till they grow to a size able to yield the firmamen
azure. As the particles grow larger under the continu

kTHE SKY. 139

stion of the light, the azure becomes less deep ; while
ter on a milkiness, such as we often observe in nature,
^ takes the place of the purer blue. Finally the particles
become large enough to reflect all the light- waves, and
then the suspended ' actinic cloud ' diffuses white light.
It must occur to the reader that even in the absence
of definite clouds there are considerable variations in
the hue of the firmament. Everybody knows, moreover,
that as the t^ky bends towards the horizon, the purer
blue is im[)aired. To measure the intensity of the colour
De Saussure invented a cyanometer, and Humboldt has
given us a mathematical formula to express the diminu-
tion of the blue, in arcs drawn east and west from the
zenith downwards. This diminution is a natural con-
sequence of the predominance of coarser particles in the
lower regions of the atmosphere. Were the particles
which produce the purer celestial vault all swept away,
we should, unless helped by what has been called
* cosmic dust,' look into the blackness of celestial space.
And were the whole atmosphere abolished along with its
suspended matter, we should have the 'blackness*
spangled with steady stars; for the twinkling of the
stars is caused by our atmosphere. Now, the higher we
ascend, the more do we leave behind us the particles
which scatter the light; the nearer, in fact, do we ap-
proach to that vision of celestial space mentioned a
moment ago. Viewed, therefore, from the loftiest
Alpine summits, the firmamental blue is darker than it
is ever observed to be from the plains.

It is thus shown that by the scattering action of
minute particles the blue of the sky can be produced ;
but there is yet more to be said upon the subject. Let
the natural sky be looked at on a fine day through a
piece of transparent Iceland spar cut into the form
known as a Nicol prism. It may be well to begin by

140 FRAGMENTS OF SCIENCE.

looking through the prism at a snow slope, or a white
wall. Turning the prism round its axis, the light com-
ing from these objects does not undergo any sensible
change. But when the prism is directed towards the
sky the great probability is that, on turning it, variations
in the amount of light reaching the eye will be observed.
Testing various portions of the sky with due diligence,
we at length discover one particular direction where the
difference of illumination becomes a maximum. Here
the Nicol, in one position, seems to offer no impediment
to the passage of the sky light ; while, when turned
through an arc of ninety degrees from this position, the
light is almost entirely quenched. We soon discern that
the particular line of vision in which this maximum
difference is observed is perpendicular to the direction
of the solar rays. The Nicol acts thus upon sky light
because that light is polarised, while the light from the
white wall or the white snow, being unpolarised, is not
affected by the rotation of the prism.

In the case of our manufactured sky not only is the
azure of the firmament reproduced, but these phenomena
of polarisation are observed even more perfectly than
in the natural sky. When the air-space from which
our best artificial azure is emitted is examined with the
Nicol prism, the blue light is found to be completely
polarised at right angles to the illuminating beam.
The artifical sky may, in fact, be employed as a second
Nicol, between which and a prism held in the han<
many of the beautiful chromatic phenomena observe
in an ordinary polariscope may be reproduced.

Let us now complete our thesis by following th<
larger light-waves, which have been able to pass amonj
the aerial particles with comparatively little fractionj
loss. Without going beyond inferential considerations
we can state what must occur. The action of the

THE- SKY. 141

particles upon the solar light increases with the atmo-
spheric distances traversed by the sun's rays. The
lower the sun, therefore, the greater the action. The
shorter waves of the spectrum being more and more
withdrawn, the tendency is to give the longer waves
an enchanced predominance in the transmitted light.
The tendency, in other words, of this light, as the rays
traverse ever-increasing distances, is more and more
towards red. This, I say, might be stated as an in-
ference, but it is borne out in the most impressive
manner by facts. When the Alpine sun is setting, or,
better still, some time after he has set, leaving the limbs
and shoulders of the mountains in shadow, while their
snowy crests are bathed by the retreating light, the
snow glows with a beauty and solemnity hardly equalled
by any other natural phenomenon. So, also, when first
illumined by the rays of the unrisen sun, the moun-
tain heads, under favourable atmospheric conditions,
shine like rubies. And all this splendour is evoked by
the simple mechanism of minute particles, themselves
without colour, suspended in the air. Those who
referred the extraordinary succession of atmospherie
glows, witnessed some years ago, to a vast and violent
discharge of volcanic ashes, were dealing with * a true
cause.' The fine floating residue of such ashes would,
undoubtedly, be able to produce the effects ascribed to
it. Still, the mechanism necessary to produce the
morning and the evening red, though of variable effi-
ciency, is always present in the atmosphere. I have
seen displays, equal in magnificence to the finest of those
above referred to, when there was no special volcanic
outburst to which they could be referred. It was the
long-continued repetition of the glows which rendered
the volcanic theory highly probable.

142 FEAGMENTS OP SCIENCB.

'

VI.

VOYAGE TO ALGERIA TO OBSERVE THE
ECLIPSE.

1870.

THE opening of the Eclipse Expedition was not pro-
pitious. Portsmouth, on Monday, December 5.
1 870, 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 Light-
house. 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

VOYAGE TO ALGERIA. 143

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 morn-
ing 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 some-
times dropped from the distant clouds to the sea.
These were falling showers, and they sometimes occu-
pied 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 coiu-se, avoided a heavy shower. From
time to time perfect rainbows spanned the heavens from
side to side. At times a bow would appear in frag-
ments, 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 conflagratipn. On Friday morn-
ing 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 diffi-

144 FRAGMENTS OF SCIENCE.

cult 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 rendc^red
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. Furni-
ture crashed, glasses rang, and alarmed enquiries
immediately followed. Amid the noises 1 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 stead-
fast adherence to an object can accomplish, and what]
large effects are heaped up by the addition of infinitesi-

VOYAaE TO ALaEEIA. 145

mals. The tiller-rope, as the blue-jackets strained in
concert, seemed hardly to niove ; 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
liad previously gone on deck. Bound 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. • Round it I coiled my arms. With the excep-
tion 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 steady-
ing 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

' The cleat is a T-shaped masa of metal employed for th»
fastening of ropes.

146 FRAGMENTS OF SCIENCE.

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

k

-VOYAaE TO ALGERIA. 147

lOon we accomplished sixty-six miles, or an average of
s 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.*

All through Saturday the wind, though somewhat
sobered, blew dead against us. The atmospheric effects
were exceedingly fine. The cumuli resembled moun-
tains 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 ti^v^ 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. Kain-
bows 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 de-
clared to be those of Cadiz itself. Out of deference to

' There is, it will be seen, a fair agreement between these im-
pressions and those so vigorously described by a scientific corre*
Bpondent of the < Times.'

148 FRAGMENTS OF SCIENCE.

these statements, the navigating lieutenant changed hia
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 Cipiona. 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 singu-
larly 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 permis-
sion 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 permis-
sion had been granted. We landed our party, and in
the afternoon weighed anchor. Thauks to the kindness
of oiu* 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

VOYAGE TO ALGEEIA. 149

id shadow being almost horizontal. A pillar of reflected

rht shimmered up to us from the slightly rippled sea.
had previously noticed the phosphorescence of the

iter, but to night it was stronger than usual, especially
long the foam at the bows. A bucket let down into

16 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 strik-
ingly 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 phosphor-
escence, 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 phospho-
rescence 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 Eock 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,

IdO FRAGMENTS OF SCIENCE.

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 my-
self 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, when-
ever 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 miaute 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 Grovernor was pro-
posed, as an act of necessary courtesy, and I accom-
panied 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 re-
ceived 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 witb

VOYAGE TO ALGEEIA. 151

fruit, in all of which he took manifest delight. Evi-
dently '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 intelli-
gent 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 embrasiu-es,
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, for-
getful of the laws of terrestrial curvature, thought h©
11

152 FRAGMENTS OP SCIENCE.

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 de-
scended 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 * Grovernor'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 an-
tagonist in arms, Mouravieff. Chivalry in war is con-
sistent 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 debris 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 ad-
jacent 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

VOYAGE TO ALGERIA. 153

Atlantic. Possibly the quantity of organisms may
have modified the colour. At night the phosphores-
cence 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 phosphor-
escent 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, shel-
tered by dominant hills. The sun was shining bright-
ly ; 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 stern 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 ei'uptions of in-
candescent 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 subse-
quently, in his own phaeton, to the place. It bore the

154 FRAGMENTS OF SCIENCE.

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.' He, in
the most liberal manner, placed his ground at the dis-
position 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 morn-
ing 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 pictur-

* Espjurto is a kind of grass now much used in the manufacture
of paper.

I

VOYAGE TO ALGERIA. 155

esque 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 indi-
viduality, 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 intelli-
gence that the tents had been blown down, and the
instruments overturned. Among these was a large and
valuable equatorial from the Eoyal Observatory, Green-
wich. 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 over-
turned 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 suc-
cessive swallowing up of the solar spots, the breaking
of the last line of crescent by the lunar mountains into

156 FKAGMENTS OF SCIENCE.

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 streamers 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 solar limb
through Bailey's beads, and, finally, the retreat of the
lunar shadow through the air.

I was provided with a telescope of admirable defi-
nition, 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 consider-
able 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 ex-
cellent 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 em-
brace in it the entire series of observations. These,
while the sun remaiued visible, were to be made with
an unsilvered diagonal eye-piece, which reflected but a
small fraction of the sun's ligbt, 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 maxi-
mum of light from the corona and prominences. The
time of totality was distributed as follows :

1. Observe approach of shadow through the air : totality,

2. Telescope . . . .30 seconds.
8. Finder . . , .30 seconds.
4. Double image prism . . 15 seconds.
6. Naked eye . . , .10 siconds.

6. Finder or binocular . , 20 seconds.

7. Telescope . . .20 seconds.
8p Observe retreat of shadow.

VOYACfE TO ALGERIA. • 157

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 mechan-
ical 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 waa tried severely,
hxt it stood firm^

158 FRAGMENTS OF SCIENCE.

The clouds and blue spaces fought for a time with
varying success. The sun was hidden 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 ap-
proached 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 thick-
ened more and more, and made the intervals of clear-
ness scantier. During those 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 Eoyal Engineers, had
the kindness to take charge of my note-book. I men-
tioned, 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

VOYAGE TO ALGEEIA. 159

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, cover-
ing 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 ap-
proached, 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 dark-
ness took possession of the ridge referred to, lowered
upon M. Janssen's observatory, passed over the southejn
heavens, blotting out the beams as if a sponge had
been drawn across them. It then took successive pos-
session 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
I 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 scat-
tering light into the shadow, rendered the darkness less
intense than it would have been had the atmosphere beep

a

160 FRAGMENTS OF SCIENCE.

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 after-
wards, 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. Groodman, observed the following tem-
peratures during the progress of the eclipse :

Hour

Degr.

Hour

Deg.

11.45

. 56

12.43

. 51

11.55

55

1.5

. 52

12.10

54

1.27

. 53

12.37

53

1.44

. 66

12.39

52

2.10

. 67

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.
Groodman, I visited a cave in a remarkable stratum
of shell-breccia, and, thanks to my guide, secured speci-
mens. 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.
A.bout four or five hundred fine-looking dragoons were

VOYAOE TO ALGERIA. 161

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 ter-
mination 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 pro-
montory 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 en-
durance, 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

162 FEAGMENTS OF SCIENCE.

seemed eroded in a remarkable manner. It has ita
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 Gribraltar.

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 pos-
sibility 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
qiinute, at first slowly, but soon with quickened pace

VOYAGE TO ALGERIA. 163

We turned to the left, so as to cut across her bows.
Five minutes' pull would have brought us up to her.
The ofiScer 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 at-
tention. I returned to the ' Urgent,' discomfited, but
grateful to the fine fellows who had wrought so hard to
carry out my wishes.

Grlad 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 Go-
vernor 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 bicar-
bonate 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 Red bourne
water exposed by Professors Graham, Miller, and Hof-
mann, 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

164 FRAGMENTS OF SCIENCE.

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 under-
neath, 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 solidi-
fication and the consequent architectiu-e 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

VOYAGE TO ALGERIA. 165

at the time, haunts me still ; for, though our requisi-
tions 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 :

I wiped away the weeds and foam,
And brought my sea-born treasures home :
But the poor, unsightly, noisome things
Had left their beauty on the shore,
With the sun, and the sand, and the wild uproar.

The promontory of Gribraltar is so burrowed with
caverns that it has been called the Hill of Caves. They
are apparently related to the geologic disturbances
which the rock has undergone. The earliest of these is
the tilting of the once horizontal strata. Suppose a
force of torsion to act upon the promontory at its
southern extremity near Europa Point, and suppose the
rock to be of a partially yielding character; such a
force would twist the strata into screw-surfaces, the
greatest amount of twisting being endured near the
point of application of the force. Such a twisting the
rock appears to have suffered ; but instead of the twist
fading gradually and uniformly off, in passing from
south to north, the want of uniformity in the material
has produced lines of dislocation where there are abrupt
changes in the amount of twist. Thus, at the northern
end of the rock the dip to the west is nineteen degrees;
in the Middle Hill, it is thirty-eight degrees ; in the
centre of the South hill, or Sugar Loaf, it is fifty-seven
degrees. At the southern extremity of the Sugar Loaf
the strata are vertical, while farther to the south they
actually turn over and dip to the east.

The rock is thus divided into three sections, sepa-

166 FRAGMENTS OF SCIENCE.

rated from each other by places of dislocation, where the
strata are much wrenched and broken. These are
called the Northern and Southern Quebrada, from the
Spanish ' Tierra Quebrada,' or broken ground. It is at
these places that the inland caves of Gibraltar are
almost exclusively found. Based on the observations of
Dr. Falconer and himself, an excellent and most in-
teresting account of these caves, and of the human
remains and works of art which they contain, was com-
municated by Mr. Busk to the meeting of the Congress
of Prehistoric Archaeology at Norwich, and afterwards
printed in the ' Transactions ' of the Congress.^ Long
subsequent to the operation of the twisting force just
referred to, the promontory underwent various changes
of level. There are sea- terraces and layers of shell-
breccia along its flanks, and numerous caves which, unlike
the inland ones, are the product of marine erosion. The
Ape's Hill, on the j^frican side of the strait, Mr. Busk
informs me has undergone similar disturbances.*

In the harbour of Gribraltar, on the morning of our
departure, I resumed a series of observations on the
colour of the sea. On the way out a number of
specimens had been collected, with a view to subsequent
examination. But the bottles were claret bottles, of
doubtful purity. At Gribraltar, therefore, I purchased
fifteen white glass bottles, with ground glass stoppers,
and at Cadiz, thanks to the friendly guidance of Mr.
Cameron, I secured a dozen more. These seven-and-j

* In this essay Mr. Busk refers to the previous labours of
Smith, of Jordan Hill, to whom we owe most of our knowledge
the geology of the rock.

' No one can rise from the perusal of Mr. Busk's paper withoul
a feeling of admiration for the principal discoverer and indef
tigable explorer of the Gibraltar caves, the late Captain Frederi(

VOYAGE TO ALGERIA. 167

twenty bottles were filled with water, taken at different
places between Oran and Spithead.

And here let me express my warmest acknowledg-
ments to Captain Henderson, the commander of H.M.S.
* Urgent,' who aided me in my observations in every
possible way. Indeed, my thanks are due to all the
ofificers for their unfailing courtesy and help. The
captain placed at my disposal his own coxswain, an
intelligent fellow named Thorogood, who skilfully
attached a cord to each bottle, weighted it with lead,
cast it into the sea, and, after three successive rinsings,
filled it under my own eyes. The contact of jugs,
buckets, or other vessels was thus avoided ; and even
the necessity of pouring out the water, afterwards,
through the dirty London air.

The mode of examination applied to these bottles
has been already described.^ The liquid is illuminated
by a powerfully condensed beam, its condition being
revealed through the light scattered by its suspended
particles. ' Care is taken to defend the eye from the
access of all other light, and, thus defended, it becomes
an organ of inconceivable delicacy.' Were water of
uniform density perfectly free from suspended matter,
it would, in my opinion, scatter no light at all. The
track of a luminous beam could not be seen in such
water. But ' 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.'

The results of the examination of nineteen bottles
filled at various places between Gibraltar and Spithead
are here tabulated :

* * Floating Matter of the Air,' Art. * Dust and Disease.'
12

168

FKAGMENTS OF SCIENCE.

No.

Locality

Colour of Sea

Appearance in Luminous Beam

1

Gibraltar Harbour . . .

Green . . .

Thick with fine partides

2

Two miles from Gibraltar

Cleai-er green .

QTiick with very fine particleB

3

Off Cabreta Point . . .

Bripht pi-een .

Still thick, but less so

4

Off Cabreta Point . . .

Black-indigo .

Much less thick, very pure

6

Off Tarifa

Undecided . .

Thicker than No. 4

6

Beyond Taiifa ....

Cobalt-blue .

Much purer than No. 6

7

Twelve miles from Cadiz .

Yellow-green .

Very thick

8

Cadiz Harbour ....

Yellow-green .

Exceedingly thick

9

Fourteen milts from Cmliz

Yellow-green .

Thick, but less so

10

Fourteen miles from Cadiz

Bright green .

Much less thick

11

Between Capes St. Mai-y

and Vincent ....

Deep indigo .

Very little matter, very pure

12

Off theBiu-lings. . . .

Stroug green .

Thick, with fine matter

13

Beyond the Burlings . .

Indigo . . .

Very little matter, pure

14

Off Cape Finisterre. . .

Undecided . .

Less pure

15

Bay of Biscay . , . .

Black-indigo .

Very little matter, very pure

16

Bay of Biscay ....

Indigo . . .

Veiy fine matter. Iridescent

17

OffUshant

Dark green . .

A good deal of matter

18

Off St. Catherine's . . .

Yellow-green .

Exceedingly thick

19

Spithead

Green . . .

Exceedingly thick

Here we have three specimens of water, described as
green, a clearer green, and bright green,^ taken in
Gibraltar Harbour, at a point two miles from the
harbour, and off Cabreta Point. The home examina-
tion showed the first to be thick with suspended matter,
the second less thick, and the third still less thick.
Thus the green brightened as the suspended matter
diminished in amount.

Previous to the fourth observation our excellent
navigating lieutenant, Mr. Brown, steered along the
coast, thus avoiding the adverse current which sets in,
through the Strait, from the Atlantic to the Mediter-
ranean. He was at length forced to cross the boundary
of the Atlantic current, which was defined with extra-
ordinary sharpness. On the one side of it the water
was a yivid green, on the other a deep blue. Standing
at the bow of the ship, a bottle could be filled with
blue water, while at the same moment a bottle cast
from the stern could be filled with green water. Tw(
bottles were secured, one on each side of this remarkable
boundary. In the distance the Atlantic had the hi

VOYAaE TO ALaERIA. 169

called ultra-marine ; but looked fairly down upon, it
was of almost inky blackness — black qualified by a
I trace of indigo.

What change does the home examination here
reveal ? In passing to indigo, the water becomes sud-
denly augmented in purity, the suspended matter
becoming suddenly less. Off Tarifa, the deep indigo
disappears, and the sea is undecided in colour. Accom-
panying this change, we have a rise in the quantity of
suspended matter. Beyond Tarifa, we change to cobalt-
blue, the suspended matter falling at the same time in
quantity. This water is distinctly purer than the
green. We approach Cadiz, and at twelve miles from
the city get into yellow-green water ; this the London
examination shows to be thick with suspended matter.
The same is true of Cadiz harbour, and also of a point
fourteen miles from Cadiz in the homeward direction.
Here there is a sudden change from yellow-green to a
bright emerald-green, and accompanying the change
a sudden fall in the quantity of suspended matter.
Between Cape St. Mary and Cape St. Vincent the
water changes to the deepest indigo, a further diminution
of the suspended matter being the concomitant pheno-
menon.

We now reach the remarkable group of rocks called
the Burlings, and find the water between the shore and
the rocks a strong green ; the home examination shows
it to be thick with fine matter. Fifteen or twenty miles
beyond the Burlings we come again into indigo water,
from which the suspended matter has in great part dis-
appeared. Off Cape Finisterre, about the place where
the ' Captain ' went down, the water becomes green, and
the home examination pronounces it to be thicker. Then
we enter the Bay of Biscay, where the indigo resumes
its power, and where the home examination shows the

170 FRAGMENTS OF SCIENCE.

greatly augmented purity of the water. A second
specimen of water, taken from the Bay of Biscay, held
in suspension fine particles of a peculiar kind ; the size
of them was such as to render the water richly
iridescent. It showed itself green, blue, or salmon-
coloured, according to the direction of the line of vision.
Finally, we come to our last two bottles, the one taken
opposite St. Catherine's lighthouse, in the Isle of Wight,
the other at Spithead. The sea at both these places
was green, and both specimens, as might be expected,
were pronounced by the home examination to be thick
Trith suspended matter.

Two distinct series of observations are here referred
to — the one consisting of direct observations of the
colour of the sea, conducted during the voyage from
Gibraltar to Portsmouth : the other carried out in the
laboratory of the Royal Institution. And here i't is to
be noted that in the home examination I never knew
what water was placed in my hands. The labels, with
the names of the localities written upon them, had been
tied up, all information regarding the source of the
water being thus held back. The bottles were simply
numbered, and not till all of them had been examined,
and described, were the labels opened, and the locality
and sea-colour corresponding to the various specimens
ascertained. The home observations, therefore, must
have been perfectly unbiassed, and they clearly
establish the association of the green colour with fine
suspended matter, and of the ultramarine colour, and
more especially of the black-indigo hue of the Atlantic,
with the comparative absence of such matter.

So much for mere observation ; but what is the
cause of the dark hue of the deep ocean ? ^ A prelimi^

* A note, written to me on October 22, by my friend Canoi
Kingsley, contains the following reference to this point : ' I hat

VOYAGE TO ALaERIA. 171

nary remark or two will clear our way towards an ex-
planation. Colour resides in white light, appearing
when any constituent of the white light is with-
drawn. The hue of a purple liquid, for example, is
immediately accounted for by its action on a spectrum.
It cuts out the yellow and green, and allows the red
and blue to pass through. The blending of these two
colours produces the purple. But while such a liquid
attacks with special energy the yellow and green, it
enfeebles the whole spectrum. By increasing the thick-
ness of the stratum we may absorb the whole of the
light. The colour of a blue liquid is similarly accounted
for. It first extinguishes the red ; then, as the thick-
ness augments, it attacks the orange, yellow, and green
in succession ; the blue alone finally remaining. But
even it might be extinguished by a sufficient depth of
the liquid.

And now we are prepared for a brief, but tolerably
complete, statement of that action of sea- water upon
light, to which it owes its darkness. The spectrum
embraces three classes of rays — the thermal, the visual,
and the chemical. These divisions overlap each other;
the thermal rays are in part visual, the visual rays in
part chemical, and vice versa. The vast body of thermal
rays lie beyond the red, being invisible. These rays are
attacked with exceeding energy by water. They are
absorbed close to the surface of the sea, and are the
great agents in evaporation. At the same time the
whole spectrum suffers enfeeblement ; water attacks all
its rays, but with different degrees of energy. Of the

aever seen the Lake of Geneva, but I thought of the brilliant
dazzling dark blue of the mid- Atlantic under the sunlight, and its
black-blue under cloud, both so solid that one might leap off the
Bponson on to it without fear ; this was to me the most wonderful
thing which I saw on my voyages to and from the West Indies.'

172 FRAGMENTS OF SCIENCE.

nsyal rays, the red are first extinguished. As the solai
beam plunges deeper into the sea, orange follows red,
yellow follows orange, green follows yellow, and the
various shades of blue, where the water is deep enough,
follow green. Absolute extinction of the solar beam
would be the consequence if the water were deep and
uniform. If it contained no suspended matter, such
water would be as black as ink. A reflected glimmer of
ordinary light would reach us from its surface, as it
would from the surface of actual ink; but no light,
hence no colour, would reach us from the body of the
water.

In very clear and deep sea-water this condition is
approximately fulfilled, and hence the extraordinary
darkness of such water. The indigo, already referred
to, is, I believe, to be ascribed in part to the suspended
matter, which is never absent, even in the purest natural
water ; and in part to the slight reflection of the light
from the limiting surfaces of strata of different densi-
ties. A modicum of light is thus thrown back to the
eye, before the depth necessary to absolute extinction
has been attained. An effect .precisely similar qccurs
under the moraines of glaciers. The ice here is ex-
ceptionally compact, and, owing to the absence of the
internal scattering common in bubbled ice, the light
plunges into the mass, where it is extinguished, the
perfectly clear ice presenting an appearance of pitchy
blackness.'

The green colour of the sea has now to be accounted
for; and here, again, let us fall back upon the sure
basis of experiment. A strong white dinner-plate had
a lead weight seciu-ely fastened to it. Fifty or sixty
yards of strong hempen line were attached to the plate.

' I learn from a correspondent that certain Welsh tarns, which
are reputed bottomless, have thia inky hue.

VOYAGE TO ALGEKIA. 173

My assistant, Thorogood, occupied a boat, fastened as
usual to the davits of the ' Urgent,' while I occupied a
second boat nearer the stern of the ship. He cast the
plate as a mariner heaves the lead, and by the time it
reached me it had sunk a considerable depth in the
water. In all cases the hue of this plate was green.
Even when the sea was of the darkest indigo, the green
was vivid and pronounced. I could notice the gradual
. deepening of the colour as the plate sank, but at its
greatest depth, even in indigo water, the colour was still
a blue-green.^

Other observations confirmed this one. The
' Urgent ' is a screw steamer, and right over the blades
of the screw was an orifice called the screw-well, through
which one could look from the poop down upon the
screw. The surface-glimmer, which so pesters the eye,
was here in a great measure removed. Midway down,
a plank crossed the screw-well from side to side; on
this I placed myself and observed the action of the
screw underneath. The eye was rendered sensitive by
the moderation of the light ; and, to remove still further
all disturbing causes. Lieutenant Walton had a sail and
tarpaulin thrown over the mouth of the well. Under-
neath this I perched myself on the plank and watched
the screw. In an indigo sea the play of colour was
indescribably beautiful, and the contrast between the
water, which had the screw-blades, and that which had
the bottom of the ocean, as a background, was extra-
ordinary. The one was of the most brilliant green, the
other of the deepest ultramarine. The surface of the
water above the screw-blade was always rufiSed. Liquid
lenses were thus formed, by which the coloured light
was withdrawn from some places and concentrated upon

» In no case, of course, is the green pure, but a mixture of green
and blue.

174 FKAGMENTS OF SCIENCE.

others, the water flashing with metallic lustre. The
screw-blades in this case played the part of the dinner-
plate in the former case, and there were other instances
of a similar kind. The white bellies of porpoises
showed the green hue, varying in intensity as the
creatures swung to and fro between the surface and
the deeper water. Foam, at a certain depth below the
surface, was also green. In a rough sea the light which
penetrated the summit of a wave sometimes reached the
eye, a beautiful green cap being thus placed upon the
wave, even in indigo water.

But how is this colour to be connected with the sus-
pended particles ? Thus. Take the dinner-plate which
showed so brilliant a green when thrown into indigo
water. Suppose it to diminish in size, until it reaches
an almost microscopic magnitude. It would still behave
substantially as the larger plate, sending to the eye its
modicum of green light. If the plate, instead of being
a large coherent mass, were ground to a powder suffi-
ciently fine, and in this condition diffused through the
clear sea- water, it would also send green light to the eye.
In fact, the suspended particles which the home exami-
nation reveals, act in all essential particulars like the
plate, or like the screw-blades, or like the foam, or like
the bellies of the porpoises. Thus I think the green-
ness of the sea is physically connected with the matter
which it holds in suspension.

We reached Portsmouth on January 5, 1871. Then
ended a voyage which, though its main object was not
realised, has left behind it pleasant memories, both of
the aspects of nature and the kindliness of men.

I

I

NIAGABA.^

ris one of the disadvantages of reading books about
natural scenery that they fill the mind with pictures,
often exaggerated, often distorted, often blurred, and,
even when well drawn, injurious to. the freshness of
first impressions. Such has been the fate of most of us
with regard to the Falls of Niagara. There was little
accuracy in the estimates of the first observers of the
cataract. Startled by an exhibition of power so novel
and so grand, emotion leaped beyond the control of the
judgment, and gave currency to notions which have
often led to disappointment,

A record of a voyage in 1 535 by a French mariner
named Jacques Cartier, contains, it is said, the first
printed allusion to Niagara. In 1603 the first map of
the district was constructed by a Frenchman named
Champlain. In 1648 the Jesuit Eageneau, in a letter
to his superior at Paris, mentions Niagara as ' a cataract
of frightful height.' 2 In the winter of 1678 and 1679
the cataract was visited by Father Hennepin, and
described in a book dedicated ' to the King of Great
Britain.' He gives a drawing of the waterfall, which

* A Discourse delivered at the Royal Institution of Great
Britain, April 4, 1873.

* From an interesting little book presented to me at Brooklyn
by its author, Mr. Holly, some of these data are derived : Hennepin,
Kalm, Bakewell, Lyell, Hall, and others I have myself consulted.

176 FKAGMENTS OF SCIENCE.

shows that serious changes have taken place since his
time. He describes it as ' a e^reat and prodigious
cadence of water, to which the universe does not offer a
parallel.' The height of the fall, according to Hennepin,
was more than 600 feet. ' The waters,' he says, ' which
fall from this great precipice do foam and boil in the
most astonishing manner, making a noise more terrible
than that of thunder. When the wind blows to the
south its frightful roaring may be heard for more than
fifteen leagues.' The Baron la Hontan, who visited
Niagara in 1687, makes the height 800 feet. In 1721
Charlevois, in a letter to Madame de Maintenon, after
referring to the exaggerations of his predecessors, thus
states the result of his own observations : ' For my part,
after examining it on all sides, I am inclined to think
that we cannot allow it less than 140 or 150 feet,' — a
remarkably close estimate. At that time, viz. a hundred
and fifty years ago, it had the shape of a horseshoe, and
reasons will subsequently be given for holding that this
has been always the form of the cataract, from its origin
to its present site.

As regards the noise of the fall, Charlevois declares
the accounts of his predecessors, which, I may say, are
repeated to the present hour, to be altogether extrava-
gant. He is perfectly right. The thunders of Niagai*a
are formidable enough to those who really seek them
at the base of the Horseshoe Fall ; but on the banks of
the river, and particularly above the fall, its silence,
rather than its noise, is surprising. This arises, in
part, from the lack of resonance; the surrounding
country being flat, and therefore furnishing no echoing
surfaces to reinforce the shock of the water. The
resonance from the surrounding rocks causes the Swiss
Reuss at the Devil's Bridge, when full, to thunder more
loudly than the Niagara.

fl

NIAGARA. 177

On Friday, November 1, 1872, just before reaching
the village of Niagara Falls, I caught, from the railway-
train, my first glimpse of the smoke of the cataract.
Immediately after my arrival I went with a friend to
the northern end of the American Fall. It may be
that my mood at the time toned down the impression
produced by the first aspect of this grand cascade ; but
I felt nothing like disappointment, knowing, from old
experience, that time and close acquaintanceship, the
gradual interweaving of mind and nature, must power-
fully influence my final estimate of the scene. After
dinner we crossed to Goat Island, and, turning to the
right, reached the southern end of the American Fall.
The river is here studded with small islands. Crossing
a wooden bridge to Luna Island, and clasping a tree
which grows near its edge, I looked long at the cataract,
which here shoots down the precipice like an avalanche
of foam. It grew in power and beauty. The channel
spanned by the wooden bridge was deep, and the river
there doubled over the edge of the precipice, like the
swell of a muscle, unbroken. The ledge here over-
hangs, the water being poured out far beyond the base
of the precipice. A space, called the Cave of the
Winds, is thus enclosed between the wall of rock and
the falling water.

Goat Island ends in a sheer dry precipice, which
connects the American and Horseshoe Falls. Midway
between both is a wooden hut, the residence of the guide
to the Cave of the Winds, and from the hut a winding
staircase, called Biddle's Stair, descends to the base of
the precipice. On the evening of my arrival I went
down this stair, and wandered along the bottom of the
cliff. One well-known factor in the formation and
retreat of the cataract was immediately observed. A
thick layer of limestone formed the upper portion of

178 FEAGMENTS OF SCIENCE.

the cliff. This rested upon a bed of soft shale, which
extended round the base of the cataract. The violent
recoil of the water against this yielding substance
crumbles it away, undermining the ledge above, which,
unsupported, eventually breaks off, and produces the
observed recession.

At the southern extremity of the Horseshoe is a
promontory, formed by the doubling back of the gorge
excavated by the cataract, and into which it plunges.
On the promontory stands a stone building, called the
Terrapin Tower, the door of which had been nailed up
because of the decay of the staircase within it. Through
the kindness of Mr. Townsend, the superintendent of
Goat Island, the door was opened for me. From this
tower, at all hours of the day, and at some hours of the
night, I watched and listened to the Horseshoe Fall.
The river here is evidently much deeper than the
American branch ; and instead of bursting into foam
where it quits the ledge, it bends solidly over, and falls
in a continuous layer of the most vivid green. The
tint is not uniform ; long stripes of deeper hue alter-
nating with bands of brighter colour. Close to the
ledge over which the water rolls, foam is generated, the
light falling upon which, and flashing back from it, is
sifted in its passage to and fro, and changed from white
to emerald-green. Heaps of superficial foam are also
formed at intervals along the ledge, and are imme-
diately drawn into long white striae.^ Lower down,
the surface, shaken by the reaction from below, in-
cessantly rustles into whiteness. The descent finally
resolves itself into a rhythm, the water reaching the
bottom of the fall in periodic gushes. Nor is the

* The direction of the wind with reference to the course of a
ship may be inferred with accuracy from the foam-streaks on the
surface of the sea.

II

NIAGARA. 179

spray uniformly diffused through the air, but is wafted
through it in successive veils of gauze-like texture.
From all this it is evident that beauty is not absent
from the Horseshoe Fall, but majesty is its chief
attribute. The plunge of the water is not wild, but
deliberate, vast, and fascinating. From the Terrapin
Tower, the adjacent arm of the Horseshoe is seen
projected against the opposite one, midway down ; to
the imagination, therefore, is left the picturing of the
gulf into which the cataract plunges.

The delight which natural scenery produces in some
minds is difficult to explain, and the conduct which it
prompts can hardly be fairly criticised by those who
have never experienced it. It seems to me a deduction
from the completeness of the celebrated Thomas Young,
that he was unable to appreciate natural scenery. ' He
had really,' says Dean Peacock, ' no taste for life in the
country ; he was one of those who thought that no one
who was able to live in London would be content to
live elsewhere.' Well, Dr. Young, like Dr. Johnson,
had a right to his delights ; but I can understand a
hesitation to accept them, high as they were, to the
exclusion of

That o'erflowing joy which Nature yields
To her true lovers.

To all who are of this mind, the strengthening of
desire on my part to see and know Niagara Falls, as
far as it is possible for them to be seen and known, will
a be intelligible.

K On the first evening of my visit, I met, at the head

B of Biddle's Stair, the guide to the Cave of the Winds.
I He was in the prime of manhood — large, well built,
m firm and pleasant in mouth and eye. My interest in
M the scene stirred up his, and made him communicative.

L

180 FRAGMENTS OF SCIENCE.

Turning to a photograph, he described, by reference tc
it, a feat which he had accomplished some time pre-
viously, and which had brought him almost under the
green water of the Horseshoe Fall. ' Can you lead me
there to-morrow ? ' I asked. He eyed me enquiringly,
weighing, perhaps, the chances of a man of light build,
and with grey in his whiskers, in such an undertaking.
' I wish,' I added, ' to see as much of the fall as can be
seen, and where you lead I will endeavour to follow.'
His scrutiny relaxed into a smile, and he said, ' Very
well ; I shall be ready for you to-morrow.'

On the morrow, accordingly, I came. In the hut
at the head of Biddle's Stair I stripped wholly, and
re-dressed according to instructions, — drawing on two
pairs of woollen pantaloons, three woollen jackets, two
pairs of socks, and a pair of felt shoes. Even if wet,
my guide assured me that the clothes would keep me
from being chilled ; and he was right. A suit and
hood of yellow oilcloth covered all. Most laudable pre-
cautions were taken by the young assistant who helped
to dress me to keep the water out; but his devices
broke down immediately when severely tested.

We descended the stair ; the handle of a pitchfoi'k
doing, in my case, the duty of an alpenstock. At the
bottom, the guide enquired whether we should go first
to the Cave of the Winds, or to the Horseshoe, remark-
ing that the latter would try us most. I decided on
getting the. roughest done first, and he turned to the
left over the stones. They were sharp and trying. The
base of the first portion of the cataract is covered with
huge boulders, obviously^ the ruins of the limestone
ledge above. The water does not distribute itself uni-
formly among these, but seeks out channels through
which it pours torrentially. We passed some of
these with wetted feet, but without difficulty. At

NIAGAEA. 181

length we came to the side of a more formidable
current. My guide walked along its edge until he
reached its least turbulent portion. Halting, he said,
' This is our greatest difficulty ; if we can cross here,
we shall get far towards the Horseshoe.'

He waded in. It evidently required all his strength
to steady him. The water rose above his loins, and it
foamed still higher. He had to search for footing,
amid unseen boulders, against which the torrent rose
violently. He struggled and swayed, but he struggled
successfully, and finally reached the shallower water at
the other side. Stretching out his arm, he said to me,
' Now come on.' I looked down the torrent, as it
rushed to the river below, which was seething with the
tumult of the cataract. De Saussure recommended
the inspection of Alpine dangers, with the view of
making them familiar to the eye before they are en-
countered ; and it is a wholesome custom in places of
difficulty to put the possibility of an accident clearly
before the mind, and to decide beforehand what ought
to be done should the accident occur. Thus wound up
in the present instance, I entered the water. Even
where it was not more than knee-deep, its power was
manifest. As it rose around me, I sought to split the
torrent by presenting a side to it ; but the insecurity
of the footing enabled it to grasp my loins, twist me
fairly round, and bring its impetus to bear upon my
back. Further struggle was impossible ; and feeling
my balance hopelessly gone, I turned, flung myself
towards the bank just quitted, and was instantly, as
expected, swept into shallower water.

The oilcloth covering was a great incumbrance ; it
had been made for a much stouter man, and, standing
upright after my submersion, my legs occupied the
centre of two bags of water. My guide exhorted me to

182 FRAGMENTS OF SCIENCE.

try again. Prudence wad at my elbow, whispering
dissuasion; but, taking everything into account, it
appeared more immoral to retreat than to proceed.
Instructed by the first misadventure, I once more
entered the stream. Had the alpenstock been of iron
it might have helped me ; but, as it was, the tendency
of the water to sweep it out of my hands rendered it
worse than useless. I, however, clung to it by habit.
Again the torrent rose, and again I wavered ; but, by
keeping the left hip well against it, I remained upright,
and at length grasped the hand of my leader at the
other side. He laughed pleasantly. The first victory
was gained, and he enjoyed it. ' No traveller,' he said,
' was ever here before.' Soon afterwards, by trusting
to a piece of drift-wood which seemed firm, I was again
taken off my feet, but was immediately caught by a
protruding rock.

We clambered over the boulders towards the thickest
spray, which soon became so weighty as to cause us to
stagger under its shock. For the most part nothing
could be seen ; we were in the midst of bewildering
tumult, lashed by the water, which sounded at times
like the cracking of innumerable whips. Underneath
this was the deep resonant roar of the cataract. I
tried to shield my eyes with my hands, and look up-
wards ; but the defence was useless. The guide con-
tinued to move on, but at a certain place he halted,
desiring me to take shelter in his lee, and observe
the cataract. The spray did not come so much from
the upper ledge, as from the rebound of the shattered
water when it struck the bottom. Hence the eyeai
could be protected from the blinding shock of the]
Bpray, while the line of vision to the upper ledgesl
remained to some extent clear. On looking upwards!
over the guide's shoulder I could see the water bending^]

NIAGARA. 183

over the ledge, while the Terrapin Tower loomed fitfully
through the intermittent spray-gusts. We were right
under the tower. A little farther on the cataract, after
its first plunge, hit a protuberance some way down,
and flew from it in a prodigious burst of spray ; through
this we staggered. We rounded the promontory on
which the Terrapin Tower stands, and moved, amid
the wildest commotion, along the arm of the Horse-
shoe, until the boulders failed us, and the cataract fell
into the profound gorge of the Niagara Eiver.

Here the guide sheltered me again, and desired me
to look up; I did so, and could see, as before, the
green gleam of the mighty curve sweeping over the
upper ledge, and the fitful plunge of the water, as
the spray between us and it alternately gathered and
disappeared. An eminent friend of mine often speaks
of the mistake of those physicians who regard man's
ailments as purely chemical, to be met by chemical
remedies only. He contends for the psychological
element of cure. By agreeable emotions, he says,
nervous currents are liberated which stimulate blood,
brain, and viscera. The influence rained from ladies'
eyes enables my friend to thrive on dishes which would
kill him if eaten alone. A sanative effect of the same
order I experienced amid the spray and thunder of
Niagara. Quickened by the emotions there aroused,
the blood sped exultingly through the arteries, abolish-
ing introspection, clearing the heart of all bitterness,
and enabling one to think with tolerance, if not with
tenderness, on the most relentless and unreasonable foe.
Apart from its scientific value, and purely as a moral
agent, the play was worth the candle. My companion
knew no more of me than that I enjoyed the wildness
of the scene ; but as I bent in the shelter of his large
frame he said, * I should like to see you attempting to
13

184 FEAGMENTS OF SCIENCE.

describe all this.' He rightly thought it indescribable.
The name of this gallant fellow was Thomas Conroy.

We returned, clambering at intervals up and down,
so as to catch glimpses of the most impressive portions
of the cataract. We passed under ledges formed by
tabular masses of limestone, and through some curious
openings formed by the falling together of the summits
of the rocks. At length we found ourselves beside our
enemy of the morning. Conroy halted for a minute or
two, scanning the torrent thoughtfully. I said that, as
a guide, he ought to have a rope in such a place ; but
he retorted that, as no traveller had ever thought of
coming there, he did not see the necessity of keeping
a rope. He waded in. The struggle to keep himself
erect was evident enough ; he swayed, but recovered
himself again and again. At length he slipped, gave
way, did as I had done, threw himself towards the
bank, and was swept into the shallows. Standing in
the stream near its edge, he stretched his arm towards
me. I retained the pitchfork handle, for it had been
useful among the boulders. By wading some way in,
the staff could be made to reach him, and I proposed
his seizing it. ' If you are sure,' he replied, ' that, in
case of giving way, you can maintain your grasp, then
I will certainly hold you.' Kemarking that he might
count on this, I waded in, and stretched the staff to
my companion. It was firmly grasped by both of us.
Thus helped, though its onset was strong, I moved
safely across the torrent. All .danger ended here. We
afterwards roamed sociably among the torrents and
boulders below the Cave of the Winds. The rocks
were covered with organic slime, which could not have
been walked over with bare feet, but the felt shoes
effectually prevented slipping. We reached the cave
and entered it, first by a wooden way carried over the

NIAGABA. 185

boulders, and then along a narrow ledge, to the point
eaten deepest into the shale. When the wind is from
the south, the falling water, I am told, can be seen
tranquilly from this spot ; but when we were there, a
blinding hurricane of spray was whirled against us.
On the evening of the same day, I went behind the
water on the Canada side, which, after the experiences
of the morning, struck me as an imposture.

Still even this latter is exciting to some nerves.
Its effect upon himself is thus vividly described by
Mr. Bakewell, jun. : ' On turning a sharp angle of the
rock, a sudden gust of wind met us, coming from the
hollow between the fall and the rock, which drove the
spray directly in our faces, with such force that in an
instant we were wet through. When in the midst of
this shower-bath the shock took away my breath : I
turned back and scrambled over the loose stones to
escape the conflict. The guide soon followed, and told
me that I had passed the worst part. With that
assurance I made a second attempt ; but so wild and
disordered was my imagination that when I had
reached half way I could bear it no longer.' *

To complete my knowledge I desired to see the fall
from the river below it, and long negotiations were
necessary to secure the means of doing so. The only
boat fit for the undertaking had been laid up for the
winter ; but this difficulty, through the kind interven-
tion of Mr. Townsend, was overcome. The main one
was to secure oarsmen sufficiently strong and skilful to
urge the boat where I wished it to be taken. The son
of the owner of the boat, a finely-built young fellow,
but only twenty, and therefore not sufficiently hardened,
was willing to go ; and up the river, it was stated, there
lived another man who could do anything with the
• « Mag. of Nat. Hist./ 1830, pp. 121, 122.

186 FKAGMENTS OF SCIENCB.

boat which strength and daring could accomplish. He
came. His figure and expression of face certainly
indicated extraordinary firmness and power. On Tues-
day, November 5, we started, each of us being clad in
oilcloth. The elder oarsman at once assumed a tone
of authority over his companion, and struck imme-
diately in amid the breakers below the American Fall.
He hugged the cross freshets instead of striking out
into the smoother water. I asked him why he did so,
and he replied that they were directed outwards, not
downwards. The struggle, however, to prevent the
bow of the boat from being turned by them, was often
very severe.

The spray was in general blinding, but at times it
disappeared and yielded noble views of the fall. The
edge of the cataract is crimped by indentations which
exalt its beauty. Here and there, a little below the
highest ledge, a secondary one juts out; the water
strikes it and bursts from it in huge protuberant masses
of foam and spray. We passed Groat Island, came to the
Horseshoe, and worked for a time along its base, the
boulders over which Conroy and myself had scrambled
a few days previously lying between us and the
cataract. A rock was before us, concealed and revealed
at intervals, as the waves passed over it. Our leader
tried to get above this rock, first on the outside of it.
The water, however, was here in violent motion. The
men struggled fiercely, the older one ringing out an
incessant peal of command and exhortation to the
younger. As we were just clearing the rock, the bo\
came obliquely to the surge ; the boat was turned sud^
denly round and shot with astonishing rapidity do\
the river. The men returned to the charge, noi
trying to get up between the half-concealed rock
the boulders to the left. But the torrent set in strongly

h

NIAGARA. 187

through this channel. The tugging was quick and
violent, but we made little way. At length, seizing a
rope, the principal oarsman made a desperate attempt
to get upon one of the boulders, hoping to be able to
drag the boat through the channel ; but it bumped so
violently against the rock, that the man flung himself
back and relinquished the attempt.

We returned along the base of the American P'ail,
running in and out among the currents which rushed
from it laterally into the river. Seen from below the
American Fall is certainly exquisitely beautiful, but it
is a mere frill of adornment to its nobler neighbour the
Horseshoe. At times we took to' the river, from the
centre of which the Horseshoe Fall appeared especially
magnificent. A streak of cloud across the neck of
Mont Blanc can double its apparent height, so here
the green summit of the cataract shining above the
smoke of spray appeared lifted to an extraordinary
elevation. Had Hennepin and La Hontan seen the
fall from this position, their estimates of the height
would have been perfectly excusable.

From a point a little way below the American Fall,
a ferry crosses the river, in summer, to the Canadian
side. Below the ferry is a suspension bridge for
carriages and foot-passengers, and a mile or two lower
down is the railway suspension bridge. Between ferry
and bridge the river Niagara flows unruffled ; but at
the suspension bridge the bed steepens and the river
quickens its motion. Lower down the gorge narrows,
and the rapidity and turbulence increase. At the place
called the ' Whirlpool Kapids ' I estimated the width of
the river at 300 feet, an estimate confirmed by the
dwellers on the spot. When it is remembered that the
drainage of nearly half a continent is compressed into

188 FRAGMENTS OF SCIENCE.

this space, the impetuosity of the river's rush may be
imagined. Had it not been for Mr. Bierstadt, the
distinguished photographer of Niagara, I should have
quitted the place without seeing these rapids; for
this, and for his agreeable company to the spot, I have
to thank him. From the edge of the cliflf above the
rapids, we descended, a little, I confess, to a climber's
disgust, in an ' elevator,' because the effects are best
seen from the water level.

Two kinds of motion are here obviously active, a
motion of translation and a motion of undulation — the
race of the river through its gorge, and the great waves
generated by its collision with, and rebound from, the
obstacles in its way. In . the middle of the river the
rush and tossing are most violent ; at all events, the
impetuous force of the individual waves is here most
strikingly displayed. Vast pyramidal heaps leap inces-
santly from the river, some of them with such energy
as to jerk their summits into the air, where they hang
momentarily suspended in crowds of liquid spherules.
The sun shone for a few minutes. At times the wind,
coming up the river, searched and sifted the spray, carry-
ing away the lighter drops, and leaving the heavier ones
behind. Wafted in the proper direction, rainbows
appeared and disappeared fitfully in the lighter mist.
In other directions the common gleam of the sunshine
from the waves and their shattered crests was exqui-
sitely beautiful. The complexity of the action was still
further illustrated by the fact, that in some cases, as if
by the exercise of a local, explosive force, the drops were
shot radially from a particular centre, forming around
it a kind of halo.

The first impression, and, indeed, the current ex-
planation of these rapids is, that the central bed of
the river is cumbered with large boulders, and that the

NIAGABA. 189

jostling, tossing, and wild leaping of the water there,
are due to its impact against these obstacles* I doubt
this explanation. At all events, there is another suffi-
cient reason to be taken into account. Boulders de-
rived from the adjacent cliffs visibly cumber the sides
of the river. Against these the water rises and sinks
rhythmically but violently, large waves being thus
produced. On the generation of each wave, there is
an immediate compounding of the wave-motion with
the river-motion. The ridges, which in still water
would proceed in circular curves round the centre of
disturbance, cross the river obliquely, and the result is
that at the centre waves commingle, which have really
been generated at the sides. In the first instance, we
had a composition of wave-motion with river-motion ;
here we have the coalescence of waves with waves.
Where crest and furrow cross each other, the motion is
annulled ; where furrow and furrow cross, the river is
ploughed to a greater depth ; and where crest and crest
aid each other, we have that astonishing leap of the
water which breaks the cohesion of the crests, and
tosses them shattered into the air. From the water
level the cause of the action is not so easily seen ; but
from the summit of the cliff the lateral generation of
the waves, and their propagation to the centre, are
perfectly obvious. If this explanation be correct, the
phenomena observed at the Whirlpool Rapids form one
of the grandest illustrations of the principle of inter-
ference. The Nile ' cataract,' Mr. Huxley informs me,
offers more moderate examples of the same action.

At some distance below the Whirlpool Rapids we
have the celebrated whirlpool itself. Here the river
makes a sudden bend to the north-east, forming nearly
a right angle with its previous direction. The water
strikes the concave bank with great force, and scoops it

190 FEAGMENTS OF SCIENCE.

incessantly away. A vast basin has been thus formed,
in which the sweep of the river prolongs itself in
gyratory currents. Bodies and trees which have come
over the falls, are stated to circulate here for days with-
out finding the outlet. From various points of the
cliffs above, this is curiously hidden. The rush of the
river into the whirlpool is obvious enough ; and though
you imagine the outlet must be visible, if one existed,
you cannot find it. Turning, however, round the bend
of the precipice to the north-east, the outlet comes into
view.

The Niagara season was over ; the chatter of sight-
seers had ceased, and the scene presented itself as one
of holy seclusion and beauty. I went down to the
river's edge, where the weird loneliness seemed to in-
crease. The basin is enclosed by high and almost
precipitous banks — covered, at the time, with russet
woods. A kind of mystery attaches itself to gyrating
water, due perhaps to the fact that we are to some
extent ignorant of the direction of its force. It is said
that at certain points of the whirlpool, pine-trees are
sucked down, to be ejected mysteriously elsewhere.
The water is of the brightest emerald -green. The
gorge through which it escapes is narrow, and the
motion of the river swift though silent. The surface is
steeply inclined, but it is perfectly unbroken. There
are no lateral waves, no ripples with their breaking
bubbles to raise a murmur; while the depth is here too
great to allow the inequality of the bed to ruffle the
surface. Nothing can be more beautiful than this
sloping liquid mirror formed by the Niagara, in sliding
from the whirlpool.

The green colour is, I think, correctly accounted for
in the last Fragment. While crossing the Atlantic in
1872-73 I had frequent opportunities of testing the ex-

NIAaAEA. 191

planation there given. Looked properly down upon, there
are portions of the ocean to which we should hardly ascribe
a trace of blue ; at the most, a mere hint of indigo reaches
the eye. The water, indeed, is practically black, and this
is an indication both of its depth and of its freedom from
mechanically suspended matter. In small thicknesses
water is sensibly transparent to all kinds of light ; but,
as the thickness increases, the rays of low refrangibility
are first absorbed, and after them the other rays.
Where, therefore, the water is very deep and very pure,
all the colours are absorbed, and such water ought to
appear black, as no light is sent from its interior to the
eye. The approximation of the Atlantic Ocean to this
condition is an indication of its extreme purity.

Throw a white pebble into such water ; as it sinks
it becomes greener and greener, and, before it disap-
pears, it reaches a vivid blue-green. Break such a
pebble into fragments, each of these will behave like
the unbroken mass ; grind the pebble to powder, every
particle will yield its modicum of green ; and if the
particles be so fine as to remain suspended in the water,
the scattered light will be a uniform green. Hence the
greenness of shoal water. You go to bed with the black
Atlantic arouud you. You rise in the morning, find it
a vivid green, and correctly infer that you are crossing
the bank of Newfoundland. Such water is found
charged with fine matter in a state of mechanical
suspension. The light from the bottom may sometimes
come into play, but it is not necessary. A storm can
render the water muddy, by rendering the particles too
numerous and gross. Such a case occurred towards the
close of my visit to Niagara. There had been rain and
storm in the upper lake-regions, and the quantity of
suspended matter brought down quite extinguished the
fascinating gjreen of the Horseshoe,

192 FRAGMENTS OF SCIENCE.

Nothing can be more superb than the green of the
Atlantic waves, when the circumstances are favourable
to the exhibition of the colour. As long as a wave
remains unbroken no colour appears; but when the
foam just doubles over the crest, like an Alpine snow-
cornice, under the cornice we often see a display of the
most exquisite green. It is metallic in its brilliancy.
But the foam is necessary to its production. The foam
is first illuminated, and it scatters the light in all direc-
tions; the light which passes through the higher portion
of the wave alone reaches the eye, and gives to that
portion its matchless colour. The folding of the wave,
producing as it does a series of longitudinal protuber-
ances and furrows which act like cylindrical lenses,
introduces variations in the intensity of the light, and
materially enhances its beauty.

