^^A^^^

FRAGMENTS OF
SCIENCE

BY

JOHN TYNDALL, F.R.S.

PART ONE

NEW YORK

P. F. COLLIER & SON

M C M V

5

lor

SCIENCE

'»:Li/s/

PREFACE TO THE SIXTH EDITION

To AVOID unwieldiness of bulk this edition of the
*' Fragments" is published in two volumes, instead of,
as heretofore, in one.

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

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

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

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

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

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CONTENTS

FRAGMENTS OF SCIENCE

VOLUME ONE

OBAP. PA€m

I. The Constitution of Nature 7

II. Radiation 83

III. On Rajjiant Heat in Relation to the Colo.r and

Chemical Constitution op Bodies .... 80

rv. New Chemical Reactions Produced by Light . . . 103

V. The Sky 140

VI. Voyage to Algeria to Observe the Eclipse . , . 153

VII. NLA.GARA 187

VIII. The Parallel Roads of Glen Roy 218

IX. Alpine Sculpture 243

X. Recent Experiments on Fog-Signals . • . . 268

XI. On the Study of Physics 297

XII. On Crystalline and Slaty Cleavage , . • .331

XIII. On Paramagnetic and Diamagnetic Forces . . .338

XIV. Physical Basis of Solar Chemistry 347

XV. Elementary Magnetism .. ^ ..... 362

XVI. On Force 389

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6

CONTENTS

CHAP.

XVII. Contributions to Molecular Physics
XVIII. Life and Letters of Faraday
XIX. The Copley Medalist of 1870
XX. The Copley Medalist of 1871
XXI. Death by Lightning .
XXII. Science and the "Spirits" •

PAOB

. 407

. 420

. 444

e 451

. 462

. 467

FRAGMENTS OF SCIENCE

INORGANIC NATURE

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 re-
mains. 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 inces-
santly shot through space." The reply of modern science
is also negative, but on different grounds. It has the best
possible reasons for rejecting the idea of luminiferous par-
ticles; but, in support of the conclusion that the celestial

» * 'Fortnightly Review," 1865, vol. iii., p. 129.

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8 FRAG31ENTS OF SCIENCE

spaces are occupied by matter, it is able to offer proofs
almost as cogent as those wbicb. can be adduced of the
existence of an atmosphere round the earth. Men's minds,
indeed, rose to a conception of the celestial and universal
atmosphere through the study of the terrestrial and local
one. From the phenomena of sound, as displayed in the
air, they ascended to the phenomena of light, as displayed
in the 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 phenom-
ena. 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 dis-
tant visible stars. In fact it is the vehicle of their light,
and without it they could not be seen. This all-pervad-
ing substance takes up their molecular tremors, and con-
veys 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 splendor 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

THE CONSTITUTION OF NATURE 9

barn; there, under proper conditions, combustion might
be carried on; fuel might consume unseen, and metals be
fused in invisible fires. A body, moreover, once heated
there, would continue forever heated; a sun or planet
once molten, would continue forever molten. For, the
loss of heat being simply the abstraction 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 dis-
turbance would be met by stronger and stronger waves.
This gradual augmentation of the impression made upon
the wader is exactly analogous to the augmentation of
light when we approach a luminous source. In the one
case, however, the coarse common nerves of the body suf-
fice; for the other we must have the finer optic nerve.
But suppose the water withdrawn; the action at a dis-
tance 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 prop-
agation of waves through the liquid is mechanically similar
to the propagation of light and radiant heat.

As far as our knowledge of space extends, we are to

10 FRAGMENTS OF SCIENCE

conceive it as tlie 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 round it, each again
rotating on its own axis; and, finally, associated with some
of these planets we have dark bodies of minor note — the
moons. Whether the other fixed stars have similar plan-
etary companions or not is to us a matter of pure con-
jecture, 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
inquiry whether things were so created at the beginning.
Was space furnished at once, by the fiat of Omnipotence,
with these burning orbs ? In presence of the revelations
of science this view is fading more and more. Behind the
orbs we now discern the nebulae from which they have
been condensed. And without going so far back as the
nebulae, the man of science can prove that out of common
non-luminous matter this whole pomp of stars might have
been evolved.

The law of gravitation enunciated by Newton is, that
every particle of matter in the universe attracts every
other particle with a force which diminishes as the square
of the distance increases. Thus the sun and the earth
mutually pull each other; thus the earth and the moon
are kept in company; the force which holds every re-
spective pair of masses together being the integrated force

THE CONSTITUTION OF NATURE 11

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 atmos-
phere and rise to incandescence. Showers of such meteors
doubtless fall incessantly upon the sun. Acted on by this
force, the earth, w^ere it stopped in its orbit to-morrow,
would rush toward, and finally combine with, the sun.
Heat would also be developed by this collision. Mayer
first, and Helmholtz and Thomson afterward, have calcu-
lated its amount. It would equal that produced by the
combustion of more than 6,000 worlds of solid coal, all
this heat being generated at the instant of collision. In
the attraction of gravity, therefore, acting upon non-lumi-
nous matter, we have a source of heat more powerful than
could be derived from any terrestrial combustion. And
were the matter of the universe thrown in cold detached
fragments into space, and there abandoned to the mutual
gravitation of its own parts, the collision of the fragments
would in the end produce the fires of the stars.