We have now to consider the genesis and proximate
destiny of the Falls of Niagara. We may open our
way to this subject by a few preliminary remarks upon
erosion. Time and intensity are the main factors of
geologic change, 3,nd they are in a certain sense conver-
tible. A feeble force acting through long periods, and
an intense force acting through short ones, may produce
approximately the same results. To Dr. Hooker I have
been indebted for some specimens of stones, the first
examples of which were picked up by Mr. Hackworth
on the shores of Lyell's Bay, near Wellington, in New
Zealand. They were described by Mr. Travers in the
' Transactions of the New Zealand Institute.' Un-
acquainted with their origin, you would certainly ascribe
their forms to human workmanship. They resemble
knives and spear-heads, being apparently chiselled off
into facets, with as much attention to symmetry as if a
tool, guided by human intelligence, had passed over

NIAGAKA. 193

them. But no human instrument has been brought to
bear upon these stones. Thej have been wrought into
their present shape by the wind-blown sand of Lyell's
Bay. Two winds are dominant here, and they in
succession urged the sand against opposite sides of the
stone ; every little particle of sand chipped away its
infinitesimal bit of stone, and in the end sculptured these
singular forms.'

The Sphynx of Egypt is nearly covered up by the
sand of the desert. The neck of the Sphynx is partly
cut across, not, as I am assured by Mr. Huxley, by
ordinary weathering, but by the eroding action of the
fine sand blown against it. In these cases Nature
furnishes us with hints which may be taken advantage
of in art; and this action of sand has been recently
turned to extraordinary account in the United States.
When in Boston, I was taken by my courteous and help-
ful friend, Mr. Josiah Quincey, to see the action of the
sand-blast. A kind of hopper containing fine silicious

* * These stones, which have a strong resemblance to works of
human art, occur in great abundance, and of various sizes, from
half-an-inch to several inches in length. A large number were
exhibited showing the various forms, which are those of wedges,
knives, arrow-heads, &c., and all with sharp cutting edges.

*Mr. Travers explained that, notwithstanding their artificial
appearance, these stones were formed by the cutting action of the
wind-driven sand, as it passed to and fro over an exposed boulder-
bank. He gave a minute account of the manner in which the
varieties of form are produced, and referred to the effect which
the erosive action thus indicated would have on railway and other
works executed on sandy tracts.

* Dr. Hector stated that although, as a group, the specimens on
the table could not well be mistaken for artificial productions, still
the foims are so peculiar, and the edges, in a few of them, so
perfect, that if they were discovered associated with human works,
there is no doubt that tliey would have been referred to the so-
called "stone period."' — Extracted from the Minutes of the Welling-
ton Philosophical Society, February i), 1 869.

194 FEAGMENTS OF SCIENCE.

sand was connected with a reservoir of compressed air,

the pressure being variable at pleasure. The hopper
ended in a long slit, from which the sand was blown,
A plate of glass was placed beneath this slit, and caused
to pass slowly under it ; it came out perfectly depolished,
with a bright opalescent glimmer, such as could only be
produced by the most careful grinding. Every little
particle of sand urged against the glass, having all its
energy concentrated on the point of impact, formed
there a little pit, the depolished surface consisting of
innumerable hollows of this description.

But this was not all. By protecting certain portions
of the surface, and exposing others, figures and tracery
of any required form could be etched upon the glass.
The figiu-es of open iron-work could be thus copied;
while wire-gauze placed over the glass produced a reti-
culated pattern. But it required no such resisting
substance as iron to shelter the glass. The patterns of
the finest lace could be thus reproduced ; the delicate
filaments of the lace itself offering a sufficient protection.
All these effects have been obtained with a simple
model of the sand-blast devised by my assistant. A
fraction of a minute suffices to etch upon glass a rich
and beautiful lace pattern. Any yielding substance
may be employed to protect the glass. By difi'using
the shock of the particle, such substances practically
destroy the local erosive power. The hand can bear,
without inconvenience, a sand-shower which would
pulverise glass. Etchings executed on glass with suit-
able kinds of ink are accurately worked out by the sand-
blast. In fact, within certain limits, the harder the
surface, the greater is the concentration of the shock,
and the more effectual is the erosion. It is not neces-
sary that the sand should be the harder substance of
the two ; corundum, for example, is much harder thap

NIAGAEA. 195

quartz; still, quartz-sand can not only depolish, but
actually blow a bole through a plate of corundum.
Nay, glass may be depolished by the impact of fine shot;
the grains in this case bruising the glass, before they
have time to flatten and turn their energy into heat.

And here, in passing, we may tie together one or
two apparently unrelated facts. Supposing you turn
on, at the lower part of a house, a cock which is fed by
a pipe fr(^m a cistern at the top of the house, the column
of water, from the cistern downwards, is set in motion.
By turning off the cock, this motion is stopped ; and
when the turning off is very sudden, the pipe, if not
strong, may be burst by the internal impact of the
water. By distributing the turning of the cock over half a
second of time, the shock and danger of rupture maybe
entirely avoided. We have here an example of the con-
centration of energy in time. The sand-blast illustrates
the concentration of energy in space. The action of
flint and steel is an illustration of the same principle.
The heat required to generate the spark is intense ;
and the mechanical action, being moderate, must, to
produce fire, be in the highest degree concentrated.
This concentration is secured by the collision of hard
substances. Calc-spar will not supply the place of flint,
nor lead the place of steel, in the production of fire by
colli'sion. With the softer substances, the total heat
produced may be greater than with the hard ones, but,
to produce the spark, the heat must be intensely loca-
lised.

We can, however, go far beyond the mere depolishing
of glass ; indeed I have already said that quartz-sand can
wear a hole through corundum. This leads me to ex-
press my acknowledgments to General Tilghman,* who

' The absorbent power, if I may use the phrase, exerted by the
industrial arts in the United States, is forcibly illustrated by the

196 FKAGMENTS OF SCIENCE.

is the inventor of the sand-blast. To his spontaneous
kindness I am indebted for some beautiful illustrations
of his process. In one thick plate of glass a figure has
been worked out to a depth of f ths of an inch. A
second plate, Jths of an inch thick, is entirely per-
forated. In a circular plate of marble, nearly half
an inch thick, open work of most intricate and
elaborate desciiption has been executed. It would pro-
bably take many days to perform this work by any-
ordinary process ; with the sand-blast it was accom-
plished in an hour. So much for the strength of the
blast ; its delicacy is illustrated by this beautiful
example of line engraving, etched on glass by means
of the blast.

This power of erosion, so strikingly displayed when
sand is urged by air, renders us better able to conceive
its action when urged by water. The erosive power of
a river is vastly augmented by the solid matter carried
along with it. Sand or pebbles, caught in a river
vortex, can wear away the hardest rock ; ' potholes ' and
deep cylindrical shafts being thus produced. An extra-
ordinary instance of this kind of erosion is to be seen
in the Val Tournanche, above the village of this name.
The gorge at Handeck has been thus cut out. Such
waterfalls were once frequent in the valleys of Switzer-
land ; for hardly any valley is without one or more
transverse barriers of resisting material, over which the
river flowing through the valley once fell as a cataract.
Near Pontresina, in the Engadin, there is such a case ;

rapid transfer of men like Mr. Tilghman from the life of the soldier
to that of the civilian. General McClellan, now a civil engineer,
whom I had the honour of frequently meeting in New York, is a
most eminent example of the same kind. At the end of the war,
indeed, a million and a half of men were thus drawn, in an as<
tonishingly short time, from military to civil life.

NIAGARA. 197

a hard gneiss being there worn away to form a gorge,
through which the river from the Morteratsch glacier
rushes. The barrier of the Kirchet above Meyringen is
also a case in point. Behind it was a lake, derived
from the glacier of the Aar, and over the barrier the
lake poured its excess of water. Here the rock, being
limestone, was in part dissolved ; but added to this we
had the action of the sand and gravel carried along by
tlie water, which, on striking the rock, chipped it
■ away like the particles of the sand-blast. Tims, by
solution and mechanical erosion, the great chasm of the
I Finsteraarschlucht was formed. It is demonstrable
I that the water which flows at the bottoms of such deep
: fissures once flowed at the level of their present edges,
and tumbled down the lower faces of the bairiers.
Almost every valley in Switzerland furnishes examples
of this kind ; the untenable hypothesis of earthquakes,
once so readily resorted to in accounting for these
gorges, being now for the most part abandoned. To
produce the Canons of Western America, no other cause
is needed than the integration of effects individually
infinitesimal.

And now we come to Niagara. Soon after Euro-
peans had taken possession of the country, the con-
viction appears t^) have arisen that the deep channel of
the river Niagara below the falls had been excavated
by the catara,ct. In JVlr. Bakewell's ' Introduction to
Geology,' the prevalence of this belief has been referred
to. It is expressed thus by Professor Joseph Henry in
the ' Transactions of the Albany Institute : ' * 'In view-
ing the position of the falls, and the features of the
country round, it is impossible not to be impressed with
the idea that this great natural raceway has been formed

• Quoted by Bakewell.

198 FBAGMENTS OF SCIENCE.

by the continued action of the irresistible Niagara, and
that the falls, beginning at Lewiston, have, in the course
of ages, worn back the rocky strata to their present
site.' The same view is advocated by Sir Charles
Lyell, by Mr. Hall, by M. Agassiz, by Professor Kam-
say, indeed by most of those who have inspected the
place.

A connected image of the origin and progress of
the cataract is easily obtained. Walking northward
from the village of Niagara Falls by the side of the
river, we have to our left the deep and comparatively
narrow gorge, through which the Niagara flows. The
bounding cliffs of this gorge are from 300 to 350 feet
high. We reach the whirlpool, trend to the north-east,
and after a little time gradually resume our northward
course. Finally, at about seven miles from the present
falls, we come to the edge of a declivity, which informs
us that we have been hitherto walking on table-land, i
At some hundreds of feet below us is a comparatively i
level plain, which stretches to Lake Ontario. The de- 1
clivity marks the end of the precipitous gorge of the
Niagara. Here the river escapes from its steep mural
boundaries, and in a widened bed pursues its way to
the lake which finally receives its waters.

The fact that in historic times, even within the
memory of man, the fall has sensibly receded, prompts
the question. How far has this recession gone ? At
what point did the ledge which thus continually creeps
backwards begin its retrograde course ? To minds
disciplined in such researches the answer has been, and
will be — At the precipitous declivity which crossed the
Niagara from Lewiston on the American to Queenston
on the Canadian side. Over this transverse barrier the
united affluents of all the upper lakes once poured their

I

waters, and here the work of erosion began. The dam,
moreover, was demonstrably of suflScient height to cause
the river above it to submerge Goat Island ; and this
would perfectly account for the finding by Sir Charles
Lyell, Mr. Hall, and others, in the sand and gravel of
the island, the same fluviatile shells as are now found in
the Niagara Eiver higher up. It would also account for
those deposits along the sides of the river, the discovery
of which enabled Lyell, Hall, and Kamsay to reduce to
demonstration the popular belief that the Niagara
once flowed through a shallow valley.

The physics of the problem of excavation, which I
made clear to my mind before quitting Niagara, are re-
vealed by a close inspection of the present Horseshoe Fall.
We see evidently that the greatest weight of water bends
over the very apex of the Horseshoe. In a passage in
liis excellent chapter on Niagara Falls, Mr. Hall alludes
to this fact. Here we have the most copious and the
most violent whirling of the shattered liquid ; here the
most powerful eddies recoil against the shale. From
this portion of the fall, indeed, the spray sometimes
rises without solution of continuity to the region of
clouds, becoming gradually more attenuated, and passing
finally through the condition of true cloud into invisible
vapour, which is sometimes reprecipitated higher up.
All the phenomena point distinctly to the centre of
the river as the place of greatest mechanical energy,
and from the centre the vigour of the fall gradually
dies away towards the sides. The Horseshoe form, with
the concavity facing downwards, is an obvious and
necessary consequence of this action. Eight along
the middle of the river the apex of the curve pushes
its way backwards, cutting along the centre a deep
and comparatively narrow groove, and draining the

U

200 FRAGMENTS Ot SClENCfi.

sides as it passes them.* Hence the remarkable dis-
crepancy between the widths of the Niagara above and
below the Horseshoe. All along its course, from Lewis-
ton Heights to its present position, the form of the fall
was probably that of a horseshoe ; for this is merely
che expression of the greater depth, and consequently
greater excavating power, of the centre of the river.
The gorge, moreover, varies in width, as the depth of
the centre of the ancient river varied, being narrowest
where that depth was greatest.

The vast comparative erosive energy of the Horse-
shoe Fall comes strikingly into view when it and the
American Fall are compared together. The Ameri(;an
branch of the river is cut at a right angle by the
gorge of the Niagara. Here the Horseshoe Fall was
the real excavator. It cut the rock, and formed the
precipice, over which the American Fall tumbles. But
since its formation, the erosive action of the American
Fall has been almost nil, while the Horseshoe has cut
its way for 500 yards across the end of Goat Island, and
is now doubling back to excavate its channel parallel to
the length of the island. This point, which impressed
me forcibly, has not, I have just learned, escaped the
acute observation of Professor Ramsay.^ The river
bends; the Horseshoe immediately accommodates it-
self to the bending, and will follow implicitly the direc-
tion of the deepest water in the upper stream. The

• In the discourse the excavation of the centre and drainage of
the sides action was illustrated by a model devised by my assistant,
Mr. John Cottrell.

'His words are: * Where the body of water is small in the
American Fall, the edge has only receded a few yards (where most
eroded) during the time that the Canadian Fall has receded from
the north corner of Goat Island to the innermost curve of the
Horseshoe Fall.' — Qu<iHerly Journal of Geological Society, May
1859.

NIAGARA. 201

flexures of the gorge are determined by those of the river
channel above it. Were the Niagara centre above the fall
sinuous, the gorge would obediently follow its sinuosities.
Once suggested, no doubt geographers will be able to
point out many examples of this action. The Zambesi
is thought to present a great difficulty to the erosion
theory, because of the sinuosity of the chasm below the
Victoria Falls. But, assuming the basalt to be of toler-
ably uniform texture, had the river been examined
before the formation of this sinuous channel, the present
zigzag course of the gorge below the fall could, I am
persuaded, have been predicted, while the sounding of
the present river would enable us to predict the course
to be pursued by the erosion in the future.

But not only has the Niagara Eiver cut the gorge ;
it has carried away the chips of its o^vn workshop. The
shale, being probably crumbled, is easily carried away.
But at the base of the fall we find the huge boulders
already described, and by some means or other these are
removed down*the river. The ice which fills the gorge
in winter, and which grapples with the boulders, has
been regarded as the transporting agent. Probably it
is so to some extent. But erosion acts without ceasing
on the abutting points of the boulder-s, thus withdrawing
their support and urging them gradually down the
river. Solution also does its portion of the work.
That solid matter is carried down is proved by the differ-
ence of depth between the Niagara Eiver and Lake
Ontario, where the river enters it. The depth falls from
72 feet to 20 feet, in consequence of the deposition of
solid matter caused by the diminished motion of the
river.*

* Near the mouth of the gorge at Queenston, the depth, ac-
cording to the Admiralty Chart, is 180 feet; well within the gorge
it U 132 feet.

202

FRAGMENTS OF SCIENCE.

NIAGABA. 203

* The annexed highly instructive map has been re-
duced from one published in Mr. Hall's ' Geology of
New York.' It is based on surveys executed in 1 842, by
Messrs. Gibson and Evershed. The ragged edge of the
American Fall north of Goat Island marks the amount
of erosion which it has been able to accomplish, while
the Horseshoe Fall was cutting its way southward across
the end of Goat Island to its present position. The
American Fall is 168 feet high, a precipice cut down,
not by itself, but by the Horseshoe Fall. The latter in
1842 was 159 feet high, and, as shown by the map, is
already turning eastward, to excavate its gorge along
the centre of the upper river, p is the apex of the
Horseshoe, and t marks the site of the Terrapin Tower,
with the promontory adjacent, round which I was con-
ducted by Conroy Probably since 1842 the Horse-
shoe has worked back beyond the position here assigned
to it.

In conclusion, we may say a word regarding the
proximate future of Niagara. At the rate of excavation
assigned to it by Sir Charles Lyell, namely, a foot a year,
five thousand years or so will carry the Horseshoe Fall
far higher than Goat Island. As the gorge recedes it
will drain, as it has hitherto done, the banks right and
left of it, thus leaving a nearly level terrace between
Goat Island and the edge of the gorge. Higher up it
will totally drain the American branch of the river ; the
channel of which in due time will become cultivable
land. The American Fall will then be transformed into
a dry precipice, forming a simple continuation of the
cliffy boundary of the Niagara gorge. At the place
occupied by the fail at this moment we shall have the
gorge enclosing a right angle, a second whirlpool being
the consequence. To those who visit Niagara a few
millenniums hence I leave the verification of this pre-

204 FRAGMENTS OF SCIENCE.

diction. All that can be said is, that if the causes
now in action continue to act, it will prove itself liter-
ally true.

Postscript.

A year or bo after I had quitted the United States,
a man sixty years of age, while engaged in painting one
of the bridges which connect Goat Island with the Three
Sisters, slipped through the rails of the bridge into the
rapids, and was carried impetuously towards the Horse-
shoe FaU. He was urged against a rock which rose
above the water, and with the grasp of desperation he
clung to it. The population of the village of Niagara .
Falls was soon upon the island, and ropes were brought,
but there was none to use them. In the midst of the
excitement, a tall powerful young fellow was observed
making his way silently through the crowd. He reached
a rope ; selected from the bystanders a number of men,
and placed one end of the rope in their hands. The
other end he fastened round himself, and choosing a
point considerably above that to which the man clung,
he plunged into the rapids. He was carried violently
downwards, but he caught the rock, secured the old
painter and saved him. Newspapers from all parts of
the Union poured in upon me, describing this gallant
act of my guide Conroy,

vni.

THE PARALLEL ROADS OF GLEN ROY^

The first published allusion to the Parallel Roads of
Grlen Roy occurs in the appendix to the third volume of
Pennant's ' Tour in Scotland,' a work published in 1776.
* In the face of these hills,' says this writer, ' both sides
of the glen, there are three roads at small distances
from each other and directly opposite on each side.
These roads have been measured in the complete parts
of them, and found to be 26 paces of a man 5 feet 10
inches high. The two highest are pretty near each
other, about 50 yards, and the lowest double that
distance from the nearest to it. They are carried along
the sides of the glen with the utmost regularity, nearly
as exact as drawn with a line of rule and compass.'

The correct heights of the three roads of Grlen Roy
are respectively 1150, 1070, and 860 feet above the
sea. Hence a vertical distance of 80 feet separates the
two highest, while the lowest road is 210 feet below the
middle one.

These ' roads ' are usually shelves or terraces formed
in the yielding drift which here covers the slopes of the
mountains. They are all sensibly horizontal and there-
fore parallel. Pennant accepted as reasonable the
explanation of them given by the country people in his

• A discourse delivered at the Boyal Institution of Great Britain
on June 9, 1876.

206 FRAGMENTS OF SCIENCE.

time. They thought that the roads * were designed for
the chase, and that the terraces were made after the
spots were cleared in lines from wood, in order to tempt
the animals into the open paths after they were rouzed,
in order that they might come within reach of the
bowmen who might conceal themselves in the woods
above and below.'

In these attempts of ' the country people ' we have
an illustration of that impulse to which all scientific
knowledge is due — the desire to know the causes of
things ; and it is a matter of surprise that in the case of
the parallel roads, with their weird appearance chal-
lenging enquiry, this impulse did not make itself more
rapidly and energetically felt. Their remoteness may
perhaps account for the fact that until the year 1817
no systematic description of them, and no scientific
attempt at an explanation of them, appeared. In that
year Dr. MacCulloch, who was then President of the
Geological Society, presented to that Society a memoir,
in which the roads were discussed, and pronoimced to
be the margins of lakes once embosomed in Grlen Roy.
Why there should be three roads, or why the lakes
should stand at these particular levels, was left unex-
plained.

To Dr. MacCulloch succeeded a man, possibly not so
learned as a geologist, but obviously fitted by nature to
grapple with her facts and to put them in their
proper setting. I refer to Sir Thomas Dick-Lauder,
who presented to the Royal Society of Edinburgh, on
the 2nd of March, 1818, his paper on the Parallel Roads
of Glen Roy. In looking over the literature of this
subject, which is now copious, it is interesting to observe
the differentiation of minds, and to single out those
who went by a kind of instinct to the core of the ques-
tion, from those who erred in it, or who learnedly

THE PARALLEL ROADS OF GLEN ROY. 207

occupied fchemselves with its analogies, adjuncts, and
details. There is no man, in my opinion, connected
with the history of the subject, who has shown, in
relation to it, this spirit of penetration, this force of
scientific insight, more conspicuously than Sir Thomas
Dick-Lauder. Two distinct mental processes are in-
' volved in the treatment of such a question. Firstly,
the faithful and suflScient observation of the data ; and
secondly, that higher mental process in which the con-
structive imagination comes into play, connecting the
separate facts of observation with their common cause,
and weaving them into an organic whole. In neither
of these requirements did Sir Thomas Dick-Lauder fail.

Adjacent to Grien Eoy is a valley called Glen Gluoy,
along the sides of which ran a single shelf, or terrace,
formed obviously in the same manner as the parallel
roads of Grlen Roy. The two shelves on the opposing
sides of the glen were at precisely the same level, and
Dick-Lauder wished to see whether, and how, they
became united at the head of the glen. He followed
the shelves into the recesses of the mountains. The
bottom of the valley, as it rose, came ever nearer to
them, until finally, at the head of Glen Gluoy, he
reached a col, or watershed, of precisely the same
elevation as the road which swept round the glen.

The correct height of this col is 1170 feet above
the sea ; that is to say, 20 feet above the highest road
in Glen Roy.

jp From this col a lateral branch-valley — Glen Turrit
— led down to Glen Roy. Our explorer descended from
the col to the highest road of the latter glen, and
pursued it exactly as he had pursued the road in Glen
Gluoy. For a time it belted the mountain sides at a
considerable height above the bottom of the valley ; but
this rose as he proceeded, coming ever nearer to the

208 FRAGMENTS OF SCIENCE.

highest shelf, until finally he reached a col, or water-
shed, looking into Grlen Spey, and of precisely the same
elevation as the highest road of Glen Koy.

PARALLEL ROADS OF GLEN ROY. j

After a Sketch by Sir Thomas Dick-Lauder. <

He then dropped down to the lowest of these roads, '
and followed it towards the mouth of the glen. Its :
elevation above the bottom of the valley gradually
increased ; not because the shelf rose, but because it
remained level while the valley sloped downwards. He
found this lowest road doubling round the hills at the
mouth of Glen Roy, and running along the sides of the
mountains which flank Glen Spean. He followed it
eastwards. The bottom of the Spean Valley, like the
others, gradually rose, and therefore gradually ap-
proached the road on the adjacent mountain-side. He
came to Loch Laggan, the surface of which rose almost

%

THE PARALLEL ROADS OF GLEN ROY. 209

to the level of the road, and beyond the head of this
lake ne found, as in the other two cases, a col, or water-
shed, at Makul, of exactly the same level as the single
road in Glen Spean, which, it will be remembered, is a
continuation of the lowest road in Glen Roy.

Here we have a series of facts of obvious significance
as regards the solution of this problem. The effort of
the mind to form a coherent image from such facts
may be compared with the effort of the eyes to cause
the pictures of a stereoscope to coalesce. For a time
we exercise a certain strain, the object remaining vague
and indistinct. Suddenly its various parts seem to run
together, the object starting forth in clear and definite
relief. Such, I take it, was the effect of his ponderings
upon the mind of Sir Thomas Dick-Lauder. His
solution was this : Taking all their features into account,
he was convinced that water only could have produced
the terraces. But how had the water been collected ?
He saw clearly that, supposing the mouth of Glen Gluoy
to be stopped by a barrier sufficiently liigh, if the
waters from the mountains flanking the glen were
allowed to collect, they would form behind the barrier
a lake, the surface of which would gradually rise until
it reached the level of the col at the head of the glen.
The rising would then cease ; the superfluous water of
Glen Gluoy discharging itself over the col into Glen
Roy. As long as the barrier stopping the mouth of
Glen Gluoy continued high enough, we should have in
that glen a lake at the precise level of its shelf, which
lake, acting upon the loose drift of the flanking moun-
tains, would form the shelf revealed by observation.

So much for Glen Gluoy. But suppose the mouth
of Glen Roy also stopped by a similar barrier. Behind
it also the water from the adjacent mountains would
collect. The surface of the lake thus formed would

210 FKAGMENTS OF SCIENCE.

gradually rise, until it had reached the level of the
col which divides Glen Eoy fiom Glen Spey. Here
the rising of the lake would cease ; its superabundant
water being poured over the col into the valley of the
Spey. This state of things would continue as long as
a sufficiently high barrier remained at the mouth of
Glen Eoy. The lake thus dammed in, with its surface
at the level of the highest parallel road, would act, as in
Glen Gluoy, upon the friable drift overspreading the
mountains, and would form the highest road or terrace
of Glen Eoy.

And now let us suppose the barrier to be so far
removed from the mouth of Glen Eoy as to establish a
connection between it and the upper part of Glen Spean,
while the lower part of the latter glen still continued
to be blocked up. Upper Glen Spean and Glen Eoy
would then be occupied by a continuous lake, the level
of which would obviously be determined by the col at
the head of Loch Laggan. The water in- Glen Eoy
would sink from the level it had previously maintained,
to the level of its new place of escape. This new lake-
surface would correspond exactly with the lowest parallel
road, and it would form that road by its action upon the
drift of the adjacent mountains.

In presence of the observed facts, this solution com-
mends itself strongly to the scientific mind. The
question next occurs, What was the character of the
assumed barrier which stopped the glens ? There are
at the present moment vast masses of detritus in certain
portions of Glen Spean, and of such detritus Sir
Thomas Dick-Lauder imagined his barriers to have
been formed. By some unknown convulsion, this
detritus had been heaped up. But, once given, and
once granted that it was subsequently removed in the
manner indicated, the single road of Glen Gluoy and

THE PAKALLEL ROADS OF GLEN BOY. 211

•
the highest and lowest roads of Grlen Eoy would be ex-
plained in a satisfactory manner.

To account for the second or middle road of Glen
Roy, Sir Thomas Dick-Lauder invoked a new agency.
He supposed that at a certain point in the breaking
down or waste of his dam, a halt occurred, the barrier
holding its ground at a particular level sufficiently long
to dam a lake rising to the height of, and forming the
second road. This point of weakness was at once de-
tected by Mr. Darwin, and adduced by him as proving
that the levels of the cols did not constitute an essential
feature in the phenomena of the parallel roads. Though
not destroyed. Sir Thomas Dick-Lauder's theory was
seriously shaken by this argument, and it became a
point of capital importance, if the facts permitted, to
remove such source of weakness. This was done in
1847 by Mr. David Milne, now Mr. Milne-Home. On
walking up Grlen Roy from Roy Bridge, we pass the
mouth of a lateral glen, called Glen Glaster, running
eastward from Glen Roy. There is nothing in this
lateral glen to attract attention, or to suggest that it
could have any conspicuous influence in the production
of the parallel roads. Hence, probably, the failure of
Sir Thomas Dick-Lauder to notice it. But Mr. Milne-
Home entered this glen, on the northern side of which
the middle and lowest roads are fairly shown. The
principal stream running through the glen turns at a
certain point northwards and loses itself among hills
too high to offer any outlet. But another branch of
the glen turns to the south-east ; and, following up
this branch, Mr. Milne- Home reached a col, or water-
shed, of the precise level of the second Glen Roy road.
When the barrier blocking the glens had been so far
removed as to open this col, the water in Glen Roy
would sink to the level of the second road. A new

2i2 FRAGMENTS OF SCIENCE.

lake of diminished depth would be thus formed, the
surplus water of which would escape over the Glen
Glaster col into Glen Spean. The margin of this new
lake, acting upon the detrital matter, would form the
second road. The theory of Sir Thomas Dick-Lauder,
as regards the part played by the cols, was re-riveted
by this new and imexpected discovery.

I have referred to Mr. Darwin, whose powerful
mind swayed for a time the convictions of the scientific
world in relation to this question. His notion was —
and it is a notion which very naturally presents itself —
that the parallel roads were formed by the sea ; that
this whole region was once submerged and subsequently
upheaved ; that there were pauses in the process of up-
heaval, during which these glens constituted so many
fiords, on the sides of which the parallel terraces were
formed. This theory will not bear close criticism ; nor
is it now maintained by Mr. Darwin himself. It would
not account for the sea being 20 feet higher in Glen
Gluoy than in Glen Roy. It would not account for the
absence of the second and third Glen Eoy roads from
Glen Gluoy, where the mountain flanks are quite as im-
pressionable as in Glen Roy. It would not account for
the absence of the shelves from the other mountains in
the neighbourhood, all of which would have been
clasped by the sea had the sea been there. Here then,
and no doubt elsewhere, Mr. Darwin has shown himself
to be fallible ; but here, as elsewhere, he has shown
himself equal to that discipline of surrender to evidence
which girds his intellect with such unassailable moral
strength.

But, granting the significance of Sir Thomas Dick-
Lauder's facts, and the reasonableness, on the whole, of
the views which he has founded on them, they will not
bear examination in detail. No such barriers of

THE PARALLEL ROADS OF GLEN ROY. 21b

detritus as he assumed could have existed without
leaving traces behind them ; but there is no trace left.
There is detritus enough in Grlen Spean, but not where
it is wanted. The two highest parallel roads stop
abruptly at different points near the mouth of Grlen
Eoy, but no remnant of the barrier against which they
abutted is to be seen. It might be urged that the sub-
sequent invasion of the valley by glaciers has swept
the detritus away ; but there have been no glaciers in
these valleys since the disappearance of the lakes.
Professor Greikie has favoured me with a drawing of
the Grlen Spean ' road ' near the entrance to Glen
Trieg. The road forms a shelf round a great mound
of detritus which, had a glacier followed the formation
of the shelf, must have been cleared away. Taking all
the circumstances into account, you may, I think, with
safety dismiss the detrital barrier as incompetent to
account for the present condition of Glen Gluoy and
Glen Roy.

Hypotheses in science, though apparently trans-
cending experience, are in reality experience modified
by scientific thought and pushed into an ultra experien-
tial region. At the time that he wrote. Sir Thomas
Dick-Lauder could not possibly have discerned the
cause subsequently assigned for the blockage of these
glens. A knowledge of the action of ancient glaciers
was the necessary antecedent to the new explanation,
and experience of this nature was not possessed by the
distinguished writer just mentioned. The extension of
Swiss glaciers far beyond their present limits, was first
made known by a Swiss engineer named Venetz, who
established, by the marks they had left behind them,
their former existence in places which they had long
forsaken. The subject of glacier extension was subse^
quently followed up with distinguished success by

214 FRAGMENTS OF SCIENCE.

Charpentier, Studer, and others. With characteristic
vigour Agassiz grappled with it, extending his obser-
vations far beyond the domain of Switzerland. He
came to this country in 1840, and found in various
places indubitable marks of ancient glacier action.
England, Scotland, Wales, and Ireland he proved to
have once given birth to glaciers. He visited Grlen
Koy, surveyed the surrounding neighbourhood, and
pronounced, as a consequence of his investigation, the
barriers which stopped the glens and produced the
parallel roads to have been barriers of ice. To Mr.
Jamieson, above all others, we are indebted for the
thorough testing and confirmation of this theory.

And let me here say that Agassiz is only too likely
to be misrated and misjudged by those who, though
accurate within a limited sphere, fail to grasp in their
totality the motive powers invoked in scientific inves-
tigation. True he lacked mechanical precision, but he
abounded in that force and freshness of the scientific
imagination which in some sciences, and probably in
some stages of aU sciences, are essential to the creator
of knowledge. To Agassiz was given, not the art of
the refiner, but the instinct of the discoverer, and the
strength of the delver who brings ore from the recesses
of the mine. That ore may contain its share of dross,
but it also contains the precious metal which gives
employment to the refiner, and without which his
occupation would depart.

Let us dwell for a moment upon this subject of
ancient glaciers. Under a flask containing water, in
which a thermometer is immersed, is placed a Bunsen's
lamp. The water is heated, reaches a temperature of
212% and then begins to boil. The rise of the ther-
mometer then ceases, although heat continues to be
poured by the lamp into the water. What becomes of

THE PAKALLEL ROADS OF GLEN ROY. 215

that heat ? We know that it is consumed in the mo-
lecular work of vaporization. In the experiment here
arranged, the steam passes from the flask through a
tube into a second vessel kept at a low temperature.
Here it is condensed, and indeed congealed to ice, the
second vessel being plunged in a mixture cold enough
to freeze the water. As a result of the process we
obtain a mass of ice. That ice has an origin very
antithetical to its own character. Though cold, it is
the child of heat. If we removed the lamp, there
would be no steam, and if there were no steam there
would be no ice. The mere cold of the mixture sur-
rounding the second vessel would not produce ice. The
cold must have the proper material to work upon ; and
this material — aqueous vapour — is, as we here see, the
direct product of heat.

It is now, I suppose, fifteen or sixteen years since I
found myself conversing with an illustrious philosopher
regarding that glacial epoch which the researches of
Agassiz and others had revealed. This profoundly
thoughtful man maintained the fixed opinion that, at a
certain stage in the history of the solar system, the
sun's radiation had suffered diminution, the glacial
epoch being a consequence of this solar chill. The
celebrated French mathematician Poisson had another
theory. Astronomers have shown that the solar system
moves through space, and ' the temperature of space ' is
a familiar expression with scientific men. It was con-
sidered probable by Poisson that our system, during its
motion, had traversed portions of space of different
temperatures ; and that, during its passage through
one of the colder regions of the universe, the glacial
epoch occurred. Notions such as these were more or
less current everywhere not many years ago, and I
therefore thought it worth while to show how incom-

16

216 FRAGMENTS OF SCIENCE.

plete they were. Suppose the temperature of our
planet to be reduced, by the subsidence of solar heat,
the cold of space, or any other cause, say one hundred
degrees. Four-and-twenty hours of such a chill would
bring down as snow nearly all the moisture of our
atmosphere. But this would not produce a glacial
epoch. Such an epoch would require the long-continued
generation of the material from which the ice of glaciers
is derived. Mountain snow, the nutriment of glaciers,
is derived from aqueous vapour raised mainly from the
tropical ocean by the sun. The solar fire is as neces-
sary a factor in the process as our lamp in the experi-
ment referred to a moment ago. Nothing is easier
than to calculate the exact amount of heat expended
by the sun in the production of a glacier. It would,
as I have elsewhere shown,^ raise a quantity of cast
iron five times the weight of the glacier not only to a
white heat, but to its point of fusion. If, as I have
already urged, instead of being filled with ice, the
valleys of the Alps were filled with white-hot metal, of
quintuple the mass of the present glaciers, it is the
heat, and not the cold, that would arrest our attention
and solicit our explanation. The process of glacier
making is obviously one of distillation, in which the
fire of the sun, which generates the vapour, plays as
essential a part as the cold of the mountains which
condenses it.^

It was their ascription to glacier action that first

* * Heat a Mode of Motion,' fifth edition, chap. vi. : Forms of
Water, §§55 and 56.

' In Lyell's excellent * Principles of Geology,' the remark occurs
that * several writers have fallen into the strange error of supposing
that the glacial period must have been one of higher mean tempe-
rature than usual.' The really strange error was the forgetfulness
of the fact that without the heat the substance necessary to the
production of glaciers would be wanting.

THE PARALLEL EOADS OF GLEN ROY. 217

gave the parallel roads of Glen Key an interest in my
eyes; and in 1867, with a view to self-instruction, 1
made a solitary pilgrimage to the place, and explored
pretty thoroughly the roads of the principal glen. I
traced the highest road to the col dividing Glen Eoy
from Glen Spey, and, thanks to the civility of an
Ordnance surveyor, I was enabled to inspect some of
the roads with a theodolite, and to satisfy myself re-
garding the common level of the shelves at opposite
sides of the valley. As stated by Pennant, the width
of the roads amounts sometimes to more than twenty
yards ; but near the head of Glen Roy the highest road
ceases to have any width, for it runs along the face of
a rock, the effect of the lapping of the water on the
more friable portions of the rock being perfectly
distinct to this hour. My knowledge of the region
was, however, far from complete, and nine years had
dimmed the memory even of the portion which had been
thoroughly examined. Hence my desire to see the
roads once more before venturing to talk to you about
them. The Easter holidays of 1876 were to be devoted
to this purpose; but at the last moment a telegram
from Roy Bridge informed me that the roads were
snowed up. Finding books and memories poor substi-
tutes for the flavour of facts, I resolved subsequently
to make another effort to see the roads. Accordingly
last Thursday fortnight, after lecturing here, I packed
up, and started (not this time alone) for the North.
Next day at noon my wife and I found ourselves at
Dalwhinnie, whence a drive of some five-and- thirty miles
brought us to the excellent hostelry of Mr. Macintosh,
at the mouth of Glen Roy.

We might have found the hills covered with mist,
which would have wholly defeated us ; but Nature was
good-natured, and we had two successful working dayi

218 FRAGMENTS OF SCIENCE.

among the hills. Guided by the excellent ordnance map
of the region, on the Saturday morning we went up the
glen, and on reaching the stream called Allt Bhreac
Achaidh faced the hills to the west. At the watershed
between Grlen Roy and Glen Fintaig we bore northwards,
struck the ridge above Glen Gluoy, came in view of its
road, which we persistently followed as long as it con-
tinued visible. It is a feature of all the roads that they
vanish before reaching the cols over which fell the waters
of the lakes which formed them. One reason doubtless
is that at their upper ends the lakes were shallow, and
incompetent on this accoimt to raise wavelets of any
strength to act upon the mountain drift. A second
reason is that they were land-locked in the higher
portions and protected from the south-westerly winds,
the stillness of their waters causing them to produce but
a feeble impression upon the mountain sides. From
Glen Gluoy we passed down Glen Turrit to Glen
Roy, and through it homewards, thus accomplishing
two or three and twenty miles of rough and honest
work.

Next day we thoroughly explored Glen Glaster,
following its two roads as far as they were visible. We
reached the col discovered by Mr. Milne-Home, which
stands at the level of the middle road of Glen Roy.
Thence we crossed southwards over the mountain Creag
Dhubh, and examined the erratic blocks upon its sides,
and the ridges and mounds of moraine matter which
cumber the lower flanks of the mountain. The obser-
vations of Mr. Jamieson upon this region, including the
mouth of Glen Trieg, are in the highest degree interest-
ing. We entered Glen Spean, and continued a search
begun on the evening of our arrival at Roy Bridge — •
the search, namely, for glacier polishings and markings.
We did not find them copious, but they are indubitable.

THE PARALLEL ROADS OF GLEN ROY. 219

One of the proofs most convenient for reference, is a
great rounded rock by the roadside, 1,000 yards east of
the milestone marked three-quarters of a mile from
Eoy Bridge. P^rther east other cases occur, and they
leave no doubt upon the mind that Glen Spean was at
one time filled by a great glacier. To the disciplined
eye the aspect of the mountains is perfectly conclusive
on this point ; and in no position can the observer more
readily and thoroughly convince himself of tliis than at
the head of Glen Glaster. The dominant hills here are
all intensely glaciated.

But the great collecting ground of the glaciers
which dammed the glens and produced the parallel
roads, were the mountains south and west of Glen
Spean. The monarch of these is Ben Nevis, 4,370 feet
high. The position of Ben Nevis and his colleagues, in
reference to the vapour-laden winds of the Atlantic, is
a point of the first importance. It is exactly similar to
that of Carrantual and the Macgillicuddy Keeks in the
south west of Ireland. These mountains are, and were,
the first to encounter the south-western Atlantic winds,
and the precipitation, even at present, in the neighbour-
hood of Killarney, is enormous. The winds, robbed of
their vapour, and charged with the heat set free by its
precipitation, pursue their direction obliquely across
Ireland ; and the effect of the drying process may be
understood by comparing the rainfall at Cahirciveen
with that at Portarlington. As found by Dr. Lloyd,
the ratio is as 59 to 21 — fifty-nine inches annually at
Cahirciveen to twenty-one at Portarlington. During
the glacial epoch this vapour fell as snow, and the con-
sequence was a system of glaciers which have left traces
and evidences of the most impressive character in the
region of the Killarney Lakes. I have referred in other
places to the great glacier which, descending from the

220 FKAGMENTS OF SCIENCE.

Reeks, moved through the Black Valley, took posses*
sion of the lake-basins, and left its traces on every rock
and island emergent from the waters of the upper lake
They are all conspicuously glaciated. Not in Switze:
land itself do we find clearer traces of ancient glaciei
action.

What the Macgillicuddy Reeks did in Ireland, Be:
Nevis and the adjacent mountains did, and continue
do, in Scotland. We had an example of this on the
morning we quitted Roy Bridge. From the bridge
westward rain fell copiously, and the roads were wet ;
but the precipitation ceased near Loch Laggan, whence
eastward the roads were dry. Measured by the gauge,
the rainfall at P'ort William is 86 inches, while at
Laggan it is only 46 inches annually. The difference
between west and east is forcibly brought out by obser-
vations at the two ends of the Caledonian Canal. Fort
William at the south-western end has, as just stated,
86 inches, while Culloden, at its north-eastern end, has
only 24. To the researches of that able and accom-
plished meteorologist, Mr. Buchan, we are indebted for
these and other data of the most interesting am
valuable kind.

Adhering to the facts now presented to us, it is n
difficult to restore in idea the process by which the
glaciers of Lochaber were produced and the glens
dammed by ice. When the cold of the glacial epoch
began to invade the Scottish hills, the sun at the same
time acting with sufficient power upon the tropical
ocean, the vapours raised and drifted on to these
northern mountains were more and more converted
into snow. This slid down the slopes, and from every
valley, strath, and corry, south of Glen Spean, glaciers
were poured into that glen. The two great factors
here brought into play are the nutrition of the glaciers

i

THE PAEALLEL EOADS OF GLEN ROY. 221

by the frozen material above, and their consumption in
the milder air below. For a period supply exceeded
consumption, and the ice extended, filling Grien Spean
to an ever-increasing height, and ab'itting against the
mountains to the north of that glen. But why, it may
be asked, should the valleys south of Glen Spean be
receptacles of ice at a time when those north of it were
receptacles of water? The answer is to be found in
the position and the greater elevation of the mountains
south of Glen Spean. They first receive 1 the loads of
moisture carried by the Atlantic winds, ard not until
they had been in part dried, and also warmed by the
liberation of their latent heat, did these winds touch
the hills north of the Glen.

An instructive observation bearing upon this point
is here to be noted. Had our visit been in th»» winter
we should have found all the mountains covered ; had
it been in the summer we should have found the ^now
all gone. But happily it was at a season when hhe
aspect of the mountains north and south of Glen Spean
exhibited their relative powers as snow collectors.
Scanning the former hills from many points of view»
we were hardly able to detect a fleck of snow, while
heavy swaths and patches loaded the latter. Were the
glacial epoch to return, the relation indicated by this
observation would cause Glen Spean to be filled with
glaciers from the south, while the hills and valleys on
the noith, visited by warmer and drier winds, would
remain comparatively free from ice. This flow from
the south would be reinforced from the west, and as
long as the supply was in excess of the consumption
the glaciers would extend, the dams which closed the
glens increasing in height. By-and-by supply and con-
sumption becoming approximately equal, the height of
the glacier barriers would remain constant. Then, if

222 FRAGMENTS OF SCIENCE.

milder weathei set in, consumption would be in excess,
a lowering of the barriers and a retreat of the ice being
the consequence. But for a long time the conflict
between supply and consumption would continue, re-
tarding indefinitely the disappearance of the barriers,
and keeping the imprisoned lakes in the northern
glens. But however slow its retreat, the ice in the
long run would be forced to yield. The dam at the
mouth of Glen Roy, which probably entered the glen
sufficiently far to block up Glen Glaster, would
gradually retreat. Glen Glaster and its col being
opened, the subsidence of the lake eighty feet, from
the level of the highest to that of the second parallel
road, would follow as a consequence. I think this the
most probable course of things, but it is also possible
that Glen Glaster may have been blocked by a glacier
from Glen Trieg. The ice dam continuing to retreat,
at length permitted Glen Koy to connect itself with
upper Glen Spean. A continuous lake then filled both
glens, the level of which, as already explained, was
determined by the col at Makul, above the head of
Loch Laggan. The last to yield was the portion of the
glacier which derived nutrition from Ben Nevis, and
probably also from the mountains north and south of
Loch Arkaig. But it at length yielded, and the waters
in the glens resumed the courses which they piu*sue
to-day.

For the removal of the ice barriers no cataclysm is
to be invoked; the gradual melting of the dam would
produce the entire series of phenomena. In sinking
from col to col the water would flow over a gradually
melting barrier, the surface of the imprisoned lake not
remaining sufficiently long at any particular level to
produce a shelf comparable to the parallel roads. By
temporary halts in the process of melting due to atmo-

TUE PAEALLEL ROADS OF GLEN ROY. 223

spheric conditions or to the character of the dam itself,
or through local softness in the drift, small pseudo-
terraces would be formed, which, to the perplexity of
some observers, are seen upon the flanks of the glens
to-day.

In presence then of the fact that the barriers which
stopped these glens to a height, it may be, of 1,500
feet alcove the bottom of Glen Spean, have dissolved
and left not a wreck behind ; in presence of the fact,
insisted on by Professor Geikie, that barriers of detritus
would undoubtedly have been able to maintain them-
selves had they ever been there ; in presence of the fact
that great glaciers once most certainly filled these
valleys — that the whole region, as proved by Mr.
Jamieson, is filled with the traces of their action ; the
theory which ascribes the parallel roads to lakes
dammed by barriers of ice has, in my opinion, a
degree of probability on its side which amounts to a
practical demonstration of its truth.

Into the details of the terrace formation I do not
enter. Mr. Darwin and Mr. Jamieson on the one side,
and Sir John Lubbock on the other, deal with true
causes. The terraces, no doubt, are due in part to the
descending drift arrested by the water, and in part to
the fretting of the wavelets, and the rearrangement of
the stirred detritus, along the belts of contact of lake
and hill. The descent of matter must have been
frequent when the drift was unbound by the rootlets
which hold it together now. In some cases, it may be
remarked, the visibility of the roads is materially aug-
mented by differences of vegetation. The grass upon
the terraces is not always of the same character as that
above and below them, wliile on heather-covered hilla
the absence of the dark shrub from the roads greatly
enhances their conspicuousness.

224 FRAGMENTS OF SCIENCE.

The annexed sketch of a model (p. 225) will enable
the reader to grasp the essential features of the problem
and its solution. Glen Gluoy and Glen Koy are lateral
valleys which open into Glen Spean. Let us suppose
Glen Spean filled from y to w with ice of a uniform
elevation of 1,500 feet above the sea, the ice not
filling the upper part of that glen. The ice would
thrust itself for some distance up the lateral valleys,
closing all their mouths. The streams from the moun-
tains right and left of Glen Gluoy would pour their
waters into that glen, forming a lake, the level of which
would be determined by the height of the col at a, 1170
feet above the sea. Over this col the water would flow
into Glen Eoy. But in Glen Roy it could not rise higher
than 1150 feet, the height of the col at B, over which it
would flow into Glen Spey.

The water halting at these levels for a sufficient
time, would form the single road in Glen Gluoy and the
highest road in Glen Roy. This state of things would
continue as long as the ice dam was sufficiently high to
dominate the cols at a and b ; but when through change
of climate the gradually sinking dam reached, in succes-
sion, the levels of these cols, the water would then begin
to flow over the dam instead of over the cols. Let us
suppose the wasting of the ice to continue until a con-
nection was established between Glen Roy and Glen
Glaster, a common lake would then fill both these glens,
the level of which would be determined by that of the
col c, over which the water would pour for an indefinite
period into Glen Spean. During this period the second
Glen Roy road and the highest road of Glen Glaster
would be formed. The ice subsiding still further, a
connection would eventually be established between
Glen Roy, Glen Glaster, and the upper part of Glen
Spean, A common lake would fill all three glens, the

THE PAEALLEL ROADS OF OLEN ROY. 225

226 FRAGMENTS OF SCIENCE.

level of which would be that of the col d, over which for
an indefinite period the lake would pour its water.
During this period the lowest Glen Roy road, which is
common also to Grien Glaster and Glen Spean, would be
formed. Finally, on the disappearance of the ice from
the lower part of Glen Spean the waters would flow
down their respective valleys as they do to-day.

Reviewing our work, we find three considerable
steps to have marked the solution of the problem of the
Parallel Roads of Glen Roy. The first of these was taken
by Sir Thomas Dick-Lauder, the second was the pregnant
conception of Agassiz regarding glacier action, and the
third was the testing and verification of this concep-
tion by the very thorough researches of Mr. Jamieson.
No circumstance or incident connected with this dis-
course gives me greater pleasure than the recognition
of the value of these researches. They are marked
throughout by unflagging industry, by novelty and
acuteness of observation, and by reasoning power of a
high and varied kind. These pages had been returned
' for press ' when I learned that the relation of Ben
Nevis and his colleagues to the vapour-laden winds of
the Atlantic had not escaped Mr. Jamieson. To him
obviously the exploration of Lochaber, and the develop-
ment of the theory of the Parallel Roads, has been a
labour of love.

Thus ends our rapid survey of this brief episode
in the physical history of the Scottish hills, — brief,
that is to say, in comparison with the immeasurable
lapses of time through which, to produce its varied
structure and appearances, our planet must have
passed. In the survey of such a field two things are
specially worthy to be taken into account — the widen-
ing of the intellectual horizon and the reaction of ex-
panding knowledge upon the intellectual organ itself

THE PARALLEL ROADS OF GLEN ROY. 227

At first, as in the case of ancient glaciers, through
sheer want of capacity, the mind refuses to take in
revealed facts. But by degrees the steady contem-
plation of these facts so strengthens and expands the
intellectual powers, that where truth once could not
find an entrance it eventually finds a home.*

A map of the district, with the parallel roads shown
in red, is annexed.

LITERATURE OF THE SUBJECT.

Thomas Pennant. — A Tour in Scotland. VoL iii. 1776, p. 394.
John MacGulloch. — On the Parallel Roads of Glen Roy. Geol.

Soc. Trans, vol. iv. 1817, p. 314.
Thomas Laudee Dick (afterwards Sir Thomas Dick-Laudeb,

Bart.)— On the Parallel Roads of Lochaber. Edin. Roy. Soc.

Trans. 1818, vol. ix. p. 1.
Charles Darwin. — Observations on the Parallel Roads of Glen

Roy, and of the other parts of Lochaber in Scotland, with an

attempt to prove that they are of marine origin. Phil. Trans.

1839, vol. cxxix. p. 30.
Sir Charles Lyell. — Elements of Geology. Second edition,

1841.
Louis AoASSiz. — The Glacial Theory and its Recent Progress —

Parallel Terraces. Edin. New Phil. Journal, 1842, vol. xxxiii.

p. 236.

' The formation, connection, successive subsidence, and final
disappearance of the glacial lakes of Lochaber were illuslraled in
the discourse here reported by the model just described, constructed
under the supervision of my assistant, Mr. John Cottrell. Glen
Gluoy with its lake and road and the cataract over its col ; Glen
Roy and its three roads with their respective cataracts at the head
of Glen Spey, Glen Glaster, and Glen Spean, were all represented.
The successive shif tings of the barriers, which were formed of plate
glass, brought each successive lake and its corresponding road into
view, while the entire removal of the barriers caused the streams
to flow down the glens of the model as they flow down the real
glens of to-day.

228 FEAGMENTS OF SCIENCE.

David Milne (afterwards David MiLNE-HoMB).--On the Parallel

Roads of Lochaber ; with Eemarks on the Change of Relative

Levels of Sea and Land in Scotland, and on the Detrital

Deposits in that Country. Edin. Roy. Soc. Trans. 1847, vol.

xvi. p. 395.
Robert Chambers.— Ancient Sea Margins. Edinburgh, 1848.
H. D. Rogers. — On the Parallel Roads of Glen Roy. Royal Inst.

Proceedings, 1861, vol. ill. p. 341.
Thomas F. Jamieson. — On the Parallel Roads of Glen Roy, and

their Place in the History of the Glacial Period. Quart.

Journal Geol. Soc. 1863, vol. xix. p. 235.
Sir Charles Lyell. — Antiquity of Man. 1863, p. 263.
Rev. R. B. Watson. — On the Marine Origin of the Parallel Roads

of Glen Roy. Quart. Joum. Geol. Soc. 1865, vol. xxii. p. 9.
Sir John Lubbock.— On the Parallel Roads of Glen Roy. Quart.

Journ. Geol. Soc. 1867, vol. xxiv. p. 83.
Charles Babbage. — Observations on the Parallel Roads of Glen

Roy. Quart. Joum. Geol. Soc. 1868, vol. xxiv. p. 273.
James Nicol. — On the Origin of the Parallel Roads of Glen Boy.

1869. Geol. Soc. Journal, vol. xxv. p. 282.
James Nicol. — How the Parallel Roads of Glen Roy were formed.

1872. Geol. Soc. Journal, vol. xxviii. p. 237.
Major-General Sir Henry James, R.B. — Notes on the Parallel

Roads of Lochaber. 4to. 1874.

rx.

ALPINE SCULPTURE.
1864.

TO account for the conformation of the Alps, two
hypotheses have been advanced, which may be
respectively named the hypothesis of fracture and the
^hypothesis of erosion. The former assumes that the
forces by which the mountains were elevated produced
fissures in the earth's crust, and that the valleys of the
Alps are the tracks of these fissures ; while the latter
maintains that the valleys have been cut out by the
action of ice and water, the mountains themselves being
the residual forms of this grand sculpture. I had heard
the Via Mala cited as a conspicuous illustration of the
fissure theory — the profound chasm thus named, and
through which the Hinter-Rhein now flows, could, it
was alleged^ be nothing else than a crack in the earth's
crust. To the Via Mala I therefore went in 1864 to
instruct myself upon the point in question.

The gorge commences about a quarter of an hour
above Tusis ; and, on entering it, the first impression
certainly is that it must be a fissure. This conclusion
in my case was modified as I advanced. Some distance
up the gorge I found upon the slopes to my right
quantities of rolled stones, evidently rounded by water-
action. Still fuither up, and just before reaching the
first bridge which spans the chasm, I found more rolled

230 FRAGMENTS OF SCIENCE.

stones, associated with sand and gravel. Through this
mass of detritus, fortunately, a vertical cutting had
been made, which exhibited a section showing perfect
stratification. There was no agency in the place to roll
these stones, and to deposit these alternating layers of
sand and pebbles, but the river which now rushes some
hundreds of feet below them. At one period of the Via
Mala's history the river must have run at this high
level. Other evidences of water-action soon revealed
themselves. From the parapet of the first bridge I
could see the solid rock 200 feet above the bed of the
river scooped and eroded.

It is stated in the guide-books that the river, which
usually runs along the bottom of the gorge, has been
known almost to fill it during violent thunder-storms; and
it may be urged that the marks of erosion which the sides
of the chasm exhibit are due to those occasional floods.
In reply to this, it may be stated that even the exist-
ence of such floods is not well authenticated, and that
if the supposition we-re true, it would be an additional
argument in favour of the cutting power of the river.
For if floods operating at rare intervals could thus
erode the rock, the same agency, acting without ceasing
upon the river's bed, must certainly be competent to
excavate it.

I proceeded upwards, and from a point near another
bridge (which of them I did not note) had a fine view
of a portion of the gorge. The river here runs at the
bottom of a cleft of profound depth, but so narrow that
it might be leaped across. That this cleft must be a
crack is the impression first produced ; but a brief in-
spection suffices to prove that it has been cut by the
river. From top to bottom we have the unmistakable
marks of erosion. This cleft was best seen on looking
downwards from a point near the bridge ; but looking

ALPINE SCULPTUKE. 231

upwards from the bridge itself, the evideuce of aqueous
erosion was equally conviucing.

The character of the erosion depends upon the rock
as well as upon the river. The action of water upon
some rocks is almost purely mechanical ; they are
simply ground away or detached in sensible masses.
Water, however, in passing over limestone, charges it
self with carbonate of lime without damage to its trans
parency ; the rock is dissolved in the water ; and the
gorges cut by water in such rocks often ret^emble those
cut in the ice of glaciers by glacier streams. To the
solubility of limestone is probably to be ascribed the
fantastic forms which peaks of this rock usually assume,
and also the grottos and caverns whicli interpenetrate
limestone formations. A rock capable of being thus
dissolved will expose a smooth surface after the water
has ({uitted it ; and in the case of the Via Mala it is the
polish of the surfaces and the curved hollows scooped in
the sides of the gorge, wliich assure us that the chasm
has been the work of the river.

About four miles from Tusis, and not far from the
little village of Zillis, the Via Mala opens into a plain
bounded by high terraces. It occurred to me the
moment I saw it that the plain had been the bed of an
ancient lake ; and a farmer, who was my temporary
companion, immediately informed me that such was
the tradition of the neighbourhood. This man con-
versed with intelligence, and as I drew his attention to
the rolled stones, which rest not only above the river,
but above the road, and inferred that the river must
once have been there to have rolled those stones, he saw
the force of the evidence perfectly. In fact, in former
times, and subsequent to the retreat of the great
glaciers, a rocky barrier crossed the valley at this place,
damming the river which came from the mountains

16

232 FRAGMENTS OF SCIENCE.

higher up. A lake was thus formed which poured its
waters over the barrier. Two actions were here at work,
both tending to obliterate the lake — the raising of its
bed by the deposition of detritus, and the cutting of its
dam by the river. In process of time the cut deepened
into the Via Mala ; the lake was drained, and the river
now flows in a definite channel through the plain which
its waters once totally covered.

From Tusis I crossed to Tiefenkasten by the Schien
Pass, and thence over the JuHer Pass to Pontresina.
There are three or four ancient lake-beds between Tiefen-
kasten and the summit of the Julier. They are all of
the same type — a more or less broad and level valley-
bottom, with a barrier in front through which the
river has cut a passage, the drainage of the lake being
the consequence. These lakes were sometimes dammed
by barriers of rock, sometimes by the moraines of
ancient glaciers.

An example of this latter kind occurs in the Eosegg
valley, about twenty minutes below the end of the
Eosegg glacier, and about an hour from Pontresina.
The valley here is crossed by a pine-covered moraine
of the noblest dimensions; in the neighbourhood of
London it might be called a mountain. That it is a
moraine, the inspection of it from a point on the Surlei
slopes above it will convince any person possessing an
educated eye. Where, moreover, the interior of the
mound is exposed, it exhibits moraine-matter — detritus
pulverised by the ice, with boulders entangled in it.
It stretched quite across the valley, and at one time
dammed the river up. But now the barrier is cut
through, the stream having about one-fourth of the
moraine to its right, and the remaining three-fourths
to its left. Other moraines of a more resisting charac-
ter hold their ground as barriers to the present day.

ALPINE SCULPTURE. 233

In the Val di Campo, for example, about three-quarters
of an hour from Pisciadello, there is a moraine com-
posed of large boulders, which interrupt the course of a
river and compel the water to fall over them in cascades.
They have in great part resisted its action since the
retreat of the ancient glacier which formed the moraine.
])cliind the moraine is a lake-bed, now converted into a
level meadow, which rests on a deep layer of mould.

At Pontr^sina a very fine and instructive gorge is to
l)e seen. The river from the Morteratsch glacier rushes
through a deep and narrow chasm which is spanned at
one place by a stone bridge. The rock is not of a charac-
ter to preserve smooth polishing ; but the larger features
of water-action are perfectly evident from top to bottom.
Those features are in part visible from the bridge, but
still better from a point a little distance from the
bridge in the direction of the upper village of Pont-
resina. The hollowing out of the rock by the eddies of
the water is here quite manifest. A few minutes' walk
upwards brings us to the end of the gorge ; and behind
it we have the usual indications of an ancient lake, and
terraces of distinct water origin. From this position
indeed the genesis of the gorge is clearly revealed.
After the retreat of the ancient glacier, a transverse
ridge of comparatively resisting material crossed the
valley at this place. Over the lowest part of this ridge
the river flowed, rushing steeply down to join at the
bottom of the slope the stream which issued from the
Rosegg glacier. On this incline the water became a
powerful eroding agent, and finally cut the channel to
its present depth.

Geological writers of reputation assume at this
place the existence of a fissure, the ' washing out ' of
which resulted in the formation of the gorge. Now
no examination of the bed of the river ever proved the

234 FRAGMENTS OF SCIENCE.

existence of this fissure ; and it is certain that water,
particularly when charged with solid matter in suspen-
sion, can cut a channel through unfissured rock. Cases
of deep cutting can be pointed out where the clean bea
of the stream is exposed, the rock which forms the
floor of the river not exhibiting a trace of fissure. An
example of this kind on a small scale occurs near the
Bernina Gasthaus, about two hours from Pontresina. A
little way below the junction of the two streams from the
Bernina Pass and the Heuthal the river flows through a
channel cut by itself, and 20 or 30 feet in depth. At some
places the river-bed is covered with rolled stones ; at
other places it is bare, but shows no trace of fissure. The
abstract power of water, if I may use the term, to cut J I
through rock is demonstrated by such instances. But if
water be competent to form a gorge without the aid of
a fissure, why assume the existence of such fissures in
cases like that at Pontresina ? It seems far more
philosophical to accept the simple and impressive history
written on the walls of those gorges by the agent which
produced them.

Numerous cases might be pointed out, varying in I
magnitude, but all identical in kind, of barriers which i
crossed valleys and formed lakes having been cut through
by rivers, narrow gorges being the consequence. One
of the most famous examples of this kind is the Finster-
aarschlucht in the valley of Hasli. Here the ridge called
the Kirchet seems split across, and the river Aar rushes
through the fissure. Behind the barrier we have the
meadows and pastures of Imhof resting on the sediment
of an ancient lake. Were this an isolated case, one
might with an apparent show of reason conclude that the
Finstcraarschlucht was produced by an earthquake, as
some suppose it to have been ; but when we find it to be
a single sample of actions which are frequent in the Alps

ALPINE SCULPTURE. 235

— when probably a hundred cases of the same kind, though
diflferent in magnitude, can be pointed out — it seems
quite unphilosophical to assume that in each particular
case an earthquake was at hand to form a channel for the
river. As in the case of the barrier at Pontresina, the
Kirchet, after the retreat of the Aar glacier, dammed the
waters flowing from it, thus forming a lake, on the bed
of which now stands the village of Imhof. Over this
harrier the x\ar tumbled towards Meyringen, cutting, as
the centuries passed, its bed ever deeper, until finally
it became deep enough to drain the lake, leaving in its
place the alluvial plain, through which the river now
flows in a definite channel.

In 1866 I subjected the Finsteraarschlucht to a close
examination. The earthquake theory already adverted
to was then prevalent regarding it, and I wished to see
whether any evidences existed of aqueous erosion. Near
the summit of the Kirchet is a signboard inviting
the traveller to visit the Aarenschlucht, a narrow
lateral gorge which runs down to the very bottom of
the principal one. The aspect of this smaller chasm
from bottom to top proves to demonstration that water
had in former ages been there at work. It is scooped,
rounded, and polished, so as to render palpable to
the most careless eye that it is a gorge of erosion.
But it was regarding the sides of the great chasm that
instruction was needed, and from its edge nothing to
satisfy me could be seen. I therefore stripped and waded
into the river until a point was reached which com-
manded an excellent view of both sides of the gorge.
The water was cutting cold, but I was repaid. Below
me on the left-hand side was a jutting cliff which bore
the thrust of the river and caused the Aar to swerve
from its direct course. From top to bottom this cliff
was polished, rounded, and scooped. There was no

236 FKAGMENTS OF SCIENCE.

room for doubt. The river which now runs so deeply
down had once been above. It has been the delver of
its own channel through the barrier of the Kirchet.

But the broad view taken by the advocates of the
fracture theory is, that the valleys themselves follow
the tracks of primeval fissures produced by the upheaval
of the land, the cracks across the barriers referred to
being in reality portions of the great cracks which
formed the valleys. Such an argument, however, would
virtually concede the theory of erosion as applied to the
valleys of the Alps. The narrow gorges, often not more
than twenty or thirty feet across, sometimes even
narrower, frequently occur at the bottom of broad
valleys. Such fissures might enter into the list of
accidents which gave direction to the real erosive agents
which scooped the valley out ; but the formation of the
valley, as it now exists, could no more be ascribed to
such cracks than the motion of a railway train could
be ascribed to the finger of the engineer which turns on
the steam.

These deep gorges occur, I believe, for the most
part in limestone strata ; and the effects which the
merest driblet of water can produce on limestone are
quite astonishing. It is not uncommon to meet chasms
of considerable depth produced by small streams the
beds of which are dry for a large portion of the year.
Eight and left of the larger gorges such secondary
chasms are often found. The idea of time must, I
think, be more and more included in our reasonings on
these phenomena. Happily, the marks which the rivers
have, in most cases, left behind them, and which refer,
geologically considered, to actions of yesterday, give us
ground and courage to conceive what may be effected in
geologic periods. Thus the modern portion of the Via
Mala throws light upon the whole. Near Bergiin, in
the valley of the Albida, there is also a little Via JNIala,

ALPINE SCULPTURE. 237

whicli is not less significant than the great one. The
river flows here through a profound limestone gorge,
and to the very edges of the gorge we have the evidences
of erosion. Rut the most striking illustration of water-
action upon limestone rock that I have ever seen is
the gorge at Pfafi'ers. Here the traveller passes
along the side of the chasm midway between top and
bottom. Whichever way he looks, backwards or for-
wards, upwards or downwards, towards the sky or to-
wards the river, he meets everywhere the irresistible
and impressive evidence that this wonderful fissure has
been sawn through the mountain by the waters of the
Tamina.

I have thus far confined myself to the consideration
of the gorges formed by the cutting tlirough of the rock-
barriers which frequently cross the valleys of the Alps ;
as far as they have been examined by me they are the
work of erosion. But the larger question still remains. To
what action are we to ascribe the formation of the valleys
themselves ? This question includes that of the forma-
tion of the mountain-ridges, for were the valleys wholly
filled, the ridges would disappear. Possibly no answer
can be given to this question which is not beset with more
or less of difficulty. Special localities might be found
which would seem to contradict every solution which
refers the conformation of the Alps to the operation of
a single cause.

Still the Alps present features of a character suffi-
ciently definite to bring the question of their origin with-
in the sphere of close reasoning. That they were in whole
or in part once beneath the sea will not be disputed ; for
they are in great part composed of sedimentary rocks
which required a sea to form them. Their present elevation
above the sea is due to one of those local changes in the
shape of the earth which have been of frequent occurrence

238 FRAGMENTS OF SCIENCE.

throughout geologic time, in some cases depressing the
land, and in others causing the sea-bottom to protrude
beyond its surface. Considering the inelastic character of
its materials, the protuberance of the Alps could hardly
have been pushed out without dislocation and fracture ;
and this conclusion gains in probability when we con-
sider the foldings, contortions, and even reversals in
position of the strata in many parts of the Alps.
Such changes in the position of beds which were once
horizontal could not have been effected without disloca-
tion. Fissures would be produced by these changes;
and such fissures, the advocates of the fracture theory
contend, mark the positions of the valleys of the Alps.

Imagination is necessary to the man of science, and
we could not reason on our present subject without the
power of presenting mentally a picture of the earth's
crust cracked and fissured by the forces which produced
its upheaval. Imagination, however, must be strictly
checked by reason and by observation. That fractures
occurred cannot, 1 think, be doubted, but that the valleys
of the Alps are thus formed is a conclusion not at all
involved in the admission of dislocations. I never met
with a precise statement of the manner in which the
advocates of the fissure theory suppose the forces to have
acted — whether they assume a general elevation of the
region, or a local elevation of distinct ridges ; or whether
they assume local subsidences after a general elevation,
or whether they would superpose upon the general up-
heaval minor and local upheavals.

In the absence of any distinct statement, I will
assume the elevation to be general — that a swelling out
of the earth's crust occurred here, sufficient to place the
most prominent portions of the protuberance three miles
above the sea-level. To fix the ideas, let us consider a
circular portion of the crust, say one hundred miles in

ALPINE SCULPTURE. 239 ^

diameter, and let us suppose, in the first instance, the
circumference of this circle to remain fixed, and that
the elevation was confined to the space within it. The
upheaval ^ould throw the crust into a state of strain ;
and, if it were inflexible, the strain must be relieved by
fracture. Crevasses would thus intersect the crust. Let
us now enquire what proportion the area of these open
fissures is likely to bear to the area of the unfissured
crust. An approximate answer is all that is here re-
quired ; for the problem is of such a character as to
lender minute precision imnecessary.

No one, I think, would affirm that the area of the
fissures would be one-hundredth the area of the land.
For let us consider the strain upon a single line drawn
over the summit of the protuberance from a point on
its rim to a point opposite. Eegarding the protuberance
as a spherical swelling, the length of the arc corre-
sponding to a chord of 100 miles and a versed sine
of 3 miles is 100*24 miles ; consequently the surface to
reach its new position must stretch 0*24 of a mile, or
be broken. A fissure or a number of cracks with this
total width would relieve the strain ; that is to say, the
sum of the widths of all the cracks over the length of
100 miles would be 420 yards. If, instead of com-
paring the width of tlie fissures with the length of the
lines of tension, we compared their areas with the area
of the unfissured land, we should of course find the
proportion much less. These considerations will help
the imagination to realise what a small ratio the area
of the open fissures must bear to the unfissured crust.
Tliey enable us to say, for example, that to assume the
area of the fissures to be one-tenth of the area of the
land would be quite absurd, while that the area of the
fissures could be one-half or more than one-half that of
the land would be in a proportionate degree unthink-

240 FRAGMENTS OF SCIENCE.

able. If we suppose the elevation to be due to the
shrinking or subsidence of the land all round our
assumed circle, we arrive equally at the conclusion
that the area of the open fissures would be altogether
insignificant as compared with that of the unfissured
crust.

To those who have seen them from a commanding
elevation, it is needless to say that the Alps themselves
bear no sort of resemblance to the picture which this
theory presents to us. Instead of deep cracks with
approximately vertical walls, we have ridges running
into peaks, and gradually sloping to form valleys.
Instead of a fissured crust, we have a state of things
closely resembling the surface of the ocean when agi-
tated by a storm. The valleys, instead of being much
narrower than the ridges, occupy the greater space. A
plaster cast of the Alps turned upside down, so as to
invert the elevations and depressions, would exhibit
blunter and broader mountains, with narrower valleys
between them, than the present ones. The valleys that
exist cannot, I think, with any correctness of language
be called fissures. It may be urged that they origi-
nated in fissures : but even this is unproved, and, were
it proved, the fissures would still play the subordinate
part of giving direction to the agents which are to be
regarded as the real sculptors of the Alps.

The fracture theory, then, if it regards the elevation
of the Alps as due to the operation of a force acting
throughout the entire region, is, in my opinion, utterly
incompetent to account for the conformation of the
country. If, on the other hand, we are compelled to
resort to local disturbances, the manipulation of the
earth's crust necessary to obtain the valleys and the
mountains will, I imagine, bring the difficulties of the
theory into very strong relief. Indeed an examination

ALPINE SCULPTUEE. 241

(if the region from many of the more accessible emi-
nences— from the Galenstock, the Grauhaupt, the Pitz
Languard, the Monte Confinale — or, better still, from
Mont Blanc, Monte Eosa, the Jungfrau, the Finsteraar-
horn, the Weisshorn, or the Matterhorn, where local
peculiarities are toned down, and the operations of the
powers which really made this region what it is are
alone brought into prominence — must, I imagine, con-
vince every physical geologist of the inability of any
fracture theory to account for the present conformation
of the Alps.

A correct model of the mountains, with an un-
exaggerated vertical scale, produces the same effect
upon the mind as the prospect from one of the highest
peaks. We are apt to be influenced by local phenomena
which, though insignificant in view of the general
question of Alpine conformation, are, with reference to
our customary standards, vast and impressive. In a
true model those local peculiarities disappear ; for on
the scale of a model they are too small to be visible ;
while the essential facts and forms are presented to the
undistracted attention.

A minute analysis of the phenomena strengthens
the conviction which the general aspect of the Alps
fixes in the mind. We find, for example, numerous
valleys which the most ardent plutonist would not
think of ascribing to any other agency than erosion.
That such is their genesis and history is as certain as
that erosion produced the Chines in the Isle of Wight.
From these indubitable cases of erosion — commencing,
if necessary, with the small ravines which run down
the flanks of the ridges, with their little working
navigators at their bottoms — we can proceed, by almost
insensible gradations, to the largest valleys of the
Alps ; and it would perplex the plutonist to fix upon

242 FRAGMENTS OF SCIENCE.

the point at which fracture begins to play a material
part.

In ascending one of the larger valleys, we enter it
where it is wide and where the eminences are gentle on
either side. The flanking mountains become higher
and more abrupt as we ascend, and at length we reach
a place where the depth of the valley is a maximum.
Continuing our walk upwards, we find ourselves flanked
by gentler slopes, and finally emerge from the valley
and reach the summit of an open col, or depression in
the chain of mountains. This is the common character
of the large valleys. Crossing the col, we descend
along the opposite slope of the chain, and through the
same series of appearances in the reverse order. If the
valleys on both sides of the col were produced by
fissures, what prevents the fissure from prolonging
itself across the col? The case here cited is repre-
sentative ; and I am not acquainted with a single
instance in the Alps where the chain has been cracked
in the manner indicated. The cols are simply de-
pressions ; in many of which the unfissured rock can
be traced from side to side.

The typical instance just sketched follows as a
natural consequence from the theory of erosion. Before
either ice or water can exert great power as an erosive
agent, it must collect in sufficient mass. On the higher
slopes and plateaus — in the region of cols — the power
is not fully developed ; but lower down tributaries
unite, erosion is carried on with increased vigour, and
the excavation gradually reaches a maximum. Lower
still the elevations diminish and the slopes become
more gentle; the cutting power gradually relaxes,
until finally the eroding agent quits the mountains
altogether, and the grand effects which it produced in
the earlier portions of its course entirely disappear.

ALPINE SCULPTURE. 243

I have hitherto confined myself to the consideration
of the broad question of the erosion theory as compared
svitli the fracture theory ; and all that I have been able
to observe and think with reference to the subject
leads me to adopt the former. Under the term erosion
I include the action of water, of ice, and of the atmo-
sphere, including frost and rain. Water and ice,
liowever, are the principal agents, and which of these
two has produced the greatest effect it is perhaps im-
possible to say. Two years ago I wrote a brief note
' On the Conformation of the Alps,' * in which I ascribed
the paramount influence to glaciers. The facts on
which that opinion was founded are, I think, un-
assailable ; but whether the conclusion then announced
fairly follows from the facts is, I confess, an open
question.

The arguments which have been thus far urged
against the conclusion are not convincing. Indeed,
the idea of glacier erosion appears so daring to some
minds that its boldness alone is deemed its sufficient
refutation. li is, however, to be remembered that a
precisely similar position was taken up by many ex-
cellent workers when the question of ancient glacier
extension was first mooted. The idea was considered
too hardy to be entertained ; and the evidences of
glacial action were sought to be explained by reference
to almost any process rather than the true one. Let
those <vho so wisely took the side of ' boldness ' in that
discussion beware lest they place themselves, with
reference to the question of glacier erosion, in the
position formerly occupied by their opponents.

Looking at the little glaciers of the present day —
mere pigmies as compared to the giants of the glacial
epoch — ^we find that from every one of them issues a
> Phil. Mag. vol. xxiv. p. 1G9.

244 FRAGMENTS OF SCIENCE.

river more or less voluminous, charged with the matter
which the ice has rubbed from the rocks. Where the
rocks are soft, the amount of this finely pulverised
matter suspended in the water is very great. The
water, for example, of the river which flows from
Santa Catarina to Bormio is thick with it. The Khine
is charged with this matter, and by it has so silted up
the Lake of Constance as to abolish it for a large
fraction of .its length. The Khone is charged with it,
and tens of thousands of acres of cultivable land are
formed by the silt above the Lake of Greneva.

In the case of every glacier we have two agents at
work — the ice exerting a crushing force on every point
of its bed which bears its weight, and either rasping
this point into powder or tearin<^ it bodily from the
rock to which it belongs ; while the water which every-
where circulates upon the bed of the glacier continually
washes the detritus away and leaves the rock clean for
further abrasion. Confining the action of glaciers to
the simple rubbing away of the rocks, and allowing
them sufficient time to act, it is not a matter of opinion,
but a physical certainty, that they will scoop out valleys.
But the glacier does more than abrade. Rocks are not
homogeneous ; they are intersected by joints and places
of weakness, which divide them into virtually detached
masses. A glacier is undoubtedly competent to root
such masses bodily away. Indeed the mere d prioH
consideration of the subject proves the competence of a
glacier to deepen its bed. Taking the case of a glacier
1,000 feet deep (and some of the older ones were
probably three times this depth), and allowing 40 feet
of ice to an atmosphere, we find that on every square
inch of its bed such a glacier presses with a weight of
375 lbs., and on every square yard of its bed with a
weight of 486,000 lbs. With a vertical pressure of

ALPINE SCULPTURE. 245

lis amount the glacier is urged down its valley by the
►ressure from behind. We can hardly, I think, deny
such a tool a power of excavation.
The retardation of a glacier by its bed has been
referred to as proving its impotence as an erosive
agent ; but this very retardation is in some measure an
expression of the magnitude of the erosive energy.
Either the bed must give way, or the ice must slide
over itself. We get indeed some idea of the crushing
pressure which the moving glacier exercises against its
bed from the fact that the resistance, and the effort to
overcome it, are such as to make the upper layers of a
glacier move bodily over the lower ones — a portion
only of the total motion being due to the progress of
the entire mass of the glacier down its valley.

The sudden bend in the valley of the Rhone at
Martigny has also been regarded as conclusive evidence
against the theory of erosion. *Why,' it has been
asked, ' did not the glacier of the Rhone go straight
forward instead of making this awkward bend ? ' But
if the valley be a crack, why did the crack make this
bend? The crack, I submit, had at least as much
reason to prolong itself in a straight line as the glacier
had. A statement of Sir John Herschel with reference
to another matter is pei'fectly applicable here : ' A
crack once produced has a tendency to run— for this
plain reason, that at its momentary limit, at the point
at which it has just arrived, the divellent force on the
molecules there situated is counteracted only by half
of the cohesive force which acted when there was no
crack, viz. the cohesion of the uncracked portion alone '
{' Proc. Roy. Soc' vol. xii. p. 678). To account, then,
for the bend, the adherent of the fracture theory must
assume the existence of some accident which turned
the crack at right angles to itself ; and he surely will

246 FKAGMENTS OF SCIENCE.

permit the adherent of the erosion theory to make a
similar assumption.

The influence of small accidents on the direction of
rivers is beautifully illustrated in glacier streams, which
are made to cut either straight or sinuous channels by
causes apparently of the most trivial character. In
his interesting paper ' On the Lakes of Switzerland,'
M. Studer also refers to the bend of the Khine at
Sargans in proof that the river must there follow a
pre-existing fissure. I made a special expedition to the
place in 1 864 ; and though it was plain that M. Studer
had good grounds for the selection of this spot, I was
unable to arrive at his conclusion as to the necessity of
a fissure.

Again, in the interesting volume recently published
by the Swiss Alpine Club, M. Desor informs us that
the Swiss naturalists who met last year at Samaden
visited the end of the Morteratsch glacier, and there
convinced themselves that a glacier had no tendency
whatever to imbed itself in the soil. I scarcely think
that the question of glacier erosion, as applied either to
lakes or valleys, is to be disposed of so easily. Let me
record here my experience of the Morteratsch glacier.
I took with me in 1864 a theodolite to Pontresina,
and while there had to congratulate myself on the
aid of my friend Mr. Hirst, who in 1857 did such
good service upon the Mer de Glace and its tribu-
taries. We set out three lines across the Morteratsch
glacier, one of which crossed the ice-stream near the
well-known hut of the painter Georgei, while the two
others were staked out, the one above the hut and the
other below it. Calling the highest line A, the line
which crossed the glacier at the hut B, and the lowest
line C, the following are the mean hourly motions of
the three lines, deduced from observations which ex*

i

ALPINE SCULPTURE. 247

tended over several days. On each line eleven stakes
were fixed, which are designated by the figures 1, 2, 3,
&c. in the Tables.

Morteratsch Glacier, Line A.

No. of Stake. Hourly Motion;

1 0-35 inch.

2 0-49 M

3 0-53 „

4 0-54 „

6 0-56 „

6 0-54 „

7 . . . . . . 0-52 „

8 0-49 „

9 0-40 „

10 0-29 „

11 0-20 „

As in all other measurements of this kind, the re-
tarding influence of the sides of the glacier is manifest :
the centre moves with the greatest velocity.

Morteratsch Glower, Line B.

No. of Stake. Hourly Motion.

1 ..... . 0-05 inch.

2 014 „

3 0-24 „

4 • • • - • • . 0'32 ,y »

5 0-41 „

6 0-44 „ ^

7 0-44 „

O • . . • . . U*40 )y

9 ..... . 0-43 „

10 0-44 „

11 0-44 „

The first stake of this line was quite close to the
edge of the glacier, and the ice was thin at the place,
hence its slow motion. Crevasses prevented us firom
carrying the line sufficiently far across to render the
retardation of the further side of the glacier fullj^
evident.

17

248 FRAGMENTS OF SCIENCE.

Morteratsch Glacier , Line C.

Ko. of stake Hourly Motion.

1 0-05 inch.

2 . , , . . , 0-09 „

3 . . . . , , 0-18 „

4 0-20 „

6 0-25 „

6 0-27 „

7 • , • , . , 0-27 „

8 0-30 „

9 0-21 „

10 0-20 „

11 c 0-16 „

Comparing the three lines together, it will be ob-
served that the velocity diminishes as we descend the
glacier. In 100 hours the maximum motion of the
three lines respectively is as follows :

Maximum Motion in 100 hours.

Line A 56 inchea

t, B , , , , , 45 „
„ 0 30 „

This deportment explains an appearance which
must strike every observer who looks upon the Mor-
teratsch from the Piz Languard, or from the new
Bernina Eoad. A medial moraine runs along the
glacier, commencing as a narrow streak, but towards
the end the moraine extending in width, until finally
it quite covers the terminal portion of the glacier.
The cause of this is revealed by the foregoing measure-
ments, which prove that a stone on the moraine where
it is crossed by the line A approaches a second stone
on the moraine where it is crossed by the line C with a
velocity of twenty-six inches per one hundred hours.
The moraine is in a state of longitudinal compression.
Its naaterials are more and more squeezed together,
and they must consequently move laterally and render

ALPINE SCULPTUEE. 249

the moraine at the terminal portion of the glacier
wider than above.

The motion of the Morteratsch glacier, then,
diminishes as we descend. The maximum motion of
the third line is thirty inches in one hundred hours, or
seven inches a day — a very slow motion ; and had we
nm a line nearer to the end of the glacier, the motion
would have been slower still. At the end itself it is
nearly insensible.* Now I submit that this is not the
place to seek for the scooping power of a glacier. The
opinion appears to be prevalent that it is the snout of
a glacier that must act the part of ploughshare ; and it
is certainly an erroneous opinion. The scooping power
will exert itself most where the weight and the motion
are greatest. A glacier's snout often rests upon matter
which has been scooped from the glacier's bed higher
up. I therefore do not think that the inspection of
what the end of a glacier does or does not accomplish
can decide this question.

The snout of a glacier is potent to remove anything
against which it can fairly abut ; and this power, not-
withstanding the slowness of the motion, manifests
itself at the end of the Morteratsch glacier. A hillock,
bearing pine-trees, was in front of the glacier when
Mr. Hirst and myself inspected its end ; and this
hillock is being bodily removed by the thrust of the
ice. Several of the trees are overturned ; and in a few
years, if the glacier continues its reputed advance, the
mound will certainly be ploughed away.

The question of Alpine conformation stands, I
think, thus : We have, in the first place, great valleys,

* The snout of the Aletsch Griacier has a diurnal motion of less
than two inches, while a mile or so above the snout the velocity is
eighteen inches. The spreading out of the moraine is here very
■triking.

250 FEAGMENTS OF SCIENCE.

^ch as those of the Rhine and the Rhone, which
we might conveniently call valleys of the first order.
The mountains which flank these main valleys are
also cut by lateral valleys running into the main
ones, and which may be called valleys of the second
order. When these latter are examined, smaller
valleys are found running into them, which may be
called valleys of the third order. Smaller ravines and
depressions, again, join the latter, which may be called
valleys of the fourth order, and so on until we reach
streaks and cuttings so minute as not to merit the
name of valleys at all. At the bottom of every valley
we have a stream, diminishing in magnitude as the
order of the valley ascends, carving the earth and
carrying its materials to lower levels. We find that
the larger valleys have been filled for untold ages by
glaciers of enormous dimensions, always moving, grind-
ing down and tearing away the rocks over which they
passed. We have, moreover, on the plains at the feet
of the mountains, and in enormous quantities, the very
matter derived from the sculpture of the mountains
themselves.

The plains of Italy and Switzerland are cumbered
by the debris of the Alps. The lower, wider, and
more level valleys are also filled to unknown depths
with the materials derived from the higher ones. In
the vast quantities of moraine-matter which cumber
many even, of the higher valleys we have also sugges-
tions as to the magnitude of the erosion which has
taken place. This moraine-matter, moreover, can only
in small part have been derived from the falling of
rocks upon the ancient glacier; it is in great part
derived from the grinding and the ploughing-out of
the glacier itself. This accounts for the magnitude of
many of the ancient moraines, which date from a

ALPINE SCULPTURK 251

period when almost all tne mountains were covered
with ice and snow, and when, consequently, the
quantity of moraine-matter derived from the nakdd
crests cannot have been considerable.

The erosion theory ascribes the formation of Alpine
valleys to the agencies here briefly referred to. It
invokes nothing but true causes. Its artificers are
still there, though, it may be, in diminished strength ;
and if they are granted sufficient time, it is demon-
strable that they are competent to produce the effects
ascribed to them. And what does the fracture theory
offer in comparison ? From no possible application of
this theory, pure and simple, can we obtain the slopes
and forms of the mountains. Erosion must in the
long run be invoked j and its power therefore conceded.
The fracture theory infers from tlie disturbances of the
Alps the existence of fissures ; and this is a probable
inference. But that they were of a magnitude sufficient
to produce the conformation of the Alps, and that
they followed, as the Alpine valleys do, the lines of
natiural drainage of the country, are assumptions which
do not appear to me to be justified either by reason or
by observation.

There is a grandeur in the secular integration of
small effects implied by the theory of erosion almost
superior to that involved in the idea of a cataclysm.
Think of the ages which must have been consumed in
the execution of this colossal sculpture. The question
may, of course, be pushed further. Think of the ages
which the molten earth required for its consolidation.
But these vaster epochs lack sublimity through our
inability to grasp them. They bewilder us, but they
fail to make a solemn impression. The genesis of the
mountains comes more within the scope of the intellect,
and the majesty of the operation is enhanced by our

252 FRAGMENTS OF SCIENCE.

partial ability to conceive it. In the falling of a rock
from a mountain-head, in the shoot of an avalanche, in
the plunge of a cataract, we often see more impressive
illustrations of the power of gravity than in the motions
of the stars. When the intellect has to intervene, and
calculation is necessary to the building up of the con-
ception, the expansion of the feelings ceases to be
proportional to the magnitude of the phenomena.

I will here record a few other measurements exe-
cuted on the Eosegg glacier : the line was staked out
across the trunk formed by the junction of the Eosegg
proper with the Tschierva glacier, a short distance
below the rocky promontory called Agaliogs,

Eosegg Glacier.

No. of Stake. Hourly Motion.

1 001 inch.

2 005 „

3 0-07 „

4 0-10 „

6 0-11 „

6 013 „

7 014 „

8 018 „

9 0-24 „

10 0-23 „

11 0-24 „

This is an extremely slowly moving glacier; the
maximum motion hardly amounts to seven inches a day.
Crevasses prevented us from continuing the line quite
across the glacier.

I

X.

RECENT EXPERIMENTS ON FOG-SIGNALS.^

THE care of its sailors is one of the first duties of a
maritime people, and one of the sailor's greatest
dangers is his proximity to the coast at night. Hence
the idea of warning him of such proximity by beacon-
fires placed sometimes on natural eminences and some-
times on towers built expressly for the purpose. Close
to Dover Castle, for example, stands an ancient Pharos
of this description.

As our marine increased greater skill was invoked,
and lamps reinforced by parabolic reflectors poured
their light upon the sea. Several of these lamps were
sometimes grouped together so as to intensify the light,
which at a little distance appeared as if it emanated
from a single source. This ' catoptric ' form of appa-
ratus is still to some extent employed in our lighthouse-
service, but for a long time past it has been more and
more displaced by the great lenses devised by the illus-
trious Frenchman, Fresnel.

In a first-class 'dioptric' apparatus the light
emanates from a lamp with several concentric wicks,
the flame of which, being kindled by a very active
draught, attains to great intensity. In fixed lights the
lenses refract the rays issuing from the lamp so as to
cause them to form a luminous sheet which grazes the

» A discourse delivered in the Royal Institution, MarcJi 22, 1Q78.

254 FRAGMENTS OF SCIENCE.

sea-horizon. In revolving lights the lenses gather up
the rays into distinct beams, resembling the spokes of a
wheel, which sweep over the sea and strike the eye of
the mariner in succession.

It is not for clear weather that the greatest strength-
ening of the light is intended^ for here it is not needed.
Nor is it for densely foggy weather, for here it is in-
effectual. But it is for the intermediate stages of hazy,
snowy, or rainy weather, in which a powerful light can
assert itself, while a feeble one is extinguished. The
usual first-order lamp is one of four wicks, but Mr.
Douglass, the able and indefatigable engineer of the
Trinity House, has recently raised the number of the
wicks to six, which produce a very noble flame. To Mr,
Wigham, of Dublin, we are indebted for the successful
application of gas to lighthouse illumination. In some
lighthouses his power varies from 28 jets to 108 jets,
while in the lighthouse of Galley Head three burners of
the largest size can be employed, the maximum number
of jets being B24. These larger powers are invoked only
in case of fog, the 28-jet burner being amply sufficient
for clear weather. The passage from the small burner
to the large, and from the large burner to the small, is
made with ease, rapidity, and certainty. This employ-
ment of gas is indigenous to Ireland, and the Board
of Trade has exercised a wise liberality in allowing
every facility to Mr. Wigham for the development of
his invention.

The last great agent employed in lighthouse illu-
mination is electricity. It was in this Institution,
beginning in 1831, that Faraday proved the existence
and illustrated the laws of those induced currents which
in our day have received such astounding development.
In relation to this subject Faraday's words have a pro-
phetic ring. * I have rather,' be writes in 1831, ' been

RECENT EXPERIMENTS ON FOG-SIGNALS. 255

desirous of discovering new facts and new relations de-
pendent on magneto-electric induction than of exalting
the force of those already obtained, being assured that
the latter would find their full development hereafter.'
The labours of Holmes, of the Paris Alliance Company,
of Wilde, and of Grramme, constitute a brilliant fulfil-
ment of this prediction.

But, as regards the augmentation of power, the
greatest step hitherto made was independently taken a
few years ago by Dr. Werner Siemens and Sir Charles
Wheatstone. Through the application of their dis-
covery a machine endowed with an infinitesimal charge
of magnetism may, by a process of accumulation at
compound interest, be caused so to enrich itself mag-
netically as to cast by its performance all the older
machines into the shade. The light now before you is
that of a small machine placed downstairs, and worked
there by a minute steam-engine. It is a light of about
1000 candles ; and for it, and for the steam-engine that
works it, our members are indebted to the liberality of
Dr. William Siemens, who in the most generous manner
has presented the machine to this Institution. After
an exhaustive trial at the South Foreland, machines on
the principle of Siemens, but of far greater power than
this one, have been recently chosen by the Elder
Brethren of the Trinity House for the two light-houses
at the Lizard Point.

Our most intense lights, including the six-wick lamp,
the Wigham gas light, and the electric light, being
intended to aid the mariner in heavy weather, may be
regarded, in a certain sense, as fog-signals. But fog,
when thick, is intractable to light. The sun cannot
penetrate it, much less any terrestrial source of illumi-
nation. Hence the necessity of employing sound-signals
in dense fogs. Bells, gongs^ horns, wliistles, guns, and

256 FRAGMENTS OF SCIENCE.

syrens have been used for this purpose ; but it is mainly,
if not wholly, with explosive signals that we have now
to deal. The gun has been employed with useful effect
at the North Stack, near Holyhead, on the Kish Bank
near Dublin, at Lundy Island, and at other points on
our coasts. During the long, laborious, and I venture
to think memorable series of observations conducted
under the auspices of the Elder Brethren of the Trinity
House at the South Foreland in 1872 and 1873, it was
proved that a , short 5^-inch howitzer, j&ring 3 lbs. of
powder, yielded a louder report than a long 18-pounder
firing the same charge. Here was a hint to be acted on
by the Elder Brethren. The effectiveness of the sound
depended on the shape of the gun, and as it could not
be assumed that in the howitzer we had hit accidentally
upon the best possible shape, arrangements were made
with the War Office for the construction of a gun spe-
cially calculated to produce the loudest sound attainable
from the combustion of 3 lbs. of powder. To prevent
the unnecessary landward waste of the sound, the gun
was furnished with a parabolic muzzle, intended to pro-
ject the sound over the sea, where it was most needed.
The construction of this gun was based on a searching
series of experiments executed at Woolwich with small
models, provided with muzzles of various kinds. A
drawing of the gun is annexed (p. 257). It was con-
structed on the principle of the revolver, its various
chambers being loaded and brought in rapid succession
into tlie firing position. The performance of the gun
proved the correctness of the prmciples on which its
construction was based.

An incidental point of some interest was decided by
the earliest Woolwich experiments. It had been a
widely spread opinion among aitillerists, that a bronze
gun produces a specially loud report. I doubted from

RECENT EXPERIMENTS ON FOG-SIGNALS. 257

the outset whether this would help us ; and in a letter
dated 22nd April, 1874, I ventured to express nayself
thus : — ' The report of a gun, as affecting an observer
close at hand, is made up of two factors — the sound due
to the shock of the air by the violently expanding gas,
and the sound derived from the vibrations of the gun,
which, to some extent, rings like a bell. This latter, I
apprehend, will disappear at considerable distances.'

Fig. 6.

Breech-loading Fog-signal Gun, with Bell Mouth,' proposed by
Major Maitland, R.A., Assistant Superintendent.

The result of subsequent trial, as reported by General
Campbell, is, ' that the sonorous qualities of bronze are
greatly superior to those of cast iron at short distances,
but that the advantage lies with the baser metal at
long ranges.' ^

' The carriage of this gnn has been modified in construction
aince this drawing was made.

* General Campbell assigns a true cause for tliis difference.
The ring of the bronze gun represents so much energy withdrawn

258 FEAGMENTS OF SCIENCE.

Coincident with tliese trials of guns at Woolwich,
gun-cotton was thought of as a probably etTective sound-
producer. From the hrst, indeed, theoretic considera-
tions caused me to fix my attention persistently on thia
substance ; for the remarkable experiments of Mr. Abel^
whereby its rapidity of combustion and violently ex-
plosive energy are demonstrated, seemed to single it
out as a substance eminently calculated to fulfil th^
conditions necessary to the production of an intense
wave of sound. What those conditions are we shall now
more particularly enquire, calling to our aid a brief but
very remarkable paper, published by Professor Stokes
in the ' Philosophical Magazine ' for 1868.

The explosive force of gunpowder is known to depenc
on the sudden conversion of a solid body into an in
tensely heated gas. Now the work which the artillerist
requires the expanding gas to perform is the displace-
ment of the projectile, besides which it has to displace
the air in front of the projectile, which is backed by
the whole pressure of the atmosphere. Such, however,
is not the work that we want our gunpowder to per-
form. We wish to transmute its energy not into the
mere mechanical translation of either shot or air, but
into vibratory motion. We want pulses to be formed
which shall propagate themselves to vast distance
through the atmosphere, and this requires a certain
choice and management of the explosive material.

A sound-wave consists essentially of two parts — s)
condensation and a rarefaction. Now air is a very
mobile fluid, and if the shock imparted to it lack due
promptness, the wave is not produced. Consider the
case of a common clock pendulum, which oscillates t

from the explosive force of the gunpowder. Further experimenta
would, however, be needed to place the superiority of the cast-iron
gun at a distance beyond question.

RECENT EXPERIMENTS ON FOa-SINGALS, 259

and fro, and which might be expected to generate cor-
responding pulses in the air. When, for example, the
bob moves to the right, the air to the right of it might
be supposed to be condensed, while a partial vacuum
might be supposed to follow the bob. As a matter of
fact, we have nothing of the kind. The air particles in
front of the bob retreat so rapidly, and those behind it
close so rapidly in, that no sound-pulse is formed. The
mobility of hydrogen, moreover, being far greater than
that of air, a prompter action is essential to the forma-
tion of sonorous waves in hydrogen than in air. It is to
this rapid power of readjustment, this refusal, so to
speak, to allow its atoms to be crowded together or to be
drawn apart, that Professor Stokes, with admirable
penetration, refers the damping power, first described
by Sir John Leslie, of hydrogen upon sound.

A tuning-fork which executes 256 complete vibra-
tions in a second, if struck gently on a pad and held in
free air, emits a scarcely audible note. It behaves to
some extent like the pendulum bob just referred to.
This feebleness is due to the prompt ' reciprocating flow '
of the air between the incipient condensations and rare-
factions, whereby the formation of sound-pulses is fore-
stalled. Stokes, however, has taught us that this flow
may be intercepted by placing the edge of a card in
close proximity to one of the corners of the fork.
An immediate augmentation of the sound of the fork is
the consequence.

The more rapid the shock imparted to the air, the
greater is the fractional part of the energy of the shock
converted into wave motion. And as different kinds of
gunpowder vary considerably in their rapidity of com-
bustion, it may be expected that they will also vary as
producers of sound. This theoretic inference is com-
pletely verified by experiment. In a series of prelimi-

260 FRAGMENTS OF SCIENCE.

nary trials conducted at Woolwich on the 4th of June,
1875, the sound-producing powers of four different
kinds of powder were determined. In the order of the
size of their grains they bear the names respectively of
Fine-grain (F. Gr.), Large-grain (L. Gr.), Eifle Large-
grain (R. L. Gr.), and Pebble- powder (P.) (See annexed
figures.) The charge in each case amounted to 4^ lbs. ;

m

L.G.

R. L. 0.

Fig. 7.

P.O.

four 24-lb. howitzers being employed to fire the respec-
tive charges. There were eleven observers, all of whom,
without a single dissentient, pronounced the sound of
the fine-grain powder loudest of all. In the opinion of
seven of the eleven the large-grain powder came next ;
seven also of the eleven placed the rifle large-grain third
on the list ; while they were again unanimous in pro-
nouncing the pebble-powder the worst ^ound-producer
These differences are entirely due to differences in the
rapidity of combustion. All who have witnessed the
performance of the 80-ton gun must have been sur-
prised at the mildness of its thunder. To avoid the
strain resulting from quick combustion, the powder
employed is composed of lumps far larger than those
of the pebble-powder above referred to. In the long
tube of the gim these lumps of solid matter gradually
resolve themselves into gas, which on issuing from the
muzzle imparts a kind of push to the air, instead of

BECENT EXPERIMENTS ON FOG-SIGNALS. 261

the sharp shock necessary to form the condensation of
an intensely sonorous wave.

These are some of the physical reasons why gun-
cotton might be regarded as a promising fog-signal.
Firing it as we have been taught to do by Mr. Abel,
its explosion is more rapid than that of gunpowder.
In its case the air particles, alert as they are, will not,
it might be presumed, be able to slip from condensation
to rarefaction with a rapidity sufficient to forestall the
formation of the wave. On a priori grounds then, we
are entitled to infer the effectiveness of gun-cotton,
while in a great number of comparative experiments,
stretching from 1874 to the present time, this inference
has been verified in the most conclusive manner.

As regards explosive material, and zealous and
accomplished help in the use of it, the resources of
Woolwich Arsenal have been freely placed at the dis-
posal of the Elder Brethren. General Campbell, Grene-
ral Younghusband, Colonel Fraser, Colonel Maitland,
and other officers, have taken an active personal part in
the investigation, and in most cases have incurred the
labour of reducing and reporting on the observations.
Gruns of various forms and sizes have been invoked for
gunpowder, while gun-cotton has been fired in free air
and in the foci of parabolic reflectors.

On the 22nd of February, 1875, a number of small
guns, cast specially for the purpose — some with plain,
some with conical, and some with parabolic, muzzles —
firing 4 oz. of fine grain powder, were pitted agriinst
4 oz. of gun-cotton detonated both in the open, and in
the focus of a parabolic reflector.^ The sound produced
by the gun-cotton, reinforced by the reflector, was
unanimously pronounced loudest of all. With equal

' For cliarj^es of this weight the reflector is of moderate size,
and may be employed without fear of fracture.

262

FEAaMENTS OF SCIENCE.

unanimity, the gun-cotton detonated in free air waa
placed second in intensity. Though the same charge
was used throughout, the guns differed notably among
themselves, but none of them came up to the gun-cotton,
either with or without the reflector. A second series,]
observed from a different distance on the same day,)
confirmed to the letter the foregoing result.

As a practical point, however, the comparative cost]
of gun-cotton and gunpowder has to be taken into]
account, though considerations of cost ought not to be j
stretched too far in cases involving the safety of human
life. In the earlier experiments, where quantities of]
equal price were pitted against each other, the results
were somewhat fluctuating. Indeed, the perfect mani-i
pulation of the gun-cotton required some preliminary]
discipline — promptness, certainty, and effectiveness of
tiring, augmenting as experience increased. As 1 lb. of]
gun-cotton costs as much as 3 lbs. of gunpowder, these
quantities were compared togetlier on the 22nd of Feb-
ruary. The guns employed to discharge the gunpowder!
were a 12-lb. brass howitzer, a 24-lb. cast-iron howitzer,]
and the long 18-pounder employed at the South Fore-j
land. The result was, that the 24-lb. howitzer, firing \
3 lbs. of gunpowder, had a slight advantage over 1 lb.
of gun-cotton detonated in the open ; while the 12-lb. i
howitzer and the 18-pounder were both beaten by the]
gun-cotton. On the 2nd of May, on the other hand, the
gun-cotton is reported as having been beaten by allj
the guns.

Meanwhile, the parabolic-muzzle gun, expressly
intended for fog-signalling, was pushed rapidly forward, j
and on March 22 and 23, 1876, its power was tested at]
Shoeburyness. Pitted against it were a 1 6-pounder, a
5J-inch howitzer, 1^ lb. of gun-cotton detonated in the
focus of a reflector (see annexed figure), and 1^ lb. of

RECENT EXPERIMENTS ON FOG-SIGNALS. 263

gun-cotton detonated in free air. On this occasion
nineteen different series of experiments were made, when
the new experimental gun, firing a 3-lb. charge, demon-
strated its superiority over all guns previously employed
to fire the same charge. As regards the comparative

Fig. 8.

Gun-cotton Slab (1^ lb.) Detonated in the Focus of a Cast-iron

Reflector.

merits of the gun-cotton fired in the open, and the gun-
powder fired from the new gun, the mean values of their
sounds were the same. Fired in the focus of'the re-
flector, the gun-cotton clearly dominated over all the
other sound-producers.'

The whole of the observations here referred to were
embraced by an angle of about 70°, of which 50° lay on
' The reflector was fractured by the explosion, but it did good
service afterwards.

18

2G4

FKAGMENTS OF SCIENCE.

the one side and 20° on the other side of the line of fire.
The shots were heard by eleven observers on board the
' Galatea,' which took up positions varying from 2 miles
to 1 3 J miles from the firing-poiDt. In all these obser-
vations, the reinforcing action of the reflector, and of
the parabolic muzzle of the gun, came into play. But
the reinforcement of the sound in one direction implies
its withdrawal from some other direction, and accord-
ingly it was found that at a distance of 5 J miles from
the firing-point, and on a line including nearly an angle
of 90° with the line of fire, the gun-cotton in the open
beat the new gun; while behind the station, at distances
of 8^ miles and 1 3 J miles respectively, the gun-cotton
in the open beat both the gun and the gun-cotton in the
reflector. This result is rendered more important by
the fact that the sound reached the Mucking Light, a
distance of 13^ miles, against a light wind which was
blowino' at the time.

o

Most, if not all, of our ordinary sound-producers
send forth waves which are not of uniform intensity
throughout. A trumpet is loudest in the direction of
its axis. The same is true of a gun. A bell, with its
mouth pointed upwards or downwards, sends forth waves
which are far denser in the horizontal plane passing
through the bell than at an angular distance of 90° from
that plane. The oldest bellhangeis must have been
aware of the fact that the sides of the bell, and not its
mouth, emitted the strongest sound, their practice being
probably determined by this knowledge. Our slabs of
gun-cotton also emit waves of different densities in differ-
ent parts. It has occurred in the experiments atShoebury-
ness thatwhen the broad side of a slab was turned towards
the suspending wire of a second slab six feet distant, the
wire was cut by the explosion, while when the edge of
the slab was turned to the wire this never occurred.

llECENt EXPEHlMENtS ON FOG-SlGNAtS. 2G5

To the circumstance that the broadsides of the slabs
faced the sea is probably to be ascribed the remarkable
fact observed on March 23, that in two directions, not
far removed from the line of fire, the gun-cotton
detonated in the open had a slight advantage over the
new gun.

Theoretic considerations rendered it probable that
the shape and size of the exploding mass would affect
the constitution of the wave of sound. I did not think
large rectangular slabs the most favourable shape, and
accordingly proposed cutting a large slab into fragments
of different sizes, and pitting them against each other.
The differences between the sounds were by no means
80 great as the differences in the quantities of explosive
material might lead one to expect. The mean values
of eighteen series of observations made on board the
'Galatea,' at distances varying from 1| mile to 4*8 miles,
were as follows: —

Weights . , 4 oz. 6 oz. 9 oz. 12 oz.
Value of sound . 3*12 3-34 4*0 403

These charges were cut from a slab of dry gun-cotton
about If inch thick: they were squares and rectangles
of the following dimensions: — 4 oz., 2 inches by 2
inches; 6 oz., 2 inches by 3 inches; 9 oz., 3 inches by
3 inches; 12 oz., 2 inches by 6 inches.

The numbers under the respective weights express
the recorded value of the sounds. They must be simply
vtaken as a ready means of expressing the approximate
relative intensity of tlie sounds as estimated by the ear.
When we find a 9-oz. charge marked 4, and a 1 2-oz.
charge marked 4*03, the two sounds may be regarded
as practically equal in intensity, thus proving that an
addition of 30 per cent, in the larger charges produces
no sensible difference in the sound. Were the sounds
estimated by some physical means, instead of by the ear.

266 FRAGMENTS OF SCIENCE.

the values of the sounds at the distances recorded wouli
not, in my opinion, show a greater advance with the in
crease of material than that indicated by the foregoing
numbers. Subsequent experiments rendered still more;
certain the effectiveness, as well as the economy, of the
smaller charges of gun-cotton.

It is an obvious corollary from the foregoing experi
ments that on our 'nesses' and promontories, where the
land is clasped on both sides for a considerable distance
by the sea — where, therefore, the sound has to propagate
itself rearward as well as forward — the use of the para-
bolic gun, or of the parabolic reflector, might be a
disadvantage rather than an advantage. Here gun-
cotton, exploded in the open, forms the most appropriate
source of sound. This remark is especially applicable
to such lightships as are intended to spread the soimd
all round them as from central foci. As a signal in
rock lighthouses, where neither syren, steam -whistle,
nor gun could be mounted ; and as a handy fleet-signal,
dispensing with the lumber of special signal-guns, the
gun-cotton will prove invaluable. But in most of these
cases we have the drawback that local damage may be
done by the explosion. The lantern of the rock light-
house might suffer from concussion near at hand, and
though mechanical arrangements might be devised,
both in the case of the lighthouse and of the ship's deck,
to place the firing-point of the gun-cotton at a safe
distance, no such arrangement could compete, as regards,
simplicity and effectiveness, with the expedient of a
gun-cotton rocket. Had such a means of signalling
existed at the Bishop's Eock lighthouse, the ill-fated
'Schiller' might have been warned of her approach to
danger ten, or it may be twenty, miles before she reached
the rock which wrecked her. Had the fleet possessed
Buch a signal, instead of the ubiquitous but ineffectual

KECENT EXPERIMENTS ON FOG-SIGNALS. 267

whistle, the 'Iron Duke' and * Vanguard' need never
have come into collision.