The action of gravity upon matter originally cold may,
in fact, be the origin of all light and heat, and also the
proximate source of such other powers as are generated
by light and heat. But we have now to inquire what is
the light and what is the heat thus produced? This ques-
tion 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 mo-
tion, taken up by the ether, and propagated through it
with a velocity of 186,000 miles a second, that comes to
ns as the light and heat of suns and stars. The atoms

12 FRAGMENTS Oil' SCIENCE

of a hot body swing with inconceivable rapidity — billions
of times in a second — but tliis power of vibration neces-
sarily implies the operation of forces between the atoms
themselves. It reveals to us that while they are held to-
gether by one force, they are kept asunder by another,
their position at any moment depending on the equilib-
rium of attraction and repulsion. The atoms beliave as
if connected by elastic springs, which oppose at the same
time their approach and their retreat, but which tolerate
the vibration called heat. The molecular vibration once
set up is instantly shared with the 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 inces-
santly forth. It is the waste of its molecular motion in
space that chills the earth upon a clear night; it is the
return of thermal motion from the clouds which prevents
the earth's temperature, on a cloudy night, from falling
so low. To the conception of space being filled we must
therefore add the conception of its being in a state of in-
cessant tremor.

The sources of this vibration are the ponderable masses
of the universe. Let us take a sample of these and exam-
ine it in detail. When we look to our planet, we find it
to be an aggregate of solids, liquids, and gases. Sub-
jected to a sujBSiciently 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 elemen-

THE CONSTITUTION OF NATURE 13

tarj 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 analyze the water, and recover from it its
two constituents. So, likewise, as regards the solid por-
tions of the earth. Our chalk hills, for example, are
formed by a combination of carbon, oxygen, and calcium.
These are the so-called elements the union of which, in
definite proportions, has resulted in the formation of
chalk. The flints within the chalk we know to be a com-
pound of oxygen and silicium, called silica ; and our ordi-
nary clay is, for the most part, formed by the union of
silicium, oxygen, and the well-known light metal, alu-
minium. By far the greater portion of the earth's crust
is compounded of the elementary substances mentioned in
these few lines.

The principle of gravitation has been already described
as an attraction which every particle of matter, however
small, exerts on every other particle. With gravity there
is no selection; no particular atoms choose, by preference,
other particular atoms as objects of attraction; the attrac-
tion of gravitation is proportional simply to the quantity
of the attracting matter, regardless of its quality. But in
the molecular world which we have now entered matters
are otherwise arranged. Here we have atoms between
which a strong attraction is exercised, and also atoms be-
tween 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 ex-
erted varies thus from atom to atom, it is still an attrac*
tion of the same mechanical quality, if I may use the term,

14 FRAGMENTS OF SCIENCE

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
82 ; because, when acting freely on a body for a second of
time, gravity imparts to the body a velocity of thirty-two
feet a second. In like manner the mutual attraction of
oxygen and hydrogen might be measured by the velocity
imparted to the atoms in their rushing together. Of course
such a unit of time as a second is not here to be thought
of, the whole interval required by the atoms to cross the
minute spaces which separate them amounting only to an
inconceivably small fraction of a second.

It has been stated that when a body falls to the earth
it is warmed by the shock. Here, to use the terminology
of Mayer, we have a mechanical combination of the earth
and the body. Let us suffer the falling body and the
earth to dwindle in imagination to the size of atoms, and
for the attraction of gravity let us substitute that of chem-
ical affinity; we have then what is called a chemical com-
bination. The effect of the union in this case also is the
development of heat, and from the amount of heat gen-
erated we can infer the intensity of the atomic pull. Meas-
ured 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 com-
bine, their atoms rushing over the llutle 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.