It was the necessity of providing a suitable signal
for rock lighthouses, and of clearing obstacles which
cast an acoustic shadow, that suggested the idea of the
gun-cotton rocket to Sir Eichard Collinson, Deputy
Master of the Trinity House. His idea was to place a
disk or short cylinder of gun-cotton in the head of a
rocket, the ascensional force of which should be employed
to carry the disk to an elevation of 1000 feet or there-
abouts, where by the ignition of a fuse associated with
a detonator, the gun-cotton should be fired, sending its
sound in all directions vertically and obliquely down
upon earth and sea. The first attempt to realise this
idea was made on July 18, 1876, at the firework manu-
factory of the Messrs. Brock, at Nunhead. Eight
rockets were then fired, four being charged with 5 oz.
and four with 7^ oz. of gim-cotton. They ascended to
a great height, and exploded with a very loud report in
the air. On July 27, the rockets were tried at Shoe-
buryness. The most noteworthy result on this occasion
was the hearing of the sounds at the Mouse Lighthouse,
SJ miles E. by S., and at the Chapman Lighthouse, 8-^
miles W. by N. ; that is to say, at opposite sides of the
firing-point. It is worthy of remark that, in the case
of the Chapman Lighthouse, land and trees intervened
between the firing-point and the place of observation
' This,' as General Younghusband justly remarked at
the time, * may prove to be a valuable consideration if
it should be found necessary to place a signal station in
a position whence the sea could not be freely observed.'
Indeed, the clearing of such obstacles was one of the
objects which the inventor of the rocket had in view.

With reference to the action of the wind, it was
thought desirable to compare the range of explosions

2G8 FRAGMENTS OF SCIENCE.

produced near the surface of the earth with otheri
produced at the elevation attainable by the gun-cotton
rockets. Wind and weather, however, are not at our
command; and hence one of the objects of a series of
experiments conducted on December 13, 1876, was not
fulfilled. It is worthy, however, of note that on this
day, with smooth water and a calm atmosphere, the
rockets were distinctly heard at a distance of 11*2 miles
from the firing-point. Tlie quantity of. gun-cotton
employed was 7^ oz. On Thursday, March 8, 1877,
these comparative experiments of firing at high and low
elevations were pushed still further. The gun-cotton
near the ground consisted of ^-Ib. disks, suspended from
a horizontal iron bar about 4^ feet above the ground.
The rockets carried the same quantity of gun-cotton in
their heads, and the height to which they attained, as
determined by a theodolite, was from 800 to 900 feet.
The day was cold, with occasional squalls of snow and
hail, the direction of the sound being at right angles to
that of the wind. Five series of observations were made
on board the ' Vestal,' at distances varying from 3 to 6
miles. The mean value of the explosions in the air
exceeded that of the explosions near the ground by
a small but sensible quantity. At Windmill Hill,
Grravesend, however, which was nearly to leeward, and
5J miles from the firing-point, in nineteen cases out of
twenty-four the disk fired near the ground was loudest;
while in the remaining five the rocket had the ad-
vantage.

Towards the close of the day the atmosphere became
very serene. A few distant cumuli sailed near the
horizon, but the zenith and a vast angular space all
round it were absolutely free from cloud. From the
deck of the ' Gralatea ' a rocket was discharged, which
reached a great elevation, and exploded with a loud

RECENT EXPERIMENTS ON FOa-sSIGNALS. 269

report. Following this solid nucleus of sound was a
continuous train of echoes, which retreated to a con-
tinually greater distance, dying gradually off into silence
after seven seconds' duration. These echoes were of
the same character as those so frequently noticed at
the South Foreland in 1872-73, and called by me 'aerial
echoes.'

On the 23rd of March the experiments were re-
sumed, the most noteworthy results of that day's obser-
vations being that the sounds were heard at Tillingham,
10 miles to the N.E. ; at West Mersea, 15| miles to the
N.E. by E. ; at Brightlingsea, 17^ miles to the N.E. ;
and at Clacton Wash, 20^ miles to the N.E. by ^ E.
The wind was blowing at the time from the S.E. Some
of these sounds were produced by rockets, some by a 24-
Ib. howitzer, and some by an 8-inch Maroon.

In December, 1876, Mr. Grardiner, the managing
director of the Cotton-powder Company, had proposed
a trial of this material against the gun-cotton. The
density of the cotton he urged was only 1 -03, while that
of the powder was 1*70. A greater quantity of explo-
sive material being thus compressed into the same
volume, Mr. Gardiner thought that a greater sonorous
effect must be produced by the powder. At the in-
stance of Mr. Mackie, who had previously gone very
thoroughly into the subject, a Committee of the Elder
Brethren visited the cotton-powder manufactory, on the
banks of the Swale, near Faversham, on the 16th of
June, 1877. The weights of cotton-powder employed
were 2 oz., 8 oz., 1 lb., and 2 lbs., in the form of rockets
and of signals fired a few feet above the ground. The
experiments throughout were arranged and conducted
by Mr. Mackie. Our desiie on this occasion was to get
as near to windward as possible, but the Swale and

270 FRAGMENTS OF SCIENCE.

other obstacles limited our distance to IJ mile. We
stood here E.S.E. from the firing-point while the wind
blew fresh from the N.E.

The cotton-powder yielded a very effective report.
The rockets in general had a slight advantage over the
same quantities of material fired near the ground. The
loudness of the sound was by no means proportional to
the quantity of the material exploded, 8 oz. yielding
very nearly as loud a report as 1 lb. The ' aerial
echoes,' which invariably followed the explosion of the
rockets, were loud and long- continued.

On the 17th of October, 1877, another series of ex-
periments with howitzers and rockets was carried out at
Shoeburyness. The charge of the howitzer was 3 lbs.
of L. G. powder. The charges of the rockets were
12 oz., 8 oz., 4 oz., and 2 oz. of gun-cotton respectively.
The gun and the four rockets constituted a series, and
eight series were fired during the afternoon of the 17th.
The observations were made from the ' Vestal ' and the
* Galatea,' positions being successively assumed which
permitted the sound to reach the observers with the
wind, against the wind, and across the wind. The dis-
tance of the ' Gralatea ' varied from 3 to 7 miles, that of
the ' Vestal,' which was more restricted in her move-
ments, being 2 to 3 miles. Briefly summed up, the
result is that the howitzer, firing a 3-lb. charge, which
it will be remembered was our best gun at the South
Foreland, was beaten by the 1 2-oz. rocket, by the 8-oz.
rocket, and by the 4-oz. rocket. The 2-oz. rocket
alone fell behind the howitzer.

It is worth while recording the distances at which
some of the sounds were heard on the day now ye*
ferred to : — '

KECENT EXPERIMENTS ON FOG-SIGNALS. 271

24 out of 40 sounds heard.

I. I^igh .

6^ miles W.N.W.

24

2. Girdler Light-

vessel .

12 „

S.B. by E.

5

3. Reculvers

m »

S.E. by S.

18

4. St. Nicholas .

20 „

S.E. .

3

5. Epple Bay

22 „

S.E. by E.

19

6. Westgate

23 „

S.E. by E.

9

7. Kingsgate

25 „

S.E. by E.

8

The day was cloudy, with occasional showers of
drizzling rain ; the wind about N.W. by N. all day ; at
times squally, rising to a force of 6 or 7 and sometimes
dropping to a force of 2 or 3. The station at Leigh
excepted, all these places were to leeward of Shoebury-
nees. At four other stations to leeward, varying in
distance from 15 J to 24^ miles, nothing was heard,
while at eleven stations to windward, varying from 8 to
26 miles, the sounds were also inaudible. It was found,
indeed, that the sounds proceeding directly against the
wind did not penetrate much beyond 3 miles.

On the following day, viz, the 18th October, we
proceeded to Dungeness with the view of making a
series of strict comparative experiments with gun-cotton
and cotton-powder. Kockets containing 8 oz., 4 oz.,
and 2 oz. of gun-cotton had been prepared at the Eoyal
Arsenal ; while others, containing similar quantities of
cotton-powder, had been supplied by the Cotton-powder
Company at Faversham. With these were compared
the ordinary 18-pounder gun, which happened to be
mounted at Dungeness, firing the usual charge of 3 lbs.

•of powder, and a syren.
ij From these experiments it appeared that the gun-
cotton and cotton-powder were practically equal as
producers of sound.

The efiectiveness of small charges was illustrated in
a very striking manner, only a single unit separating
the numerical value of the 8-oz. rocket from that of tha

272 FKAGMENTS OF SCIENCE.

2-oz. rocket. The former was recorded as 6'9 and the
latter as 5*9, the value of the 4-oz. rocket being inter-
mediate between them. These results were recorded by
a number of very practised observers on board the
'Gralatea.' They were completely borne out by the
observations of the Coastguard, who marked the value
of the 8-oz rocket 6*1, and that of the 2-oz. rocket 5*2.
The 18-pounder gun fell far behind all the rockets, a
residt, possibly, to be in part ascribed to the imperfec-
tion of the powder. The performance of the syren was,
on the whole, less satisfactory than that of the rocket.
The instrument was worked, not by steam of 70 lbs.
pressure, as at the South Foreland, but by compressed
air, beginning with 40 lbs. and ending with 30 lbs.
pressure. The trumpet was pointed to windward, and
in the axis of the instrument the sound was about as
effective as that of the 8-oz. rocket. But in a direction
at right angles to the axis, and still more in the rear of
this direction, the syren fell very sensibly behind even
the 2-oz. rocket.

These are the principal comparative trials made be-
tween the gun-cotton rocket and other fog-signals ; but
they are not the only ones. On the 2nd of August,
1877, for example, experiments were made at Lundy
Island with th(} following results. At 2 miles distant
from the firing-point, with land intervening, the 18-
pounder, firing a 3-lb. charge, was quite unheard.
Both the 4-oz. rocket and the 8-oz. rocket, however,
reached an elevation which commanded the acoustic
shadow, and yielded loud reports. When both were in
view the rockets were still superior to the gun. On
the 6th of August, at St. Ann's, the 4-oz. and 8-oz.
rockets proved superior to the syren. On the Shambles
Light- vessel, when a pressure of 13 lbs. was employed!
to sound the syren, the rockets proved greatly superior I

RECENT EXPERIMENTS ON FOG-SIGNALS.

273

to that instrument. Proceeding along the sea margin
at Flamboro' Head, Mr. Edwards states that at a dis-
tance of li mile, with the 18-pounder previously used
as a fog-signal hidden behind the cliffs, its report was
quite unheard, while the 4-oz. rocket, rising to an eleva-
tion which brought it clearly into view, yielded a power-
ful sound in the face of an opposing wind.

On the evening of February 9th, 1877, a remarkable
series of experiments were made by Mr. Prentice at
Stowmarket with the gun-cotton rocket. From the
report with which he has kindly furnished me I extract
the following, particulars. The first column in the
annexed statement contains the name of the place of
observation, the second its distance from the firing-poiut,
and the third the result observed : —

Stoke Hill, Ipswich • 10 miles Rockets clearly seen and sounds

distinctly heard 53 seconds
after the flash.

Melton . • • 15 „ Signals distinctly heard.

Thought at first that sounds
were reverberated from the

Pramlingham ,

Stratford. St. Andrews . 19

Tuddenham. St. Martin 10

Christ Church Purk.

Nettlestead Hall

Bildestone

Nacton

18 ff Signals very distinctly heard,
both in the open air and in a
closed room. Wind in favour
of sound.
Rei)orts loud ; startled pheasants

in a cover close by.
Reports very loud ; rolled away
like thunder.
11 „ Report arrived a little more
than a minute after flash.
G „ Distinct in every part of ob-
server's house. Very loud in
the open air.
6 „ Explosion very loud, wind
against sound.
14 K Reports quite distinct — mis-
taken by inhabitants for clapi
of thunder.

274 njAGMENTS OF SCIENCE.

Aldboro' . • • .25 miles Rockets seen throngh a very

hazy atmosphere ; a rumbling
detonation heard.

Capel Mills • • . 11 „ Reports heard within and with-
out the observer's house.
Wind opposed to sound.

Lawford . • • • 15 J ^ Reports distinct : attributed to

distant thunder.

In the great majority of these cases, the direction of
the sound enclosed a large angle with the direction
of the wind. In some cases, indeed, the two directions
were at right angles to each other. It is needless to
dwell for a moment on the advantage of possessing a
signal commanding ranges such as these.

The explosion of substances in the air, after having
been carried to a considerable elevation by rockets, is
a familiar performance. In 1873, moreover, the Board
of Trade proposed a Hght-and-sound rocket as a signal
of distress, which proposal was subsequently realized,
but in a form too elaborate and expensive for practical
use. The idea of a gun-cotton rocket fit for signalling
in fogs is, I believe, wholly due to Sir Richard
Collinson, the Deputy Master of the Trinity House.
Thanks to the skilful aid given by the authorities of
Woolwich, by Mr. Prentice, and Mr. Brock, that idea
is now an accomplished fact ; a signal of great power,
handiness, and economy, being thus placed at the
service of our mariners. Not only may the rocket be
applied in association with lighthouses and lightships,
but in the Navy also it may be turned to important
account. Soon after the loss of the 'Vanguard' I
ventured to urge upon an eminent naval officer the de-
sirability of having an organized code of fog-signals
for the fleet. He shook his head doubtingly, and
referred to the difficulty of finding room for signal guns.
The gun-cotton rocket completely surmounts this diffi-

BECENT EXPERIMENTS ON -FOG-SIGNALS. 275

culty. It is manipulated with ease and rapidity, while
its discharges may be so grouped and combined as to
give a most important extension to the voice of the
admiral in command. It is needless to add that at any
point upon our coasts, or upon any other coast, where
its establishment might be desirable, a fog-signal
station might be extemporised without difficulty.

I have referred more than once to the train of echoes
which accompanied the explosion of gun-cotton in free
air, speaking of them as similar in all respects to those
which were described for the first time in my Eeport on
Fog-signals, addressed to the Corporation of Trinity
House in 1874.^ To tliese echoes I attached a funda-
mental significance. There was no visible reflecting
surface from which they could come. On some days,
with hardly a cloud in the air and hardly a ripple on
the sea, they reached a magical intensity. As far as the
sense of hearing could judge, they came from the body
of the air in front of the great trumpet which produced
them. The trumpet blasts were five seconds in dura-
tion, but long before the blast had ceased the echoes
struck in, adding their strength to the primitive note
of the trumpet. After the blast had ended the echoes
continued, retreating further and further from the point
of observation, and finally dying away at great distances.
The echoes were perfeptly continuous as long as the sea
was clear of ships, ' tapering ' by imperceptible grada-
tions into absolute silence. But when a ship happened
to throw itself athwart the course of the sound, the echo
from the broadside of the vessel was returned as a shock
which rudely interrupted the continuity of the dying
atmospheric music.

* See also * Philosophical Transactions ' for 1874, p. 183.

270 FEAGMENTS OF SCIENCE.

These echoes have been ascribed to reflection from
the crests of the sea-waves. But this hypothesis is
negatived by the fact, that the echoes were produced
in great intensity and duration when no waves existed
— when the sea, in fact, was of glassy smoothness. It
has been also shown that the direction of the echoes
depended not on that of waves, real or assumed, but
on the direction of the axis of the trumpet. Causiug
that axis to traverse an arc of 210°, and the trumpet
to sound at various points of the arc, the echoes were
always, at all events in calm weather, returned from
that portion of the atmosphere towards which the
trumpet was directed. They could not, under the
circumstances, come from the glassy sea; while both
their variation of direction and their perfectly con-
tinuous fall into silence, are irreconcilable with the
notion that they came from fixed objects on the land.
They came from that portion of the atmosphere into
which the trumpet poured its maximum sound, and fell
in intensity as the direct sound penetrated to greater
atmospheric distances.

The day on which our latest observations were made
was particularly fine. Before reaching Duugeness, the
smoothness of the sea and the serenity of the air caused
me to test the echoing power of the atmosphere. A
single ship layabout half a mile distant between us and
the land. The result of the proposed experiment was
clearly foreseen. It was this. The rockpt being sent
up, it exploded at a great height; the echoes retreated
in their usual fashion, becoming less and less intense as
the distances of the invisible surfaces of reflection from
the observers increased. About five seconds after the
explosion, a single loud shock was sent back to us from
the side of the vessel lying between us and the land.
Obliterated for a moment by this more intense echo.

EECENT EXPERIMENTS ON FOG-SIGNALS. 277

the aerial reverberation continued its retreat, dying
away into silence in two or three seconds afterwards.*

I have referred to the firing of an 8-oz. rocket from
the deck of the ' Gralatea' on March 8, 1877, stating the
duration of its echoes to be seven seconds. Mr. Prentice,
who was present at the time, assured me that in his
experiments similar echoes had been frequently heard
of more than twice this duration. The ranges of his
sounds alone would render this result in the highest
degree probable.

To attempt to interpret an experiment which I have
not had an opportunity of repeating, is an operation of
some risk ; and it is not without a consciousness of this
that I refer here to a result announced by Professor
Joseph Henry, which he considers adverse to the notion
of aerial echoes. He took the trouble to point the
trumpet of a syren towards the zenith, and found that
when the syren was sounded no echo was returned.
Now the reflecting surfaces which give rise to these
echoes are for the most part due to differences of tem-
perature between sea and air. If, through any cause,
the air above be chilled, we have descending streams —
it the air below be warmed, we have ascending streams
as the initial cause of atmospheric flocculence. A
sound proceeding vertically does not cross the streams,
nor impinge upon the reflecting surfaces, as does a
sound proceeding horizontally across them. Aerial
echoes, therefore, will not accompany the vertical sound
as they accompany the horizontal one. The experiment,
as I interpret it, is not opposed to the theory of these
echoes which I have ventured to enunciate. But, as I
have indicated, not only to see but to vary such an

' The echoes of the gxm fired on shore this day were very brief ;
those of the 12-oz. gun-cotton rocket were 12" and those of the 8-oz.
ootton-powder rocket 11" in duration.

278 FEAGMENTS OF SCIENCE.

experiment is a necessary prelude to grasping its full
significance.

In a paper published in the ' Philosophical Trans- •.
actions' for 1876, Professor Osborne Reynolds refers
to these echoes in the following terms : — ' Without
attempting to explain the reverberations and echoes
which have been observed, I will merely call attention
to the fact that in no case have I heard any attending
the reports of the rockets,^ although they seem to have,,
been invariable with the guns and pistols. These facts!
suggest that the echoes are in some way connected with
the direction given to the sound. They are caused by
the voice, trumpets, and the syren, all of which give
direction to the sound ; but I am not aware -that they
have ever been observed in the case of a sound which has
no direction of greatest intensity.' The reference to the
voice, and other references in his paper, cause me to think
that, in speaking of echoes. Professor Osborne Reynolds
and myself are dealing with different phenomena. Be
that as it may, the foregoing observations render it
perfectly certain that the condition as to direction here
laid down is not necessary to the production of the
echoes.

There is not a feature connected with the aerial
echoes which cannot be brought out by experiments in
the air of the laboratory. I have recently made the
following experiment : — A rectangle, x y (p. 279), 22
inches by 12, was crossed by twenty-three brass tubes
(half the number would suffice and only eleven are
shown in the figure), each having a slit along it from
which gas can issue. In this way twenty-three low
flat flames were obtained. A sounding reed a fixed in a

' These carried 12 oz. of gunpowder, which has been found by
Col. Fruder to require an iron case to produce an effective explosion, ,

RECENT EXPERIMENTS ON FOG-SIGNALS.

279

short tube was placed at one end of the rectangle, and
a ' sensitive flame,' ^ /, at some distance beyond the
other end. When the reed sounded, the flame in front
of it was violently agitated, and roared boisterously.
Turning on the gas, and lighting it as it issued from
the slits, the air above the flames became so hetero-
geneous that the sensitive flame was instantly stilled,
rising from a height of 6 inches to a height of 18
inches. Here we had the acoustic opacity of the air
in front of the South Foreland strikingly imitated.^
Turning off the gas, and removing the sensitive flame
to /', some distance behind the reed, it burned there
tranquilly, though the reed was sounding. Again

Fig. 9.

lighting the gas as it issued from the brass tubes,
the sound reflected from the heterogeneous air threw

» Fully described in my ' Lectures on Soun^,' 3rd edition, p. 227.
" Lectures on Sound, 3rd ed., p. 268.

19

280 FRAGMENTS OF SCIENCE.

the sensitive flame into violent agitation. Here we
had imitated the aerial echoes heard when standing
behind the syren-trumpet at the South Foreland. The
experiment is extremely simple, and in the highest
degree impressive.

The explosive rapidity of dj^naraite marks it as a
substance specially suitable for the production of sound.
At the suggestion of Professor Dewar, Mr. McEoberts
has carried out a series of experiments on dynamite,
with extremely promising results. Immediately after
the delivery of the foregoing lecture I was informed
that Mr. Brock proposed the employment of dynamite
in the Collinson rocket.

r^mJI

XL

ON THE STUDY OF PHYSICS}

I HOLD in my hand an uncorrected proof of the sylla-
bus of this course of lectures, and the title of
the present lecture is there stated to be ' On the Import-
ance of the Study of Physics as a Means of Education.*
The corrected proof, however, contains the title : — ' On
the Importance of the Study of Physics as a Branch
of Education.' Small as this editorial alteration may
seem, the two words suggest two radically distinct
modes of viewing the subject before us. The term
Education is sometimes applied to a single faculty or
organ, and if we know wherein the education of a
single faculty consists, this will help us to clearer
notions regarding the education of the sum of all the
faculties, or of the mind. When, for example, we speak
of the education of the voice, what do we mean?
There are certain membranes at the top of the
Vv'indpipe which throw into vibration the air forced
between them from the lungs, thus producing musical
sounds. These membranes are, to some extent, under
the control of the will, and it is found that they can be
so modified by exercise as to produce notes of a clearer
and more melodious character. This exercise we call
the education of the voice. We may choose for our

* From a lecture delivered in the Boyal Institution of Great
Britain in the Spring of 1854.

28^ FEAGMENTS OF SCIENCE.

exercise songs new or old, festive or solemn ; the edu-
cation of the voice being the object aimed at, the songs
may be regarded as the means by which this education
is accomplished. I think this expresses the state of
the case more clearly than if we were to call the songs
a branch of education. Regarding also the education
of the human mind as the improvement and develop-
ment of the mental faculties, 1 shall consider the study
of Physics as a means towards the attainment of this
end. From this point of view, I degrade Physics into
an implement of culture, and this is my deliberate
design.

The term Physics, as made use of in the present
Lecture, refers to that portion of natural science which
lies midway between astronomy and chemistry. The
former, indeed, is Physics applied to ' masses of enor-
mous weight,' while the latter is Physics applied to
atoms and molecules. The subjects of Physics proper
are therefore those which lie nearest to human per-
ception:— light and heat, colour, sound, motion, the
loadstone, electrical attractions and repulsions, thunder
and lightning, rain, snow, dew, and so forth. Our
senses stand between these phenomena and the reasoning
mind. We observe the fact, but are not satisfied with
the mere act of observation : the fact must be accounted
for — fitted into its position in the line of cause and
effect. Taking our facts from Nature we transfer
them to the domain of thouo'ht : look at them, compare
them, observe their mutual relations and connexions,
and bringing them ever clearer before the mental eye,
finally alight upon the cause which unites them. This
is the last act of the mind, in this centripetal direction —
in its progress from the multiplicity of facts to the
central cause on which they depend. But, having
guessed the cause, we are not yet contented. We set

ON THE STUDY OF PHYSICS. 283

out from the centre and travel in the other direction.
If the guess be true, certain consequences must follow
from it, and we appeal to the law and testimony of
experiment whether the thing is so. Thus is the
circuit of thought completed,— from without inward,
from multiplicity to unity, and from within outward,
from unity to multiplicity. In thus traversing both
ways the line between cause and effect, all our reason-
ing powers are called into play. The mental effort
involved in these processes may be compared to those
exercises of the body which invoke the co-operation of
every muscle, and thus confer upon the whole frame the
benefits of healthy action.

The first experiment a child makes is a physical
experiment : the suction-pump is but an imitation of
the first act of every new-born infant. Nor do I think
it calculated to lessen that infant's reverence, or to
make him a worse citizen, when his riper experience
shows him that the atmosphere was his helper in ex-
tracting the first draught from his mother's breast.
The child grows, but is still an experimenter : he grasps
at the moon, and his failure teaches him to respect
distance. At length his little fingers acquire sufficient
mechanical tact to lay hold of a spoon. He thrusts
the instrument into his mouth, hurts his gums, and
thus learns the impenetrability of matter. He lets the
spoon fall, and jumps with delight to hear it rattle
against the table. The experiment made by accident
is repeated with intention, and thus the young student
receives his first lessons upon sound and gravitation.
There are pains and penalties, however, in the path of
the enquirer : he is sure to go wrong, and Nature is
just as sure to inform him of the fact. He falls down-
stairs, burns his fingers, cuts his hand scalds his tongue,
and in this way learns the conditions of his physical

284 FRAGMENTS OF SCIENCE.

well being. This is Nature's way of proceeding, and it
is wonderful what progress her pupil makes. His
enjoyments for a time are physical, and the con-
fectioner's shop occupies the foreground of human
happiness ; but the blossoms of a finer life are already
beginniDg to unfold themselves, and the relation of
cause and effect dawns upon the boy. He begins to
see that the present condition of thin^ is not final,
but depends upon one that has gone before, and will be
succeeded by another. He l>ecomes a puzzle to himself;
and to satisfy his newly-awakened curiosity, asks all
manner of inconvenient questions. The needs and
tendencies of human nature express themselves through
these early yearnings of the child. As thought ripens,
he desires to know the character and causes of the
phenomena presented to his observation; and unless
this desire has been granted for the express purpose of
having it repressed, unless the attractions of natural
phenomena be like the blush of the forbidden fruit,
conferred merely for the purpose of exercising our self-
denial in letting them alone ; we may fairiy claim for
the study of Physics the recognition that it answers to an
impulse implanted by nature in the constitution of man.
A few days ago, a Master of Arts, who is still a
young man, and therefore the recipient of a modem
education, stated to me that until he had reached the
age of twenty years he had never been taught anything
whatever regarding natural phenomena, or natural law.
Twelve years of his life previously had been spent
exclusively among the ancients. The case, I regret to
say, is typical. Now, we cannot, without prejudice to
humanity, separate the present from the past. The
nineteenth century strikes its roots into the centuries
gone by, and draws nutriment from them. The world
cannot afford to lose the record of any great deed or

' ON THE STUDY OF PHYSICS. 285

utterance ; for such are prolific throughout all time.
We cannot yield the companionship of our loftier
brothers of antiquity, — of our Socrates and Cato, —
whose lives provoke us to sympathetic greatness across
the interval of two thousand years. As long as the
ancient languages are the means of access to the ancient
mind, they must ever be of priceless value to humanity ;
but surely these avenues might be kept open without
making such sacrifices as that above referred to, uni-
versal. We have conquered and possessed ourselves of
continents of land, concerning which antiquity knew
nothing ; and if new continents of thought reveal them-
selves to the exploring human spirit, shall we not
possess them also ? In these latter days, the study of
Physics has given us glimpses of the methods of Nature
which were quite hidden from the ancients, and we
should be false to the trust committed to us, if we
were to sacrifice the hopes and aspirations of the
Present out of deference to the Past,

The bias of my own education probably manifests
itself in a desire I always feel to seize upon every
possible opportunity of checking my assumptions and
conclusions by experience. In the present case, it is true,
your own consciousness might be appealed to in proof
of the tendency of the human mind to inquire into the
phenomena presented to it by the senses ; but I trust you
will excuse me if, instead of doing this, I take advan-
tage of the facts which have fallen in my way through
life, referring to your judgment to decide whether such
facts are truly representative and general, and not
merely individual and local.

At an agricultural college in Hampshire, with which
I was connected for some time, and which is now con-
verted into a school for the general education of youth,
a Society was formed among the boys, who met weekly

286 FllAGMENTS OF SCIENCE.

for the purpose of reading reports and papers upon
various subjects. The Society had its president and
treasurer; and abstracts of its proceedings were pub-
lished in a little monthly periodical issuing from the
school press. One of the most remarkable features
of these weekly meetings was, that after the general
business had been concluded, each member enjoyed the
right of asking questions on any subject on which he
desired information. The questions were either written
out previously in a book, or, if a question happened to
suggest itself during the meetingj it was written upon
a slip of paper and handed in to the Secretary, who
afterwards read all the questions aloud. A number of
teacliers were usually present, and they and the boys
made a common stock of their wisdom in furnishing
replies. As might be expected from an assemblage of
eighty or ninety boys, varying from eighteen to eight
years old, many odd questions were proposed. To the
mind which loves to detect in the tendencies of the
young the instincts of humanity generally, such ques-
tions are not without a certain philosophic interest,
and I have therefore thought it not derogatory to the
present course of Lectures to copy a few of them, and
to introduce them here. They run as follows : —

What are the duties of the Astronomer Koyal ?

What is frost ?

Why are thunder and lightning more frequent in
summer than in winter ?

What occasions falling stars ?

What is the cause of the sensation called ' pins and
needles ' ?

What is the cause of waterspouts ?

What is the cause of hiccup ?

If a towel be wetted with water, why does the wet
portion become darker than before ?

ON THE STUDY OF PHYSICS. 287

What is meant by Lancashire witches ?

Does the dew rise or fall ?

"What is the principle of the hydraulic press ?

Is there more oxygen in the air in summer than in
winter ?

What are those rings which we see round the. gas
and sun ?

What is thunder ?

How is it that a black hat can be moved by forming
round it a magnetic circle, while a white hat remains
stationary ?

What is the cause of perspiration ?

Is it true that men were once monkeys ?

What is the difference between the soul and the
mind?

Is it contrary to the rules of Vegetarianism to eat
eggs?

In looking over these questions, which were wholly
unprompted, and have been copied almost at random
from the book alluded to, we see that many of them
are suggested directly by natural objects, and are not
such as had an interest conferred on them by previous
culture. Now the fact is beyond the boy's control, and
80 certainly is the desire to know its cause. The sole
question then is, whether this desire is to be gratified or
not. Who created the fact ? Who implanted the desire ?
Certainly not man. Who then will undertake to place
himself between the desire and its fulfilment, and pro-
claim, a divorce between them ? Take, for example, the
case of the wetted towel, which at first sight appears to
be one of the most unpromising questions in the list.
Shall we tell the proposer to repress his curiosity, as
the subject is improper for him to know, and thus
interpose our wisdom to rescue the boy from the con-
sequences of a wish which acts to his prejudice ? Or,

288 FRAGMENTS OF SCIENCE.

recognising the propriety of the question, how shall we
answer it? It is impossible to answer it without
reference to the laws of optics — without making the
boy to some extent a natural philosopher. You may
say that the effect is due to the reflection of light
at the common surface of two media of different
refractive indices. But this answer presupposes on the
part of the boy a knowledge of what reflection and
refraction are, or reduces you to the necessity of
explaining them.

On looking more closely into the matter, we find
that our wet towel belongs to a class of phenomena
which have long excited the interest of philosophers.
The towel is white for the same reason that snow is
white, that foam is white, that pounded granite or
glass is white, and that the salt we use at table is
white. -On quitting one medium and entering another,
a portion of light is always reflected, but on this condi-
tion— ^the media must possess different refractive indices.
Thus, when we immerse a bit of glass in water, light
is reflected from the common surface of both, and it is
this light which enables us to see the glass. But when
a transparent solid is immersed in a liquid of the same
refractive index as itself, it immediately disappears. I
remember once dropping the eyeball of an ox into
water ; it vanished as if by magic, with the exception
of the crystalline lens, and the surprise was so great
as to cause a bystander to suppose that the vitreous
humour had been instantly dissolved. This, however,
was not the case, and a comparison of the refractive
index of the humour with that of water cleared up
the whole matter. The indices were identical, and
hence the light pursued its way through both as if they
formed one continuous mass.

In the case of snow, powdered quartz, or salt, we

ON THE STTTDY OF PHYSICS. 289

have a transparent solid mixed with air. At every
transition from solid to air, or fiom air to solid, a por-
tion of light is reflected, and this takes place so often
that the light is wholly intercepted. Thus from the
mixture of two transparent bodies we obtain an opaque
one. Now the case of the towel is precisely similar.
The tissue is composed of semi-transparent vegetable
fibres, with the interstices between them filled with
air ; repeated reflection takes place at tlie limiting
surfaces of air and fibre, and hence the towel becomes
opaque like snow or salt. But if we fill the interstices
with water, we diminish the reflection ; a portion of
the light is transmitted, and the darkness of the towel
is due to its increased transparency. Thus the deport-
ment of various minerals, such as hydrophane and
tabasheer, the transparency of tracing paper used by
engineers, and many other considerations of the highest
scientific interest, are involved in the simple enquiry of
this unsuspecting little boy.

Again, t^ke the question regarding the rising or
falling of the dew — a question long agitated, and finally
set at rest by the beautiful researches of Wells. I do
not think that any boy of average intelligence will be
satisfied with the simple answer that the dew falls.
He will wish to learn how you know that it falls, and,
if acquainted with the notions of the middle ages, he
may refer to the opinion of Father Laurus, that a goose
egg filled in the morning with dew and exposed to the
sun, will rise like a balloon — a swan's egg being better
for the experiment than a goose egg. It is impossible
to give the boy a clear notion of the beautiful pheno-
menon to which his question refers, without first
making him acquainted with the radiation and con-
duction of heat. Take, for example, a blade of grass,
from which one of these orient pearls is depending,

290 FRAGMENTS OF SCIENCE.

During the day the grass, and the earth beneath it,
possess a certain amount of warmth imparted by the
sun ; during a serene night, heat is radiated from the
surface of the grass into space, and to supply the loss,
there is a flow of heat from the earth to the blade.
Thus the blade loses heat by radiation, and gains heat
by conduction. Now, in the case before us, the power
of radiation is great, whereas the power of conduction
is small ; the consequence is that the blade loses more
than it gains, and hence becomes more and more
refrigerated. The light vapour floating around the
surface so cooled is condensed upon it, and there accu-
mulates to form the little pearly globe which we call a
dew-drop.

Thus the boy finds the simple and homely fact
which addressed his senses to be the outcome and flower
of the deepest laws. The fact becomes, in a mea«ure,
sanctiHed as an object of thought, and invested for him
with a beauty for evermore. He thus learns that
things which, at first sight, seem to stand ^isolated and
without apparent brotherhood in Natiw-e axe organically
united, and finds the detection of such analogies a source
of perpetual delight. To enlist pleasure on the side of
intellectual performance is a point of the utmost im-
portance ; for the exercise of the mind, like that of the
body, depends for its value upon the spirit in which it
is accomplished. Every physician knows that some-
thing more than mere mechanical motion is compre-
hended under the idea of healthfid exercise — that,
indeed, being most healthful which makes us forget all
ulterior ends in the mere enjoyment of it. What, for
example, could be substituted for the action of the
playground, where the boy plays for the mere love of
playing, and without reference to physiological laws;
while kindly Nature accomplishes her ends uncon-

ON TRE STUDY OF PHYSICS. 291

sciously, and makes his very indifference beneficial to
him. You may have more systematic motions, you
may devise means for the more perfect traction of each
particular must le, but you cannot create the joy and
gladness of the game, and where these are absent, tlie
charm and the health of the exercise are gone. The
case is similar with the education of the mind.

The study of Physics, as already intimated, consists
of two processes, which are complementary to each
other — the tracing of facts to their causes, and the
logical advance from the cause to the fact. In the
former process, called induction, certain moral qualities
come into play. The first condition of success is patient
industry, an honest receptivity, and a willingness to
abandon all preconceived notions, however cherished, if
they be found to contradict the truth. Believe me, a
self-renunciation which has something lofty in it, and
of which the world never hears, is often enacted in the
private experience of the true votary of science. And
if a man be not capable of this self-renunciation — this
loyal surrender of himself to Nature and to fact, he
lacks, in my opinion, the first mark of a true philo-
sopher. Thus the earnest prosecutor of science, who
does not work with the idea of producing a sensation
in the world, who loves the truth better than the
transitory blaze of to-day\j fame, who comes to his task
with a single eye, finds in that task an indirect means
of the highest moral culture. And although the virtue
of the act depends upon its privacy, this sacrifice of
self, this upright determination to accept the truth, no
matter how it may present itself — even at the hands of
a scientific foe, if necessary — carries with it its own
reward. When prejudice is put under foot and the
stains of personal bias have been washed away — when a
man consents to lay aside his vanity and to become

292 FRAGMENTS OF SCIENCE.

Nature's organ— his elevation is the instant consequence
of his humility. I should not wonder if my remarks
provoked a smile, for they seem to indicate that I
regard the man of science as a heroic, if not indeed
an angelic, character; and cases may occur to you
which indicate the reverse. You may point to the
quarrels of scientific men, to their struggle's for priority,
to that unpleasant egotism which screams around its
little property of discovery like a scared plover about
its young. I will not deny all this ; but let it be set
down to its proper account, to the weakness — or, if
you will — to the selfishness of Man, but not to the
charge of Physical Science.

The second process in physical investigation is de-
duction, or the advance of the mind from fixed prin-
ciples to the conclusions which flow from them. The
rules of logic are the formal statement of this process,
which, however, was practised by every healthy mind
before ever such rules were written. In the study of
Physics, induction and deduction are perpetually wedded
to each other. The man observes, strips facts of their
peculiarities of form, and tries to unite them by their
essences ; having effected this, he at once deduces, and
thus checks his induction. Here the grand difference
between the methods at present followed, and those of
the ancients, becomes manifest. They were one-sided
in these matters : they omitted the process of induction,
and substituted conjecture for observation. They could
never, therefore, fulfil the mission of Man to ' replenish
the earth, and subdue it.' The subjugation of Nature
is only to be accomplished by the penetration of her
secrets and the patient mastery of her laws. This not
only enables us to protect ourselves from the hostile action
of natural forces, but makes them our slaves. By the
study of Physics we have indeed opened to us treasuriei

ON THE STUDY OF PHYSICS. 293

of power of which antiquity never dreamed. But while
we lord it over Matter, we have thereby become better
acquainted with the laws of Mind ; for to the mental
philosopher the study of Physics furnishes a screen
agfiinst which the human spirit projects its own image,,
and thus becomes capable of self-inspection.

Thus, then, as a means of intellectual culture, the
study of Physics exercises and sharpens observation : it
brings the most exhaustive logic into play : it compares,
abstracts, and generalizes, and provides a mental scenery
appropriate to these processes. The stricte.->t precision
of thought is everywhere enforced, and prudence, fore-
sight, and sagacity are demanded. By its appeals to
experiment, it continually checks itself, and thus walks
on a foundation of facts. Hence the exercise it invokes
does not end in a mere game of intellectual gymnastics,
such as the ancients delighted in, but tends to the
mastery of Nature. This gradual conquest of the ex-
ternal world, and the consciousness of augmented
strength which accompanies it, render the study of
Physics as delightful as it is important.

With regard to the effect on the imagination, certain
it is that the cool results of physical induction furnish
conceptions which transcend the most daring flights of
tl]at faculty. Take for example the idea of an all-
pervading ether which transmits a tingle, so to speak,
to the finger ends of the universe every time a street
lamp is lighted. The invisible billows of this ether can
be measured with the same ease and certainty as that
with which an engineer measures a base and two angles,
and from these finds tlie distance across the Thames.
Now it is to be confessed that there may be just as little
poetry in the measurement of an ethereal undulation
as in that of the river ; for the intellect, during the acts
of measurement and calculation, destroys those notions

294 FRAGMENTS OF SCIENCE.

of size which appeal to the poetic sense. It is a mia-
take to suppose, with Dr. Young, that

An undevout astronomer is mad ;

there being no necessary connexion between a devout
state of mind and the observations and calculations of
a practical astronomer. It is not until the man
withdraws from his calculation, as a painter from his
work, and thus realizes the great idea on which he has
been engaged, that imagination and wonder are excited.
There is, I admit, a possible danger here. If the arith-
metical processes of science be too exclusively pursued,
they may impair the imagination, and thus the study
of Physics is open to the same objection as philological,
theological, or political studies, when carried to excess.
But even in this case, the injury done is to the in-
vestigator himself: it does not reach the mass of
mankind. Indeed, the conceptions furnished by his
cold unimaginative reckonings may furnish themes for
the poet, and excite in the highest degree that senti-
ment of wonder which, notwithstanding all its foolish
vagaries, table-turning included, I, for my part, should
be sorry to see banished from the world.

I have thus far dwelt upon the study of Physics as
an agent of intellectual culture ; but like other things
in Nature, this study subserves more than a single end.
The colours of the clouds delight the eye, and, no
doubt, accomplish moral purposes also, but the self-
same clouds hold within their fleeces the moisture by
which our fields are rendered fruitful. The sunbeams
excite our interest and invite our investigation; but
they also extend their beneficent influences to our fruits
and corn, and thus accomplish, not only intellectual
ends, but minister, at the same time, to our material
necessities. And so it is with scientific research.

Mi

ON THE STUDY OF PHYSICS. 295

While the love of science is a sufficient incentive to
the pursuit of science, and the investigator, in the
prosecution of his enquiries, is raised above all material
considerations, the results of his labours may exercise a
potent influence upon the physical condition of the
community. This is the arrangement of Nature, and
not that of the scientific investigator himself; for he
usually pursues his object without regard to its practical
applications.

And let him who is dazzled by such applications —
who sees in the steam-engine and the electric telegraph
the highest embodiment of human genius and the only
legitimate object of scientific research, beware of pre-
scribing conditions to the investigator. Let him be-
ware of attempting to substitute for that simple love
with which the votary of science pursues his task, the
calculations of what he is pleased to call utility. The
professed utilitarian is unfortunately, in most cases, the
very last man to see the occult sources from which
useful results are derived. He admires the flower, but
is ignorant of the conditions of its growth. The scien-
tific man must approach Nature in his own way ; for
if you invade his freedom by your so-called practical
considerations, it may be at the expense of those
qualities on which his success as a discoverer depends.
Let the self-styled practical man look to those from
the fecundity of whose thought he, and thousands like
him, have sprung into existence. Were they inspired
in their first enquiries by the calculations of utility ?
Not one of them. They were often forced to live low
and lie hard, and to seek compensation for their
penury in the delight which their favourite pursuits
afforded them. In the words of one well qualified to
speak upon this subject, ' I say not merely look at the
pittance of men like John Dalton, or the voluntary

20

296 FRAGMENTS OF SCIENCE.

starvation of the late Graff; but compare what ia
considered as competency or affluence by your Faradays,
Liebigs, and Herschels, with the expected results of a
life of successful commercial enterprise : then compare
the amount of mind put forth, the work done for society
in either case, and you will be constrained to allow that
the former belong to a class of workers who, properly
speaking, are not paid, and cannot be paid for their
work, as indeed it is of a sort to which no payment
could stimulate.'

But while the scientific investigator, standing upon
the frontiers of human knowledge, and aiming at
the conquest of fresh soil from the surrounding region
of the unknown, makes the discovery of truth his ex-
clusive object for the time, he cannot but feel the
deepest interest in the practical application of the
truth discovered. There is something ennobling in the
triumph of Mind over Matter. Apart even from its
uses to society, there is something elevating in the idea
of Man having tamed that wild force which flashes
through the telegraphic wire, and made it the minister
of his will. Our attainments in these directions appear
to be commensurate with our needs. We had already
subdued horse and mule, and obtained from them all
the service which it was in their power to render : we
must either stand still, or find more potent agents to
execute our purposes. At this point the steam-engine
appears. These are still new things; it is not long
since we struck into the scientific methods which have
produced these results. We cannot for an instant
regard them as the final achievements of Science, bufcl
rather as an earnest of what she is yet to do. They
mark our first great advances upon the dominion ot
Nature. Animal strength fails, but here are the forces
which hold the world together, and the instincts and

ON THE STUDY OF PHYSICS. 297

successes' of Man assure him that these forces are his
when he is wise enough to command them.

As an instrument of intellectual culture, the study
of Physics is profitable to all : as bearing upon special
functions, its value, though not so great, is still more
tangible. Wliy, for example, should Members of Par-
liament be ignorant of the subjects concerning which
they are called upon to legislate? In this land of
practical physics, why should they be unable to form
an independent opinion upon a physical question?
Why should the member of a parliamentary committee
be left at the mercy of interested disputants when a
scientific question is discussed, imtil he deems the nap
a blessing which rescues him from the bewilderments
of the committee-room ? The education which does
not supply the want here referred to, fails in its duty
to England. With regard to our working people, in
the ordinary sense of the term working, the study of
Physics would, I imagine, be profitable, not only as a
means of intellectual culture, but also as a moral
influence to woo them from pursuits which now
degrade them. A man's reformation oftener depends
upon the indirect, than upon the direct action of the
will. The will must be exerted in the choice of em-
ployment which shall break the force of temptation
by erecting a barrier against it. The drunkard, for
example, is in a perilous condition if he content himself
merely with saying, or swearing, that he will avoid strong
drink. His thoughts, if not attracted by another force,
will revert to the public-house, and to rescue him per-
manently from this, you must give him an equivalent.

By investing the objects of hourly intercourse with
an interest which prompts reflection, new enjoyments
would be opened to the working man, and every one of
these would be a point of force to protect him against

298 FKAGMENTS OF SCIENCE.

temptation. Besides this, our factories and our foun-
dries present an extensive field of observation, and
were those who work in them rendered capable, by
previous culture, of observing what thej see^ the
results might be incalculable. Who can say what
intellectual Samsons are at the present ipoment toiling
with closed eyes in the mills and forges of Manchester
and Birmingham? Grant these Samsons sight, and
you multiply the chances of discovery, and with them
the prospects of national advancement. In our multi-
tudinous technical operations we are constantly playing
with forces our ignorance of which is often the cause of
our destruction. There are agencies at work in a
locomotive of which the maker of it probably never
dreamed, but which nevertheless may be sufficient
to convert it into an engine of death. When we
reflect on the intellectual condition of the people who
work in our coal mines, those terrific explosions which
occur from time to time need not astonish us. If these
men possessed sufficient physical knowledge, from the
operatives themselves would probably emanate a system
by which these shocking accidents might be avoided.
Possessed of the knowledge, their personal interests
would furnish the necessary stimulus to its practical
application, and thus two ends would be served at the
same time — the elevation of the men and the diminu-
tion of the calamity.

Before the present Course of Lectures was publicly
announced, I had many misgivings as to the propriety
of my taking a part in them, thinking that my place
might be better filled by an older and more experienced
man. To my experience, however, such as it was, I
resolved to adhere, and I have therefore described
things as they revealed themselves to my own eyes, and
have been enacted in my own limited practice. There

ON THE STUDY OF PHYSICa 299

is one mind common to us all ; and the true expression
of this mind, even in small particulars, will attest
itself by the response which it calls forth in the
convictions of my hearers. I ask your permission to
proceed a little further in this fashion, and to refer to
a fact or two in addition to those already cited, which
presented themselves to my notice durinj^^ my brief
career as a teacher in the college already alluded to.
The facts, though extremely humble, and deviating in
some slight degree from the strict subject of the present
discourse, may yet serve to illustrate an educational
principle.

One of the duties which fell to my share was the
instruction of a class in mathematics, and I usually
found that Euclid and the ancient geometry generally,
when properly and sympathetically addressed to the
understanding, formed a most attractive study for
youth. But it was my habitual practice to with-
draw the boys from the routine of the book, and
to appeal to their self- power in the treatment of
questions not comprehended in that routine. At
first, the change from the beaten track usually excited
aversion : the youth felt like a child amid strangers ;
but in no single instance did this feeling continue.
When utterly dit^heartened, I have encouraged the
boy by the anecdote of Newton, where he attributes
the difference between him and other men, mainly
to his own patience ; or of Mirabeau, when he ordered
his servant, who had stated something to be impos-
sible, never again to use that blockhead of a word.
Thus cheered, the boy has returned to his task with
a smile, which perhaps had something of doubt in it,
but which, nevertheless, evinced a resolution to try
again. I have seen his eye brighten, and, at length,
with a pleasm-e of which the ecstasy of Archimedes was

300 FRAGMENTS OF SCIENCE.

but a simple expansion, heard him exclaim, * I have it,
sir.* The consciousness of self-power, thus awakened,
was of immense value ; and, animated by it, the pro-
gress of the class was astonishing. It was often my
custom to give the boys the choice of pursuing their
propositions in the book, or of trying their strength
at others not to be found there. Never in a
single instance was the book chosen. I was ever
ready to assist when help was needful, but my offers
of assistance were habitually declined. The boys
had tasted the sweets of intellectual conquest and
demanded victories of their own. Their diagrams
were scratched on the walls, cut into the beams upon
the playground, and numberless other illustrations were
afforded of the living interest they took in the subject.
For my own part, as far as experience in teaching goes,
I was a mere fledgling- -knowing nothing of the rules of
pedagogics, as the Germans name it ; but adhering to
the spirit indicated at the commencement of this dis-
course, and endeavouring to make geometry a means
rather than a branch of education. The experiment
was successful, and some of the most delightful hours
of my existence have been spent in marking the vigorous
and cheerful expansion of mental power, when appealed
to in the manner here described.

Our pleasure was enhanced when we applied our
mathematical knowledge to the solution of physical
problems. Many objects of hourly contact had thus a
new interest and significance imparted to them. The
swing, the see-saw, the tension of the giant- stride ropes,
the fall and rebound of the football, the advantage of a
small boy over a large one when turning short, par-
ticularly in slippy weather ; all became subjects of
investigation. A lady stands before a looking-glass, of
her own height ; it Was required to know how much of

ON THE STUDY OF PHYSICS. 301

the glass was really useful to her ? We learned with
pleasure the economic fact that she might dispense
with the lower half and see her whole figure notwith-
standing. It was also pleasant to prove by mathe-
matics, and verify by experiment, that the angular
velocity of a reflected beam is twice that of the mirror
which reflects it. From the hum of a bee we were
able to determine the number of times the insect flaps
its wings in a second. Following up our researches
upon: the pendulum, we learned how Colonel Sabine
had made it the means of determining the figure of
the earth ; and we were also startled by the inference
which the pendulum enabled us to draw, that if the
diurnal velocity of the earth were seventeen times its
present amount, the centrifugal force at the equator
would be precisely equal to the force of gravitation, so
that an inhabitant of those regions would then have
the same tendency to fall upwards as downwards. All
these things were sources of wonder and delight to us :
and when we remembered that we were gifted with the
powers which had reached such results, and that the
same great field was ours to work in, om* hopes arose
that at some future day we might possibly push the
subject a little further, and add our own victories to
the conquests already won.

I ought to apologise to you for dwelling so long
upon this subject ; but the days spent among these
young philosophers made a deep impression on me. I
learned among them something of myself and of human
nature, and obtained some notion of a teacher's vocation.
If there be one profession in England of paramount
importance, I believe it to be that of the schoolmaster ;
and if there be a position where selfishness and in-
competence do most serious mischief, by lowering the
moral tone and exciting irreverence and cunning where

302 FRAGMENTS OF SCIENCE.

reverence and noble truthfulness ought to be the
feelings evoked, it is that of the principal of a school.
When a man of enlarged heart and mind comes among
boys, — when he allows his spirit to stream through
them, and observes the operation of his own character
evidenced in the elevation of theirs, — it woull be idle
to talk of the position of such a man being honourable.
It is a blessed position. The man is a blessing to
himself and to all around him. Such men, I believe,
are to be found in England, and it behoves those who
busy themselves with the mechanics of education at
the present day, to seek them out. For no matter
what means of culture may be chosen, whether physical
or philological, success must ever mainly depend upon
the amount of life, love, and earnestness, which the
teacher himself brings with him to his vocation.

Let me again, and finally, remind you that the claims
of that science which finds in me to-day its unripened
advocate, are those of the logic of Nature upon the reason
of her child — that its disciplines, as an agent of cultiu*e,
are based upon the natural relations subsisting between
Man and the universe of which he forms a part. On
the one side, we have the apparently lawless shifting of
phenomena ; on the other side, mind, which requires
law for its equilibrium, and through its own indestruc-
tible instincts, as well as through the teachings of
experience, knows that these phenomena are reducible
to law. To chasten this apparent chaos is a problem
which man has set before him. The world was built in
order: and to us are trusted the will and power to
discern its harmonies, and to make them the lessons of
our lives. From the cradle to the grave we are sur-
rounded with objects which provoke inquiry. Descend-
ing for a moment from this high plea to considerations
which lie closer to us as a nation — as a land of gas

ON THE STUDY OF PHYSICS. 303

and furnaces, of steam and electricity : as a land wbioh
science, practically applied, has made great in peace
and mighty in war: — I ask you whether this *land of
old and just renown ' has not a right to expect from her
institutions a culture more in accordance with her
present needs than that supplied by declension and
conjugation? And if the tendency should be to lower
the estimate of science, by regarding it exclusively as
the instrument of material prosperity, let it be the
high mission of our universities to furnish the proper
counterpoise by pointing out its nobler uses — lifting
the national mind to the contemplation of it as the last
development of that ' increasing purpose ' which runs
through the ages and widens the thoughts of men.

304 FRAGMENTS OF SCIENCE.

xn.

ON CRYSTALLINE AND 8LATT CLEAVAGE}

WHEN the student of physical science has to investi-
gate the character of any natural force, his first
care must be to purify it from the mixture of other forces,
and thus study its simple action. If, for example, he
wishes to know how a mass of liquid would shape itself if
at liberty to follow the bent of its own molecular forces,
he must see that these forces have free and undisturbed
exercise. We might perhaps refer him to the dew-
drop for a solution of the question ; but here we have
to do, not only with the action of the molecules of
the liquid upon each other, but also with the action of
gravity upon the mass, which pulls the drop downwards
and elongates it. If he would examine the problem in
its purity, he must do as Plateau has done, detadh the
liquid mass from the action of gravity ; he would then
find the shape to be a perfect sphere. Natural processes
come to us in a mixed manner, and to the uninstructed
mind are a mass of unintelligible confusion. Suppose J
half-a-dozen of the best musical performers to be placed
in the same room, each playing his own instrument to
perfection, but no two playing the same tune ; though
each individual instrument might be a source of perfect
music, still the mixture of all would produce mere noise.

' From a discourse delivered in the Royal Institution of Great
Britain, June 6, 1856.

SLATES. 305

Thus it is with the processes of natnre, where mecha-
nical and molecular laws intermingle and create ap-
parent confusion. Their mixture constitutes what may
be called the noise of natural laws, and it is the vocation
of the man of science to resolve this noise into its com-
ponents, and thus to detect the underlying music.

The necessity of this detachment of one force from
all other forces is nowhere more strikingly exhibited
than in the phenomena of crystallisation. Here, for
example, is a solution of common sulphate of soda or
Glauber salt. Looking into it mentally, we see the
molecules of that liquid, like disciplined squadrons
under a governing eye, arranging themselves into bat-
talions, gathering round distinct centres, and forming
themselves into solid masses, which after a time assume
the visible shape of the crystal now held in my hand.
I may, like an ignorant meddler wishing to hasten
matters, introduce confusion into this order. This may
be done by plunging a glass rod into the vessel ; the
consequent action is not the pure expression of the crys-
talline forces ; the molecules rush together with the
confusion of an unorganised mob, and not with the steady
accuracy of a disciplined host. In this mass of bismuth
also we have an example of confused crystallisation ; but
in the crucible behind me a slower process is going on :
here there is an architect at work ' who makes no chips,
no din,' and who is now building the particles into
crystals, similar in shape and structure to those beauti-
ful masses which we see upon the table. By permitting
alum to crystallise in this slow way, we obtain these
perfect octahedrons ; by allowing carbonate of lime to
crystallise, nature produces these beautiful rhomboids ;
when silica crystallises, we have formed these hexagonal
prisms capped at the ends by pyramids ; by allowing
saltpetre to crystallise we have these prismatic masses,

306 FRAGMENTS OF SCIENCE.

and when carbon crystallises, we have the diamond. If
we wish to obtain a perfect crystal we must allow the
molecular forces free play ; if the crystallising mass be
permitted to rest upon a surface it will be flattened,
and to prevent this a small crystal must be so suspended
as to be surrounded on all sides by the liquid, or, if it
rest upon the surface, it must be turned daily so as to
present all its faces in succession to the working builder.

In building up crystals these little atomic bricks
often arrange tliemselves into layers which are perfectly
parallel to each other, and which can be separatf^d by
mechanical means ; this is called the cleavage of the
crystal. The crystal of sugar I hold in my hand has
thus far escaped the solvent and abrading forces which
sooner or later determine the fate of sugar-candy. I
readily discover that it cleaves with peculiar facility in
one direction. Again I lay my knife upon this piece of
rocksalt, and with a blow cleave it in one direction.
Laying the knife at right angles to its former position,
the crystal cleaves again ; and finally placing the knife
at right angles to the two former positions, we find a
third cleavage. Eocksalt cleaves in three directions,
and the resulting solid is this perfect cube, which may
be broken up into any number of smaller cubes. Ice-
land spar also cleaves in three directions, not at right
angles, but oblique to each other, the resulting solid
being a rhomboid. In each of these cases the mass
cleaves with equal facility in all three directions. For
the sake of completeness I may say that many crystals
cleave with unequal facility in dififerent directions :
heavy spar presents an example of this kind of cleavage.

Tm-n we now to the consideration of some other
phenomena to which the term cleavage may be applied.
Beech, deal, and other woods cleave with facility along
the fibre, and this cleavage is most perfect when the

SLATES. 307

edge of the axe is laid across the rings which mark the
growth of the tree. If you look at this bundle of hay
severed from a rick, you will see a sort of cleavage in it
also ; the stalks lie in horizontal planes, and only a small
force is required to separate them laterally. But we
cannot regard the cleavage of the tree as the same in
character as that of the hayrick. In the one case it is
the molecules arranging themselves according to organic
laws which produce a cleavable structure, in the other
case the easy separation in one direction is due to the
mechanical arrangement of the coarse sensible stalks
of hay.

This sandstone rock was once a powder held in
mechanical suspension by water. The powder was com-
posed of two distinct parts, fine grains of sand and
small plates of mica. Imagine a wide strand covered
by a tide, or an estuary with water which holds such
powder in suspension : how will it sink ? The rounded
grains of sand will reach the bottom first, because they
encounter least resistance, the mica afterwards, and
when the tide recedes we have the little plates shining
like spangles upon the surface of the sand. Each
successive tide brings its charge of mixed powder,
deposits its duplex layer day after day, and finally
masses of immense thickness are piled up, which by
preserving the alternations of sand and mica tell the
tale of their formation. Take the sand and mica, mix
them together in water, and allow them to subside ;
they will arrange themselves in the manner indicated,
and by repeating the process you can actually build
up a mass which shall be the exact counterpart of that
presented by nature. Now this structure cleaves with
readiness along the planes in which the particles of
mica are strewn. Specimens of such a rock sent to me
from Halifax, and other masses from the quarries of

308 FRAGMENTS OF SCIENCE.

Over Darwen in Lancashire, are here before you. With
a hammer and chisel I can cleave them into flags;
indeed these flags are employed for roofing purposes in
the districts from which the specimens have come, and
receive the name of slatestone.' But you will discern
without a word from me, that this cleavage is not a
crystalline cleavage any more than that of a hayrick is.
It is molar, not molecular.

This, so far as I am aware of, has never been
imagined, and it has been agreed among geologists not
to call such splitting as this cleavage at all, but to
restrict the term to a phenomenon of a totally different
character.

Those who have visited the slate quarries of Cumber-
land and North Wales will have witnessed the pheno-
menon to which I refer. We have long drawn our
supply of roofing-slates from such quarries ; school-boys
ciphered on these slates, they were used for tombstones
in churchyards, and for billiard-tables in the metropolis ;
but not until a comparatively late period did men begin
to enquire how their wonderful structure was produced.
What is the agency which enables us to split Honister
Crag, or the cliffs of Snowdon, into laminae from crown
to base ? This question is at the present moment one
of the great difficulties of geologists, and occupies their
attention perhaps more than any other. You may
wonder at this. Looking into the quarry of Penrhyn,
you may be disposed to offer the explanation I heard
given two years ago. 'These planes of cleavage,' said a
friend who stood beside me on the quarry's edge, ' are
the planes of stratification which have been lifted by
some convulsion into an almost vertical position.' But
this was a mistake, and indeed here lies the grand
difficulty of the problem. The planes of cleavage stand
in most cases at a high angle to the bedding. Thanka

SLATES. 309

to Sir Koderick Murchison, I am able to place the proof
of this before you. Here is a specimen of slate in which
both the planes of cleavage and of bedding are distinctly
marked, one of them making a large angle with the
other. This is common. The cleavage of slates then
is not a question of stratification ; what then is its
cause?

In an able and elaborate essay published in 1835,
Prof. Sedgwick proposed the theory that cleavage is due
to the action of crystalline or polar forces subsequent to
the consolidation of the rock. ' We may affirm,' he says,
*that no retreat of the parts, no contraction of dimen-
sions in passing to a solid state, can explain such
phenomena. They appear to me only resolvable on the
supposition that crystalline or polar forces acted upon
the whole mass simultaneously in one direction and with
adequate force.' And again, in another place : ' Crys-
talline forces have re-arranged whole mountain masses,
producing a beautiful crystalline cleavage, passing alike
through all the strata.'^ The utterance of such a man
struck deep, as it ought to do, into the minds of
geologists, and at the present day there are few who do
not entertain this view either in whole or in part.^ The
boldness of the theory, indeed, has, in some cases, caused
speculation to run riot, and we have books published on

* Transactions of the Geological Society ^ ser. ii. vol. iii. p. 477.

* In a letter to Sir Charles Lyell, dated from the Cape of Good
Hope February 20, 1836, Sir John Herschel writes as follows:— * If
rocks have been so heated as to allow of a commencement of crys-
tallisation, that is to say, if they have been heated to a point at
which the particles can begin to move amongst themselves, or at
least on their own axes, some general law must then determine the
position in which these particles will rest on cooling. Probably
that position will have some relation to the direction in which the
heat escapes. Now when all or a majority of particles of the same
nature have a general tendency to one position, that must of course
determine a cleavage plane.'

310 FEAGMENTS OF SCIENCE.

the action of polar forces and geologic magnetism, whick
rather astonish those who know something about the
subject. According to this theory whole districts of
North Wales and Cumberland, mountains included, are
neither more nor less than the parts of a gigantic crystal.
These masses of slate were originally fine mud, composed
of the broken and abraded particles of older rocks.
They contain silica, alumina, potash, soda, and mica
mixed mechanically together. In the course of ages the
mixtiire became consolidated, and the theory before us
assumes that a process of crystallisation afterwards re-
arranged tlie particles and developed in it a single plane
of cleavage. Though a bold, and I think inadmissible,
stretch of analogies, this hypothesis has done good
service. Eight or wrong, a thoughtfully uttered theory
has a dynamic power which operates against intellectual
stagnation ; and even by provoking opposition is event-
ually of service to the cause of truth. It would, however,
have been remarkable if, among the ranks of geologists
themselves, men were not found to seek an explanation
of slate-cleavage involving a less hardy assumption.

The first step in an enquiry of this kind is to seek
facts. This has been done, and the labours of Daniel
Sharpe (the late President of the Greological Society,
who, to the loss of science and the sorrow of all who
knew him, has so suddenly been taken away from us),
Mr. Henry Clifton Sorby, and others, have furnished us
with a body of facts associated with slaty cleavage, and
having a most important bearing upon the question.

Fossil shells are found in these slate-rocks. I ha\'e
here several specimens of such shells in the actual rock,
and occupying various positions in regard to the cleavage
planes. They are squeezed, distorted, and crushed ; in
all cases the distortion leads to the inference that the
rock which contains these shells has been subjected to

SLATES. 311

feUonnous pressure in a direction at right angles to the
planes of cleavage. The sliells are all flattened and
spread out in these planes. Compare this fossil trilobite
of normal proportions with these others which have
suffered distortion. Some have lain across, some along,
and some oblique to the cleavage of the slate in which
they are found ; but in all cases the distortion is such
as required for its production a compressing force acting
at right angles to the planes of cleavage. As the trilo-
bites lay in the mud, the jaws of a gigantic vice appear
to have closed upon them and squeezed them into the
shapes you see.

We sometimes find a thin layer of coarse gritty
(material, between two layers of finer rock, through
which and across the gritty layer pass the planes of
lamination. The coarse layer is found bent by the
pressure into sinuosities like a contorted ribbon. Mr.
Sorby has described a striking case of this kind. This
^crumpling can be experimentally imitated ; the amount
of compression might, moreover, be roughly estimated
by supposing the contorted bed to be stretched out, its
•length measured and compared with the shorter distance
into which it has been squeezed. We find in this way
'that the yielding of the mass has been considerable.

iLet me now direct your attention to another proof
•of pressure ; you see the varying colours which indicate
tthe bedding on this mass of slate. The dark portion is
;gritty, being composed of comparatively coarse par-
'ticles, which, owing to their size, shape and gravity,
sink first and constitute the bottom of each layer.
<G-radually, from bottom to top the coarseness diminishes,
;and near the upper surface we have a layer of exceed-
lingly fine grain. It is the fine mud thus consolidated
'from which are derived the Grerman razor-stones, so
?much prized for the sharpening of surgical instruments.
21

312 FRAGMENTS OF SCIENCE

Wlien a bed is thin, the fine-grain slate is permitted to
rest upon a slab of the coarse slate in contact with it ;
when the fine bed is thick, it is cut into slices which are
cemented to pieces of ordinary slate, and thus rendered
stronger. The mud thus deposited is, as might be
expected, often rolled up into nodular masses, carried
forward, and deposited among coarser material by the
rivers from which the slate-mud has subsided. Here
are such nodules enclosed in sandstone. Everybody,
moreover, who has ciphered upon a school-slate must
remember the whitish-green spots which sometimes
dotted the surface of the slate, and over which the
pencil usually slid as if the spots were greasy. Now
these spots are composed of the finer mud, and they
could not, on account of their fineness, bite the pencil
like the surrounding gritty portions of the slate.
Here is a beautiful example of these spots : you observe
them, on the cleavage surface, in broad round patches.
But turn the slate edgeways and the section of each
nodule is seen to be a sharp oval with its longer axis
parallel to the cleavage. This instructive fact has
'been adduced by Mr. Sorby. I have made excursions
to the quarries of Wales and Cumberland, and to many
•of the slate yards of London, and found the fact
general. Thus we elevate a common experience of our
boyhood into evidence of the highest significance as
regards a most important geological problem. From
the magnetic deportment of these slates, I was led to
infer that these spots contain a less amount of iron
than the surrounding dark slate. An analysis was
made for me by Mr. Hambly in the laboratory of Dr.
Percy at the School of Mines with the following
result : —

SLATEa 313

Analysis of Slate.
DcMfJt Slate, two analyses,

1. Percentage of iron ••••,, 5*85
a. n n £13

Mean • , 5*99

Whitish Oreen Slate,

1. Percentage of iron . t • , « i 3*24

2. „ • „ 312

Mean . ,3-18

According to these analyses the quantity of iron in the
dark slate immediately adjacent to the greenish spot
is nearly double the quantity contained in the spot
itself. This is about the proportion which the mag-
netic experiments suggested.

Let me now remind you that the facts brought
before you are typical — each is the representative of
a class. We have seen shells crushed ; the trilo-
bites squeezed, beds contorted, nodules of greenish
marl flattened ; and all these sources of independent
testimony point to one and the same conclusion,
namely, that slate-rocks have been subjected to
enormous pressure in a direction at right angles to the
planes of cleavage.

In reference to Mr. Sorby's contorted bed, I have
said that by supposing it to be stretched out and its
length measured, it would give us an idea of the
amount of yielding of the mass above and below the bed.
Such a measurement, however, would not give the
exact amount of yielding. I hold in my hand a specimen
of slat/C with its bedding marked upon it ; the lower
portions of each layer being composed of a com-
paratively coarse gritty material something like what
you may suppose the contorted bed to be composed of.
Now in crossing these gritty portions, the cleavage turns,

S14 FRAGMENTS OF SCIENCE.

as if tending to cross the bedding at another angle.
When the pressure began to act, the intermediate bed,
which is not entirely unyielding, suffered longitudinal
pressure; as it bent, the pressure became gradually
more transverse, and the direction of its clea\Tige is
exactly such as you would infer from an action of this
kind — it is neither quite across the bed, nor yet in the
same direction as the cleavage of the slate above and
below it, but intermediate between both. Supposing
the cleavage to be at right angles to the pressure, this
is the direction which it ought to take across these
more unyielding strata.

Thus we have established the concurrence of the
phenomena of cleavage and pressure — that they accom-
pany each other; but the question still remains. Is the
pressure sufficient to account for the cleavage ? A
single geologist, as far as I am aware, answers boldly in
the affirmative. This geologist is Sorby, who has attacked
the question in the true spirit of a physical investigator.
Call to mind the cleavage of the flags of Halifax and
Over Darwen, which is caused by the interposition of
layers of mica between the gritty strata. Mr. Sorby
finds plates of mica to be also a constituent of slate-
rock. He asks himself, what will be the effect of
pressure upon a mass containing such plates confusedly
mixed up in it ? It will be, he argues, and he argues
rightly, to place the plates with their flat surfaces
more or less perpendicular to the direction in which
the pressure is exerted. He takes scales of the oxide
of iron, mixes them with a fine powder, and on squeez-
ing the mass finds that the tendency of the scales is to
Bet themselves at right angles to the line of pressure.
Along the planes of weakness produced by the scales
the mass cleaves.

By tests of a different character from those applied

SLATES. 315

by Mr. Sorby, it migbt be shown how true his con-
clusion is — that the effect of pressure on elongated
particles, or plates, will be such as he describes it.
But while the scales must be regarded as a true cause,
I should not ascribe to them a large share in the pro-
duction of the cleavage. I believe that even if the
plates of mica were wholly absent, the cleavage of
slate-rocks would be much the same as it is at present.

Here is a mass of pure white wax : it contains no
mica particles, no scales of iron, or anything analogous
to them. Here is the selfsame substance submitted to
pressure. I would invite the attention of the eminent
geologists now before me to the structure of this wax.
No slate ever exhibited so clean a cleavage ; it splits
into laminae of surpassing tenuity, and proves at a
single stroke that pressure is sufficient to produce
cleavage, and that this cleavage is independent of inter-
mixed plates or scales. I have purposely mixed this
wax with elongated particles, and am unable to say at
the present moment that the cleavage is sensibly
affected by their presence — if anything, I should say
they rather impair its fineness and clearness than pro-
mote it.

The finer the slate is the more perfect will be the
resemblance of its cleavage to that of the wax. Com-
pare the surface of the wax with the surface of this
slate from Borrodale in Cumberland. You have pre-
cisely the same features in both : you see flakes clinging
to the surfaces of each, which have been partially torn
away in cleaving. Let ^any close observer compare
these two effects, he will, I am persuaded, be led to
the conclusion that they are the product of a common
cause.'

' I have usually softened the wax by warming it, kneaded it
with the fingers, and pressed it between thick plates of glass pre*

316 PEAGMENTS OF SCIENCE.

But you will ask me how, according to my view,
does pressure produce this remarkable result? This
may be stated in a very few words.

There is no such thing in nature as a body of
perfectly homogeneous structure. I break this clay
which seems so uniform, and find that the fracture
presents to my eyes innumerable surfaces along which
it has given way, audit has yielded along those surfaces
because in them the cohesion of the mass is less than
elsewhere. I break this marble, and even this wax,
and observe the same result ; look at the mud at the
bottom of a dried pond ; look at some of the ungravelled
walks in Kensington Gardens on drying after rain, —
they are cracked and split, and other circumstances
being equal, they crack and split where the cohesion is
a minimum. Take then a mass of partially consolidated
mud. Such a mass is divided and subdivided by interior
surfaces along which the cohesion is comparatively
small. Penetrate the mass in idea, and you will see it
composed of numberless irregular polyhedra bounded
by surfaces of weak cohesion. Imagine such a mass
subjected to pressure, — it yields and spreads out in the
direction of least resistance ; ^ the little polyhedra be-
come converted into laminaB, separated from each other
by surfaces of weak cohesion, and the infallible result

viously wetted. At the ordinary snmmer temperature the pressed
wax is soft, and tears rather than cleaves ; on this account I cool
my compressed specimens in a mixture of pounded ice and salt,
and when thus cooled they split cleanly.

' It is scarcely necessary to say that if the mass were squeezed
equally in all directions no laminated structure could be produced ;
it must have room to yield in a lateral direction. Mr. Warren De
la Rue informs me that he once wislie(? to obtain white-lead in a
fine granular state, and to accomplish this he first compressed it.
The mould was conical, and permitted the lead to spread out a
little laterally. The lamination was as perfect as that of slate, and
it quite defeated him in his effort to obtain a granular powder.

II

SLATES. 317

vdll be a tendency to cleave at right angles to the line
of pressure.

Further, a mass of dried mud is full of cavities and
fissures. If you break dried pipe-clay you see them in
great numbers, and there are multitudes of them so
small that you cannot see them. A flattening of these
cavities must take place in squeezed mud, and this
must to some extent facilitate the cleavage of the mass
in the direction indicated.

Although the time at my disposal has not permitted
me duly to develope these thoughts, yet for the last
twelve months the subject has presented itself to me
almost daily under one aspect or another. I have
never eaten a biscuit during this period without re-
marking the cleavage developed by the rolling-pin.
You have only to break a biscuit across, and to look at
the fracture, to see the laminated structure. We have
here the means of pushing the analogy further. I in-
vite you to compare the structure of this slate, which
was subjected to a high temperature during the confla-
gration of Mr. Scott Eussell's premises, with that of a
biscuit. Air or vapour within the slate has caused it
to swell, and the mechanical structure it reveals is pre-
cisely that of a biscuit. During these enquiries I have
received much instruction in the manufacture of puff-
paste. Here is some such paste baked under my own
superintendence. The cleavage of our hills is acci-
dental cleavage, but this is cleavage with intention.
The volition of the pastrycook has entered into its
formation. It has been his aim to preserve a series of
surfaces of structural weakness, along which the dough
divides into layers. Puff-paste in preparation must
not be handled too much ; it ought, moreover, to be
rolled on a cold slab, to prevent the butter from melt-

318 FKAGMENTS OF SCIENCE.

ing, and diffusing itself, thus rendering the paste more

homogeneous and less liable to split. Pufif-paste is,,
then, simply an exaggerated case of slaty cleavage.

The principle here enunciated is so simple as to-
be almost trivial ; nevertheless, it embraces not only
the cases mentioned, but, if time permitted, it might
be shown you that the principle has a much wider
range of application. When iron is taken from the
puddling furnace it is more or less spongy, an aggre-
gate n fact of small nodules : it is at a welding heat,
and at this temperature is submitted to the process of
rolling. Bright smooth bars are the result. But not-
withstanding the high heat the nodules do not perfectly
blend together. The process of rolling draws them into
fibres. Here is a mass acted upon by dilute sulphuric
acid, which exhibits in a striking manner this fibrous
structure. The experiment was made by my friend Dr.
Percy, without any reference to the question of cleavage.

Break a piece of ordinary iron and you have a granu-
lar fracture ; beat the iron, you elongate these granules,
and finally render the mass fibrous. Here are pieces of
rails along which the wheels of locomotives have slid-
den ; the granules have yielded and become plates.
They exfoliate or come off in leaves ; all these effects
belong, I believe, to the great class of phenomena
of which slaty cleavage forms the most prominent
example.^

We have now reached the termination of our task.
You have witnessed the phenomena of crystallisation,
and have had placed before you "the facts which are found
associated with the cleavage of slate rocks. Such facts,
as expressed by Helmholtz, are so many telescopes
to our spiritual vision, by which we can see backward

• For some further observations on this subject by Mr, Sorbj^
and myself, see Phihsoj/hical Magazine for August, 1856,

SLATES. 319

through the night of antiquity, and discern the forces
which have been in operation upon the earth's surface

Ere the lion roared.
Or the eagle soared.

From evidence of the most independent and
trustworthy character, we come to the conclusion
that these slaty masses have been subjected to enormous
pressure, and by the sure method of experiment we
have shown — and this is the only really new point
which has been brought before you — how the pressure
is sufficient to produce the cleavage. Expanding our
field of view, we find the self-same law, whose footsteps we
trace amid the crags of Wales and Cumberland, extend-
ing into the domain of the pastrycook and ironfounder ;
nay, a wheel cannot roll over the half-dried mud of our
streets without revealing to us more or less of the features
of this law. Let me say, in conclusion, that the spirit
in which this problem has been attacked by geologists,
indicates the dawning of a new day for their science.
The great intellects who have laboured at geology, and
who have raised it to its present pitch of grandeur, were
compelled to deal with the subject in mass ; they had
no time to look after details. But the desire for more
exact knowledge is increasing; facts are fio.vnj in
which, while they leave untouched the intrinsic wonders
of geology, are gradually supplanting by solid truths the
uncertain speculations which beset the subject in its in-
fancy. Geologists now aim to imitate, as far as possible,
the conditions of nature, and to produce her results ;
they are approaching more and more to the domain of
physics, and I trust the day will soon come when we
shall interlace our friendly arms across the common
boundary of our sciences, and pursue our respective tasks

320 FKAGMENTS OF SCIENCE.

in a spirit of mutual helpfulness, encouragement and
goodwill.

[I would now lay more stress on the lateral yielding,
referred to in the note at the bottom of page 316,
accompanied as it is by tangential. sliding, than I was
prepared to do when this lecture was given. This sliding
is, I think, the principal cause of the planes of weak-
ness, both in pressed wax and slate rock. J. T. 1871.]

I

kni.

ON PARAMAGNETIC AND DIAMAGNETIC FORCES}

THE notion of an attractive force, which draws bodies
towards the centre of the earth, was entertained by
Anaxagoras and his pupils, by Democritus, Pythagoras,
and Epicurus ; and the conjectures of these ancients were
renewed by Galileo, Huyghens, and others, who stated
that bodies attract each other as a magnet attracts iron.
Kepler applied the notion to bodies beyond the surface
of the earth, and afi&rmed the extension of this force to
the most distant stars. Thus it would appear, that in
the attraction of iron by a magnet originated the con-
ception of the force of gravitation. Nevertheless, if we
look closely at the matter, it will be seen that the mag-
netic force possesses characters strikingly distinct from
those of the force which holds the universe together.
The theory of gravitation is, that every particle of
matter attracts every other particle ; in magnetism also
we have attraction, but we have always, at the same
time, repulsion, the final effect being due to the differ-
ence of these two forces. A body may be intensely
acted on by a magnet, and still no motion of translation
will follow, if the repulsion be equal to the attraction.
Previous to magnetization, a dipping needle, when its
centre of gravity is supported, stands accurately level ;
but, after magnetization, one end of it, in our latitude,

' Abstract of a discourse delivered in the Koyal Institution,
February 1, 1856.

322 FRAGMENTS OF SCIENCE.

is pulled towards the north pole of the earth. The
needle, however, being suspended from the arm of a fine
balance, its weight is found unaltered by its magneti-
zation. In like manner, when the needle is permitted
to float upon a liquid, and thus to follow the attraction of
the north magnetic pole of the earth, there is no motion
of the mass towards that pole. The reason is known
to be, that although the marked end of the needle is
attracted by the north pole, the unmarked end is
repelled by an equal force, the two equal and opposite
forces neutralizing each other.

When the pole of an ordinary magnet is brought
to act upon the swimming needle, the latter is at-
tracted,— the reason being that the attracted end of
the needle being nearer to the pole of the magnet
than the repelled end, the force of attraction is the!
more powerful of the two. In the case of the earth,
its pole is so distant that the length of the needle is
practically zero. In like manner, when a piece of iron
is presented to a magnet, the nearer parts are attracted,
while the more distant parts are repelled ; and because
the attracted portions are nearer to the magnet than the
repelled ones, we have a balance in favour of attraction.
Here then is the special characteristic of the magnetic
force, which distinguishes it from that of gravitation.
The latter is a simple un polar force, while the former
is duplex or polar. Were gravitation like magnetism,
a stone would no more fall to the ground than a piece
of iron towards the north magnetic pole : and thus,
however rich in consequences the supposition of Kepler
and others may have been, it is clear that a force like
that of magnetism would not be able to transact the
business of the universe.

The object of this discourse is to enquire whether
the force of diamagnetism, which manifests itself as a

ON PARAMAaNETIC AND DIAMAaNETIC FORCES. 323

repulsion of certain bodies by the poles of a magnet,
is to be ranged as a polar force, beside that of magne-
tism ; or as an unpolar force, beside that of gravitation.
When a cylinder of soft iron is placed within a wire

i helix, and surrounded by an electric current, the anti-

|l theuis of its two ends, or, in other words, its polar ex-
citation, is at once manifested by its action upon a
magnetic needle ; and it may be asked why a cylinder

( of bismuth may not be substituted for the cylinder of
iron, and its state similarly examined. The reason is,

I that the excitement of the bismuth is so feeble, that it
would be quite masked by that of the helix in which it
'is enclosed ; and the problem that now meets us is, so
to excite a Jiamagnetic body that the pure action of the
body upon a magnetic needle may be observed, un-
mixed with the action of the body used to excite the
diamagnetic.

How this has been effected may be illustrated in the
following manner : — When through an upright helix
of covered copper wire, a voltaic current is sent, the top
of the helix attracts, while its bottom repels, the same
pole of a magnetic needle ; its central point, on the con-
trary, is neutral, and exhibits neither attraction nor
repulsion. Such a helix is* caused to stand between the

NC

3N^

jS'

Fig. 10.

two poles n's' of an astatic system. ^ The two magnets

sn' and s'n are united by a rigid cross piece at

> The reversal of the poles of the two magnets, which were of
the same strength, completely annulled the action of the earth as a
magnet.

324 FRAGMENTS OF SCIENCE.

their centres, and are suspended from the point a, so that
both magnets swing in the same horizontal plane. It
is so arranged that the poles n' s' are opposite to the
central or neutral point of the helix, so that when a
current is sent through the latter, the magnets, as
before explained, are unaffected. Here then we have ,
an excited helix which itself has no action upon the!
magnets, and we are thus enabled to examine the
action of a body placed within the helix and excited
by it, undisturbed by the influence of the latter. The
helix being 12 inche? bigh, a cylinder of soft iron 6
inches long, suspended from a string and passing over
a pulley, can be raised or lowered within the helix.
When it is so far sunk that its lower end rests upon
the table, the upper end finds itself between the poles
n' s' of the astatic system. The iron cylinder is thus
converted into a strong magnet, attracting one of the
poles, and repelling the other, and consequently deflect-
ing the entire astatic system. When the cylinder is
raised so that the upper end is at the level of the top
of the helix, its lower end comes between the poles
n' s' ; and a deflection opposed in direction to the
former one is the immeditite consequence. To render
these deflections more easily visible, a mirror m is
attached to the system of magnets ; a beam of light
thrown upon the mirror being reflected and projected
as a bright disk against the wall. The distance of
this image from the mirror being considerable, and its
angular motion double that of the latter, a very slight
motion of the magnet is sufficient to produce a dis-
placement of the image through several yards.

This then is the principle of the beautiful apparatus*

' Devised by Prof. W. Weber, and constructed by M. Leyser,
of Leipzig.

ON PAEAMAGNETIC AND DIAMAGNETIC FORCES. 325

by which the investigation was conducted. It is mani-
fest that if a second helix be placed between the poles
SN with a cylinder within it, the action upon the
astatic magnet may be exalted. This was the arrange-
ment made use of in the actual enquiry. Thus to
intensify the feeble action, which it is here our object
to seek, we have in the first place neutralized the action
of the earth upon the magnets, by placing them asta-
itically. Secondly, by making use of two cylinders, and
permitting them to act simultaneously on the four poles
of the magnets, we have rendered the deflecting force
four times what it would be, if only a single pole
were used. Finally, the whole apparatus was en-
closed in a suitable case which protected the magnets
from air-currents, and the deflections were read off
through a glass plate in the case, by means of a tele-
scope and scale placed at a considerable distance from
the instrument.

A pair of bismuth cylinders was first examined.
Sending a current through the helices, and observing
that the magnets swung perfectly free, it was first ar-
ranged that the bismuth cylinders within the helices
had their central or neutral points opposite to the poles
of the magnets. All being at rest the number on the
scale marked by the cross wire of the telescope was 572.
The cylinders were then moved, one up the other down,
so that two of their ends were brought to bear simul-
taneously upon the magnetic poles : the magnet moved
promptly, and after some oscillations^ came to rest at
the number 612; thus moving from a smaller to a
larger number. The other two ends of the bars were
next brought to bear upon the magnet : a prompt deflec-
tion was the consequence, and the final position of
• To lessen these a copper damper was made use of.

326 FEAGMENTS OF SCIENCE.

equilibrium was 526 : the movement being from a larget
to a smaller number. We thus observe a manifest polar
action of the bismuth cylinders upon the magnet ; one. .
pair of ends deflecting it in one direction, and the other i
pair deflecting it in the opposite direction.

Substituting for the cylinders of bismuth thin
cylinders of iron, of magnetic slate, of sulphate of iron,
carbonate of iron, protochloride of iron, red ferrocyanide
of potassium, and other magnetic bodies, it was found
that when the position of the magnetic cylinders was
the same as that of the cylinders of bismuth, the deflec-
tion produced by the former was always opposed in
direction to that produced by the latter ; and hence the
disposition of the force in the diamagnetic body must
have been precisely antithetical to its disposition in the
magnetic ones.

But it will be urged, and indeed has been urged
against this inference, that the deflection produced by
the bismuth cylinders may be due to induced currents
excited in the metal by its motion within the helices.
In reply to this objection, it may be stated, in the
first place, that the deflection is permanent, and can-
not therefore be due to induced cun-ents, which are only
of momentary duration. It has also been m-ged that
such experiments ought to be made with other metals,
and with better conductors than bismuth ; for if due
to currents of induction, the better the conductor, the
more exalted will be the effect. This requirement waaJi
complied with. '

Cylinders of antimony were substituted for those of
bismuth. This metal is a better conductor of elec-
tricity, but less strongly diamagnetic than bismuth.
If therefore the action referred to be due to induced
currents we ought to have it greater in the case of anti-
mony than with bismuth ; but if it springs from a true

0^ PARAMAGNETIC ANt) DlAMAGNETiC FORCES. 327

diamagnetic polarity, the action of the bismuth ought
to exceed that of the antimony. Experiment proves
this to be the case. Hence the deflection produced by
these metals is due to their diamagnetic, and not to
their conductive capacity. Copper cylinders were next
examined : here we have a metal which conducts elec-
tricity fifty times better than bismuth, but its diamag-
netic power is nearly null; if the effects be due to
induced currents we ought to have them here in an
enormously exaggerated degree, but no sensible deflec^
tion was produced by the two cylinders of copper.

It has also been proposed by the opponents of! dia-
magnetic polarity to coat fragments of bismuth with
some insulating substance, so as to render the formation
of induced currents impossible, and to test the question
with cylinders of these fragments. This requirement
was also fidfiUed. It is only necessary to reduce the
bismuth to powder and expose it for a short time to the
air to cause the particles to become so far oxidised as
to render them perfectly insulating. The insulating
power of the powder was exhibited experimentally;
nevertheless, this powder, enclosed in glass tubes, ex-
hibited an action scarcely less powerful than that of the
massive bismuth cylinders.

But the most rigid proof, a proof admitted to be
conclusive by those who have denied the antithesis of
magnetism and diamagnetism, remains to be stated.
Prisms of the same heavy glass as that with which the
diamagnetic force was discovered, were substituted for
the metallic cylinders, and their action upon the mag-
net was proved to be precisely the same in kind as that
of the cylinders of bismuth. The enquiry was also
extended to other insulators : to phosphorus, sulphur,
nitre, calcareous spar, statuary marble, with the same
invariable result : each of these substances was proved
22.

328 FRAGMENTS OF SCIENCE.

to be polar, the disposition of the force being the same as
that of bismuth and the reverse of that of iron. When
a bar of iron is set erect, its lower end is known to be
a north pole, and its upper end a south pole, in virtue
of the earth's induction. A marble statue, on the con-
trary, has its feet a south pole, and its head a north
pole, and there is no doubt that the same remark applies
to its living archetype ; each man walking over the
earth's surface is a true diamagnet, with its poles the
reverse of those of a mass of magnetic matter of the
same shape and position.

An experiment of practical value, as affording a
ready estimate of the different conductive powers of
two metals for electricity, was exhibited in the lecture,
for the purpose of proving experimentally some of the
statements made in reference to this subject. A cube
of bismuth was suspended by a twisted string #bet ween
the two poles of an electro-magnet. The cube was
attached by a short copper wire to a little square
pyramid, the base of which was horizontal, and its sides
formed of four small triangular pieces of looking-glass.
A beam of light was suffered to fall upon this reflector,
and as the reflector followed the motion of the cube
the images cast from its sides followed each other in
succession, each describing a circle about thirty feet
in diameter. As the velocity of rotation augmented,
these images blended into a continuous ring of light.
At^a particular instant the electro-magnet was excited,
currents were evolved in the rotating cube, and the
strength of these currents, which increases with the
conductivity of the cube for electricity, was practically
estimated by the time required to bring the cube and
its associated mirrors to a state of rest. With bismuth
this time amounted to a score of seconds or more : a
cube of copper, on the contrary, was struck almost
instantly motionless when the circuit was established.

I

xnr.

PHYSICAL BASIS OFfSOLAM CHEMISTRY.^

OMITTING- all preface, attention was first drawn to
an experimental arrangement intended to prove
that gaseous bodies radiate heat in different degrees.
Near a double screen of polished tin was placed an
ordinary ring gas-burner, and on this was placed a hot
copper ball, from which a column of heated air ascended.
Behind the screen, but so situated that no ray from the
ball could reach the instrument, was an excellent
thermo-electric pile, connected by wires with a very
delicate galvanometer. The pile was known to be an
instrument whereby heat is applied to the generation
of electric currents ; the strength of the current being
an accurate measure of the quantity of the heat. As
long as both faces of the pile are at the same tempera-
ture, no current is produced ; but the slightest difference
in the temperature of the two faces at once declares
itself by the production of a current, which, when
carried through the galvanometer, indicates by the
deflection of the needle both its strength and its
direction.

The two faces of the pile were in the first instance
brought to the same temperature ; the equilibriiftn
being shown by the needle of the galvanometer standing

* From a discourse delivered at the Eoyal Institution of Great
Britain, June 7, 1861.

J

330 FRAGMENTS OF SCIENCE.

at zero. The rays emitted by the current of hot air
already referred to were permitted to fall upon one of
the faces of the pile ; and an extremely slight move-
ment of the needle showed that the radiation from the
hot air, though sensible, was extremely feeble. Con-
nected with the ring-burner was a holder containing
oxygen gas ; and by turning a cock, a stream of this
gas was permitted to issue from the burner, strike the
copper ball, and ascend in a heated column in front of
the pile. The result was, that oxygen showed itself, as a
radiator of heat, to be quite as feeble as atmospheric air.

A second holder containing defiant gas was then
connected with the ring-burner. Oxygen and air had
already flowed over the ball and cooled it in some degree.
Hence the defiant gas laboured under a disadvantage.
But on permitting the gas to rise from the ball, it casts
an amount of heat against the adjacent face of the pile
sufficient to impel the needle of the galvanometer almost
to 90°. This experiment proved the vast difference
between two equally invisible gases with regard to their
power of emitting radiant heat.

The converse experiment was now performed. The
thermo-electric pile was removed and placed between
two cubes filled with water kept in a state of constant
ebullition ; and it was so arranged that the quantities
of heat falling from the cubes on the opposite faces of
the pile were exactly equal, thus neutralising each
other. The needle of the galvanometer being at zero,
a sheet of oxygen gas was caused to issue from a slit
between one of the cubes and the adjacent face of the
pile. If this sheet of gas possessed any sensible power
oT intercepting the thermal rays from the cube, one
face of the pile being deprived of the heat thus inter-
cepted, a difference of temperature between its two
faces would instantly set in, and the result would be

PHYSICAL BASIS OF SOLAR CHEMISTKY. 331

declared by the galvanometer. The quantity absorbed
by the oxygen under those circumstances was too feeble
to affect the galvanometer ; the gas, in fact, proved
perfectly transparent to the rays of heat. It had but
a feeble power of radiation : it had an equally feeble
power of absorption.

The pile remaining in its position, a sheet of olefiant
gas was caused to issue from the same slit as that through
which the oxygen had passed. No one present could
see the gas ; it was quite invisible, the light went
through it as freely as through oxygen or air ; but its
effect upon the thermal rays emanating from the cube
was what might be expected from a sheet of metal. A
quantity so large was cut off, that the needle of the
galvanometer, promptly quitting the zero line, moved
with energy to.its stops. Thus the olefiant gas, so light
and clear and pervious to luminous rays, was proved to
be a most potent destroyer of the rays emanating from
an obscure source. The reciprocity of action established
in the case of oxygen comes out here ; the good radiator
is found by this experiment to be the good absorber.

This result, now exhibited before a public audience
for the first time, was typical of what had been obtained
with gases generally. Going through the entire list of
gases and vapours in this way, we find radiation and
absorption to be as rigidly associated as positive and
negative in electricity, or as north and south polarity
in magnetism. So that if we make the number which
expresses the absorptive power the numerator of a
fraction, and that which expresses its radiative power
the denominator, the result would be, that on account
of the numerator and denominator varying in the same
proportion, the value of that fraction would always
remain the same, whatever might be the gas or vapour
experimented with.

332 FRAGMENTS OF SCIENCE.

But why should this reciprocity exist? WTiat is
the meaning of absorption ? what is the meaning of
radiation? When you cast a stone into still water,
rings of waves surround the place where it falls; motion
is radiated on all sides from the centre of disturbance.
When a hammer strikes a bell, the latter vibrates ; and
sound, which is nothing more than an undulatory motion
of the air, is radiated in all directions. Modern philo-
sophy reduces light and heat to the same mechanical
category. A luminous body is one with its atoms in
a state of vibration ; a hot body is one with its atoms
also vibrating, but at a rate which is incompetent
to excite the sense of vision ; and, as a sounding body
has the air around it, through which it propagates its
vibrations, so also the luminous or heated body has a
medium, called ether, which accepts its motions and
carries them forward with inconceivable velocity.
Radiation, then, as regards both light and heat, is the
transference of motion from the vibrating body to the
ether in which it swings : and, as in the case of sound,
the motion imparted to the air is soon transferred to
surrounding objects, against which the aerial undu-
lations strike, the sound being, in technical language,
absorbed ; so also with regard to light and heat, absorp-
tion consists in the transference of motion from the
agitated ether to the molecules of the absorbing body.

The simple atoms are found to be bad radiators;
the compound atoms good ones : and the higher the
degree of complexity in the atomic^ grouping, the more
potent, as a general rule, is the radiation and absorption.
Let us get definite ideas here, however gross, and purify
them afterwards by the process of abstraction. Imagine
our simple atoms swinging like single spheres in the
ether ; they cannot create the swell which a group of
them united to form a system can produce. An oaj

PHYSICAL BASIS OF SOLAR CHEMISTRY. 333

runs freely edgeways through the water, and imparts far
less of its motion to the water than when its broad flat
side is brought to bear upon it. In our present language
the oar, broad side vortical, is a good radiator ; broad
I side horizontal, it is a bad radiator. Conversely the
waves of water, impinging upon the flat face of the oar-
blade, will impart a greater amount of motion to it than
when impinging upon the edge. In the position in
which the oar radiates well, it also absorbs well. Simple
atoms glide through the ether without much resistance;
compound ones encounter resistance, and hence yield up
more speedily their motion to the ether. Mix oxygen
and nitrogen mechanically, they absorb and radiate a
certain amount of heat. Cause these gases to combine
chemically and form nitrous oxide, both the absorption
and radiation are thereby augmented hundreds of times !
In this way we look with the telescope of the in-
tellect into atomic systems, and obtain a conception of
processes which the eye of sense can never reach. But
gases and vapoiu-s possess a power of choice as to the
rays which they absorb. They single out certain groups
of rays for destruction, and allow other groups to pass
unharmed. This is best illustrated by a famous experi-
ment of Sir David Brewster's, modified to suit present
requirements. Into a glass cylinder, with its ends
stopped by discs of plate-glass, a small quantity of
nitrous acid gas is introduced ; the presence of the
gas being indicated by its rich brown colour. The
beam from an electric lamp being sent through two
prisms of bisulphide of carbon, a spectrum seven feet
long and eighteen inches wide is cast upon the screen.
Introducing the cylinder containing the nitrous acid
into the path of the beam as it issues from the lamp,
the splendid and continuous spectrum becomes instantly
furrowed by numerous darx bands, the rays answering

334 FEAGMENTS OF SCIENCE.

to which are intercepted by the nitric gas, while the
light which falls upon the intervening spaces is per-
mitted to pass with comparative impunity.

Here also the principle of reciprocity, as regards
radiation and absorption, holds good ; and could we,
without otherwise altering its physical character, render
that nitrous gas luminous, we should find that the very
rays which it absorbs are precisely those which it would
emit. When atmospheric air and other gases are brought
to a state of intense incandescence by the passage of an
electric spark, the spectra which we obtain from them
consist of a series of bright bands. But such spectra
are produced with the greatest brilliancy when, instead
of ordinary gases, we make use of metals heated so
highly as to volatilise them. This is easily done by
the voltaic current. A capsule of carbon filled with
mercury, which formed the positive electrode of the
electric lamp, has a carbon point brought down upon
it. On separating the one from the other, a brilliant
arc containing the mercury in a volatilised condition
passes between them. The spectrum of this arc is
not continuous like that of the solid carbon points,
but consists of a series of vivid bands, each corre-
sponding in colour to that particular portion of the
spectrum to which its rays belong. Copper gives its
system of bands; zinc gives its system; and brass, which
is an alloy of copper and zinc, gives a spectrum made
up of the bands belonging to both metals.

Not only, however, when metals are united like zinc
and copper to form an alloy, is it possible to obtain
the bands which belong to them. No matter how
we may disguise the metal— allowing it to unite with
oxygen to form an oxide, and this again with an acid to
form a salt ; if the heat applied be sufficiently intense,
the bands belonging to the metal reveal themselves with

PHYSICAL BASIS OF SOLAR CHEMISTRY. 335

perfect definition. Into holes drilled in a cylinder of
retort carbon, pure culinary salt is introduced. When
the carbon is made the positive electrode of the
lamp, the resultant spectrum shows the brilliant yellow
lines of the metal sodium. Similar experiments made
with the chlorides of strontium, calcium, lithium,* and
other metals, give the bands due to the respective
metals. When different salts are mixed together, and
rammed into holes in the carbon ; a spectrum is ob-
tained which contains the bands of them all.

The position of these bright bands never varies, and
each metal has its own system. Hence -the competent
observer can infer from the bands of the spectrum the
metals which produce it. It is a language addressed to
the eye instead of the ear ; and the certainty would not
be augmented if each metal possessed the power of
audibly calling out, ' I am here ! * Nor is this language
afifected by distance. If we find that the sun or the
stars give us the bands of our terrestrial metals, it is a
declaration on the part of these orbs that such metals
enter into their composition. Does the sun give us any
such intimation ? Does the solar spectrum exhibit bright
lines which we might compare with those produced by
our terrestrial metals, and prove either their identity
or difference ? No. The solar spectrum, when closely
examined, gives us a multitude of fine dark lines instead
of bright ones. They were first noticed by Dr. Wollas-
ton, but were multiplied and investigated with profound
skill by Fraunhofer, and named after him Fraunhofer's

* The vividness of the colours of the lithium spectrum is extra-
ordinaxy; the spectrum, moreover, contained a blue band of in-
describable splendour. It was thought by many, during the
discourse, that I had mistaken strontium for lithium, as this blue
band had never before been seen. I have obtained it many ti mei
since ; and my friend Dr. Miller, having kindly analysed the sub-
Bt^nce made use of, pronounces it pure chloride of lithium. — J. T,

336 FRAGMENTS OF SCIENCE.

lines. They had been long a standing puzzle to philo-
sophers. The bright lines yielded by metallic vapours
had been also known to us for years ; but the connec-
tion between both classes of phenomena was wholly
unknown, until Kirchhoff, with admirable acuteness,
revealed the secret, and placed it at the same time in
our power to chemically analyse the sun.

We have now some difficult work before us. Hither-
to we have been delighted by objects which addressed
themselves as much to our aesthetic taste as to our scien-
tific faculty ; we have ridden pleasantly to the base of
the final cone of Etna, and must now dismount and
march through ashes and lava, if we would enjoy the
prospect from the summit. Our problem is to connect
the dark lines of Fraunhofer with the bright ones of the
metals. The white beam of the lamp is refracted in
passing through our two prisms, but its different com-
ponents are refracted in different degrees, and thus its
colours are drawn apart. Now the colour depends solely
upon the rate of oscillation of the atoms of the lumi-
nous body ; red light being produced by one rate, blue
light by a much qidcker rate, and the colours between
red and blue by the intermediate rates. The solid in-
candescent coal-points give us a continuous spectrum ;
or in other words they emit rays of all possible periods
between the two extremes of the spectrum. Colour, as
many of you know, is to light what jpitch is to sound.
When a violin-player presses his finger on a string he
makes it shorter and tighter, and thus, causing it to
vibrate more speedily, heightens the pitch. Imagine
such a player to move his fingers slowly along the string,
shortening it gradually as he draws his bow, the note
would rise in pitch by a regular gradation ; there would
be no gap intervening between note and note. Here
we have the analogue to the continuous spectrum, whose
colours insensibly blend together without gap or inter-

PHYSICAL BASIS OF SOLAR CHEMISTRY. 337

ruption, from the red of the lowest pitch to the violet of
the highest. But suppose the player, instead of gradu-
J ally shortening his string, to press his finger on a cer-
tain point, and to sound the corresponding note ; then
to pass on to another point more or less distant, and
sound its note ; then to another, and so on, thus sounding
particular notes separated from each other by gaps which
correspond to the intervals of the string passed over ; we
should then have the exact analogue of a spectrum com-
posed of separate bright bands with intervals of darkness
between them. But this, though a perfectly true and in-
telligible analogy, is not sufficient for our purpose ; we
must look with the mind's eye at the oscillating atoms of
the volatilised metal. Figure these atoms as connected
together by springs of a certain tension, which, if the
atoms are squeezed together, push them again asunder,
and if the atoms are drawn apart, pull them again
together, causing them, before coming to rest, to quiver
for a certain time at a certain definite rate determined
by the strength of the spring. Now the volatilised metal
which gives us one bright band is to be figured as having
its atoms united by springs all of the same tension, its
vibrations are all of one kind. The metal which gives
us two bands may be figured as having some of its atoms
united by springs of one tension, and others by springs
of a different tension. Its vibrations are of two distinct
kinds ; so also when we have three or more bands we are
to figure as many distinct sets of springs, each capable
of vibrating in its own particular time and at a difierent
rate from the others. If we seize this idea definitely, we
shall have no difficulty in dropping the metaphor of
springs, and substituting for it mentally the forces by
which the atoms act upon each other. Having thus far
cleared our way, let us make another effort to advance.
A heavy ivory ball is here suspended from a string,
I blow against tiiis ball; a single puff of my breath

S38 FRAGMENTS OF SCIENCE.

moves it a little way from its position of rest ; it swings
back towards me, and when it reaches the limit of its
swing I puff again. It now swings further ; and thus by
timing the puffs I can so accumulate their action as to
produce oscillations of large amplitude. The ivory ball
here has absorbed the motion which my breath com-
municated to the air. I now bring the ball to rest.
Suppose, instead of the breath, a wave of air to strike
against it, and that this wave is followed by a series of
others which succeed each other exactly in the same
intervals as my puffs ; it is obvious that these waves
would communicate their motion to the ball and cause
it to swing as the puffs did. And it is equally manifest
that this would not be the case if the impulses of the
waves were not properly timed ; for then the motion
imparted to the pendulum by one wave would be neutra-
lised by another, and there could not be the accumula-
tion of effect obtained when the periods of the waves
correspond with the periods of the pendulum. So much
for the particular impulses absorbed by the pendulum.
But if such a pendulum set oscillating in air could pro-
duce waves in the air, it is evident that the waves it
would produce would be of the same period as those
whose motions it would take up or absorb most com-
pletely, if they struck against it.

Perhaps the most curious effect of these timed
impulses ever described was that observed by a watch-
maker, named Ellicott, in the year 1741. He left two
clocks leaning against the same rail; one of them,
which we may call A, was set going ; the other, B, not.
Some time afterwards he foimd, to his surprise, that B
was ticking also. The pendulums being of the same
length, the shocks imparted by the ticking of A to the
rail against which both clocks rested were propagated
to B, and were so timed as to set B going. Other

PHYSICAL BASIS OF SOLAR CHEMISTRY. 339

curious eflfects were at the same time observed. When
the pendulums differed from each other a certain amount,
A set B going, but the reaction of B stopped A. 'Then
B set A going, and the reaction of A stopped B. When
the periods of oscillation were close to each other, but
still not quite alike, the clocks mutually controlled
each other, and by a kind of compromise they ticked
in perfect unison.

But what has all this to do with our present subject?
The varied actions of the universe are all modes of
motion ; and the vibration of a ray claims strict brother-
hood with the vibrations of our pendulum. Suppose
ethereal waves striking upon atoms which oscillate in
the same periods as the waves, the motion of the waves
will be absorbed by the atoms ; suppose we send our
beam of white light through a sodium flame, the atoms
of that flame will be chiefly affected by those undula-
tions which are synchronous with their own periods of
vibration. There will be on the part of those particular
rays a tranpference of motion from the agitated ether
to the atoms of the volatilised metal, which, as already
defined, is absorption.

The experiment justifying this conclusion is now for
the first time to be made before a public audience. I pass
a beam through our two prisms, and the spectrum spreads
its colours upon the screen. Between the lamp and the
prism I interpose a snapdragon light. Alcohol and
water are here mixed with common salt, and the metal
dish that holds them is heated by a spirit-lamp. The
vapour from the mixture ignites and we have a mono-
chromatic flame. Through this flame the beam from the
lamp is now passing ; and observe the result upon the
spectrum. You see a shady band cut out of the yellow,
— not very dark, but sufficiently so to be seen by every-
body present.

^6 FRAGMENTS OF SCIENCE.

But let me exalt this effect. Placing in front of
the electric lamp the intense flame of a large Bunsen's
burner, a platinum capsule containiDg a bit of sodium
less than a pea in magnitude is plunged into the flame.
The sodium soon volatilises and bums with brilliant
incandescence. The beam crosses the flame, and at the
same time the yellow band of the spectrum is clearly
and sharply cut out, a band of intense darkness occupy-
ing its place. On withdrawing the sodium, the brilliant
yellow of the spectrum takes its proper place, while the
reintroduction of the flame causes the band to reappear.

Let me be more precise : — The yellow colour of the
spectrum extends over a sensible space, blending on one
side with the orange and on the other with the green.
The term ' yellow band ' is therefore somewhat indefi-
nite. This vagueness may be entirely removed. By
dipping the carbon-point used for the positive electrode
into a solution of common salt, and replacing it in the
lamp, the bright yellow band produced by the sodium
vapour stands out from the spectrum. When the
sodium flame is caused to act upon the beam it is that
particular yellow band that is obliterated, an intensely
black streak occupying its place.

An additional step of reasoning leads to the con-
clusion that if, instead of the flame of sodium alone, we
were to introduce into the path of the beam a flame in
which lithium, strontium, magnesium, calcium, &c., are '
in a state of volatilisation, each metallic vapour would
cut out a system of bands, corresponding exactly in
position with the bright bands of the same metallic
vapour. The light of our electric lamp shining through
such a composite flame would give us a spectrum cut
up by dark lines, exactly as the solar spectrum is cut
up by the lines of Fraunhofer.