THE CONSTITUTION OF NATURE 15

It is sometimes stated tliat gravity is distinguislied from
all other forces by the fact of its resisting conversion into
other forms of force. Chemical affinity, it is said, can be
converted into heat and light, and these again into mag-
netism and electricity: but gravity refuses to be so con-
verted; being a force maintaining itself under all circum-
stances, and not capable of disappearing to give place to
another. The statement arises from vagueness of thought.
If by it be meant that a particle of matter can never be
deprived of its weight, the assertion is correct; but the
law which affirms the convertibility of natural forces was
never intended, in the minds of those who understood it,
to affirm that such a conversion as that here implied oc-
curs 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 inde-
structible as in the other. Nobody affirms that when a
stone rests upon the surface of the earth, the mutual at-
traction of the earth and stone is abolished; nobody means
to affirm that the mutual attraction of oxygen for hydro-
gen 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 toward
the other. This motion is not heat, nor is the force that
produces it heat. But when the atoms strike and recoil,
the motion of translation is converted into a motion of
vibration, which is heat. The vibration, however, so far
from causing the extinction of the original attraction, is
in part carried on by that attraction. The atoms recoil,
in virtue of the elastic force which opposes actual contact,
and in the recoil they are driven too far back. The orig-

16 FRAGMENTS OF SCIENCE

inal attraction then triumplis over the force of recoil, and
urges the atoms once more together. Thus, like a pen-
dulum, they oscillate, until their motion is imparted to the
surrounding ether; or, in other words, until their heat
becomes radiant heat.

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

It was formerly universally supposed that by the 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 mo-
tion possessed by the masses, prior to impact, was more
or less annihilated. They believed in an absolute destruc-
tion of the force of impact. Until recent times, indeed,
no difficulty was experienced in believing this, whereas,
at present, the ideas of force and its destruction refuse to
be united in most philosophic minds. In the collision of
elastic bodies, on the contrary, it was observed that the
motion with which they clashed together was in great
part restored by the resiliency of the masses, the more

THE CONSTITUTION OF NATURE 17

perfect the elasticity tlie more complete being the restitu-
tion. This led to the idea of perfectly elastic bodies —
bodies competent to restore by their recoil the whole of
the motion which they possessed before impact — and this
again to the idea of the conservation of force, as opposed
to that destruction of force which was supposed to occur
when unelastic bodies met in collision.

We now know that the principle of conservation holds
equally good with elastic and unelastic bodies. Perfectly
elastic bodies would develop no heat on collision. They
would retain their motion afterward, though its direction
might be changed^ and it is only when sensible motion
is wholly or partly destroyed that heat is generated. This
always occurs in unelastic collision, the heat developed
being the exact equivalent of the sensible motion extin-
guished. 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 princi-
ple 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 toward the earth. We applied the term force also
to that molecular attraction which we called chemical affin-
ity. 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 measurCj

18 FRAGMENTS OF SCIENCE

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

But a body may be caused to reach a certain elevation
in opposition to the force of gravity, without being actu-
ally 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 pre-
cisely the same as if he had slowly carried up the brick.
The initial velocity to be imparted, in this case, is well
known. To reach a height of sixteen feet, the brick must
quit the man's hand with a velocity of thirty-two feet a
second. It is needless to say, that a body starting with
any velocity, would, if wholly unopposed or unaided, con-
tinue to move forever with the same velocity. But when,
as in the case before us, the body is thrown upward, it

THE CONSTITUTION OF NATURE 19

moves in opposition to gravity, which, incessantly retards
its motion, and finally brings it to rest at an elevation of
sixteen feet. If not here canght 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 compe-
tent to impart to the brick, at starting, a velocity of sixty-
four feet a second, or twice its former velocity, would the
amount of work performed be twice what it was in the
first instance ? No ; it would be four times that quantity ;
for a body starting with twice the velocity of another will
rise to four times the height. In like manner, a three-
fold velocity will give a nine-fold elevation, a four-fold
velocity will give a sixteen-fold elevation, and so on.
The height attained, then, is not proportional to the ini-
tial velocity, but to the square of the velocity. As before,
the work is also proportional to the weight elevated.
Hence the work which any moving mass whatever is com-
petent to perform, in virtue of the motion which it at any
moment possesses, is jointly proportional to its weight and
the square of its velocity. Here, then, we have a second
measure of work, in which we simply translate the idea
of height into its equivalent idea of motion.

In mechanics, the product of the mass of a moving
body into the square of its velocity, expresses what is
called the vis viva^ or living force. It is also sometimes
called the ** mechanical effect." If, for example, a cannon
pointed to the zenith urge a ball upward with twice the
velocity imparted to a second ball, the former will rise to
four times the height attained by the latter. If directed

20 FRAGMENTS OF SCIENCE

against a target, it will also do four times the execution.
Hence the importance of imparting a high velocity to pro-
jectiles in war. Having thus cleared our way to a per-
fectly definite conception of the vis viva of moving masses,
we are prepared for the announcement that the heat gen-
erated by the shock of a falling body against the earth is
proportional to the vis viva annihilated. The heat is pro-
portional to the square of the velocity. In the case, there-
fore, of two cannon-balls of equal weight, if one strike
a target with twice the velocity of the other, it will gen-
erate four times the heat, if with three times the velocity,
it will generate nine times the heat, and so on.