Thus by the combination of the strictest reasoning

PHYSICAL BASIS OF SOLAR CHEMISTRY* 341

with the most conclusive experiment, we reach the
solution of one of the grandest of scientific problems —
the constitution of the sun. The sun consists of a
nucleus surrounded by a flaming atmosphere. The light
of the nucleus would give us a continuous spectrum,
like that of our common carbon-points ; but having to
pass through the photosphere, as our beam had to pass
through the flame, those rajs of the nucleus which the
photosphere can itself emit are absorbed, and shaded
spaces, corresponding to the particular rays absorbed,
occur in the spectrum. Abolish the solar nucleus, and
we should have a spectrum showing a bright line in the
place of every dark line of Fraunhofer. These lines are
therefore not absolutely dark, but dark by an amount
corresponding to the difference between the light of
the nucleus intercepted by the photosphere, and the
light which issues from the latter.

The man to whom we owe this noble generalisation
is Kirchhoff, Professor of Natural Philosophy in the
University of Heidelberg ; * but, like every other great
discovery, it is compounded of various elements. Mr.
Talbot observed the bright lines in the spectra of
coloured flames. Sixteen years ago Dr. Miller gave
drawings and descriptions of the spectra of various
coloured flames. Wheatstone, with his accustomed
ingenuity, analysed the light of the electric spark, and
showed that the metals between which the spark passed
determined the bright bands in the spectrum of the
spark. Masson published a prize essay on these bands;
Van der Willigen, and more recently Pliicker, have
given us beautiful drawings of the spectra, obtained
from the discharge of Kuhmkorfif's coil. But none of
these distinguished men betrayed the least knowledge
of the connection between the bright bands of the
* Now Professor in the University of Berlin.

S42 FRAGMENTS OF SCIENCE.

metals and the dark lines of the solar spectrum. Thd
man who came nearest to the philosophy of the subject
was Angstrom. In a paper translated from Poggen-
dorff 's * Annalen ' by myself, and published in the
'Philosophical Magazine' for 1855, he indicates that
the rays which a body absorbs are precisely those which
it can emit when rendered luminous. In another place,
he speaks of one of his spectra giving the general im-
pression of a reversal of the solar spectrum. Foucault,
Stokes, and Thomson, have all been very close to the
discovery; and, for my own part, the examination of
the radiation and absorption of heat by gases and
vapours, some of the results of which I placed before
you at the commencement of this discourse, would have
led me in 1859 to the law on which all Kirchhoff's
speculations are founded, had not an accident withdrawn
me from the investigation. But Kirchhoff's claims are
unaffected by these circumstances. True, much that I
have referred to formed the necessary basis of his dis-
covery ; so did the laws of Kepler furnish to Newton
the basis of the theory of gravitation. But what
Kirchhoff has done carries us far beyonii all that had*
before been accomplished. He has introduced the order
of law amid a vast assemblage of empirical observa-
tions, and has ennobled our previous knowledge by
showing its relationship to some of the most sublime of
natural phenomena*

XV.

ELEMENTARY MAGNETISM.

A LECTUBB TO SCHOOLMASTERS.

WE have no reason to believe that the sheep osr tlie
dog, or indeed any of the lower animals, feel an
interest in the laws by which natural phenomena are
regulated. A herd may be terrified by a thunder-
storm ; birds may go to roost, and cattle return to
their stalls, during a solar eclipse ; but neither birds
nor cattle, as far as we know, ever think of enquiring
into the causes of these things. It is otherwise with
man. The presence of natural objects, the occurrence
of natural events, the varied appearances of the universe
in which he dwells penetrate beyond his organs of sense,
and appeal to an inner power of which the senses are
the mere instruments and excitants. No fact is to him
either original or final. He cannot limit himself to the
contemplation of it alone, but endeavours to ascertain
its position in a series to which uniform experience
assures him it must belong. He regards all that he
witnesses in the present as. the efflux and sequence of
something that has gone before, and as the source of
a system of events which is to follow. The notion of
spontaneity, by which in his ruder state he accounted
for natural events, is abandoned ; the idea that nature
is an aggregate of independent parts also disappears, as
the connection and mutual dependence of physical
23

L

344 FEAGMENTS OF SCIENCE.

powers become more and more manifest: until he ia
finally led to regard Nature as an organic whole — as a
body each of whose members sympathises with the rest,
changing, it is true, from age to age, but changing
without break of continuity in the relation of cause
and effect.

The system of things which we call Nature is, how-
ever, too vast and various to be studied first-hand by
any single mind. As knowledge extends there is always
a tendency to subdivide the field of investigation. Its
various parts are taken up by different minds, and thus
receive a greater amount of attention than could pos-
sibly be bestowed on them if each investigator aimed
at the mastery of the whole. The centrifugal form in
which knowledge, as a whole, advances, spreading ever
wider on all sides, is due in reality to the exertions of
individuals, each of whom directs his efforts, more or
less, along a single line. Accepting, in man}^ respects,
his culture from his fellow-men — taking it from spoken
words or from written books — in some one direction,
the student of Nature ought actually to touch his work.
He may otherwise be a distributor of knowledge, but
not a creator, and he fails to attain that vitality of
thought, and correctness of judgmept, which direct
and habitual contact with natural truth can alone im-
part.

One large department of the system of Nature
which forms the chief subject of my own studies, and
to which it is my duty to call your attention this
evening, is that of physics, or natural philosophy. This
term is large enough to cover the study of Nature gen
rally, but it is usually restricted to a department which,
perhaps, lies closer to our perceptions than any other.
It deals with the phenomena and laws of light and
heat — with the phenomena and laws of magnetism and

ELEMENTARY MAGNETISM. 345

electricity — with those of sound — with the pressures and
motions of liquids and gases, whether at rest or in a state
of translation or of undulation. The science of mechanics
is a portion of natural philosophy, though at present
so large as to need the exclusive attention of him who
would cultivate it profoundly. Astronomy is the ap-
plication of physics to the motions of the heavenly
bodies, the vastness of the field causing it, however, to
be regarded as a department in itself. In chemistry
physical agents play important parts. By heat and
light we cause atoms and molecules to unite or to fall
asunder. Electricity exerts a similar power. Through
their ability to separate nutritive compounds into their
constituents, the solar beams build up the whole vege-
table world, and by it the animal world. The touch of
the self-same beams causes hydrogen and chlorine to
unite with sudden explosion, and to form by their com-
bination a powerful acid. Thus physics and chemistry
intermingle. Physical agents are, however, employed
by the chemist as a means to an end ; while in physics
proper the laws and phenomena of the agents them-
selves, both qualitative and quantitative, are the pri-
mary objects of attention,

My duty here to-night is to spend an hour in telling
how this subject is to be studied, and how a knowledge
of it is to be imparted to others. From the domain of
physics, which would be unmanageable as a whole, T
select as a sample the subject of magnetism. I might
readily entertain you on the present occasion with an
account of what natural philosophy has accomplished.
I might point to those applications of science of which
we hear so much in the newspapers, and which are
so often mistaken for science itself. I might, of course,
ring changes on the steajn-engine and the telegraph,
the electrotype and the photograph, the medical appli-

346 FRAGMENTS OF SCIENCE.

cations of physics, and the various other inlets by which
scientific thought filters into practical life. That would
be easy compared with the task of informing you how
you are to make the study of physics the instrument of
your pupil's culture ; how you are to possess its facts
and make them living seeds which shall take root and
grow in the mind, and not lie like dead lumber in the
storehouse of memory. This is a task much heavier
than the mere recounting of scientific achievements ;
and it is one which, feeling my own want of time to
execute it aright, I might well hesitate to accept.

But let me sink excuses, and attack the work before
me. First and foremost, then, I would advise you to
get a knowledge of facts from actual observation.
Facts looked at directly are vital ; when they pass into
words half the sap is taken out of them. You wish,
for example, to get a knowledge of magnetism ; well,
provide yourself with a good book on the subject, if
you can, but do not be content with what the book tells
you ; do not be satisfied with its descriptive woodcuts ;
see the operations of the force yourself. Half of our
book writers describe experiments which they never
made, and their descriptions often lack both force and
truth ; but, no matter how clever or conscientious they
,may be, their written words cannot supply the place of
actual observation. Every fact has numerous radia-
tions, which are shorn off by the man who describes it.
Go, then, to a philosophical instrument maker, and
give a shilling or half a crown for a straight bar-
magnet, or, if you can afford it, purchase a pair of
them ; or get a smith to cut a length of ten inches
from a bar of steel an inch wide and half an inch
thick ; file its ends smoothly, harden it, and get some-
body like myself to magnetisait. Procure some darn-
ing needles, and also a little unspun silk, which will

^ ELEMENTAKY MAGNETISM. 347

give you a suspending fibre void of torsion. Make a
little loop of paper, or of wire, and attach your fibre
to it. Do it neatly. In the loop place a darning-
needle, and bring the two ends or poles, as they are
called, of your bar-magnet successively up to the ends
of the needle. Both the poles, you find, attract both
ends of the needle. Eeplace the needle by a bit of
annealed iron wire ; the same effects ensue. Suspend
successively little rods of lead, copper, silver, brass,
wood, glass, ivory, or whalebone ; the magnet produces
no sensible effect upon any of the substances. You
thence infer a special property in the case of steel and
iron. Multiply your experiments, however, and you
will find that some other substances, besides iron and
steel, are acted upon by your magnet. A rod of the
metal nickel, or of the metal cobalt, from which the
blue colour used by painters is derived, exhibits
powers similar to those observed with the iron and steel.
In studying the character of the force you may,
however, Confine yourself to iron and steel, which are
always at hand. Make your experiments with the
darning-needle over and over again ; operate on both
ends of the needle ; try both ends of the magnet. Do
not think the work dull ; you are conversing with
Nature, and must acquire over her language a certain
grace and mastery, which practice can alone impart.
Let every movement be made with care, and avoid
slovenliness from the outset. Experiment, as I have
said, is the language by which we address Nature, and
through which she sends her replies ; in the use of tliis
language a lack of straightforwardness is as possible,
and as prejudicial, as in the spoken language of the
tongue. If, therefore, you wish to become acquainted
with the truth of Nature, you must from the first re-
solve to deal with her sincerely.

348 FRAGMENTS OF SCIENCE. •

Now remove your needle from its loop, and draw it
from eye to point along one of the ends of the magnet ;
resuspend it, and repeat your former experiment. You
now find that each extremity of the magnet attracts one
end of the needle, and repels the other. The simple
attraction observed in the first instance, is now replaced
by a dual force. Repeat the experiment till you have
thoroughly observed the ends which attract and those
which repel each other.

Withdraw the magnet entirely from the vicinity of
your needle, and leave the latter freely suspended by
its fibre. Shelter it as well as you can from currents
of air, and if you have iron buttons on your coat, or a
steel penknife in your pocket, beware of their action.
If you work at night, beware of iron candlesticks, or
of brass ones with iron rods inside. Freed from such
disturbances, the needle takes up a certain determinate
position. It sets its length nearly north and south.
Draw it aside and let it go. After several oscillations it
will again come to the same position. If you have
obtained your magnet from a philosophical instrument
maker, yon will see a mark on one of its ends. Suppos-
ing, then, that you drew your needle along the end thus
marked, and tliat the point of your needle was the last
to quit the magnet, you will find that the point turns to
the south, the eye of the needle turning towards the
north. Make sure of this, and do not take the state-
ment on my authority.

Now take a second darning-needle like the first,
and magnetise it in precisely the same manner : freely
suspended it also will turn its eye to the north and
its point to the south. Your next step is to examine
the action of the two needles which you have thus mag-
netised upon each other.

Take one of them in your hand, and leave the other

ELEMENTARY MAGNETISM. 349

suspended ; bring the eye-end of the former near the
eye-end of the latter ; the •suspended needle retreats :
it is repelled. Make the same experiment with the
two points ; you obtain the same result, the suspended
needle is repelled. Now cause the dissimilar ends to
act on each other — you have attraction — point attracts
eye, and eye attracts point. Prove the reciprocity of
this action by removing the suspended needle, and
putting the other in its place. You obtain the same
result. The attraction, then, is mutual, and the repulsion
is mutual. You have thus demonstrated in the clearest
manner the fundamental law of magnetism, that like
poles repel, and that unlike poles attract, each other.
You may say that this is all easily understood without
doing ; but do it, and your knowledge will not be con-
fined to what I have uttered here.

I have said that one end of your bar magnet has a
mark upon it ; lay several silk fibres together, so as to
get sufiBcient strength, or employ a thin silk ribbon,
and form a loop large enough to hold your magnet.
Suspend it ; it turns its marked end towards the north.
This marked end is that which in England is called the
north pole. If a common smith has made your magnet,
it will be convenient to determine its north pole yourself,
and to mark it with a file. Vary your experiments by
causing your magnetised darning-needle to attract and
repel your large magnet ; it is quite competent to do
so. In magnetising the needle, I have supposed the
point to be the last to quit the marked end of the
magnet ; the point of the needle is a south pole. The
end which last quits the magnet is always opposed in
polarity to the end of the magnet with which it has been
last in contact.

You may perhaps learn all this in a single hour ; but
spend several at it, if necessary ; aud remember, under-

350 FRAGMENTS OF SCIENCE.

standing it is not sufficient : you must obtain a manunl
aptitude in addressing Nature. If you speak to your
fellow-man you are not entitled to use jargon. Bad
experiments are jargon addressed to Nature, and just
as much to be deprecated. Manual dexterity in illus-
trating the interaction of magnetic poles is of the
utmost importance at this stage of your progress ; and
you must not neglect attaining this power over your
implements. As you proceed, moreover, you will be
tempted to do more than I can possibly suggest.
Thoughts will occur to you wliich you will endeavour
to follow out : questions will arise which you will try
to answer. The same experiment may be twenty
different things to twenty people. Having witnessed
the actiou of pole on pole, through the air, you will
perhaps try whether the magnetic power is not to be
screened off. You use plates of glass, wood, slate,
pasteboard, or gutta-percha, but find them all pervious
to this wondrous force. One magnetic pole acts upon
another through these bodies as if they were not present.
Should you ever become a patentee for the regulation
of ships' compasses, you will not fall, as some projectors
have done, into the error of screening off the magnetism
of the ship by the interposition of such substances.

If you wish to teach a class you must contrive that
the effects which you have thus far witnessed for your-
self shall be witnessed by twenty or thirty pupils. And
here your private ingenuity must come into play. You
will attach bits of paper to your needles, so as to render
their movements visible at a distance, denoting the
north and south poles by different colours, say green and
red. You may also improve upon your darning-needle.
Take a strip of sheet steel, heat it to vivid redness and
plunge it into cold water. It is thereby hardened ;
rendered, in fact, almost as brittle as glass. Six inches
of this, magnetised in the manner of the darning-

ELEMENTABY MAGNETISM. 351

needle, will be better able to carry your paper indexes.
Having secured such a strip, you proceed thus : —

Magnetise a small sewing-needle and determine its
poles ; or, break half an inch, or an inch, off your magnet-
ised darning-needle and suspend it by a fine silk fibre.
The sewing-needle, or the fragment of the darning
needle, is now to be used as a test-needle, to examine
the distribution of the magnetism in your strip of steel.
Hold the strip upright in your left hand, and cause the
test-needle to approach the lower end of your strip;
one end of the test-needle is attracted, the other is
repelled. Kaise your needle along the strip ; its oscil-
lations, which at first were quick, become slower;
opposite the middle of the strip they cease entirely ;
neither end of the needle is attracted ; above the
middle the test-needle turns suddenly round, its other
end being now attracted. Go through the experi-
ment thorovighly : you thus learn that the entire lower
half of the strip attracts one end of the needle, while
the entire upper half attracts the opposite end. Sup-
posing the north end of your little needle to be that
attracted below, you infer that the entire lower half of
your magnetised strip exhibits south magnetism, while
the entire upper half exhibits north magnetism. So
far, then, you have determined the distribution of
magnetism in your strip of steel.

You look at this fact, you think of it ; in its sug-
gestiveness the value of an experiment chiefly consists.
The thought naturally arises : ' What will occur if I
break my strip of steel across in the middle ?^ Shall I
obtain two magnets each possessing a single pole?'
Try the experiment ; break your strip of steel, and test
each half as you tested the whole. The mere presenta-
tion of its two ends in succession to your test-needle,
suffices to show that you have Tiot a magnet with a

852 FRAGMENTS OF SCIENCE.

single pole — that each half possesses two poles with a
neutral point between them. And if you again break
the half into two other halves, you will find that each
quarter of the original strip exhibits precisely the same
magnetic distribution as the whole strip. You may
continue the breaking process: no matter how small
your fragment may be, it still possesses two opposite
poles and a neutral point between them. Well, your
hand ceases to break where breaking becomes a mecha-
nical impossibility; but does the mind stop there?
No : you follow the breaking process in idea when you
can no longer realise it in fact ; your thoughts wander
amid the very atoms of your steel, and you conclude
that each atom is a magnet, and that the force exerted
by the strip of steel is the mere summation, or resultant,
of the forces of its ultimate particles.

Here, then, is an exhibition of power which we can
call forth at pleasure or cause to disappear. We mag-
netise our strip of steel by drawing it along the pole of
a magnet; we can demagnetise it, or reverse its mag-
netism, by properly drawing it along the same pole in
the opposite direction. What, then, is the real nature
of this wondrous change ? What is it that takes place
among the atoms of the steel when the substance is
magnetised ? The question leads us beyond the region
of sense, and into that of imagination. This faculty,
indeed, is the divining-rod of the man of science.
Not, however, an imagination which catches its crea-
tions from the air, but one informed and inspired by
facts ; capable of seizing firmly on a physical image
as a principle, of discerning its consequences, and of,
devising means whereby these forecasts of thought
may be brought to an experimental test. If such a
principle be adequate to account for all the phenomena
— if from an assumed cause the observed acts neces^

ELEMENTARY MAGNETISM. 353

sarily follow, we call the assumption a theory, and,
once possessing it, we can not only revive at pleasure
facts already known, but we can predict others which
we have never seen. Thus, then, in the prosecution
of physical science, oiu* powers of observation, memory,
imagination, and inference, are all drawn upon. We
observe facts and store them up ; the constructive imagi-
nation broods upon these memories, tries to discern their
interdependence and weave them to an or<;anic whole.
The theoretic principle flashes or slowly dawns upon
the mind; and then* the deductive faculty interposes
to carry out the principle to its logical consequences.
A perfect theory gives dominion over natural facts;
and even an assumption which can only partially stand
the test of a comparison with facts, may be of eminent
use in enabling us to connect and classify groups of
phenomena. The theory of magnetic fluids is of this
latter character, and with it we must now make our-
selves familiar.

With the view of stamping the thing more firmly
on your minds, I will make use of a strong and vivid
image. In optics, red and green are called comple-
mentary colours ; their mixture produces ivhite. Now
I ask you to imagine each of these colours to possess
a self-repulsive power ; that red repels red, that green
repels green ; but that red attracts green and green
attracts red, the attraction of the dissimilar colours
being equal to the repulsion of the similar ones.
Imagine the two colours mixed so as to produce white,
and suppose two strips of wood painted with this white ;
what will be their action upon each other ? Suspend
one of them freely as we suspended our darning needle,
and bring the other near it ; what will occur ? The
red component of the strip you hold in your hand will
repel the red component of your suspended strip ; but

354 FRAGMENTS OF SCIENCE.

then it will attract the green, and, the forces being
equal, they neutralise each other. In fact, the least
reflection shows you that the strips will be as indifferent
to each other as two unmagnetised darning-needles
would be under the same circumstances.

But suppose, instead of mixing the colours, we
painted one half of each strip from centre to end red, and
the other half green, it is perfectly manifest that the
two strips would now behave towards each other exactly
as our two magnetised darning-needles — the red end
wouldrepel the red and attract the. green, the green would
repel the green and attract the red ^ so that, assuming
two colours thus related to each other, we could by
their mixture produce the neutrality of an immagnetised
body, while by their separation we could produce the
duality of action of magnetised bodies.

But you have already anticipated a defect in my
conception ; for if we break one of our strips of wood
in the middle we have one half entirely red, and the
other entirely green, and with these it would be im-
possible to imitate the action of our broken magnet.
How, then, must we modify our conception ? We
must evidently suppose each molecule of the wood
painted green on one face ,and red on the opposite one.
The resultant action of all the atoms would then exactly
resemble the action of a magnet. Here also, if the two
opposite colours of each atom could be caused to mix
so as to produce white, we should have, as before, per-
fect neutrality.

For these two self- repellent and mutually attractive
colours, substitute in your minds two invisible self-
repellent and mutually attractive fluids, which in ordi-
nary steel are mixed to form a neutral compound, but
which the act of magnetisation separates from each
other, placing the opposite fluids on the opposite face

ELEMENTARY MAGNETISM. 355

of each molecule. You have then a perfectly distinct
conception of the celebrated theory of magnetic fluids.
The strength of the magnetism excited is supposed to
be proportional to the quantity of neutral fluid decom-
posed. According to this theory nothing is actually
transferred from the exciting magnet to the excited
steel. The act of magnetisation consists in the forcible
separation of two fluids which existed in the steel before
it was magnetised, but which then neutralised each
other by their coalescence. And if you test your
magnet, after it has excited a hundred pieces of steel,
you will find that it has lost no force — no more, in-
deed, than I should lose, had my words such a magnetic
influence on your minds as to excite in them a strong
resolve to study natural philosophy. I should rather
be the gainer by my own utterance, and by the reaction
of your fervour. The magnet also is the gainer by the
reaction of the body which it magnetises.

Look now to your excited piece of steel ; figure
each molecule with its opposed fluids spread over its
opposite faces. How can this state of things be per-
manent ? The fluids, by hypothesis, attract each other;
what, then, keeps them apart? Why do they not
instantly rush together across the equator of the atom,
and thus neutralise each other ? To meet this question
philosophers have been obliged to infer the existence
of a special force, which holds the fluids asunder. They
call it coercive force ; and it is found that those kinds
of steel which ofier most resistance to being magnetised
— which require the greatest amount of ' coercion ' to
tear their fluids asunder — are the very ones which offer
the greatest resistance to the reunion of the fluids,
after they have been once separated. Such kinds of
steel are most suited to the formation of perrttanent
magnets. It is manifest, indeed, that without coer-

356 FRAGMENTS OF SCIENCE.

cive force a permanent magnet would not be at aD
possible.

Probably long before this you will have dipped the
end of your magnet among iron filings, and observed
how they cling to it; or into a nail-box, and found
how it drags the nails after it. I know very well that
if you are not the slaves of routine, you will have by
this time done many things that I have not told you
to do, and thus multiplied your experience beyond whatl
I have indicated. You are almost sure to have caused
a bit of iron to hang from the end of your magnet, and
you have probably succeeded in causing a second bit
to attach itself to the first, a third to the second ; until
finally the force has become too feeble to bear the
weight of more. If you have operated with nails, you
may have observed that the points and edges hold to-
gether with the greatest tenacity ; and that a bit of
iron clings more firmly to the comer of your magnet
than to one of its flat surfaces. In short, you will in
all likelihood have enriched your experience in many
ways without any special direction from me.

Well, the magnet attracts the nail, and the nail
attracts a second one. This proves that the nail in
contacjt with the magnet has had the magnetic quality
developed in it by that contact. If it be withdrawn
from the magnet its power to attract its fellow nail
ceases. Contact, however, is not necessary. A sheet ot
glass or paper, or a space of air, may exist between the
magnet and the nail; the latter is still magnetised
though not so forcibly as when in actual contact. The
nail thus presented to the magnet is itself a temporary
magnet. That end which is turned towards the mag-
netic pole has the opposite magnetism of the pole which
excites it ; the end most remote from the pole has th
same magnetism as the pole itself, and between tha

ELEMENTAEY MAGNETISM. 367

two poles the nail, Uke the magnet, possesses a mag-
netic equator.

Conversant as you now are with the theory of mag-
netic fluids, you have already, I doubt not, anticipated
me in imagining the exact condition of an iron nail under
the influence of the magnet. You picture the iron as
possessing the neutral fluid in abundance ; you picture
the magnetic pole, when brought near, decomposing
the fluid ; repelling the fluid of a like kind with itself,
and attracting the unlike fluid ; thus exciting in the
parts of the iron nearest to itself the opposite polarity.
But the iron is incapable of becoming a permanent
magnet. It only shows its virtue as long as the magnet
acts upon it. What, then, does the iron lack which the
steel possesses ? It lacks coercive force. Its fluids are
separated with ease ; but, once the separating cause is
removed, they flow together again, and neutrality is
restored. Imagination must be quite nimble in pictur-
ing these changes — able to see the fluids dividing and
reuniting, according as the magnet is brought near or
withdrawn. Fixing a definite pole in your mind, you
must picture the precise arrangement of the two fluids
with reference to this pole, and be able to arouse
similar pictures in the minds of your pupils. You
will cause them to place magnets and iron in various
positions, and describe the exact magnetic state of the
iron in each particular case. The mere facts of mag-
netism will have their interest immensely augmented
by an acquaintance with the principles whereon the facts
depend. Still, while youusethis theory of magnetic fluids
to track out the phenomena and link them together,
you will not forget to tell your pupils that it is to be re-
garded as a symbol merely, — a symbol, moreover, which
is incompetent to coverall the facts,* but which does good
* This theory breaks down when applied to diama^etic bodies

358 FRAGMENTS OF SCIENCE.

practical service whilst we are waiting for tlie actual
truth.

The state of excitement into which iron is thrown
by the influence of a magnet, is sometimes called
'magnetisation by influence.' More commonly, how-
ever, the magnetism is said to be ' induced ' in the iron,
and hence this mode of magnetising is called ' mag
rietic induction.' Now, there is nothing theoretically
perfect in Nature : there is no iron so soft as not to
possess a certain amount of coercive force, and no steel
so hard as not to be capable, in some degree, of mag-
netic induction. The quality of steel is in somel
measure possessed by iron, and the quality of iron is
shared in some degree by steel. It is in virtue of this
latter fact that the unmagnetised darning-needle was
attracted in your first experiment ; and from this you
may at once deduce the consequence that, after the
steel has been magnetised, the repulsive action of a
magnet must be always less than its attractive action.
For the repulsion is opposed by the inductive action of
the magnet on the steel, while the attraction is assisted
by the same inductive action. Make this clear to your
minds, and verify it by your experiments. In some
cases you can actually make the attraction due to theJI
temporary magnetism overbalance the repulsion due to"!
the permanent magnetism, and thus cause two poles of
the same kind apparently to attract each other. When,
however, good hard magnets act on each other from a
sufficient distance, the inductive action practically
vanishes, and the repulsion of like poles is sensibly
equal to the attraction of unlike ones.

which are repelled by magnets. Like soft iron, such bodies aw
thrown into a state of temporary excitement, in virtue of which
they are repelled ; but any attempt to explain such a repulsion hf
the decomposition of a fluid will demonstrate its own futility.

ELEMENTARY MAGNETISM. 359

I dwell thus long on elementary principles, because
they are of the first importance, and it is the temptation
of this age of unhealthy cramming to neglect them.
Now follow me a little farther. In examining the
distribution of magnetism in your strip of steel you
raised the needle slowly from bottom to top, and found
what we called a neutral point at the centre. Now
does the magnet really exert no influence on the pole
presented to its centre ? Let us see.

Let s N, fig. 1 1 , be our magnet, and let n represent
a particle of north magnetism placed exactly opposite
the middle of the magnet. Of course this is an ima-
ginary case, as ^ou can never in reality thus detach
your north magnetism from its neighbour. But sup-
posing us to have done so, what would be the action of
the two poles of the magnet on n ? Your reply will of
coiu'se be that the pole s attracts n while the pole N
repels it. Let the magnitude and direction of the
attraction be expressed by the line n m, and the mag-

>

1-

1

•1

Fig. 11.

N

nitude and direction of the repulsion by the line n o.
Now, the particle n being equally distant from s and N,
the line n o, expressing the repulsion, will be equal to
m n, which expresses the attraction. Acted upon by
two such forces, the particle n must evidently move in
the dire tion np, exactly midway between m n and n o.
Hence you see that, although there is no tendency of
24

360

FRAGMENTS OF SCIENCE.

the particle n to move towards the magnetic equator,
there is a tendency on its part to move parallel to the
magnet. If, instead of a particle of north magnetism,
we placed a particle of south magnetism opposite to
the magnetic equator, it would evidently be urged
along the line n q ; and if, instead of two separate:;
particles of magnetism, we place a little magnetic
needle, containing both north and south magnetism,
opposite the magnetic equator, its south pole being
urged along n q, and its north along n p, the little
needle will be compelled to set itself parallel to the
magnet s N. Make the experiment, and satisfy your-
selves that this is a true deduction.

Substitute for your magnetic needle a bit of iron-
wire, devoid of permanent magnetism, and it will set
itself exactly as the needle does. Acted upon by the
magnet, the wire, as you know, becomes a magnet and ,
behaves as such ; it will turn its north pole towards p, \
and south pole towards q, just like the needle.

But supposing you shift the position of your particlej
of north magnetism, and bring it nearer to one end of]
your magnet than to the other ; the forces acting on
the particle are no longer equal ; the nearest pole ofj

SE

Fig. 12.

the magnet will act more powerfully on the particle
than the more distant one. Let s N, fig. 12, be the
magnet, and n the particle of north magnetism, in it

ELEMENTARY MAGNETISM. 361

new position. It is repelled by N, and attracted by s.
Let the repulsion be represented in magnitude and
direction by the line n o, and the attraction by the
shorter line n m. The resultant of these two forces
will be found by completing the parallelogram Tnnop,
and drawing its diagonal n p. Along n p, then, a
particle of north magnetism would be urged by the
simultaneous action of s and n. Substituting a particle
of south magnetism for n, the same reasoning would
lead to the conclusion that the particle would be urged
along n q. If we place at 7i a short magnetic needle,
its north pole will be urged along n p, its south pole
along n q, the only position possible to the needle,
thus acted on, being along the line p q, which is no
longer parallel to the magnet. Verify this deduction
by actual experiment.

In this way we might go round the entire magnet ;
and, considering its two poles as two centres from
which the force emanates, we could, in accordance with
ordinary mechanical principles, assign a definite direc-
tion to the magnetic needle at every particular place.
And substituting, as before, a bit of iron wire for the
magnetic needle, the positions of both will be the
same.

Now, I think, without further preface, you will be
able to comprehend for yourselves, and explain to
others, one of the most interesting effects in the whole
domain of magnetism. Iron filings you know are
particles of iron, irregular in shape, being longer in
some directions than in others. P'or the present ex-
periment, moreover, instead of the iron filings, very
small scraps of thin iron wire might be employed. I
place a sheet of paper over the magnet ; it is all the
better if the paper be stretched on a wooden frame, as
this enables us to keep it quite level. I scatter the

MAGNETIC LINES OF FORCE.
From a J'hotograph pj/ Pbofessob Maybb,

ELEMENTARY MAGNETISM. 363

filings, or the scraps of wire, from a sieve upon the
paper, and tap the latter gently, so as to liberate the
particles for a moment from its friction. The magnet
acts on the filings through the paper, and see how it
arranges them I They embrace the magnet in a series
of beautiful ciu'ves, which are technically called ' mag-
netic curves,' or * lines of magnetic force.' Does the
meaning of these lines yet flash upon you ? Set your
magnetic needle, or your suspended bit of wire, at any
point of one of the curves, and you will find the direc-
tion of the needle, or of the wire, to be exactly that of
the particle of iron, or of the magnetic curve, at that
point. Gro round and round the magnet ; the direction
of your needle always coincides with the direction of
the curve on which it is placed. These, then, are the
lines along which a particle of south magnetism, if you
could detach it, would move to. the north pole, and a
bit of north magnetism to the south pole. They are
the lines along which the decomposition of the neutral
fluid takes place. In the case of the magnetic needle,
one of its poles being urged in one direction, and the
other pole in the opposite direction, the needle must
necessarily set itself as a tangent to the curve. I will
not seek to simplify this subject further. If there be
anything obscure or confused or incomplete in my
statement, you ought now, by patient thought, to be
able to clear away the obscurity, to reduce the con-
fusion to order, and to supply wliat is needed to render
the explanation complete. Do not quit the subject
until you thoroughly understand it ; and if you are
then able to look with your mind's eye at the play of
forces around a magnet, and see distinctly the operation
of those forces in the production of the magnetic curves,
the time which we have spent together will not have
been spent in vain.

364 FKAGMENTS OF SCIENCE.

In this thorough manner we must master our ma-
terials, reason upon them, and, by determined study,
attain to clearness of conception. Facts thus dealt
with exercise an expansive force upon the intellect ;—
they widen the mind to generalisation. We soon
recognise a brotherhood between the larger phenomena
of Nature and the minute effects which we have ob-
served in our private chambers. Why, we enquire,
does the magnetic needle set north and south ? Evi-
dently it is compelled to do so by the earth ; the great
globe which we inherit is itself a magnet. Let us
learn a little more about it. By means of a bit of
wax, or otherwise, attach the end of yom* silk fibre to
the middle point of your magnetic needle ; the needle
will thus be uninterfered with by the paper loop, and
will enjoy to some extent a power of * dipping' its
point, or its eye, belpw the horizon. Lay your bar
magnet on a table, and hold the needle over the equator
of the magnet. The needle sets horizontal. Move it
towards the north end of the magnet ; the south end
of the needle dips, the dip augmenting as you approach
the north pole, over which the needle, if free to move,
will set itself exactly vertical. Move it back to the
centre, it resumes its horizontality ; pass it on towards
the south pole, its north end now dips, and directly
over the south pole the needle becomes vertical, its
north end being now turned downwards. Thus we
learn that on the one side of the magnetic ecjuator the
north end of the needle dips; on the other side the
south end dips, the dip varying from nothing to 90°.
If we go to the equatorial regions of the earth with a
suitably suspended needle we shall find there the
position of the needle horizontal. If we sail north one
end of the needle dips ; if we sail south the opposite
end dips; and over the north or south terrestrial

ELEMENTARY MAGNETISM. 365

magnetic pole the needle sets vertical. The south
magnetic pole has not yet been found, but Sir James
Eoss discovered the north magnetic pole on June 1,
1831. In this manner we establish a complete paral-
lelism between the action of the earth and that of an
ordinary magnet.

The terrestrial magnetic poles do not coincide vrith
the geographical ones; nor does the earth's magnetic
equator quite coincide with the geographical equator.
The direction of the magnetic needle in London, which
is called the magnetic meridian, encloses an angle of 24**
with the astronomical meridian, this angle being called
the Declination of the needle for London. The north
pole of the needle now lies to the west of the true meri-
dian ; the declination is westerly. In the year 1660,
however, the declination was nothing, while before that
time it was easterly. All this proves that the earth's
magnetic constituents are gradually changing their dis-
tribution. This change is very slow : it is therefore
called the secular change, and the observation of it has
not yet extended over a sufficient period to enable us to
guess, even approximately, at its laws.

Having thus discovered, to some extent, the secret of
the earth's magnetic power, we can turn it to account. In
the line of ' dip ' I hold a poker formed of good soft iron.
The earth, acting as a magnet, is at this moment con-
straining the two fluids of the poker to- separate, making
the lower end of the poker a north pole, and the upper end
a south pole. Mark the experiment : When the knob
is uppermost, it attracts the north end of a magnetic
needle ; when undermost it attracts the south end of
a magnetic needle. With such a poker repeat this ex-
periment and satisfy yourselves that the fluids shift their
position according to the manner in which the poker is
presented to the earth. It has already been stated that

366 FRAGMENTS OF SCIENCE.

the softest iron possesses a certain amount of coercive
force. The earth, at this moment, finds in this force
an antagonist which opposes the decomposition of the
neutral fluid, The component fluids may be figured as
meeting an amount of friction, or possessing an amount
of adhesion, which prevents them from gliding over the
molecules of the poker. Can we assist the earth in this
case ? If we wish to remove the residue of a powder
from the interior surface of a glass to which the powder
clings, we invert the glass, tap it, loosen the hold of
the powder, and thus enable the force of gravity to pull
it down. So also by tapping the end of the poker we
loosen the adhesion of the magnetic fluids to the mole-
cules and enable the earth to pull them apart. Butj
what is the consequence ? The portion of fluid which
has been thus forcibly dragged over the molecules
refuses to return when the poker has been removed from
the line of dip ; the iron, as you see, has become a per-
manent magnet. By reversing its position and tapping
it again we reverse its magnetism. A thoughtful and
competent teacher will know how to place these remark-
able facts before his pupils in a manner which will
excite tlieir interest. By the use of sensible images,
more or less gross, he will first give those whom he
teaches definite conceptions, purifying these conceptions
afterwards, as the minds of his pupils become more
capable of abstraction. By thus giving them a distinct
substratum for their reasonings, he will confer upon his
pupils a profit and a joy which the mere exhibition of
facts without principles, or the appeal to the bodily
senses and the power of memory alone, could never
inspire.

ELEMENTARY MAGNETISM. 367

As an expansion of the note at p. 357, the following extract may
find a place here : —

* It is well known that a voltaic current exerts an attractive
force upon a second current, flowing in the same direction ; and
that when the directions are opposed to each other the force exerted
is a repulsive one. By coiling wires into spirals, Ampere was
enabled to make them produce all the phenomena of attraction
and repulsion exhibiled by magnets, and from this it was but a
step to his celebrated theory of molecular currents. He supposed
the molecules of a magnetic body to be surrounded by such currents,
which, however, in the natural state of the body mutually neutral-
ised each other, on account of their confused grouping. The act of
magnetisation he supposed to consist in setting these molecular
currents parallel to each other; and, starting from this principle,
he reduced all the phenomena of magnetism to the mutual action
of electric currents.

* If we reflect upon the experiments recorded in the foregoing
pages from first to last, we can hardly fail to be convinced that
diamagnetic bodies operated on by magnetic forces possess a polarity
*' the same in kind as, but the reverse in direction of, that acquired
by magnetic bodies." But if this be the case, how are we to
conceive the physical mechanism of this polarity ? According to
Coulomb's and Poisson's theory, the act of magnetisation consists
in the decomposition of a neutral magnetic fluid ; the north pole of
a magnet, for example, possesses an attraction for the south fluid
of a piece of soft iron submitted to its influence, draws the said
fluid towards it, and with it the material particles with which the
fluid is associated. To account for diamagnetic phenomena this
theory seems to fail altogether ; according to it, indeed, the oft-
used phrase, " a north pole exciting a north pole, and a south pole
a south pole," involves a contradiction. For if the north fluid be
supposed to be attracted towards the influencing north pole, it is
absurd to suppose that its presence there could produce reimhum.
The theory of Ampere is equally at a loss to explain diamagnetic
action ; for if we suppose the particles of bismuth surrounded by
molecular currents, then, according to all that is known of electro-
dynamic laws, these currents would set themselves parallel to, and
in the same direct ion as, those of the magnet, and hence attraction,
and not repulsion, would be the result. The fact, however, of this
not being the case, proves that these molecular currents are not
the mechanism by which diamagnetic induction is efi'ected. The
eonsciotisness of this, I doubt not, drove M. Weber to the assumption
that the phenomena of diamagnetism are produced by molecular
earrents, not directed^ but actually excited in the bismuth by the

368 FRAGMENTS OF SCIENCE,

magnet. Such induced currents would, according to known laws,
have a direction opposed to those of the inducing magnet, and hence
would produce the phenomena of repulsion. To carry out the
assumption here made, M. Weber is obliged to suppose that the
molecules of diamagnetic bodies are surrounded by channels, in
which the induced molecular currents, once excited, continue to
flow without resistance.' ' — Biamagnetism and Magne-erygtallio
Actiont p. 136-7.

• In assuming these non-resisting channels M. Weber, it most be
ftdmitted, did not go beyond the assumptions of Amp^ro*

rvL

ON FOECE.*

A SPHERE of lead was suspended at a height of 16
feet above the theatre floor of the Royal lastitu-
tion. It was liberated, and fell by gravity. That weight
required a second to fall to the floor from that eleva-
tion ; and the instant before it touched the floor, it had
a velocity of 32 feet a second. That is to say, if at that
instant the earth were annihilated, and its attraction
annulled, the weight would proceed through space at
the uniform velocity of 32 feet a second.

If instead of being pulled downward by gravity, the
weight be cast upward in opposition to gravity, then,
to reach a height of 16 feet it must start with a
velocity of 32 feet a second. This velocity imparted
to the weigfit by the human hand, or by any other
mechanical means, would carry it to the precise height
from which we saw it fall.

Now the lifting of the weight may be regarded as
so much mechanical work performed. By means of a
ladder placed against the wall, the weight might be
carried up to a height of 1 6 feet ; or it might be drawn
up to this height by means of a string and pulley, or
it might be suddenly jerked up to a height of 16 feet.
The amount of work done in all these cases, as far as
the raising of the weight is concerned, would be abso-
' A discourse delivered in the Koyal Institution, June 6, 1862.

370 FRAGMKNTS OF SCIENCE. |

lutely the same. The work done at one and the same
place, and neglecting the small change of gravity with
the height, depends solely upon two things ; on the
quantity of matter lifted, and on the height to which
it is lifted. If we call the quantity or mass of mutter m,
and the height through which it is lifted h, then the
product of m into A, or m h, expresses, or is propor-
tional to, the amount of work done.

Supposing, instead of imparting a velocity of 32 feet
a second we impart at starting twice this velocity. To
what height will the weight rise ? You might be dis-
posed to answer, ' To twice the height ; ' but this would
be quite incorrect. Instead of twice 1 6, or 32 feet, it
would reach a height of four times 1 6, or 64 feet. So
also, if we treble the starting velocity, the weight would
reach nine times the height ; if we quadruple the speed
at starting, we attain sixteen times the height. Thus,
with a four- fold velocity of 128 feet a second at start-
ing, the weight would attain an elevation of 256 feet.
With a seven-fold velocity at starting, the weight would
rise to 49 times the height, or to an elevation of 784
feet.

Now the work done — or, as it is sometimes called,
the mechanical effect — other things being constant, is,
as before explained, proportional to the height, and
as a double velocity gives four times the height, a treble
velocity nine times the height, and so on, it is perfectly
plain that the mechanical effect increases as the square
of the velocity. If the mass of the body be represented
by the letter m, and its velocity by v, the mechanical
effect would be proportional to or represented by m v\
In the case considered, I have supposed the weight to be
cast upward, being opposed in its flight by the resist-
ance of gravity; but the same holds true if the pro-
jectile be sent into water, mud, earth, timber, or other

FORCE. 371

resisting material. If, for example, we double the
velocity of a cannon-ball, we quadruple its mechanical
effect. Hence the importance of augmenting the velo-
city of a projectile, and hence the philosophy of Sir
William Armstrong in using a large charge of powdei
in his recent striking experiments.

The measure then of mechanical effect is the mass
of the body multiplied by the square of its velocity.

Now in firing a ball against a target the projectile,
after collision, is often found hot. JNIr. Fairbairn
informs me that in the experiments at Shoeburyness it
is a common thing to see a flash, even in broad day-
light, when the ball strikes the target. And if our lead
weight be examined after it has fallen from a height
it is also found heated. Now here experiment and
reasoning lead us to the remarkable law that, like
the mechanical effect, the amount of heat generated is
proportional to the product of the mass into the scpiare
of the velocity. Double your mass, other things being
equal, and you double your amount of heat ; double
your velocity, other things remaining equal, and you
quadruple your amount of heat. Here then we have
common mechanical motion destroyed and heat pro-
duced. When a violin bow is drawn across a string,
the sound produced is due to motion imparted to the
aif, and to produce that motion muscular force has
been expended. We may here correctly say, that the
mechanical force of the arm is converted into music.
In a similar wjiy we say that the arrested motion of
our descending weight, or of the cannon-ball, is con-
verted into heat. The mode of motion changes, but
motion still continues ; the motion of the mass is con-
verted into a motion of the atoms of the mass ; and
these small motions, communicated to the nerves, pro-
duce the sensation we call heat.

372 FRAGMENTS OF SCIENCE.

We know the amount of heat which a given amount
of mechanical force can develope. Our lead ball, for
example, in falling to the earth generated a quantity of
heat sufficient to raise its own temperature three-fifths
of a Fahrenheit degree. It reached the earth with a
velocity of 32 feet a second,. and forty times this velo-
city would be small for a rifle bullet ; multiplying Jths
by the square of 40, we find that the amount of heat
developed by collision with the target would, if wholly
concentrated in the lead, raise its temperature 960
degrees. This would be more than sufficient to fuse
the lead. In reality, however, the heat developed is
divided between the lead and the body against which
it strikes ; nevertheless, it would be worth while to pay
attention to this point, and to ascertain whether rifle
bullets do not, under some circumstances, show signs
of fusion.^

From the motion of sensible masses, by gravity and
other means, we now pass to the motion of atoms towards
each other by chemical affinity. A collodion balloon
filled with a mixture of chlorine and hydrogen being
hung in the focus of a parabolic mirror, in the focus
of a second mirror 20 feet distant a strong electric
light was suddenly generated ; the instant the concen-
trated light fell upon the balloon, the gases within it
exploded, hydrochloric acid being the result. Here the
atoms virtually fell together, the amount of heat pro-
duced showing the enormous force of the collision.
The burning of charcoal in oxygen is an old . experi-
ment, but it has now a significance beyond what it
used to have ; we now regard the act of combination
on the part of the atoms of oxygen and coal as we re-

' Eight years subsequently this surmise was proved correct.
In the Franco-German War signs of fusion were observed in tlie
case of bullets impinging on bones.

1

FORCE. 373

gard the clashing of a falling weight against the
earth. The heat produced in both cases is referable to
a common cause. A diamond, which burns in oxygen
as a star of white light, glows and bums in consequence
of the falling of the atoms of oxygen against it. And
could we measure the velocity of the atoms when they
clash, and could we find their number and weights, mul-
tiplying the weight of each atom by the square of its
velocity, and adding all together, we should get a
number representing the exact amount of lieat de-
veloped by the union of the oxygen and carbon.

Thus far we have regarded the heat developed by
the clashing of sensible masses and of atoms. Work is
expended in giving motion to these atoms or masses,
and heat is developed. But we reverse this process
daily, and by the expenditure of heat execute work.
We can raise a weight by heat ; and in this agent we
possess an enormous store of mechanical power. A
pound of coal produces by its combination with oxygen
an amount of heat wliich, if mechanically applied,
would suffice to raise a weight of 100 lbs. to a height of
20 miles above the earth's surface. Conversely, 100 lbs.
falling from a height of 20 miles, and striking against
the earth, would generate an amount of heat equal to
that developed by the combustion of a pound of coal.
Wherever work is done by heat, heat disappears. A
gun which fires a ball is less heated than one which fires
blank cartridge. The quantity of heat communicated
to the boiler of a working steam-engine is greater than
that which could be obtained from the re-condensation
of the steam, after it had done its work ; and the
amount of work performed is the exact equivalent of
the amount of heat lost. Mr. Smyth informed us in
his interesting discourse, that we dig annually 84
millions of tons of coal from our pits. The amount of

374 FRAGMENTS OF SCIENCE.

mechanical force represented by this quantity of coal
seems perfectly fabulous. The combustion of a single
pound of coal, supposing it to take place in a minute,
would be equivalent to the work of 300 horses; and if
we suppose 108 millions of horses working day and
night with unimpaired strength, for a year, their
united energies would enable them to perform an
amount of work just equivalent to that which the
annual produce of our coal-fields would be able to
accomplish.

Comparing with ordinary gravity the force with
which oxygen and carbon unite together, chemical
affinity seems almost infinite. But let us give gravity
fair play by permitting it to act throughout its entire
range. Place a boily at such a distance from the earth
that the attraction of our planet is l)arely sensible, and
let it fall to the earth from this distance. It would
reach the earth with a final velocity of 36,747 feet a
second ; and on collision with the earth the body would
generate about twice the an^iount of heat generated by
the combustion of an equal weight of coal. We have
stated that by falling through a space of 16 feet our lead
bullet would be heated three-fifths of a degree ; but a
body falling frjm an infinite distance has already used
up 1,299,999 parts out of 1,300,000 of the earth's
pulling power, when it has arrived within 16 feet of the
surface ; on this space only i^sfe^^^ths of the whole force
is exerted.

Let us now turn our thoughts for a moment from
the earth to the sun. The researches of Sir John
Ilerschel and M. Pouillet have informed us of the
annual expenditure of the sun as regards heat ; and by
an easy calculation we ascertain the precise amount of
the expenditure which falls to the share of our planet.
Out of 2300 million parts of light and heat the earth

FORCE. 375

receives one. The whole l)eat emitted by the sun in a
minute would be competent to boil 12,000 millions of
cubic miles of ice-cold water. How is this enormous
loss made good — whence is the sun's heat derived, and
by what means is it maintained ? No combustion — no
chemical affinity with which we are acquainted, would
be competent to produce the temperature of the sun's
surface. Besides, were the sun a burning body merely,
its light and heat would speedily come to an end.
Supposing it to be a solid globe of coal, its combustion
would only cover 4600 years of expenditure. In this
short time it would burn itself out. What agency
then can produce the temperature and maintain the
outlay ? We have already regarded the case of a body
falling from a great distance towards the earth, and
found that the heat generated by its collision would be
twice that produced by the combustion of an equal
weight of coal. How much greater must be the heat
developed by a body falling against the sun I The
maximum velocity with which a body can strike the
earth is about 7 miles in a second ; the maximum velo-
city with which it can strike the sun is 390 miles in a
second. And as the heat developed by the collision is
proportional to the square of the velocity destroyed, an
asteroid falling into the sun with the above velocity
would generate about 10,000 times the quantity of heat
produced by the combustion of an asteroid of coal of
the same weight.

Have we any reason to believe that such bodies
exist in space, and that they may be raining down upon
the sun ? The meteorites flashing through the air are
small planetary bodies, drawn by the earth's attraction.
They enter our atmosphere with planetary velocity,
and by friction against the air they are raised to incan-
descence and caused to emit light and heat. At certain
25

376 FRAGMENTS OF SCIENCE.

seasons of the year they shower down upon us in great
numbers. In Boston 240,000 of them were observed
in nine hours. There is no reason to suppose that the
planetary system is limited to ' vast masses of enormous
weight;' there is, on the contrary, reason to believe
that space is stocked with smaller masses, which obey
the same laws as the larger ones. That lenticular
envelope which surrounds the sun, and which is known
to astronomers as the Zodiacal light, is probably a crowd
of meteors; and moving as they do in a resisting
medium, they must continually approach the sun.
Falling into it, they would produce enormous heat, and
this would constitute a source from which the annual
loss of heat might be made good. The sun, according
to this hypothesis, would continually grow larger ; but
how much larger ? Were our moon to fall into the
sun, it would develope an amount of heat sufficient to
cover one or two years' loss ; and were our earth to fall
into the sun a century's loss would be made good. Still,
our moon and our earth, if distributed over the surface
of the sun, would utterly vanish from perception. In-
deed, the quantity of matter competent to produce the
required effect would, during the range of history, cause
no appreciable augmentation in the sun's magnitude.
The augmentation of the sun's attractive force would
be more sensible. However this hypothesis may fare
as a representant of what is going on in nature, it cer-
tainly shows how a sun might be formed and main-
tained on known thermo-dynamic principles.

Our earth moves in its orbit with a velocity of
68,040 miles an hour. Were this motion stopped, an
amount of heat would be developed sufficient to raise
the temperature of a globe of lead of the same size as
the earth 384,000 degrees of the centigrade thermo-

FOEOE. 377

meter. It has been prophesied that ' the elements
shall melt with fervent heat.' The earth's own motion
embraces the conditions of fidfilment ; stop that motion,
and the greater part, if not the whole, of our planet
would be reduced to vapour. If the earth fell into the
sun, the amount of heat developed by the shock would
be equal to that developed by the combustion of a mass
of solid coal 6435 times the earth in size.

There is one other consideration connected with the
permanence of our present terrestrial conditions, which
is well worthy of our attention. Standing upon one of
the London bridges, we observe the current of the
Thames reversed, and the water poured upward twice
a-day. The water thus moved rubs against the river's
bed, and heat is the consequence of this friction.
The heat thus generated is in part radiated into
space and lost, as far as the earth is concerned. What
supplies this incessant loss ? The earth's rotation. Let
us look a little more closely at the matter. Imagine
the moon fixed, and the earth turning like a wheel
from west to east in its diurnal rotation. Suppose a
high mountain on the earth's surface approaching the
earth's meridian ; that mountain is, as it were, laid
hold of by the moon ; it forms a kind of handle by
wliich the earth is pulled more quickly round. But
when the meridian is passed the pull of the moon on
the mountain would be in the opposite direction, it
would tend to diminish the velocity of rotation as much
as it previously augmented it ; thus the action of all
fixed bodies on the earth's surface is neutralised. But
suppose the mountain to lie always to the east of the
moon's meridian, the pull then would be always exerted
against the earth's rotation, the velocity of which
would be diminished in a degree corresponding to the
strength of the pull. The tidal wave occupies this

378 FRAGMENTS OF SCIENCE.

py^sition — it lies always to the east of the moon's
meridian. The waters of the ocean are in part dragged
as a brake along the surface of the earth ; and as a
Lrake they must diminish the velocity of the earth's
rotation.^ Supposing then that we turn a mill by the
action of the tide, and produce heat by the friction
of the millstones ; that heat has an origin totally dif-
ferent from the heat produced by another mill which
is turned by a mountain stream. The former is pro-
duced at the expense of the earth's rotation, the latter
at the expense of the sun's radiation.

The sun, by the act of vaporisation, lifts mechani-
cally all the moisture of our air, which when it con-
denses falls in the form of rain, and when it freezes falls
as snow. In this solid form it is piled upon the Alpine
heights, and furnishes materials for glaciers. But the
sun again interposes, liberates the solidified liquid, and
permits it to roll by gravity to the sea. The me-
chanical force of eveiy river in the world as it rolls
towards the ocean, is drawn from the heat of the sun.
No streamlet glides to a lower level without having
been first lifted to the elevation from which it springs
by the power of the sun. The energy of winds is also
due entirely to the same power.

But there is still another work which the sun per-
forms, and its connection with which is not so obvious.
Trees and vegetables grow upon the earth, and when
biimed they give rise to heat, and hence to mechanical
energy. Whence is this power derived ? You see this
oxide of iron, produced by the falling together of the
atoms of iron and oxygen ; you cannot see this trans-
parent carbonic acid gas, formed by the falling together

> Eant surmised an action of this kind.

FORCE. 379

of carbon and oxygen. The atoms thus in close union
resemble our lead weight while resting on the earth ;
but we can wind up the weight and prepare it for
another fall, and so these atoms can be wound up and
thus enabled to repeat the process of combination. In
the building of plants carbonic acid is the material
from which the carbon of the plant is derived; and
the solar beam is the agent which tears the atoms
asunder, setting the oxygen free, and allowing the
carbon to aggregate in woody fibre. Let the solar rays
fall upon a surface of sand ; the sand is heated, and
finally radiates away as much heat as it receives ; let
the same beams fall upon a forest, the quantity of heat
given back is less than the forest receives ; for the
energy of a portion of the sunbeams is invested in
building the trees. Without the sun the reduction of
the carbonic acid cannot be effected, and an amount of
sunlight is consumed exactly equivalent to the mole-
cular work done. Thus trees are formed ; thus the
cotton on which Mr. Bazley discoursed last Friday is pro-
duced. I ignite this cotton, and it flames ; the oxygen
again unites with the carbon; but an amount of heat
equal to that produced by its combustion was sacrificed
by the sun to form that bit of cotton.

We cannot, however, stop at vegetable life, for it
is the source, mediate or immediate, of all animal
life. The sun severs the carbon from its oxygen and
builds the vegetable ; the animal consumes the vege-
table thus formed, a reunion of the severed elements
takes place, producing animal heat. The process of
building a vegetable is one of winding up ; the process
of building an animal is one of running down. The
warmth of our bodies, and every mechanical energy
which we exert, trace their lineage directly to the sun.

380 FRAGMENTS OF SCIENCE.

The fight of a pair of pugilists, the motion of an
army, or the lifting of his own body by an Alpine
climber up a mountain slope, are all cases of mechani-
cal energy drawn from the sun. A man weighing 150
pounds has 64 pounds of muscle ; but these, wlien dried,
reduce themselves to 15 pounds. Doing an ordinary
day's work, for eiglity days, this mass of muscle would
be wholly oxidiseil. Special organs which do more work
would be more quickly consumed : the heart, for ex-
ample, if entirely unsustained, would be oxidised in
about a week. Take the amount of heat due to the
direct oxidation of a given weight of food ; less heat
is developed by the oxidation of the same amount of
food in the working animal frame, and the missing
quantity is the e(|uivalent of the mechanical work ac-
complished by the muscles.

I might extend these considerations ; the work,
indeed, is done to my hand — but I am warned tl^at
you have been already kept too long. To wliom tlien
are we indebted for the most striking generalisations
of this evening's discourse ? They are the work of
a man of whom you have scarcely ever heard —
the published labours of a German doctor, named
Mayer. Without external stimulus, and pursuing his
profession as town physician in Heilbronn, this man
was the first to raise the conception of the interaction
of heat and other natural forces to clearness in his
own mind. And yet he is scarcely ever heard of, and
even to scientific men his merits are but partially
known. Led by his own beautiful researches, and quite
independent of Mayer, Mr. Joule published in 1843
his first paper on the 'Mechanical Value of Heat;'
but in 1842 Mayer had actually calculated the
mechanical equivalent of heat from data which only
a man of the rarest penetration could turn to account.

FORCE. 381

In 1845 he published his memoir on * Organic
Motion,' and applied the mechanical theory of heat
in the most fearless and precise manner to vital
processes. He also embraced the other natural agents
in his chain of conservation. In 1853 Mr. Water-
ston proposed, independently, the meteoric theory
of the sun's heat, and in 1854 Professor William
Thomson applied his admirable mathematical powers
to the development of the theory ; but six years pre-
viously the subject had been handled in a masterly
manner by Mayer, and all that I have said about it has
been derived from him. When we consider the cir-
cumstances of Mayer's life, and the period at which he
wrote, we cannot fail to be struck with astonishment at
what he has accomplished. Here was a man of genius
working in silence, animated solely by a love of his
subject, and arrivino- at the most important results in
advance of those whose lives were entirely devoted to
Natural Philosophy. It was the accident of bleeding a
feverish patient at Java in 1840 that led Mayer to
speculate on these subjects. He noticed that the venous
blood in the tropics was of a brighter red than in colder
latitudes, and his reasoning on this fact led him into
the laboratory of natural forces, where he has worked
with such signal ability and success. Well, you will
desire to know what has become of this man. His
mind, it is alleged, gave way ; it is said he became in-
sane, and he was certainly sent to a lunatic asylum. In a
hiograpliical dictionary of his country it is stated that
he died there, but this is incorrect. He recovered;
ani, I believe, is at this moment a cultivator of
vineyards in Heilbronn.

382 FRAGIMENTS OF SCIENCE.

June 20, 1 862.

While preparing for publication my last course of
lectures on Heat, I wished to make myself acquainted
with all that Dr. Mayer had done in connection with this
subject, r accordingly wrote to two gentlemen who
above all others seemed likely to give me the informa-
tion which I needed.' Both of them are Germans, and
both particularly distinguished in connection with the
Dynamical Theory of Heat. Each of them kindly fur-
nished me with the list of Mayer's publications, and one
of them [Clausius] was so friendly as to order them from
a bookseller, and to send them to me. This friend, ir
his reply to my first letter regarding Mayer, stated
his belief that I should not find anything very impor-
tant in Mayer's writings; but before forwarding the
memoirs to me he read them himself. His letter
accompanying them contains the following words: —
'I must here retract the statement in my last letter,
that you would not find much matter of importance in
Mayer's writings : I am astonished at the multitude of
beautiful and correct thoughts which they contain;' and
he goes on to point out various important subjects, in
the treatment of which Mayer had anticipated other
eminent writers. My other friend, in whose own
publications the name of Mayer repeatedly occurs, and
whose papers containing these references were translated
some years ago by myself, was, on the 10th of last month,
unacquainted with the thoughtful and beautiful essay
of Mayer's, entitled 'Beitrage zur Dynamik des Hira-
mels ,' and in 1854, when Professor William Thomson
developed in so striking a manner the meteoric theory
of the sun's heat, he was certainly not aware of the
existence of that essay, though from a recent article in

> Helmholtz and Clansius.

roRCE. 383

'Macmillan's Magazine' T infer that he is now aware of
it. Mayer's physiological writings have been referred
to by physiologists — by Dr. Carpenter, for example
— in terms of honouring recognition. We have
hitherto, indeed, obtained fragmentary glimpses of the
man, partly from physicists and partly from physiolo-
gists; but his total merit has never yet been recognised
as it assuredly would liave been had he chosen a liappier
mode of publication. I do not think a greater disservice
could be done to a man of science, than to overstate his
claims : such overstatement is sure to recoil to the dis-
advantage of him in whose interest it is made. But
when Mayer's opportunities, achievements, and fate are
taken into account, I do not think that I shall be deeply
blamed for attempting to place him in that honourable
positioa, which I believe to be his due.

Here, however, are the titles of Mayer's papers, the
perusal of which will correct any error of judgment
into which I may have fallen regarding their author.
' Bemerkungen uber die Krafte der unbelebten Natur,'
Liebig's ' Annalen,' 1842, Vol. 42, p. 231 ; 'DieOrgan-
ische Bewegung in ihrem Zusammenhange mit dem
Stoffwechsel,' Heilbronn, 1845; 'Beitrage zurDynamik
des Himmels,' Heilbronn, 1848; 'Bemerkungen uber
das Mechanische Equivalent der Warme,' Heilbronn,
1851.

In memoriam. — Dr. Julius Eobert Mayer died at
Heilbronn on March 20, 1878, aged 63 years. It gives
me pleasure to reflect that tho great position which he
will for ever occupy in the annals of science was first
virtually assigned to him in the foregoing discourse.
He was subsequently chosen by acclamation a member of
the French Academy of Sciences ; and he received from

384 FRAGMENTS OF SCIENCE.

the Royal Society the Copley medal — its highest
reward.*

November 1878.

At the meeting of the British Association at Grlas-
gow in 1876 — that is to say, more than fourteen years
after its delivery and publication — the foregoing lecture
was made the cloak for an unseemly personal attack by
Professor Tait. The anger which found this uncour-
teous vent dates from 1863,^ when it fell to my lot to
maintain, in opposition to him and a more eminent
colleague, the position which in 1862 I had assigned to
Dr. Mayer. In those days Professor Tait denied to
Mayer all originality, and he has since, I regret to say,
never missed an opportunity, however small, of carping
at Mayer's claims. The action of the Academy of
Sciences and of the Royal Society summarily disposes
of this detraction, to which its object, during his life-
time, never vouchsafed either remonstrance or reply.

Some time ago Professor Tait published a volume of
lectures entitled ' Recent Advances in Physical Science,'
which I have reason to know has evoked an amount of
censure far beyond that hitherto publicly expressed.
Many of the best heads on the continent of Europe
agree in their rejection and condemnation of the historic
portions of this book. In March last it was subjected
to a brief but pungent critique by Du Boi«-Reymond,
tlie celebrated Perpetual Secretary of the Academy of
Sciences in Berlin. Du Bois-Reymond's address was
on * National F'eeling,' and his critique is thus wound
up : — ' The author of the "Lectures" is not, perhaps,

» See « The Copley Medalist for 1871,' p. 479.

• See * Philosophical Magazine ' for this and the succeeding yearg,

FORCE. 385

sufficiently well acquainted with the history on which
he professes to throw light, and on the later phases of
which he passes so unreserved (schroff) a judgment.
He thus exposes himself to the suspicion — which, un-
happily, is not weakened by his other writings — that
the fiery Celtic blood of his country occasionally runs
away with him, converting him for the time into a scien-
tific Chauvin. Scientific Chauvinism,' adds the learned
secretary, ' from which German investigators have
hitherto kept free^ is more reprehensible (gehassig)
than political Chauvinism, inasmuch as self-control
{sUtliche Haltung) is more to be expected from men
of science, than from the politically excited mass/

In the case before this ' expectation ' would, I fear,
be doomed to disappointment. But Du Bois-Reymond
and his countrymen must not accept the writings of
Professor Tait as representative of the thought of
England. Surely no nation in the world has more
effectually shaken itself free from scientific Chauvinism.
From the day that Davy, on presenting tlie Copley
medal to Arago, scornfully brushed aside that spurious
patriotism which would run national boundaries through
the free domain of science, chivalry towards foreigners
has been a guiding principle with the Royal Society.

On the more private amenities indulged in by Pro-
fessor Tait, I do not consider it necessary to say a
word,

* Festrede, delivered before the Academy of Sciences of Berlin,
in celebration of the birthday of the iJim^eror and King, March 28|
1873.

386 FRAGMENTS OF SCIENCE.

xvn.

CONTRIBUTIONS TO MOLECULAR PHYSICS}

HAVING- on previous occasions dwelt upon the
enormous differences which exist among gaseous
bodies both as regards their power of absorbing and
emitting radiant h^^at, I have now to consider the effect
of a change of aggregation. When a gas is condensed
to a liquid, or a liquid congealed to a solid, the mole-
cules coalesce, and grapple with each other by forces
which are insensible as long as the gaseous state is
maintained. But, even in the solid and liquid con-
ditions, the luminiferous ether still surrounds the mole-
cules: hence, if the acts of radiation and absorption
depend on them individually, regardless of their state of
aggregation, the change from the gaseous to the liquid
state ought not materially to affect the radiant and
absorbent power. If, on the contrary, the mutual
entanglement of the molecules by the force of cohesion
be of paramount influence, then we may expect that
liquids will exliibit a deportment towards radiant heat
altogether different from that of the vapours from which
they are derived.

The first part of an enquiry conducted in 1863-64
was devoted to an exhaustive examination of this ques-
tion. Twelve different liquids were employed, and five

• A discourse delivered at the Royal Institution, March 18, 1864—
supplementing, though of prior date, the Rede Lecture on Radiation,

CONTKlliUTIONS TO MOLECULAR PHYSICS. 387

different layers of each, varying in thickness from 0*02 of
an inch to 0*27 of an inch. The liquids were enclosed,
not in glass vessels, which would have materially modi-
fied the incident heat, but between plates of transparent
rock-salt, which only slightly affected the radiation.
The source of heat througliout these comparative experi-
ments consisted of a platinum wire, raised to incandes-
cence by an electric current of unvarying strength. The
quantities of radiant heat absorbed and transmitted by
each of the liquids at the respective thicknesses were first
determined. The vapours of these liquids were subse-
quently examined, the quantities of vapour employed
being rendered proportional to the quantities of liquid
previously traversed by the radiant heat. The result
was that, for heat from the same source, the order of
absorption of liquids and of their vapours proved
absolutely the same. There is no known exception to
this law ; so that, to determine the position of a vapour
as an absorber or a radiator, it is only necessary to
determine the position of its liquid.

This result proves tliat the state of aggregation, as
far at all events as the liquid stage is concerned, is of
altogether subordinate moment — a conclusion which
will probably prove to be of cardinal importance in
molecular physics. On one important and contested
point it has a special bearing. If the position of a liquid
as an absorber and radiator determine that of its vapour,
the position of water fixes that of aqueous vapour.
Water has been compared with other liquids in a multi-
tude of experiments, and it has been found, both as a
radiant and as an absorbent, to transcend them all.
Thus, for example, a layer of bisulphide of carbon 0*02
of an inch in thickness absorbs 6 per cent., and allows
94 per cent, of the radiation from the red-hot platinum
spiral to pass through it ; benzol absorbs 43 and trana-

388 FRAGMENTS OF SCIENCE.

mits 57 per cent, of the same radiation ; alcohol absorbs
67 and transmits 33 per cent., and alcohol, as an absorber
of radiant heat, stands at the head of all liquids except
one. The exception is water. A layer of tins sub-
stance, of the thickness above given, absorbs 81 per
cent., and permits only 19 per cent, of the radiation to
pass through it. Had no single experiment ever been
made upon the vapour of water, its vigorous action upon
radiant heat might be inferred from the deportment o£
the liquid.

The relation of absorption and radiation to the
chemical constitution of the radiating and absorbing
substances was next briefly considered. For the first
six substances in the list of liquids examined, the radiant
and absorbent powers augment as the number of atoms
in tlie compound molecule augments. Tlius, bisulphide
of carbon has 3 atoms, chloroform 5, iodide of ethyl 8,
benzol 12, and amylene 15 atoms in their respective
molecules. The order of their power as radiants and
absorbents is that here indicated, bisulphide of carbonj
biiing the feeblest, and amylene the strongest of the six,
Alcohol, however, excels benzol as an absorber, though
it has but 9 atoms in its molecule ; but, on the other
hand, its molecule is rendered more complex by the
introduction of a new element. Benzol contains car-
bon and hydrogen, while alcohol contains carbon
hydrogen and oxygen. Thus, not only does atomic
multitude come into play in absorption and radiation
— atomic complexity must also be taken into account.
I would recommend to the particular attention of
chemists the molecule of water ; the deportment of this
substance towards radiant' heat being j>ej'fectly anoraa
lous, if the chemical formula at present ascribed to it
be correct.

Sir William Herschel made the important discovery

CONTEIBUTIONS TO MOLKCULAR PHYSICS. 389

that, beyond the limits of the red end of the solar spec-
trum, rays of high beating power exist wliich are incom-
petent to excite vision. The discovery is capable of
extension. Die^solving iodine in the bisulphide of car-
bon, a solution is obtained which entirely intercepts the
light of the most brilliant flames, while to the ultra^red
rays of such flames the same iodine is found to be per-
fectly diathermic. The transparent bisulphide, which
is highly pervious to invisible heat, exercises on it
the same absorption as the perfectly opaque solution.
A hollow prism filled with the opac^ue liquid being placed
in the path of the beam from an electiic lamp, the
light-spectrum is completely intercepted, but the heat-
spectrum may be received upon a screen and there
examined. Falling upon a thermo-electric pile, its in-
visible presence is shown by the prompt deflection of
even a coarse galvanometer.

What, then, is the physical meaning of opacity and
transparency as regards light and radiimt heat ? The
visible rays of the spectrum differ from the invisible
ones simply in peHod. The sensation of liglit is
excited by waves of ether shorter and more quickly
recurrent than the non- visual waves which fall beyond
the extreme red. But why should iodine stop the
former and allow the latter to pass? The answer to
this question no doubt is, that the intercepted waves
are those whose periods of recm-rence coincide with ihe
periods of oscillation possible to the atoms of the dis-
solved iodine. The elastic forces which keep these
atoms apart compel them to vibrate in definite periods,
and, when these periods synchronise with those of the
ethereal waves, the latter are absorbed. Briefly de-
fined, then, transparency in liquids, as well as in gases,
is synonymous with discord, while opacity is synony-
mous with accord, between the periods of the waves of

390 FEAaMENTS OF SCIENCE.

ether and those of the molecules on which ihey im-
pinge.

According to this view transparent and colourless
substances owe their transparency to the dissonance
existing between the oscillating periods of their atoms
and those of the waves of the whole visible spec-
trum. From the prevalenc3 of transparency in com- '
pound bodies, the general discord of the vibrating
periods of their atomy with the light-gi\ing waves
of the spectrum, may be inferred ; wliile their synchro-
nism with the ultra-red periods is to be inferred from
their opacity to the ultra-red rays. Water illustrates
this in a most striking manner. It is highly trans-
parent to the luminous rays, which proves that its
atoms do not readily oscillate in the periods which
excite vision. It is higlily opaque to the ultra-red
undulations, which proves the synchronism of its vibra-
ting periods with those of the longer waves.

If, then, to the radiation from any source water
shows itself eminently or perfectly ojiaque, we may
infer that the atoms whence the radiation emanates
oscillate in ultra-red periods. Let us apply this test
to the radiation from a flame of hydrogen. This
flame consists mainly of incandescent aqueous vapour^
the temperature of which, as calculated by Bunsen,
is 3259° C, so that, if the penetrative power o^
radiant heat, as generally supposed, augment with the
temperature of its source, we may expect the radia-
tion from this flame to be copiously transmitted by
water. While, however, a layer of the bisulphide
of carbon 0*07 of an inch in thickness transmits 72 per
cent, of the incident radiation, and while every other
liquid examined transmits more or less of the heat, a
layer of water of the above thickness is entirely opaque to
the radiation from the hydrogen flame. Thus we establish

CONTRIBUTIONS TO MOLECULAR PHYSICS. 391

accord between the periods of the atoms of cold water
and those of aqueous vapour at a temperature of 3259° C.
But the periods of water have ah-eady been proved to be
ultra red — hence those of the hydrogen flame must be
sensibly ultra-red also. The absorption by dry air of
the heat emitted by a platinum spiral raised to in-
candescence by electricity is insensible, while that by
the ordinary undried air is 6 per cent. Substituting
for the platinum spiral a hydrogen flame, the absorp-
tion by dry air still remains insensible, while that of
the undried air rises to 20 per cent, of the entire radia-
tion. The temperature of the hydrogen flame is, as
stated, 3259° C. ; that of the aqueous vapour of the
air 20° C. Suppose, then, the temperature of aqueous
vapour to rise from 20° C. to 3259° C, we must con-
clude that the yugraentation of temperature is applied
to an increase of amplitude or width of swing, and not
to the introduction of quicker periods into tlie radia-
tion.

The part played by aqueous vapour in the economy
of nature is far more wonderful than has been hitherto
supposed. To nourish the vegetation of the earth the
actinic and luminous rays of the sun must penetrate
our atmosphere ; and to such rays aqueous vapour is
eminently transparent. The violet and tlie ultra-violet
rays pass through it with freedom. To protect vegeta-
tion from destructive chills tlie terrestrial rays must be
checked in their transit towards stellar space ; and this
is accomplished by the aqueous vapour diffused through
the air. This substance is the great moderator of the
earth's temperature, bringing its extremes into prox-
imity, and obviating contrasts between day and night
which would render life insupportable. But we can
advance beyond this general statement, now that we
know the radiation from aqueous vapour is intercepted,

20

392 FKAGMENTS OF SCIENCE.

in a special degree, by water, and, reciprocally, the radi-
ation from water by aqueous vapour ; for it follows from
this that the very act of nocturnal refrigeration whicli
produces the condensation of aqueous vapour at the
surface Of the earth — giving, as it were, a varnish of
water to that surface — imparts to terrestrial radiation
that particular character which disqualifies it from
passing through the earth's atmosphere and losing itself
in space.

And here we come to a question in molecular physics
which at the present moment occupies attention. By
allowing the violet and ultra-violet rays of the spectrum
to fall upon sulphate of quinine and other substances
Professor Stokes has changed the periods of those rays.
Attempts have been made to produce a similar result at
the other end of the spectrum— to convert the ultra-red
periods into periods competent to excite vision — but
hitherto without success. Such a change of period, I
agree with Dr. Miller in believing, occurs when the lime-
light is produced by an oxy-hydrogen flame. In this
common experiment there is an actual breakingup of long
periods into short ones — a true rendering of unvisual
periods visual. The change of refrangibility here effected
differs from that of Professor Stokes ; firstly, by its
being in the opposite direction — that is, from a lower
refrangibility to a higher ; and, secondly, in the circum-
stance that the lime is heated by the collision of the mole-
cules of aqueous vapour, before their heat has assumed
the radiant form. But it cannot be doubted that the
same effect would be produced by radiant heat of the same
periods, provided the motion of the aether could be ren-
dered sufficiently intense.' The effect in principle is the
same, whetlier we consider the lime to be struck by

• This was soon afterwards accomplished. See pp. 48, 49.

CONTRIBUTIONS TO MOLECULAR PHYSICS. 393

a particle of aqueous vapour oscillating at a certain rate,
or by a particle of ether oscillating at the same rate.

By plunging a platinum wire into a hydrogen flame
we cause it to glow, and thus introduce shorter periods
into the radiation. These, as already stated, are in
discord with the atomic vibrations of water ; hence we
may infer that the transmission through water will be
rendered more copious by the introduction of the wire
into the flame. Experiment proves this conclusion to be
true. Water, from being opaque, opens a passage to 6
per cent, of the radiation from the spiral. A thin plate
of colourless glass, moreover, transmits 58 per cent,
of the radiation from the hydrogen flame ; but when
the flame and spiral are employed, 78 per cent, of the
heat is transmitted.

For an alcohol flame Knoblauch and Melloni found
glass to be less transparent than for the same flame
with a platinum spiral immersed in it ; but Melloni
afterwards showed that the result was not general
— that black glass and black mica were decidedly
more diathermic to the radiation from the pure
alcohol flame. Melloni did not explain this, but the
reason is now obvious. The mica and glass owe
their blackness to the carbon diffused through them.
This carbon, as first proved by Melloni, is in some
measure transparent to the ultra-red rays, and I have
myself succeeded in transmitting between 40 and 50
per cent, of the radiation from a hydrogen flame
through a layer of carbon which intercepted the
light of an intensely brilliant flame. The products
of combustion of alcohol are carbonic acid and
aqueous vapour, the heat of which is almost wholly
ultra-red. For this radiation, then, the carbon is in a
considerable degree transparent, while for the radiation
from the platinum spiral, it is in a great measure

394 . FRAGMENTS OF SCIENCE.

opaque. The platinum wire, therefore, which aug-
mented the radiation through the pure gla^^s, augmented
the absorption of the black glass and mica.

No more striking or instructive illustration of the
influence of coincidence could be adduced than that
furnished by the radiation from a carbonic oxide flame.
Here the product of combustion is carbonic acid ; and
on the radiation from this flame even the ordinary
carbonic acid of the atmosphere exerts a powerful
effect. A quantity of the gas, only one- thirtieth of an
atmosphere in density, contained in a polished brass
tube four feet long, intercepts 50 per cent, of the
radiation from the carbonic oxide flame. For the heat
emitted by lampblack, olefiant gas is a far more
powerful absorber than carbonic acid ; in fact, for such
heat, with one exception, carbonic acid is the most
feeble absorber to be found among the compound gases.
Moreover, for the radiation from a hydrogen flame f
olefiant gas possesses twice the absorbent power of
carbonic acid, while for the radiation from the carbonic
oxide flame, at a common pressure of one inch of mer-
cury, the absorption by carbonic acid is more than
twice that of olefiant gas. Thus we establish the
coincidence of period between carbonic acid at a tem-
perature of 20° C. and carbonic acid at a temperature
of over 3000° C, the periods of oscillation of both the ]
incandescent and the cold gas belonging to the ultra-
red portion of the spectrum.

It will be seen from the foregoing remarks and
experiments how impossible it is to determine the effect
of temperature pure and simple on the transmission of
radiant heat if different sources of heat be employed.
Throughout such an examination the same oscillating
atoms ought to be retained. This is done by heating a
platinum spiral by an electric current, the temperature

CONTRIBUTIONS TO MOLECULAR PHYSICS. 395

meanwhile varying between the widest possible limits.
Their comparative opacity to the ultra-red rays shows the
general accord of the oscillating periods of the vapom-s re-
ferred to at the commencement of this lecture with those
of the ultra-red undulations. Hence, by gradually heat-
ing a platinum wire from darkness up to whiteness, we
ouglit gradually to augment the discord between it and
these vapours, and thus augment the transmission. Ex-
periment entirely confirms this conclusion. Formic
ether, for example, absorbs 45 per cent, of the radiation
from a platinum spiral heated to barely visible redness ;
32 per cent, of the radiation from the same spiral at a red
heat ; 26 per cent, of the radiation from a white-hot
spiral, and only 21 per cent, when the spiral is brought
near its point of fusion. Remarkable cases of inversion
as to transparency also occur. For barely visible red-
ness formic ether is more opaque than sulphuric ; for a
bright red heat both are equally transparent; while, for
a white heat, and still more for a higher temperature,
sulphuric ether is more opaque than formic. This
result gives us a clear view of the relationship of the
two substances to the luminiferous ether. As we intro-
duce waves of shorter period the sulphuric gpther
augments most rapidly in opacity ; that is to say, its
accord with the shorter waves is greater than that of
the formic. Hence we may infer that the atoms of
formic ether oscillate, on the whole, more slowly than
those of sulphuric ether.

When the source of heat is a Leslie's cube coated
with lampblack and filled with boiling water, the
opacity of formic sether in comparison with sulphuric
is very decided. With this source also the positions
of chloroform and iodide of methyl are inverted.
For a white-hot spiral, the absorption of chloroform
vapour being 10 per cent., that of iodide of methyl is

396 FRAGMENTS OF SCIENCE.

16 ; with the blackened cube as source, the absorption
by chloroform is 22 per cent., while that by the iodide
of methyl is only 19. This inversion is not the result
of temperature merely; for when a platinum wire,
heated to the 'temperature of boiling water, is em-
ployed as a source, the iodide continues to be the most
powerful absorber. All the experiments hitherto made
go to prove that from heated lampblack an emission
takes place which synchronises in an especial manner
with chloroform. For the cube at 100° C, coated
with lampblack, the absorption by chloroform is more
than three times that by bisulphide of carbon ; for the
radiation from the most luminous portion of a gas-flame
the absorption by chloroform is also considerably in
excess of that by bisulphide of carbon ; while, for the
flame of a Bunsen's burner, from which the incan-
descent carbon particles are removed by the free ad-
mixture of air, the absorption by bisulphide of carbon
is nearly twice that by chloroform. The removal of
ike carbon 'particles Ttiore than dovMes the relative
transparency of the chloroform. Testing, moreover,
the radiation from various parts of the same flame,
it was found that for the blue base of the flame the
bisulphide of carbon was most opaque, while for all
other parts of the flame the chloroform was most opaque.
For the radiation from a very small gas flame, consisting
of a blue base and a small white tip, the bisulphide
was also most opacjue, and its opacity very decidedly
exceeded that of the chloroform when the source of
heat was the flame of bisulphide of carbon. Com-
paring the radiation from a Leslie's cube coated with
isinglass with that from a similar cube coated with
lampblack, at the common temperature of 100° C, it
was found that, out of eleven vapours, all but one ab-
sorbed the radiation from the isinglass most powerfully ;
the single exception was chloroform.

CONTRIBUTIONS TO MOLECULAR PHYSICS. 397

It is worthy of remark that whenever, throu<^h a
change of source, the position of a vapour as an ab-
sorber of radiant heat was altered, the position of the
liquid from which the vapour was derived underwent a
similar change.

It is still a point of difference between eminent
investigators whether radiant heat, up to a temperature
of 100° C, is monoL'hromatic or not. Some affirm this ;
some deny it. A long series of experiments enables
me to state that probably no two substances at a tem-
perature of 100° C. emit heat of the same quality.
The heat emitted by isinglass, for example, is different
from that emitted by lampblack, and the heat emitted
by cloth, or paper, differs from both. It is also a subject
of discussion whether rock-salt is equally diathermic to
all kinds of calorific rays ; the differences affirmed to
exist by some investigators being ascribed by others to
differences of incidence from the various sources em-
ployed. MM. de la Provostaye and Desains maintain
the former view, Melloni and M. Knoblauch maintain
the latter. I tested this point without changing any-
thing but the temperature of the source ; its size,
distance, and surroundings remaining the same. The
experiments proved rock-salt to be coloured thermally.
It is more opaque, for example, to the radiation from a
barely visible spiral than to that from a white-hot one.

In regard to the relation of radiation to conduction,
if we define radiation, internal as well as external, as
the communication of motion from the vibrating atoms
to the ether, we may, I think, by fair theoretic reason-
ing, reach the conclusion that tlie best radiators ought
to prove the worst conductors. A broad consideration
of the subject shows at once the general harmony of this
conclusion with observed facts. Organic substances
are all excellent radiators ; they are also extremely bad

398 FRAGMENTS OF SCIENCE.

conductors. The moment we pass from the metala
to their compounds we pass from good conductors to
bad ones, and from bad radiators to good ones. Water,
among liquids, is probably the worst conductor; it
is the best radiator. Silver, among solids, is the best
conductor; it is the worst radiator. The excellent
researches of MM. de la Provostaye and Desains fur-
nish a striking illustration of wliat I am inclined to
regard as a natural law — that those atoms which
transfer the greatest amount of motion to the ether,
or, in other words, radiate most powerfully, are the
least competent to communicate motion to each other,
or, in other words, to propagate by conduction readily.

xvra.

LIFE AND LETTERS OF FAR AD AT.
1870.

UNDERTAKEN and executed in a reverent and lov-
ing spirit, the work of Dr. Bence Jones makes
Faraday the virtual writer of his own life. Everybody
now knows the story of the philosopher's birth; that his
father was a smith ; that he was born at Newington Butts
in 1791; that he ran along the London pavements, a
bright-eyed errand boy, with a load of brown curls upon
his head and a packet of newspapers under his arm ; that
the lad's master was a bookseller and bookbinder — a
kindly man, who became attached to the little fellow,
and in due time made him his apprentice without fee ;
that during his apprenticesliip he found his appetite for
knowledge provoked and strengthened by the books he
stitched and covered. Thus he grew in wisdom and
stature to his year of legal manhood, when he appears
in the volumes before us as a writer of letters, which
reveal his occupation, acquirements, and tone of mind.
His correspondent was Mr. Abbott, a member of the
Society of Friends, who, with a forecast of his corre-
spondent's greatness, preserved his letters and produced
them at the proper time.

In later years Faraday always carried in his pocket
a blank card, on which he jotted down in pencil his

400 FRAGMENTS OF SCIENCE.

thoughts and memoranda. He made his notes in the
laboratory, in the theatre, and in the streets. This
distrust of his memory reveals itself in his first letter to
Abbot. To a proposition that no new enquiry should
be started between them before the old one had been
exhaustively discussed, Faraday object'^. ' Your notion,'
he says, ' I can hardly allow, for the following reason :
ideas and thoughts spring up in my mind which are
irrevocably lost for want of noting at the time.' Gentle
as he seemed, he wished to have his own way, and he
had it throughout his life. Differences of opinion
sometimes arose between the two friends, and then they
resolutely faced each other. *I accept your offer to
fight it out with joy, and shall in the battle of experi-
ence cause not pain, but, I hope, pleasure.' Faraday
notes his own impetuosity, and incessantly checks it.
There is at times something almost mechanical in his
self-restraint. In another nature it would have hardened
into mere 'correctness' of conduct; but his overflowing
affections prevented this in his case. The habit of self-
control became a second nature to him at last, and lent
serenity to his later years.

In October 1812 he was engaged by a Mr. De la
Roche as a journeyman bookbinder ; but the situation
did not suit him. His master appears to have been an
austere and passionate man, and Faraday was to the last
degree sensitive. All his life he continued so. He
suffered at times from dejection; and a certain grim-
ness, too, pervaded his moods. ' At present,' he writes
to Abbott, ' I am as serious as you can be, and would
not scruple to speak a truth to any human being, what;-
ever repugnance it might give rise to. Being in this
state of mind, I should have refrained from writing to
you, did I not conceive from the general tenor of your
letters that your mind is, at proper times, occupied upon

FARADAY. 401

serious subjects to the exclusion of those that are
frivolous.' Plainly he had fallen into that stern Puritan
mood, which not only crucifies the affections and lusts
of him who harbours it, but is often a cause of disturbed
digestion to his friends.

About three months after his engagement with
De la Eoche, Faraday quitted him and bookbinding
together. He had heard Davy, copied his lectures, and
written to him, entreating to be released from Trade,
which he hated, and enabled to pursue Science. Davy
recognised the merit of his correspondent, kept his eye
upon him, and, when occasion offered, drove to his door
and sent in a letter, offering him the post of assistant
in the laboratory of the Royal Institution. He was
engaged March 1, 1813, and on the 8th we find him
extracting the sugar from beet-root. He joined the
City Philosophical Society which had been founded by
Mr. Tatum in 1808. 'The discipline was very sturdy,
the remarks very plain, and the results most valuable.'
Faraday derived great profit from this little association.
In the laboratory he had a discipline sturdier still.
Both Davy and himself were at this time frequently cut
and bruised by explosions of chloride of nitrogen. One
explosion was so rapid ' as to blow my hand open, tear
away a part of one nail, and make my fingers so sore
that I cannot use them easily.' In another experiment
' the tube and receiver were blown to pieces, I got a cut
on the head, and Sir Humphry a bruise on his hand.'
And again speaking of the same substance, he says,
'when put in the pump and exhausted, it stood for a
moment, and then exploded with a fearful noise. Both
Sir H. and I had masks on, but I escaped this time the
best. Sir H. had his face cut in two places about the
chin, and a violent blow on the forehead struck through

402 FRAGMENTS OF SCIENCE.

a considerable thickness of silk and leather.' It was
this same substance that blew out the eye of Dulong.

Over and over again, even at this early date, we can
discern the quality which, compounded with his rare
intellectual power, made Faraday a great experimental
philosopher. This was his desire to see facts, and not
to rest contented with the descriptions of them. He
frequently pits the eye against the ear, and affirms the
enormous superiority of the organ of vision. Late in
life I have heard him say that he could never fully
understand an experiment until he had seen it. But
he did not confine himself to experiment. He aspired
to be a teacher, and reflected and wrote upon the method
of scientific exposition. 'A lecturer,' he observes,
' should appear easy and collected, undaunted and
unconcerned : ' still ' his whole behaviour should evince
respect for his audience.' These recommendations were
afterwards in great part embodied by himself. I doubt
his ' unconcern,' but his fearlessness was often manifested.
It used to rise within him as a wave, which carried both
him and his audience along with it. On rare occasions
also, when he felt himself and his subject hopelessly
unintelligible, he suddenly evoked a certain recklessness
of thought, and, without halting to extricate his bewil-
dered followers, he would dash alone through the jungle
into which he had unwittingly led them ; thus saving
them from ennui by tlie exhibition of a vigour which,
for the time being, they could neither share nor com-
prehend.

In October 1813 he quitted England with Sir Hum-
phry and Lady Davy. During his absence he kept a
journal, from which copious and interesting extracts have
been made by Dr. Bence Jones. Davy was considerate,
preferring at times to be his own servant rather than im-
pose on Faraday duties which he disliked. But Lady

FARADAY. 403

Davy was the reverse. She treated him as an underling ;
he chafed under the treatment, and was often on the point
of returning home. They halted at Geneva. De la Rive,
the elder, had known Davy in 1799, and, by his writings
in the ' Bibliotbe(|ue Britannique,' had been the first to
make the English chemist's labours known abroad. He
welcomed Davy to his country residence in 1814. Both
were sportsmen, and they often went out shooting
together. On these occasions P^araday charged Davy's
gun while De la Rive charged his own. Once the
Grenevese philosopher found himself by the side of Fara-
day, and in his frank and genial way entered into con-
versation with the young man. It was evident that a
person possessing such a charm of manner and such
high intelligence could be no mere servant. On enquiry
De la Rive was somewhat shocked to find that the soi-
disant domestique was really preparateur in the labo-
ratory of the Royal Institution ; and he immediately
proposed that PVraday thenceforth should join the
masters instead of the servants at their meals. To this
Davy, probably out of weak deference to his wife,
objected ; bat an arrangement was come to that Fara^
day thenceforward should have his food in his own room.
Rumour states that a dinner in honour of Faraday was
given by De la Rive. This is a delusion ; there was no
such banquet ; but Faraday never forgot the kindness ot
the friend who saw his merit when he was a mere gargon
de laboratoire.^

' While confined last autumn at Geneva by the effects of a fall
in the Alps, my friends, with a kindness I can never forget, did all
that friendship could suggest to render my captivity pleasant to
me. M. de la Rive tl en wrote out for me the full account, of
which the foregoing is a condensed abstract. It was at the desire
of Dr. Bence Jones that I asked him to do so. The rumour of a
banquet at Geneva illustrates the tendency to substitute for the
youth of 1814 the Faraday of later years.

404 FRAGMENTS OF SCIENCE.

He returned in 1815 to the Royal Institution. Here
he helped Davy for years ; he worked also for himself,
and lectured frequently at the City Philosophical Society.
He took lessons in elocution, happily without damage to
his natural force, earnestness, and grace of delivery. He
was never pledged to theory, and he changed in opinion
as knowledge advanced. With him life was growth.
In those early lectures we hear him say, ' In knowledge,
that man only is to be contemned and despised who is
not in a state of transition.' And again : ' Nothing is
more difficult and requires more caution than philoso-
phical deduction, nor is there anything more adverse
to its accuracy than fixity of opinion.' Not that he was
wafted about by every wind of doctrine ; but that he
united flexibility with his strength. In striking con-
trast with this intellectual expansiveness was his fixity
in religion, but this is a subject which cannot be dis-
cussed here.

Of all the letters published in these volumes none
possess a greater charm than those of Faraday to his
wife. Here, as Dr. Bence Jones truly remarks, ' he laid
open all his mind and the whole of his character, and
what can be made known can scarcely fail to charm every
one by its loveliness, its truthfulness, and its earnest-
ness.' Abbott and he sometimes swerved into word-
play about love; but up to 1820, or thereabouts, the
passion was potential merely. P^'araday's journal indeed
contains entries which show that he took pleasure in the
assertion of his contempt for love ; but these very
entries became links in his destiny. It was tlirougli
them that he became acquainted with one who inspired
him with a feeling which only ended with his life. His
biographer has given us the means of tracing the vary-
ing moods which preceded liis acceptance. They reveal
more than the common alternations of light and gloom ;

FAKADAY. 405

at one moment he wishes that his flesh might melt and
that he might become nothing ; at another he is in-
toxicated with hope. The impetuosity of his character
was then unchastened by the discipline to which it was
subjected in after years. The very strength of his
passion proved for a time a bar to its advance, suggest-
ing, as it did, to the conscientious mind of Miss Barnard,
doubts of her capability to return it with adequate force.
But they met again and again, and at each successive
meeting he found his heaven clearer, until at length he
was able to say, ' Not a moment's alloy of this evening's
happiness occurred. Everything was deliglitful to the
last moment of my stay with my companion, because
she was so.' The turbulence of doubt subsided, and a
calm and elevating confidence took its place. ' What
can I call myself,' he writes to her in a subsequent
letter, ' to convey most perfectly my affection and love
for you ? Can I or can truth say more than that for
this world T am yours ? ' Assuredly he made his pro-
fession good, and no fairer light f^dls upon his character
than that which reveals his relations to his wife. Never,
I believe, existed a manlier, purer, steadier love. Like
a burning diamond, it continued to shed, for six-and-
forty years, its white and smokeless glow.

Faraday was married on June 12, 1821 ; and up to
this date Davy appears throughout as his friend. Soon
afterwards, however, disunion occurred between them,
which, while it lasted, must have given Faraday intense
pain. It is impossible to doubt the honesty of conviction
with which this subject has been treated by Dr. Bence
Jones, and there may be facts known to him, but not ap-
pearing in these volumes, which justify his opinion that
Davy in those days had become jealous of Faraday. Tliis,
which is the prevalent belief, is also reproduced in an
excellent article in the March number of 'Fraser'g

406 FRAGMENTS OF SCIENCE.

Magazine.' But the best analysis I can make of the
data fails to present Davy in this light to me. The
facts, as I regard them, are briefly these.

In 1820, Oersted of Copenhagen made the cele-
brated discovery which connects electricity with mag-
netism, and immediately afterwards the acute mind of
Wollaston perceived that a wire carrying a current ought
to rotate round its own axis under the influence of a
magnetic pole. In 1821 he tried, but failed, to realise
this result in the laboratory of the Royal Institution.
Faraday was not present at the moment, but he came
in immediately afterwards and heard the conversation
of Wollaston and Davy about the experiment. He had
also heard a rumour of a wager that Dr. Wollaston
would eventually succeed.

This was in April. In the autumn of the same
year Faraday wrote a history of electro-magnetism, and
repeated for himself the experiments which he described.
It was while thus instructing himself that he succeeded
in causing a wire, carrying an electric current, to
rotate round a magnetic pole. This was not the result
sought by Wollaston, but it was closely related to that
result.

The strong tendency of Faraday's mind to look upon
the reciprocal actions of natural forces gave birth to
bis greatest discoveries ; and we, who know this, should
be justified in concluding that, even had Wollaston not
preceded him, the result would have been the same.
But in judging Davy we ought to transport ourselves
to his time, and carefully exclude from our thoughts
and feelings that noble subsequent life, which would
render simply impossible the ascription to Faraday of
anything unfair. It would be unjust to Davy to put
our knowledge in the place of his, or to credit him
with data which he could not have possessed. Rumour

FARADAY. 407

and fact had connected the name of Wollaston with
these supposed interactions hetween magnets and cur-
rents. When, therefore, Faraday in October published
his successful experiment, without any allusion to
Wollaston, general, though really ungrounded, criti-
cism followed. I say ungrounded because, firstly,
Faraday's experiment was not that of Wollaston, and
secondly, Faraday, before he published it, had actually
called upon Wollaston, and not finding him at home,
did not feel himself authorised to mention his name.

In December, Faraday published a second paper OD
the same subject, from which, through a misappre-
hension, the name of Wollaston was also omitted.
Warburton and others thereupon affirmed that Wol-
laston's ideas had been appropriated without acknow-
ledgment, and it is plain that Wollaston himself,
though cautious in his utterance, was also hurt.
Censure grew till it became intolerable. ' I hear,'
writes Faraday to liis fiiend Stodart, ' every day more
and more of these sounds, which, though only whispers
to me, are, I suspect, spoken aloud among scientific
men.' He might have written explanations and de-
fences, but he went straighter to the point. He wished
to see the principals face to face — to plead his cause
before them personally. There was a certain vehemence
in his desire to do this. He saw Wollaston, he saw
Davy, he saw Warburton ; and I am inclined to think
that it was the irresistible candour and truth of
character which these viva voce defences revealed, as
much as the defences themselves, that disarmed resent-
ment at the time.

As regards Davy, another cause of dissension arose
in 1823. In the spring of that year Faraday analysed
the hydrate of chlorine, a substance once believed to be
the element chlorine, but proved by Davy to be a

27

408 FRAGMENTS OF SCIENCE.

'ompound of that element and water. The analysis
was looked over by Davy, who then and there suggested
to Faraday to heat the hydrate in a closed glass tube.
This was done, the substance was decomposed, and one
of the products of decomposition was proved by Faraday
to be chlorine liquefied by its own pressure. On the
day of its discovery he communicated this result to
Dr. Paris. Davy, on being informed of it, instantly
liquefied another gas in the same way. Having struck
thus into Faraday's enquiry, ought he not to have left
the matter in Faraday's hands ? I think he ought.
But, considering his relation to both Faraday and the
hydrate of chlorine, Davy, I submit, may be excused
for thinking differently. A father is not always wise
enough to see that his son has ceased to be a boy, and
estrangement on this account is not rare ; nor was
Davy wise enough to discern that Faraday had passed
the mere assistant stage, and become a discoverer, it
is now hard to avoid magnifying this error. But had
Faraday died or ceased to work at this time, or had his
subsequent life been devoted to money-getting, instead
of to research, would anybody now dream of ascribing
jealousy to Davy ? Assuredly not. Why should he be
jealous ? His reputation at this time was almost with-
out a parallel : his glory was without a cloud. He had
added to his other discoveries that of Faraday, and
after having been his teacher for seven years, his lan-
guage to him was this : ' It gives me great pleasure to
hear that you are comfortable at the Eoyal Institution,
and I trust that you will not only do something good
and honourable for yourself, but also for science.' This
is not the language of jealousy, potential or actual.
But the chlorine business introduced irritation and
anger, to which, and not to any ignobler motive, Davy's

FARADAY. 409

opposition to the election of Faraday to the Royal
Society is, I am persuaded, to be ascribed.

These matters are touched upon with perfect candour,
and becoming consideration, in the volumes of Dr.
Bence Jones ; but in ' society ' they are not always so
handled. Here a name of noble intellectual associations
is surrounded by injurious rumours which! would will-
ingly scatter for ever. The pupil's magnitude, and
the splendour of his position, are too great and absolute
to need as a foil the humiliation of his master. Brothers
in intellect, Davy and Faraday, however, could never
have become brothers in feeling; their characters were
too unlike. Davy loved the pomp and circumstance of
fame ; Faraday the inrer consciousness that he had
fairly won renown. They were both proud men. But
with Davy pride projected itself into the outer world;
while with Faraday it became a steadying and dignifying
inward force. In one great particular they agreed.
Each of them could have turned his science to immense
commercial profit, but neither of them did so. The
noble excitement of research, and the delight of dis-
covery, constituted their reward. I commend them to
the reverence which great gifts greatly exercised ought
to inspire. They were both ours; and through the
coming centuries England wil] be able to point with
just pride to the possession of such men.

The first volume of the * Life and Letters ' reveals
to us the youth who was to be father to the man.
Skilful, aspiring, resolute, he grew steadily in know-
ledge and in power. Consciously or unconsciously, the
relation of Action to Reaction was ever present to

410 FRAGMENTS OF SCIENCE.

Faraday's mind. It had been fostered by his discovery
of Magnetic Rotations, and it planted in him more
daring ideas of a similar kind. Magnetism he knew
could be evoked by electricity, and he thought that
electricity, in its turn, ought to be capable of evolution
by magnetism. On August 29, 1831, his experiments
on this subject began. He had been fortified by
previous trials, which, though failures, had begotten
instincts directing him towards the truth. He, like
every strong worker, might at times miss the outward
object, but he always gained the inner light, education,
and expansion. Of this Faraday's life was a constant
illustration. By November he had discovered and col-
ligated a multitude of the most wonderful and un-
expected phenomena. He had generated currents by
currents ; currents by magnets, permanent and transi-
tory; and he afterwards generated currents by the
earth itself. Arago's ' Magnetism of Rotation,' which
had for years offered itself as a challenge to the best
scientific intellects of Europe, now fell into his hands.
It proved to be a beautiful, but still special, illustration
of the great principle of Magneto-electric Induction.
Nothing equal to this latter, in the way of pure experi-
mental enquiry, had previously been achieved.

Electricities from various sources were next exa-
mined, and their differences and resemblances revealed.
He thus assured himself of their substantial identity.
He then took up Conduction, and gave many striking
illustrations of the influence of Fusion on Conducting
Power. Renouncing professional work, from which at
ibis time he might have derived an income of many
thousands a year, he poured his whole momentum into
his researches. He was long entangled in Electro-
chemistry. The light of law was for a time obscured
by the thick umbrage of novel facts; but he finally

FARADAY. 411

emerged from his researches with the great principle of
Definite Electro -chemical Decomposition in his hands.
If his discovery of Magneto-electricity may he ranked
with that of the pile by Volta, this new discovery
may almost stand beside that of Definite Combining
Proportions in Chemistry. He passed on to Static
Electricity — its Conduction, Induction, and Mode of
Propagation. He discovered and illustrated the prin-
ciple of Inductive Capacity ; and, turning to theory, he
asked himself how electrical attractions and repulsions
are transmitted. Are they, like gravity, actions at a
distance, or do they require a medium ? If the former,
then, like gravity, they will act in straight lines ; if
the latter, then, like sound or light, they may turn a
corner. Faraday held — and his views are gaining
ground — that his experiments proved the fact of curvi-
linear propagation, and hence the operation of a medium.
Others denied this ; but none can deny the profound
and philosophic character of his leading thought.^ The
first volume of the Researches contains all the papers
here referred to.

Faraday had heard it stated that henceforth physical
discoveries would be made solely by the aid of mathe-
matics ; that we had our data, and needed only to work
deductively. Statements of a similar character crop
out from time to time in our day. They arise from
an imperfect acquaintance with the nature, present
condition, and prospective vastness of the field ot
physical enquiry. The tendency of natural science
doubtless is to bring all physical phenomena under the
dominion of mechanical laws ; to give them, in other
words, mathematical expression. But our approach to

' In a very remarkable paper published in PoggfendorfE's
* Annalen ' for 1857, Werner Siemens accepts and develops Faraday's
theory of Molecular Induction.

412 FEAaMENTS OF SCIENCE.

this result is asymptotic ; and for ages to come — pos-
sibly for all the ages of the human race — Nature will
find room for both the philosophical experimenter and
the mathematician. Faraday entered his protest against
the foregoing statement by labelling his investigations
' Experimental Eesearches in Electricity.' They were
completed in 1854, and three volumes of them have
been published. For the sake of reference, he numbered
every paragraph, the last number being 3362. In
1859 he collected and published a fourth volume
of papers, under the title, 'Experimental Eesearches
in Chemistry and Physics.' Thus did this apostle of
experiment illustrate its power, and magnify his
office.

The second volume of the Eesearches embraces
memoirs on the Electricity of the Grymnotus ; on the
Source of Power in the Voltaic Pile ; on the Electricity
evolved by the Friction of Water and Steam, in which
the phenomena and principles of Sir William Arm-
strong's Hydro-electric machine are described and de-
veloped ; a paper on Magnetic Eotations, and Faraday's
letters in relation to the controversy it aroused. The
contribution of most permanent value here, is that on
the Source of Power in the Voltaic Pile. By it the
Contact Theory, pure and simple, was totally over-
thrown, and the necessity of chemical action to the
maintenance of the current demonstrated.

The third volume of the Eesearches opens with a
memoir entitled ' The Magnetisation of Light,' and the
' Illumination of Magnetic Lines of Force.' It is diffi-
cult even now to affix a definite meaning to this title ;
but the discovery of the rotation of the plane of polari-
sation, which it announced, seems pregnant with great
resvdts. The writings of William Thomson on the
theoretic aspects of the discovery ; the excellent electro-

FAKADAY. 413

dynamic measurements of Wilhelm Weber, which are
models of experimental completeness and skill; Weber's
labours in conjunction with his lamented friend Kohl-
rausch — above all, the researches of Clerk Maxwell on
the Electro-magnetic Theory of Light — point to that
wonderful and mysterious medium, which is the vehicle
of light and radiant heat, as the probable basis also of
magnetic and electric phenomena. The hope of such a
connection was first raised by the discovery here referred
to.^ Faraday himself seemed to cling with particular
affection to this discovery. He felt that there was
more in it than he was able to unfold. He predicted
that it would grow in meaning with the growth of
science. This it has done ; this it is doing now. Its
right interpretation will probably mark an epoch in
scientific history.

Eapidly following it is the discovery of Diamag-
netism, or the repulsion of matter by a magnet. Brug-
mans had shown that bismuth repelled a magnetic
needle. Here he stopped. Le Bailliflf proved that
antimony did the same. Here he stopped. Seebeck,
Becquerel, and others, also touched the discovery.
These fragmentary gleams excited a momentary curiosity
and were almost forgotten, when Faraday independently
alighted on the same facts ; and, instead of stopping,
made them the inlets to a new and vast region of

• A letter addressed to me by ]*rofessor "Weber on March
18 last contains the following reference to the connection here
mentioned: *Die HofEnung einer solchen Combination ist durch
Faraday's Entdeckung der Drehung der Polarisationsebene durch
magnet ische Directionskraft zuerst, und sodann durch die Ueberein-
stimmung derjenigen Geschwindigkeit, welche das Verhiiltniss der
electro-dynamischen Einheit zur elect ro-statischen ausdriickt, mit
der Geschwindigkeit des Lichts angeregt worden ; und mir scheint
von alien Versuchen, welche zur Verwirklichung dieser Hoffniing
gemacht worden siud, das von Herrn Maxwell gemachte am
erfolgreichsten.*

414 FRAGMENTS OF SCIENCE.

research. The value of a discovery is to be measured
by the intellectual action it calls forth ; and it was
Faraday's good fortune to strike such lodes of scientific
truth as give occupation to some of the best intellects
of our age.

The salient quality of Faraday's scientific character
reveals itself from beginning to end of these volumes ;
a union of ardour and patience — the one prompting
the attack, the other holding him on to it, till defeat
was final or victory assured. Certainty in one sense or
the other was necessary to his peace of mind. The
right method of investigation is perhaps incommuni-
cable ; it depends on the individual rather than on the
system, and the mark is missed when Faraday's re-
searches are pointed to as merely illustrative of the
power of the inductive philosophy. The brain may be
filled with that philosophy ; but without the energy
and insight which this man possessed, and which with
him were personal and distinctive, we should never rise
to the level of his achievements. His power is that of
individual genius, rather than of philosophic method ;
the energy of a strong soul expressing itself after its
own fashion, and acknowledging no mediator between*
it and Nature.

The second volume of the ' Life and Letters,' like
the first, is a historic treasury as regards Faraday's
work and character, and his scientific and social rela-
tions. It contains letters from Humboldt, Herschel,
Hachette, De la Rive, Dumas, Liebig, Melloni, Bec-
querel. Oersted, Pliicker, Du Bois Reymond, Lord
Melbourne, Prince Louis Napoleon, and many other
distinguished men. I notice with particular pleasure a
letter from Sir John Herschel, in reply to a sealed
packet addressed to him by Faraday, but which he had
permission to open if he pleased. The packet referred

FAEADAY. 415

to one of the many unfulfilled hopes which spring up
in the minds of fertile investigators : —

' Gro on and prosper, " from strength to strength,"
like a victor marching with assured step to furthei
conquests ; and he certain that no voice will join more
heartily in the peans that already begin to rise, and
will speedily swell into a shout of triumph, astounding
even to yourself, than that of J. F. W. Herschel.'

Faraday's behaviour to Melloni in 1835 merits a
word of notice. The young man was a political exile
in Paris. He had newly fashioaed and applied the
thermo-electric pile, and had obtained with it results
of the greatest importance. But they were not appre-
ciated. With the sickness of disappointed hope Melloni
waited for the report of the Commissioners, appointed
by the Academy of Sciences to examine the Primier.
At length he published his researches in the ' Annales
de Chimie.' They thus fell into the hands of Faraday,
who, discerning at once their extraordinary merit,
obtained for their author the Rumford Medal of the
Royal Society. A sum of money always accompanies
this medal ; and the pecuniary help was, at this time,
even more essential than the mark of honour to the
young refugee. Melloni's gratitude was boundless : —

'Et vous, monsieur,' he writes to Faraday, 'qui
appartenez a une societe a laquelle je n'avais rien offert,
vous qui me connaissiez a peine de nom ; vous n'avez
pas demande si j 'avals des ennemis faibles ou puissants,
ni calcule quel en etait le norabre ; mais vous avez
parle pour I'opprime etranger, pour celui qui n'avait
pas le moindre droit a tant de bienveillance, et vos
paroles ont ete accueillies favorablement par des col-
leiTues consciencieux ! Je reconnais bien la des hommea
dignes de leur noble mission, les veritable represea-
tants de la science d'un pays bbre et genereux*'

416 FEAGMENTS OF SCIENCE.

"Within the prescribed limits of this article it would
be impossible to give even the slenderest summary of
Faraday's correspondence, or to carve from it more than
the merest fragments of his character. His letters,
written to Lord Melbourne and others in 1836, regard-
ing his pension, illustrate his uncompromising independ-
ence. The Prime Minister had offended him, but
assuredly the apology demanded and given was com-
plete. I think it certain that, notwithstanding the
very full account of this transaction given by Dr. Bence
Jones, motives and influences were at work which even
now are not entirely revealed. TJie minister was bit-
terly attacked, but he bore the censure of the press with
great dignity. Faraday, while he disavowed having
either directly or indirectly furnished the matter of
those attacks, did not publicly exonerate the Premier.
The Hon. Caroline Fox had proved herself Faraday's
ardent friend, and it was she who had healed the breach
between the philosopher and the minister. She mani
festly thought that Faraday ought to have come for-
ward in Lord Melbourne's defence, and there is a flavour
of resentment in one of her letters to him on the sub-
ject. No doubt Faraday had good grounds for his
reticence, but they are to me unknown.

In 1841 his health broke down utterly, and he went
to Switzerland with his wife and brother-in-law. His
bodily vigour soon revived, and he accomplished feats
of walking respectable even for a trained mountaineer.
The published extracts from his Swiss journal contain
many beautiful and touching allusions. Amid references
to the tints of the Jungfrau, the blue rifts of the glaciers,
and the noble Niesen towering over the Lake of Thun,
we come upon the charming little scrap which I have
elsewhere quoted : ' Clout-nail making goes on here
rather considerably, and is a very neat and pretty

FARADAY. 4X7

Operation to observe. I love a smith's shop and any-
thing relating to smithery. My father was a smith/
This is from his journal ; but he is unconsciously speak-
ing to somebody — perhaps to the world.

His description of the Staubbach, Giessbach, and
of the scenic effects of sky and mountain, are all fine
and sympathetic. But amid it all, and in reference to
it all, he tells his sister that ' true enjoyment is from
withiu, not from without.' In those days Agassiz was
living under a slab of gneiss on the glacier of the Aar.
Faraday met Forbes at the Grimsel, and arranged
with him an excursion to the ' Hotel des Neuchatelois ' ;
but indisposition put the project out.

From the Fort of Ham, in 1843, Faraday received
a letter addressed to him by Prince Louis Napoleon
Bonaparte. He read this letter to me many years ago,
and the desire, shown in various ways by the French
Emperor, to turn modern science to account, has often
reminded me of it since. At the age of thirty-five the
prisoner of Ham speaks of 'rendering his captivity
less sad by studying the great discoveries' which
science owes to Faraday ; and he asks a question which
reveals his cast of thought at the time : ' What is the
most simple combination to give to a voltaic battery,
in order to produce a spark capable of setting fire to
powder under water or under ground?' ShoAld the
necessity arise, the French Emperor will not lack at
the outset the best appliances of modern science ; while
we, I fear, shall have to learn the magnitude of the
resources we are now neglecting amid the pangs of
actual war.*

* The ' science ' has since been applied, with astonishing effect,
by those who had studied it far more thoroughly than the Emperor
of the French. We also, I am liappy to think, have improved th«
time since the above words were written [1878].

418 FRAGMENTS OF SCIENCE.

One turns with renewed pleasure to Faraday's letters
to his wife, published in the second volume. Here
surely the loving essence of the man appears more dis-
tinctly than anywhere else. From the house of Dr.
Percy, in Birmingham, he writes thus : —

' Here — even here — the moment I leave the table,
I wish I were with you in quiet. Oh, what happiness
is ours ! My runs into the world in this way only serve
to make me esteem that happiness the more.'
And again :

'We have been to a grand conversazione in the
town-hall, and I have now returned to. my room to
talk with you, as the pleasantest and happiest thing
that I can do. Nothing rests me so much as com-
munion with you. I feel it even now as I write, and
catch myself saying the words aloud as I write them.'
Take this, moreover, as indicative of his love for
Nature :

' After writing, I walk out in the evening hand in
hand with my dear wife to enjoy the suns'et ; for to
me who love scenery, of all that I have seen or can see,
there is none surpasses that of heaven. A glorious sim-
set brings with it a thousand thoughts that delight me.'

Of the numberless lights thrown upon him by the
' Life and Letters,' some fall upon his religion. In a
letter to Lady Lovelace, he describes himself as belong-
ing to 'a very small and despised sect of Christians,
known, if known at all, as Sandemanians, and our
hope is founded on the faith that is in Christ.' He
adds : ' I do not think it at all necessary to tie the study
of the natural sciences and religion together, and in
my intercourse with my fellow-creatures, that which
is religious, and that which is philosophical, have ever
been two distinct things.' He saw clearly the danger of
(quitting his moorings, and his science acLed indirectly

FARADAY. 419

as the safeguard of his faith. For his investigations
so filled his mind as to leave no room for sceptical
questionings, thus shielding from the assaults of philo-
sophy the creed of his youth. His religion was con-
stitutional and hereditary. It was implied in the
eddies of his blood and in the tremors of his brain ; and,
however its outward and visible form might have
changed, Faraday would still have possessed its elemental
constituents — awe, reverence, truth, and love.

It is worth enquiring how so profoundly religious a
mind, and so great a teacher, would be likely to regard
our present discussions on the subject of education.
P'araday would be a ' secularist ' were he now alive.
He had no sympathy with those who contemn know-
ledge unless it be accompanied by dogma. A lecture
delivered before the City Philosophical Society in 1818,
when he was twenty-six years of age, expresses the
views regarding education which he entertained to the
end of his life. ' First, then,' he says, ' all theological
considerations are banished from the society, and of
course from my remarks ; and whatever I may say has
no reference to a future state, or to the means which
are to be adopted in this world in anticipation of it.
Next, I have no intention of substituting anything for
religion, but I wish to take that part of human nature
which is independent of it. Morality, philosophy,
commerce, the various institutions and habits of
society, are independent of religion, and may exist
either with or without it. They are always the same,
and can dwell alike in the breasts of those who, from
opinion, are entirely opposed in the set of principles
they include in the term religion, or in those who have
none.

* To discriminate more closely, if possible, I will
observe that we have no right to judge religious

420 FKAGMENTS OF SCIENCE.

opinions ; but the human nature of this evening is
that part of man which we have a right to judge.
And I think it will be found on examination, that this
humanity — as it may perhaps be called — will accord
with what I have before described as being in our own
hands so improvable and perfectible.'

In an old journal I find the following remarks on
one of my earliest dinners with Faraday : ' At two
o'clock he came down for me. He, his niece, and
myself, formed the party, " I never give dinners," he
said. " I don't know how to give dinners, and I
never dine out. But I should not like my friends to
attribute this to a wrong cause. I act thus for the
sake of securing time for work, and not through re-
ligious motives, as some imagine." He said grace. I
am almost ashamed to call his prayer a " saying of
grace." In the language of Scripture, it might be
described as the petition of a son, into whose heart
God had sent the Spirit of His Son, and who with
absolute trust asked a blessing from his father. We
dined on roast beef, Yorkshire pudding, and potatoes ;
drank sherry, talked of research and its requirements,
and of his Iiabit of keeping himself free from the dis-
tractions of society. He was bright and joyful — boy-
like, in fact, though he is now sixty-two. His work
excites admiration, but contact with him warms and
elevates the heart. Here, surely, is a strong man. I
love strength ; but let me not forget the example of
its union with modesty, tenderness, and sweetness, in
the character of Faraday.'

Faraday's progress in discovery, and the salient
points of his character, are well brought out by the
wise choice of letters and extracts published in the
volumes before us. I will not call the labours of the
biographer final. So great a character will challenge

FAEADAY. 421

reconstruction. In the coming time some sympathetic
spirit, with the requisite strength, knowledge, and
solvent power, will, I doubt not, render these materials
plastic, give them more perfect organic form, and send
through them, with less of interruption, the currents of
Faraday's life. ' He was too good a man,' writes his
present biographer, ' for me to estimate rightly, and too
great a philosopher for me to understand thoroughly.'
That may be : but the reverent affection to which we
owe the discovery, selection, and arrangement of the
materials here placed before us, is probably a surer
guide than mere literary skill. The task of the artist
who may wish in future times to reproduce the real
though unobtrusive grandeur, the purity, beauty, and
childlike simplicity of him whom we have lost, will
find his chief treasury already provided for him b^
Dr. Bence Jones's labour of love.

422 FRAGMENTS OF SCIENCB.

XIX.

THE COPLEY MEDALIST OF 1870.

rilHIRTY years ago Electro-magnetism was looked to
i as a motive power, which might possibly com-
pete with steam. In centres of industry, such as
Manchester, attempts to investigate and apply this
power were numerous. This is shown by the scientific
literature of the time. Among others Mr. James
Prescot Joule, a resident of Manchester, took up the
subject, and, in a series of papers published in Stur-
geon's * Annals of Electricity' between 1839 and 1841,
described various attempts at the construction and per-
fection of electro-magnetic engines. The spirit in
which Mr. Joule pursued these enquiries is revealed in
the following extract : ' I am particularly anxious,' he
says, ' to communicate any new arrangement in order,
if possible, to forestall the monopolising designs^of those
who seem to regard this most interesting subject merely
in the light of pecuniary speculation.' He was natur-
ally led to investigate the laws of electro-magnetic
attractions, and in 1840 he announced the important
principle that the attractive force exerted by two electro-
magnets, or by an electro-magnet and a mass of an-
nealed iron, is directly proportional to the square
of the strength of the magnetising current; while
the attraction exerted between an electro-magnet
and the pole of a permanent steel magnet, varies

THE COPLEY MEDALIST 0^ 1870. 42S

simply as the strength of the current. These inves-
tigations were conducted independently of, though a
little subsequently to, the celebrated enquiries of
Henry, Jacobi, and Lenz and Jacobi, on the same
subject.

On December 17, 1840, Mr. Joule communicated to
the Eoyal Society a paper on the production of heat
by Voltaic electricity. In it he announced the law that
the calorific effects of equal quantities of transmitted
electricity are proportional to the resistance overcome
by the current, whatever may be the length, thickness,
shape, or character of the metal which closes the circuit ;
and also proportional to the square of the quantity of
transmitted electricity. This is a law of primary
importance. In another paper, presented to, but
declined by, the Eoyal Society, he confirmed this law
by new experiments, and materially extended it. He
also executed experiments on the heat consequent on
the passage of Voltaic electricity through electro-
lytes, and found, in all cases, that the heat evolved
by the proper action of any Voltaic current is propor-
tional to the square of the intensity of that current,
multiplied by the resistance to conduction which
it experiences. From this law he deduced a number
of conclusions of the highest importance to electro-
chemistry.

It was during these enquiries, which are marked
throughout by rare sagacity and originality, that the
great idea of establishing quantitative relations between
Mechanical Energy and Heat arose and assumed definite
form in his mind. In 1843 Mr. Joule read before the
meeting of the British Association at Cork a paper ' On
the Calorific Effects of Magneto-Electricity, and on the
Mechanical Value of Heat.' Even at the present day
this memoir is tough reading, and at the time it was

28

424 FEAGMENTS OF SCIENCE.

written it must have appeared hopelessly entangled.
This, I should think, was the reason why Faraday
advised Mr. Joule not to submit the paper to the
Eoyal Society. But its drift and results are summed
up in these memorable words by its author, written
some time subsequently : ' In that paper it was demon-
strated experimentally, that the mechanical power
exerted in turning a magneto-electric machine is con-*
verted into the heat evolved by the passage of the
currents of induction through its coils ; and, on the
other hand, that the motive power of the electro-
magnetic engine is obtained at the expense of the heat
due to the chemical reaction of the battery by which it
is worked.' * It is needless to dwell upon the weight
and importance of this statement.

Considering the imperfections incidental to a first
determination, it is not surprising that the 'mechani-
cal values of heat,' deduced from the different series
of experiments published in 1843, varied widely
from each other. The lowest limit was 587, and
the highest 1,026 foot-pounds, for 1° Fahr. of tempera-
ture.

One noteworthy result of his enquiries, which was
pointed out at the time by Mr. Joule, ha-d reference to
the exceedingly small fraction of the heat actually
converted into useful effect in the steam-engine. The
thoughts of the celebrated Julius Eobert Mayer, who
was tlien engaged in Germany upon the same question,
had moved independently in the same groove ; but to
his labours due reference will be made on a future
occasion. 2 In the memoir now referred to, Mr. Joule
also announced that he had proved heat to be evolved
during the passage of water through narrow tubes ; and
« PhiL Mag., May 1845. ■ See the next Fragment.

THE COPLEY MEDALIST OF 1870. 425

he deduced trom these experiments an equivalent of
770 foot-pounds, a figure remarkably near the one now
accepted. A detached statement regarding the origin
and convertibility of animal heat strikingly illustrates
the penetration of Mr. Joule, and his mastery of prin-
ciples, at the period now referred to. A friend had
mentioned to him Haller's liypo thesis, that animal heat
might arise from the friction of the blood in the veins
and arteries. ' It is unquestionable,' writes Mr. Joule,
' that heat is produced by such friction ; but it must be
understood that the mechanical force expended in the
friction is a part of the force of affinity which causes
the venous blood to unite with oxygen, so that the
whole heat of the system niust still be referred to the
chemical changes. But if the animal were engaged in
turning a piece of machinery, or m ascending a moun-
tain, I apprehend that in proportion to the muscular
effort put forth for the purpose, a diminution of the
heat evolved in the system by a given chemical action
would be experienced.' The italics in this memorable
passage, written, it is to be remembered, in 1843, are
Mr. Joule's own.

The concluding paragraph of this British Association
paper equally illustrates his insight and precision,
regarding the nature of chemical and latent heat. ' I
had,' he writes, ' endeavoured to prove that when two
atoms combine together, the heat evolved is exactly
that which would have been evolved by the electrical
current due to the chemical action taking place, and is
therefore proportional to the intensity of the chemical
force causing the atoms to combine. I now venture to
state more explicitly, that it is not precisely the attrac-
tion of affinity, but rather the mechanical force ex-
pended by the atoms in falling towards one another,

426 FRAGMENTS OF SCIENCE.

which determines the intensity of the current, and,
consequently, the quantity of heat evolved ; so that we
have a simple hypothesis by which we may explain why
heat is evolved so freely in the combination of gases,
and by which indeed we may account " latent heat *' as
a mechanical power, prepared for action, as a watch-
spring is when wound up. Suppose, for the sake of
illustration, that 8 lbs. of oxygen and 1 lb. of hydrogen
were presented to one another in the gaseous state, and
then exploded; the heat evolved would be about 1**
Fahr. in 60,000 lbs. of water, indicating a mechanical
force, expended in the combination, equal to a weight
of about 50,000,000 lbs. raised to the height of one foot.
Now if the oxygen and hydrogen could be presented
to each other in a liquid state, the heat of combina-
tion would be less than before, because the atoms
in combining would fall through less space.' No
words of mine are needed to point out the com-
manding grasp of molecular physics, in their relation
to the mechanical theory of heat, implied by this state-
ment.

Perfectly assured of the importance of the principle
which his experiments aimed at establishing, Mr. Joule
did not rest content with results presenting such discre-
pancies as those above referred to. He resorted in 1 844
to entirely new metliods, and made elaborate experi-
ments on the thermal changes produced in air during
its expansion : firstly, against a pressure, and therefore
performing work ; secondly, against no pressure, and
therefore performing no work. He thus established
anew the relation between the heat consumed and the
work done. From five different series of experiments
he deduced five different mechanical equivalents ; the
agreement between them being far greater than that

THE COPLEY MEDALIST OF 1870. 427

attained in his first experiments. The mean of them
was 802 foot-pounds. From experiments with water
agitated by a paddle-wheel, he deduced, in 1845, an
equivalent of 890 foot-pounds. In 1847 he again
operated upon water and sperm-oil, agitated them by a
paddle-wheel, determined their elevation of temperature,
and the mechanical power which produced it. From
the one he derived an equivalent of 781*5 foot-pounds;
from the other an equivalent of 782*1 foot-pounds.
The mean of these two very close determinations is
781*8 foot-pounds.

By this time the labours of the previous ten years
had made Mr. Joule completely master of the conditions
essential to accuracy and success. Bringing his ripened
experience to bear upon the subject, he executed in
1849 a series of 40 experiments on the friction of water,
50 experiments on the friction of mercury, and 20
experiments on the friction of plates of cast-iron. He
deduced from these experiments our present mechanical
equivalent of heat, justly recognised all over the world
as ' Joule's equivalent.'

There are labours so great and so pregnant in conse-
quences, that they are most highly praised when they
are most simply stated. Such are the labours of Mr.
Joule. They constitute the experimental foundation of
a principle of incalculable moment, not only to the
practice, hut still more to the philosophy of Science.
Since* the days of Newton, nothing more important than
the theory, of which Mr. Joule is the experimental
demonstrator, has been enunciated.

1 have omitted all reference to the numerous minor
papers with which Mr. Joule has enriched scientific
literature. Nor have I alluded to the important inves-
tigations which he has conducted jointly with Sir

428 FRAGMENTS OF SCIENCE.

m

William Thomson. But sufficient, I think, has been
here said to show that, in conferring upon Mr. Joule
the highest honour of the Eoyal Society, the Council
paid to genius not only a well-won tribute, but one
which had been fairly earned twenty years previously.*

* Lord Beaconsfield has recently honoured himself and England
by bestowing an annual pension of 200^. on Dr. Joule.

XX.

TME COPLEY MEDALIST OF 1871*

DR. JULIUS EGBERT MAYER was educated for
the medical profession. In the summer of 1840,
as he himself informs us, he was at Java, and there
observed that the venous blood of some of his patients
had a singularly bright red colour. The observation
riveted his attention ; he reasoned upon it, and came to
the conclusion that the brightness of the colour was
due to the fact that a less amount of oxidation sufficed
to keep up the temperature of the body in a hot climate
than in a cold one. The darkness of the venous blood
he regarded as the visible sign of the energy of the
oxidation.

It would be trivial to remark that accidents such as
this, appealing to minds prepared for them, have often
led to great discoveries. Mayer's attention was thereby
drawn to the whole question of animal heat. Lavoisier
had ascribed this heat to the oxidation of the food.
' One great principle,' says Mayer, ' of the physiological
theory of combustion, is that under all circumstances
the same amount of fuel yields, by its perfect combus-
tion, the same amount of heat ; that this law holds good
even for vital processes ; and that hence the living body,
notwithstanding all its enigmas and wonders, is incom-
petent to generate heat out of nothing.'

But beyond the power of generating internal heat,

430 FRAGMENTS OF SCIENCE.

the animal organism can also generate heat outside of
itself. A blacksmith, for example, by hammering can
heat a nail, and a savage by friction can warm wood to
its point of ignition. Now, unless we give up the physio-
logical axiom that the living body cannot create heat
out of nothing, ' we are driven,' says Mayer, 'to the
conclusion that it is the total heat generated within and
without that is to be regarded as the true calorific
effect of the matter oxidised in the body.'

From this, again, he inferred that the heat generated
externally must stand in a fixed relation to the work
expended in its production. For, supposing the organic
processes to remain the same ; if it were possible, by the
mere alteration of the apparatus, to generate different
amounts of heat by the same amount of work, it would
follow that the oxidation of the same amount of material
would sometimes yield a less, sometimes a greater,
quantity of heat. ' Hence,' says Mayer, * that a fixed
relation subsists between heat and work, is a postulate
of the physiological theory of combustion.'

This is the simple and natural account, given subse-
quently by Mayer himself, of the course of thought
started by his observation in Java. But the conviction
once formed, that an unalterable relation subsists between
work and heat, it was inevitable that Mayer should seek
to express it numerically. It was also inevitable that a
mind like his, having raised itself to clearness on this
important point, should push forward to consider the
relationship of natural forces generally. At the begin-
ning of 1842 his work had made considerable progress ;
but he had become physician to the town of Heilbronn,
and the duties of his profession limited the time which
he could devote to purely scientific enquiry. He thought
it wise, therefore, to secure himself against accident, and
in the spring of 1842 wrote to Liebig, asking him to

THE COPLEY MEDALIST OF 1871. 431

publish in his « Annalen ' a brief preliminary notice of
the work then accomplished. Liebig did so, and Dr.
Mayer's first paper is contained in the May number of
the < Annalen ' for 1842.

Mayer had reached his conclusions by reflecting on
the complex processes of the living body ; but his first
step in public was to state definitely the physical prin-
ciples on which his physiological deductions were to
rest. He begins, therefore, with the forces of inorganic
nature. He finds in the universe two systems of causes
which are not mutually convertible ; — the different kinds
of matter and the different forms of force. The first
quality of both he affirms to be indestructibility, A
force cannot become nothing, nor can it arise from
nothing. Forces are convertible but not destructible.
In the terminology of his time, he then gives clear ex-
pression to the ideas of potential and dynamic energy,
illustrating his point by a weight resting upon the
earth, suspended at a height above the earth, and actu-
ally falling to the earth. He next fixes his attention
on cases where motion is apparently destroyed, without
producing other motion ; on the shock of inelastic
bodies, for example. Under what form does the vanished
motion maintain itself ? Experiment alone, says Mayer,
can help us here. He warms water by stirring it ; he
refers to the force expended in overcoming friction.
Motion in both cases disappears ; but heat is generated,
and the quantity generated is the equivalent of the
motion destroyed. ' Our locomotives,' he observes with
extraordinary sagacity, ' may be compared to distilling
apparatus : the heat beneath the boiler passes into the
motion of the train, and is again deposited as heat in
the axles and wheels.'

A numerical solution of the relation between heat
and work was what Mayer aimed at, and towards the end

432 FKAGMENTS OF SCIENCK

of his first paper he makes the attempt. It was kno^vn
that a definite amount of air, in rising one degree in
temperature, can take up two different amounts of heat.
If its volume be kept constant, it takes up one amount i
if its pressure be kept constant it takes up a different
amount. These two amounts are called the specific
heat under constant volume and under constant pres-
sure. The ratio of the first to the second is as 1 : 1*421.
No man, to my knowledge, prior to Dr. Mayer, pene-
trated the significance of these two numbers. He first
saw that the excess 0*421 was not, as then universally
supposed, heat actually lodged in the gas, but heat which
had been actually consumed by the gas in expanding
against pressure. The amount of work here performed
was accurately known, the amount of heat consumed was
also accurately known, and from these data Mayer deter-
mined the mechanical equivalent of heat. Even in this
first paper he is able to direct attention to the enormous
discrepancy between the theoretic power of the fuel
consumed in steam-engines, and their useful effect.

Though this paper contains but the germ of his
further labours, I think it may be safely assumed that,
as regards the mechanical theory of heat, this obscure
Heilbronn physician, in the year 1842, was in advance
of all the scientific men of the time.

Having, by the publication of this paper, secured
himself against what he calls ' Eventualitaten,' he de-
voted every hour of his spare time to his studies, and in
1845 published a memoir which far transcends his first
one in weight and fulness, and, indeed, marks an epoch
in the history of science. The title of Mayer's first
paper was, ' Remarks on the Forces of Inorganic Nature.'
The title of his second great essay was, ' Organic Motion
in its Connection with Nutrition.' In it he expands and
illustrates the physical principles laid down in his first

THE COPLEY MEDAIJST OF 1871. 433

brief paper. He goes fully through the calculation of
the mechanical equivalent of heat. He calculates the
performances of steam-engines, and finds that 100 lbs.
of coal, in a good working engine, produce only the
same amount of heat as 95 lbs. in an un working one ;
the 5 missing lbs. having been converted into work.
He determines the useful effect of gunpowder, and finds
nine per cent, of the force of the consumed charcoal in-
vested on the moving ball. He records observations on
the heat generated in water agitated by the pulping-
engine of a paper manufactory, and calculates the equi-
valent of that heat in horse-power. He compares
chemical combination with mechanical combination— -
the union of atoms with the union of falling bodies with
the earth. He calculates the velocity with which a
body starting at an infinite distance would strike the
earth's surface, and finds that the heat generated by its
collision would raise an equal weight of water 17,356°
C. in temperature. He then determines the thermal
effect which would be produced by the earth itself falling
into the sun. So that here, in 1845, we have the germ
of that meteoric theory of the sun's heat which Mayer
develaped with such extraordinary ability three years
afterwards. He also points to the almost exclusive effi-
cacy of the sun's heat in producing mechanical motions
upon the earth, winding up with the profound remark,
that the heat developed by friction in the wheels of our
wind and water mills comes firom the sun in the form
of vibratory motion ; while the heat produced by mills
driven by tidal action is generated at the expense of the
earth's axial rotation.

Having thus, with firm step, passed through the
powers of inorganic nature, his next object is to bring
his principles to bear upon the phenomena of vegetable
and animal life. Wood and coal can burn; whence

434 FKAGMENTS OF SCIENCE.

come their heat, and the work producible by that heat ?
From the immeasm'able reservoir of the sun. Nature
has proposed to herself the task of storing up the light
which streams earthward from the sun, and of caating
into a permanent form the most fugitive of all powers.
To this end she has overspread the earth with organisms
which, while living, take in the solar light, and by its
consumption generate forces of another kind. These
organisms are plants. The vegetable world, indeed,
constitutes the instrument whereby the wave-motion of
the sun is changed into the rigid form of chemical ten-
sion, and thus prepared for future use. With this pre-
vision, as shall subsequently be shown, the existence
of the human race itself is inseparably connected. It
is to be observed that Mayer's utterances are far from
being anticipated by vague statements regarding the
' stimulus ' of light, or regarding coal as ' bottled sun-
light.' He first saw the full meaning of De Saussure's
observation as to the reducing power of the solar rays, and
gave that observation its proper place in the doctrine of
conservation. In the leaves of a tree, the carbon and
oxygen of carbonic acid, and the hydrogen and oxygen
of water, are forced asunder at the expense of the sun,
and the amount of power thus sacrificed is accurately
restored by the combustion of the tree. The heat and
work potential in our coal strata are so much strength
withdrawn from the sun of former ages. Maj^er lays the
axe to the root of the notions regarding ' vital force '
which were prevalent when he wrote. With the plain
fact before us that in the absence of the solar rays
plants cannot perform the work of reduction, or generate
chemical tensions, it is, he contends, incredible that these
tensions should be caused by the mystic play of the vital
force. Such an hypothesis would cut off" all investiga-
tion ; it would land us in a chaos of unbridled phantasy

THE COPLEY MEDALIST OF 1871. 435

' I count,' he says, ' therefore, upon your agreement with
me when I state, as an axiomatic truth, that during
vital processes the conversion only, and never the
creation of matter or force occurs.'

Having cleared his way through the vegetable world,
as he had previously done through inorganic nature,
Mayer passes on to the other organic kingdom. The
physical forces collected by plants become the property
of animals. Animals consume vegetables, and cause
them to reunite with the atmospheric oxygen. Animal
heat is thus produced ; and not only animal heat, but
animal motion. There is no indistinctness about Mayer
here; he grasps his subject in all its details, and reduces
to figures the concomitants of muscular action. A
bowler who imparts to an 8-lb. ball a velocity of 30 feet,
consumes in the act ^ of a grain of carbon. A man
weighing 150 lbs., who lifts his own body to a height
of 8 feet, consumes in the act 1 grain of carbon. In
climbing a mountain 10,000 feet high, the consumption
of the same man would be 2 oz. 4 drs. 50 grs. of carbon.
Boussingault had determined experimentally the ad-
dition to be made to the food of horses when actively
working, and Liebig had determined the addition to be
made to the food of men. Employing the mechanical
equivalent of heat, which he had previously calculated,
Mayer proves the additional food to be amply sufficient
to cover the increased oxidation.

But- he does not content himself with showing, in a
general way, that the human body burns according to
definite laws, when it performs mechanical work. He
seeks to determine the particular portion of the body
consumed, and in doing so executes some noteworthy
calculations. The muscles of a labourer 150 lbs. in
weight weigh 64 lbs. ; but when perfectly desiccated they
fall to 15 lbs. Were the oxidation corresponding to

436 FRAGMENTS OF SCIENCE.

that labourer's work exerted on the muscles alone, they
would be utterly consumed in 80 days. The heart
furnishes a still more striking example. "Were the
oxidation necessary to sustain the heart's action exerted
upon its own tissue, it would be utterly consumed in 8
days. And if we confine our attention to the two
ventricles, their action would be sufficient to consume
the associated muscular tissue in S^ days. Here, in his
own words, emphasised in his own way, is Mayer's
pregnant conclusion from these calculations : ' The
muscle is only the apparatus by means of which the
conversion of the force is effected ; hut it is not the
substance consumed in the 'production of the mecha-
nical effect,'' He calls the blood ' the oil of the lamp of
life;' it is the slow-burning fluid whose chemical force,
in the furnace of the capillaries, is sacrificed to produce
animal motion. This was Mayer's conclusion twenty-six
years ago. It was in complete opposition to the scien-
tific conclusions of his time ; but eminent investigators
have since amply verified it.

Thus, in baldest outline, I have sought to give some
notion of the first half of this marvellous essay. The
second half is so exclusively physiological that I do not
wish to meddle with it. I will only add the illustration
employed by Mayer to explain the action of the nerves
upon the muscles. As an engineer, by the motion of his
finger in opening a valve or loosing a detent, can liberate
an amount of mechanical motion almost infinite com-
pared with its exciting cause, so the nerves, acting upon
the muscles, can unlock an amount of activity, wholly
out of proportion to the work done by the nerves them-

As regards these questions of weightiest import to the
science of physiology. Dr. Mayer, in 1 845, was assuredly
far in advance of all living men.

THE COPLEY MEDALIST OP 1871. 437

Mayer grasped the mechanical theory of heat with
commanding power, illustrating it and applying it in
the most diverse domains. He began, as we have seen,
with physical principles ; he determined the numerical
relation between heat and work ; he revealed the source
of the energies of the vegetable world, and showed the
relationship of the heat of our j&res to solar heat. He
followed the energies which were potential in the vege-
table, up to their local exhaustion in the animal. But
in 1845 a new thought was forced upon him by his
calculations. He then, for the first time, drew attention
to the astounding amount of heat generated by gravity
where the force has sufficient distance to act through.
He proved, as I have before stated, the heat of collision
of a body falling from an infinite distance to the earth,
to be sufficient to raise the temperature of a quantity
of water, equal to the falling body in weight, 17,356° C.
He also found, in 1845, that the gravitating force
between the earth and sun was competent to generate
an amount of heat equal to that obtainable from the
combustion of 6,000 times the weight of the earth of
solid coal. With the quickness of genius he saw that
we had here a power sufficient to produce the enormous
temperature of the sun, and also to account for the
primal molten condition of our own planet. Mayer
shows the utter inadequacy of chemical forces, as we
know them, to produce or maintain the solar temperature.
He shows that were the sun a lump of coal it would be
utterly consumed in 5,000 years. He shows the diffi-
culties attending the assumption that the sun is a cooling
body; for, supposing it to possess even the high specific
heat of water, its temperature would fall 15,000° in
5,000 years. He finally concludes that the light and
heat of the sun are maintained by the constant impact
of meteoric matter. I never ventured an opinion aa to

438 FRAGMENTS OF SCIENCE,

"IHB

the truth of this theory ; that is a question which
may still have to be fought out. But I refer to it as
an illustration of the force of genius with which Mayer
followed the mechanical theory of heat through all its
applications. Whether the meteoric theory be a matter
of fact or not, with him abides the honour of proving
to demonstration that the light and heat of suns and
stars TYiay be originated and maintained by the collisions
of cold planetary matter.

It is the man who with the scantiest data could
accomplish all this in six short years, and in the hours
snatched from the duties of an arduous profession, that
the Royal Society, in 1871, crowned with its highest
honour.

Comparing this brief history with that of the Copley
Medalist of 1870, the differentiating influence of
'environment,' on two minds of similar natural cast
and endowment, comes out in an instructive manner.
Withdrawn from mechanical appliances, Mayer fell back
upon reflection, selecting with marvellous sagacity, from
existing physical data, the single result on which could
be founded a calculation of the mechanical equivalent
of heat. In the midst of mechanical appliances. Joule
resorted to experiment, and laid the broad and firm
foundation which has secured for the mechanical theory
the acceptance it now enjoys. A great portion of Joule's
time was occupied in actual manipulation; freed from
this, Mayer had time to follow the theory into its most
abstruse and impressive applications. With their places
reversed, however, Joule might have become Mayer, and
Mayer might have become Joule.

It does not lie vvitliin the scope of these brief articles
to enter upon the developments of the Dynamical
Theory accomplished since Joule and Mayer executed
their memorable labours.

XXL

DEATH BY LIGHTNING.

PEOPLE in general imagine, when tbb^ think at «ll
about the matter, that an impression upon the
nerves — a blow, for example, or the prick of a pin — is
felt at the moment it is inflicted. But this is not the case.
The seat of sensation being the brain, to it the intelli-
gence of any impression made upon the nerves has to be
transmitted before this impression can become manifest
as consciousness. The transmission, moreover, requires
time, and the consequence is, that a wound inflicted on
a portion of the body distant from the brain is more
tardily appreciated than one inflicted adjacent to the
brain. By an extremely ingenious experimental arrange-
ment, Helmholtz has determined the velocity of this
nervous transmission, and finds it to be about eighty
feet a second, or less than one-thirteenth of the velocity
of sound in air. If therefore, a whale forty feet long
were wounded in the tail, it would not be conscious of
the injury till half a second after the wound had been
inflicted.^ But this is not the only ingredient in the
delay. There can scarcely be a doubt that to every act
of consciousness belongs a determinate molecular arrange-
ment of the brain — that every thought or feeling has

> A most admirable lecture on the velocity of nervous trans-
mission has been published by Dr. Du Bois Keymond in the *PrO'
cjeedings of the Royal Institution ' for 1866, vol. iv. p. 576.

440 ' FRAGMENTS OF SCIENCE.

its physical correlative in that organ ; and nothing can
be more certain than that every physical change, whe-
ther molecular or mechanical, requires time for its
accomplishment. So that, besides the interval of trans-
mission, a still further time is necessary for the brain
to put itself in order — for its molecules to take up the
motions or positions necessary to the completion of
consciousness. Helmholtz considers that one-tenth of
a second is demanded for this purpose. Thus, in the
case of the whale above supposed, we have first half a
second consumed in the transmission of the intelligence
through the sensor nerves to the head, one-tenth of a
second consumed by the brain in completing the arrange-
ments necessary to consciousness, and, if the velocity of
transmission through the motor be the same as that
through the sensor nerves, half a second in sending a
command to the tail to defend itself. Thus one second
and a tenth would elapse before an impression made
upon its caudal nerves could be responded to by a whale
forty feet long.

Now, it is quite conceivable that an injury might
be inflicted so rapidly that within the time required by
the brain to complete the arrangements necessary to
consciousness, its power of arrangement might be des-
troyed. In such a case, though the injury might be of
a nature to cause death, this would occur without pain.
Death in this case would be simply the sudden negation
of life, without any intervention of consciousness what-
ever.

The time required for a rifle-bullet to pass clean
through a man's head may be roughly estimated at a
thousandth of a second. Here, therefore, we should
have no room for sensation, and death would be pain-
less. But there are other actions which far transcend
in rapidity that of the rifle-bullet, A flash of light-

DEATH BY LIGHTNING. 441

ning cleaves a cloud, appearing and disappearing in
less than a hundred-thousandth of a second, and the
velocity of electricity is such as would carry it in a
single second over a distance almost equal to that which
separates the earth and moon. It is well known that a
luminous impression once made upon the retina endures
for about one- sixth of a second, and that this is the
reason why we see a continuous band of light when a
glowing coal is caused to pass rapidly through the air.
A body illuminated by an instantaneous flash continues
to be seen for the sixth of a second after the flash has
become extinct ; and if the body thus illuminated be in
motion, it appears at rest at the place where the flash
falls upon it. When a colour-top with differently-coloured
sectors is caused to spin rapidly the colours blend together.
Such a top, rotating in a dark room and illuminated
by an electric spark, appears motionless, each distinct
colour being clearly seen. Professor Dove has foirad
that a flash of lightning produces the same effect.
During a thunderstorm he put a colour-top in exceed-
ingly rapid motion, and found that every flash revealed
the top as a motionless object with its colours distinct.
If illuminated solely by a flash of lightning, the motion
of all bodies on the earth's surface would, as Dove has
remarked, appear suspended. A cannon-ball, for ex-
ample, would have its flight apparently arrested, and
would seem to hang motionless in space as long as the
luminous impression which revealed the ball remained
upon the eye.

If, then, a riflt>-bullet move with sufficient rapidity
to destroy life without the interposition of sensation,
much more is a flash of lightning competent to produce
this effect. Accordingly, we have well-authenticated
cases of people being struck senseless by lightning who.

442 FRAGMENTS OF SCIENCE.

on recovery, had no memory of pain. The following
circumstantial case is described by Hemmer : —

On June 30, 1788, a soldier in the neighbourhood
of Mannheim, being overtaken by rain, placed himself
under a tree, beneath which a woman had previously
taken shelter. He looked upwards to see whether the
branches were thick enough to afford the required pro-
tection, and, in doing so, was struck by lightning, and
fell senseless to the earth. The woman at his side ex-
perienced the shock in her foot, but was not struck
down. Some hours afterwards the man revived, but
remembered nothing about what had occurred, save the
fact of his looking up at the branches. This was his
last act of consciousness, and he passed from the con-
scious to the unconscious condition without pain. The
visible marks of a lightning stroke are usually insigni-
ficant : the hair is sometimes burnt ; slight wounds are
observed ; while, in some instances, a red streak marks
the track of the discharge over the skin.

Under ordinary circumstances, the discharge from
a small Leyden jar is exceedingly unpleasant to me. I
Some time ago I happened to stand in the presence of ]
a numerous audience, with a battery of fifteen large
Leyden jars charged beside me. Through some awk-
wardness on my part, I touched a wire leading from the
battery, and the discharge went through my body.
Life was absolutely blotted out for a very sensible
interval, without a trace of pain. In a second or so con-
sciousness returned ; I vaguely discerned the audience
and apparatus, and, by the help of these external
appearances, immediately concluded that I had received
the battery discharge. The intellectual consciousness
of my position was restored with exceeding rapidity,
but not so the optical consciousness. To prevent the
audience from being alarmed, I observed that it had

DEATH BY LIGHTNING. 443

often been my desire to receive accidentally such a
shock, and that my wish had at length been fulfilled.
But, while making this remark, the appearance which
my body presented to my eyes was that of a number
of separate pieces. The arms, for example, were
detached from the trunk, and seemed suspended in the
air. In fact, memory and the power of reasoning
appeared to be complete long before the optic nerve
was restored to healthy action. But what I wish chiefly
to dwell upon here is, the absolute painlessness of the
shock ; and there cannot, I think, be a doubt that, to a
person struck dead by lightning, the passage from life
to death occurs without consciousness being in the least
degree implicated. It is an abrupt stoppage of sensa'
tion, unaccompanied by a pang.

I

444 FRAGMENTS OF SCIENCE.

XXIL

SCIENCE AND THE * SPIRITS.'

THEIR refusal to investigate ' spiritual phenomena
is oflen urged as a reproach against scientific men.
I here propose to give a sketch of an attempt to apply
to the ' phenomena ' those methods of enquiry which
are found available in dealing with natural truth.

Some years ago, when the spirits were particularly
active in this country, Faraday was invited, or rather
entreated, by one of his friends to meet and question
them. He had, however, already made their acquaint-
ance, and did not wish to renew it. I had not been
80 privileged, and he therefore kiudly arranged a
transfer of the invitation to me. The spirits themselves
named the time of meeting, and I was conducted to
the place at the day and hour appointed.

Absolute unbelief in the facts was by no means my
condition of mind. On the contrary, I thought it pro-
bable that some physical principle, not evident to the
spiritualists themselves, might underlie their manifes-
tations. Extraordinary effects are produced by the
accumulation of small impulses. Galileo set a heavy
pendulum in motion by the well-timed puffs of his
breath. Ellicot set one clock going by the ticks of
another, even when the two clocks were separated by a
wall. Preconceived notions can, moreover, vitiate, to
an extraordinary degree, the testimony of even veracious

i

SCIENCE AND THE 'SPIRITS.' 445

persons. Hence my desire to witness those extra-
ordinary phenomena, the existence of which seemed
placed beyond a doubt by the known veracity of those
who had witnessed and described them. The meeting
took place at a private residence in the neighbourhood
of London. My host, his intelligent wife, and a gentle-
man who may be called X., were in the house when I
arrived. I was informed that the ' medium ' had not
yet made her appearance ; that she was sensitive, and
might resent suspicion. It was therefore requested
that the tables and chairs should be examined before
her arrival, in order to be assured that there was n©
trickery in the furniture. This was done ; and I then
first learned that my hospitable host had arranged that
the seance should be a dinner-party. This was to me
an unusual form of investigation ; but I accepted it,
as one of the accidents of the occasion.

The ' medium ' arrived — a delicate-looking young
lady, who appeared to have suffered much from ill-
health. I took her to dinner and sat close beside her.
Facts were absent for a considerable time, a series of
very wonderful narratives supplying their place. The
duty of belief on the testimony of witnesses was frequently
insisted on. X. appeared to be a chosen spiritual agent,
and told us many surprising things. He affirmed that,
when he took a pen in his hand, an influence ran from
his shoulder downwards, and impelled him to write oracu-
lar sentences. I listened for a time, offering no observa-
tion. *And now,' continued X., 'this power has so
risen as to reveal to me the thoughts of others. Only
this morning I told a friend what he was thinking of,
and what he intended to do during the day.' Here, I
thought, is something that can be at once tested. I
said immediately to X. : ' If you wish to win to your
cause an apostle, who will proclaim your principles to

446 FKAGMENTS OF SCIENCE.

the world from the housetop, tell me what I am now
thinking of.' X. reddened, and did not tell me my
thought.

Some time previously I had visited Baron Keichen-
bach, in Vienna, and I now asked tlie young lady who
sat beside me, whether she could see any of the curious
things which he describes — the light emitted by crystals,
for example ? Here is the conversation which followed,
as extracted from my notes, written on tlie day fol-
lowing the seance.

Medium. — 'Oh, yes; but I see light around all
bodies.'

7. — 'Even in perfect darkness ?'

Medium. — ' Yes ; I see luminous atmospheres
round all people. The atmosphere which surrounds
Mr. E. C. would fill this room with light.'

/. — ' You are aware of the effects ascribed by Baron
Reichenbach to magnets ? '

Medium. — ' Yes ; but a magnet makes me terribly
ill.'

T. — *Am I to understand that, if this room were
perfectly dark, you could tell whether it contained a
magnet, without being informed of the fact ? '

Medium. — 'I should know of its presence on
entering the room.'

/.—'How?'

Medium. — ' I should be rendered instantly ilL

I. — ' How do you feel to-day ? '

Medium..— ' Particularly well ; I have not been so
well for months.'

I. — -^ Then, may I ask you whether there is, at the
present moment, a magnet in my possession ? '

The young lady looked at me, blushed, and stam-
mered,

' No ; I am not en rajpjport with you.'

SCIENCE AND THE SPIRITS.' 447

I sat at her right hand, and a left-hand pocket,
within six inches of her person, contained a magnet.

Our host here deprecated discussion, as it ' exhausted
the medium.' The wonderful narratives were resumed ;
but I had narratives of my own quite as wonderful.
These spirits, indeed, seemed clumsy creations, com-
pared with those with which my own work had made
me familiar. I therefore began to match the wonders
related to me by other wonders. A lady present dis-
coursed on spiritual atmospheres, which she could see
as beautiful colours when she closed her eyes, I pro-
fessed myself able to see similar colours, and, more
than that, to be able to see the interior of my own
eyes. The medium affirmed that she could see actual
waves of light coming from the sun. I retorted that
men of science could tell the exact number of waves
emitted in a second, and also their exact length. The
medium spoke of the performances of the spirits on
musical instruments. I said that such performance
was gross, in comparison with a kind of music which
had been discovered some time previously by a scientific
man. . Standing at a distance of twenty feet from a
jet of gas, he could command the flame to emit a
melodious note ; it would obey, and continue its song
for hours. So loud was the music emitted by the gas-
flame, that it might be heard by an assembly of a
thousand people. These were acknowledged to be as
great marvels as any of those of spiritdom. The spirits
were then consulted, and I was pronounced to be a
first-class medium.

During this conversation a low knocking was heard
from time to time under the table. These, I was told,
were the spirits' knocks. I was informed that one knock,
in answer to a question, meant ' No ; ' that two knocks
meant ' Not yet , ' and that three knocks meunt ' Yes.'

448 FRAGMENTS OF SCIENCE.

In answer to a question whether I was a medium, the
response was three brisk and vigorous knocks. I noticed
that the knocks issued from a particular locality, and
therefore requested the spirits to be good enough to
answer from another corner of the table. They did
not comply ; but I was assured that they would do it,
and much more, by-and-by. The knocks continuing,
I turned a wine-glass upside down, and placed my ear
upon it, as upon a stethoscope. The spirits seemed dis-
concerted by the act ; they lost their playfulness, and
did not recover it for a considerable time.

Somewhat weary of the proceedings, I once threw
myself back against my chair and gazed listlessly out
of the window. While thus engaged, the table was
rudely pushed. Attention was drawn to the wine, still
oscillating in the glasses, and I was asked whether that
was not convincing. I readily granted the fact of
motion, and began to feel the delicacy of my position.
There were several pairs of arms upon the table, and
several pairs of legs under it ; but how was I, without
offence, to express the conviction which I really enter-
tained ? To ward off the difficulty, I again turned a
wine-glass upside down and rested my ear upon it.
The rim of the glass was not level, and my hair, on
touching it, caused it to vibrate, and produce a peculiar
buzzing sound. A perfectly candid and warm-hearted
old gentleman at the opposite side of the table, whom
I may call A., drew attention to the sound, and
expressed his entire belief that it was spiritual. 1,
however, infonned him that it was the moving hair
acting on the glass. The explanation was not well
received; and X., in a tone of severe pleasantry,
demanded whether it was the hair that had moved the
table. The promptness of my negative probably
satisfied him that my notion was a very different one.

SCIENCE AND THE 'SPIRITS.' 449

Tlie superhuman power of the spirits was next
dwelt upon. The strength of man, it was stated, was
unavailing in opposition to theirs. No human power
could prevent the table from moving when they pulled
it. During the evening this pulling of the table
occurred, or rather was attempted, three times. Twice
the table moved when my attention was withdrawn
from it ; on a third occasion, I tried whether the act
could be provoked by an assumed air of inattention.
Grasping the table firmly between my knees, I threw
myself back in the chair, and waited, with eyes fixed
on vacancy, for the pull. It came. For some seconds
it was pull spirit, hold muscle ; the muscle, however,
prevailed, and the table remained at rest. Up to the
present moment, this interesting fact is known only to
the particular spirit in question and myself.

A species of mental scene-painting, with which my
own pursuits had long rendered me familiar, was
employed to figure the changes and distribution of
spiritual power. The spirits, it was alleged, were pro-
vided with atmospheres, which combined with and
interpenetrated each other, and considerable ingenuity
was shown in demonstrating the necessity of tiine in
effecting the adjustment of the atmospheres. A re-
arrangement of our positions was proposed and carried
out; and soon afterwards my attention was drawn to a
scarcely sensible vibration on the part of the table.
Several persons were leaning on the table at the time,
and I asked permission to touch the medium's hand.
' Oh ! I know I tremble,' was her reply. Throwing one
leg across the other, I accidentally nipped a muscle, and
produced thereby an involuntary vibration of the free
leg. This vibration, I knew, must be communicated
to the floor, and thence to the chairs of all present.
I therefore intentionally promoted it. My attention

450 FRAGMENTS OF SCIENCE.

was promptly drawn to the motion ; and a gentleman
beside me, whose value as a witness I was particularly
desirous to test, expressed his belief that it was out of
the compass of human power to produce so strange a
tremor. ' I believe,' he added, earnestly, ' that it is
entirely the spirits' work.' ' So do I,' added, with heat,
the candid and warmhearted old gentleman A. 'Why,
sir,' he continued, ' I feel them at this moment shaking
my chair.' I stopped the motion of the leg. ' Now,
sir,' A. exclaimed, ' they are gone.' I began again, and
A. once more affirmed their presence. I could, how-
ever, notice that there were doubters present, who did
not quite know what to think of the manifestations.
I saw their perplexity; and, as there was sufficient
reason to believe that the disclosure of the secret
would simply provoke anger, I kept it to myself.

Again a period of conversation intervened, during
which the spirits became animated. The evening was
confessedly a dull one, but matters appeared to brighten
towards its close. The spirits were requested to spell
the name by which I was known in the heavenly wortd.
Our host commenced repeating the alphabet, and when
he reached the letter 'P' a knock was heard. He
began again, and the spirits knocked at the letter * 0.*
I was puzzled, but waited for the end. The next
letter knocked down was ' E.' I laughed, and remarked
that the spirits were going to make a poet of me.
Admonished for my levity, I was informed that the
frame of mind proper for the occasion ought to have
been superinduced by a perusal of the Bible imme-
diately before the seance. The spelling, however, went
on, and sure enough I came out a poet. But matters
did not end here. Our host continued his repetition
of the alphabet, and the next letter of the name proved
to be * 0.' Here was manifestly an unfinislied word i

SCIENCE AND THE 'SPIRIXa' 451

and the spirits were apparently in their most communi-
cative mood. The knocks came from under the table,
but no person present evinced the slightest desire to
look under it. I asked whether I might go under-
neath ; the permission was granted ; so I crept under
the table. Some tittered ; but the candid old A. ex-
claimed, * He has a right to look into the very dregs of
it, to convince himself.' Having pretty well assured
myself that no sound could be produced under the table
without its origin being revealed, I requested our host
to continue his questions. He did so, but in vain.
He adopted a tone of tender entreaty ; but the ' dear
spirits ' had become dumb dogs, and refused to be
entreated. I continued under that table for at least a
quarter of an hour, after which, with a feeling of
despair as regards the prospects of humanity never be-
fore experienced, I regained my chair. Once there, the
spirits resumed their loquacity, and dubbed me ' Poet
of Science.'

This, then, is the result of an attempt made by a
scientific man to look into these spiritual phenomena.
It is not encouraging ; and for this reason. The present
promoters of spiritual phenomena divide themselves
into two classes, one of which needs no demonstration,
while the other is beyond the reach of proof. The
victims like to believe, and they do not like to be un-
deceived. Science is perfectly powerless in the presence
of this frame of mind. It is, moreover, a state per-
fectly compatible with extreme intellectual subtlety
and a capacity for devising hypotheses which only
require the hardihood engendered by strong conviction,
or by callous mendacity, to render them impregnable.
The logical feebleness of science is not sufficiently
borne in mind. It keeps down the weed of superstition,
not by logic but by slowly rendering the mental soil

452 FRAGMENTS OF SCIENCE.

unfit for its cultivation. When science appeals to
uniform experience, the spiritualist will retort, ' How
do you know that a uniform experience will continue
uniform ? You tell me that the sun has risen for six
thousjand years : that is no proof that it will rise to-
morrow ; within the next twelve hours it may be puffed
out by the Almighty.' Taking this ground, a man
may maintain the story of ' Jack and the Beanstalk ' in
the face of all the science in the world. You urge, in
vain, that science has given us all the knowledge of the
universe which we now possess, while spiritualism has
added nothing to that knowledge. The drugged soul
is beyond the reach of reason. It is in vain that im-
postors are exposed, and the special demon cast out.
He has but slightly to change his shape, return to his
house, and find it ' einpty, swept, and garnished/

Since the time when the foregoing remarks were
written I have been more than once among the spirits,
at their own invitation. They do not improve on ac-
quaintance. Surely no baser delusion ever obtained
dominance over the weak mind of man«

Bin) OV THE HBST VOLUlfS.

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