Mr. Joule has shown that a pound weight falling from
a height of 772 feet, or 772 pounds falling through one
foot, will generate by its collision with the earth an
amount of heat sufficient to raise a pound of water one
degree Fahrenheit in temperature. 772 "foot-pounds'*
constitute the mechanical equivalent of heat. Now, a body
falling from a height of 772 feet, has, upon striking the
earth, a velocity of 223 feet a second; and if this velocity
were imparted to the body, by any other means, the quan-
tity 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, gen-
erate an amount of heat competent to raise its own weight
of water 36 degrees Fahrenheit in temperature. If com-
posed of iron, and if all the heat generated were concen-
trated 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

THE CONSTITUTION OF NATURE 21

case of iron. In artillery practice, the heat generated is
usually concentrated upon the front of the bolt, and on
the portion of the target first struck. By this concentra-
tion 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 burned 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 equiva-
lent 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 con-
nected.

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

22 FRAGMENTS OF SCIENCE

For the sake of simplicity we will consider a particle
of matter, wliicli we may call F, to be perfectly fixed, and
a second movable particle, D, placed at a distance from F.
We will assume that these two particles attract each other
according to the Newtonian law. At a certain distance,
the attraction is of a certain definite amount, which might
be determined by means of a spring balance. At half this
distance the attraction would be augmented four times; at
a third of the distance, nine times; at one-fourth of the
distance, sixteen times, and so on. In every case, the at-
traction might be measured by determining, with the
spring balance, the amount of tension just sufficient to
prevent D from moving toward F. Thus far we have noth-
ing whatever to do with motion ; we deal with statics, not
with dynamics. We simply take into account the distance
of D from F, and the pull exerted by gravity at that
distance.

It is customary in mechanics to represent the 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 D to F, and at D erect a perpendicular to this line
which shall represent the amount of the attraction exerted
on D. If D be at a very great distance from F, the attrac-
tion will be very small, and the perpendicular consequently
very short. If the distance be practically infinite, the at-
traction 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 perpen-
diculars, of gradually increasing length, as D approaches F.

THE CONSTITUTION OF NATURE 23

Uniting the ends of all these perpendiculars, we obtain a
curve, and between this curve and the straight line join-
ing F and D we have an area containing all the perpen-
diculars placed side by side. Each one of this infinite
series of perpendiculars representing an attraction, or ten-
sion, as it is sometimes called, the area just referred to
represents the sum of the tensions exerted upon the par-
ticle 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 in-
finitely 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 to-
ward F the particle D consumes, as it were, the tensions.
Let us fix our attention on D, at any point of the path
over which it is moving. Between that point and F there
is a quantity of unused tensions; beyond that point the
tensions have been all consumed, but we have in their
place an equivalent quantity of vis viva. After d has
passed any point, the tension previously in store at that
point disappears, but not without having added, during
the infinitely small duration of its action, a due amount
of motion to that previously possessed by D. The nearer
D approaches to F, the smaller is the sum of the tensions
remaining, but the greater is the vis viva ; the further D is
from F, the greater is the sum of the unconsumed ten-

24 FRAGMENTS OF SCIENCE

Sions, and the less is the living force. Now the principle
of conservation affirms, not the constancy of the value of
the tensions of gravity, nor yet the constancy of the vis
viva, taken separately, but the absolute constancy of the
value of both taken together. At the beginning the vis
viva was zero, and the tension area was a maximum; close
to F the vis viva is a maximum, while the tension area is
zero. At every other point, the work- producing power
of the particle D consists in part of vis viva, and in part
of tensions.

If gravity, instead of being attraction, were repulsion,
then, with the particles in contact, the sum of the tensions
between D and F would be a maximum, and the vis viva
zero. If, in obedience to the repulsion, D moved away
from F, vis viva would be generated; and the further D
retreated from F the greater would be its vis viva, and the
less the amount of tension still available for producing
motion. Taking repulsion as well as attraction into ac-
count, the principle of the conservation of force affirms
that the mechanical value of the tensions and vires vivce of
the material universe, so far as we know it, is a constant
quantity. The universe, in short, possesses two kinds of
property which are mutually convertible. The diminution
of either carries with it the enhancement of the other, the
total valae 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 applica-
tion of proper means they may be caused to rush together
across the space that separates them. While this space
exists, and as long as the atoms have not begun to move
toward each other, we have tensions and nothing else.

THE CONSTITUTION OF NATURE TO

During their motion toward each other the tensions, as
in the case of gravity, are converted into vis viva. After
they clash we have still vis viva, but in another form. It
was translation, it is vibration. It was molecular transfer,
it is heat.

It is possible to reverse these processes, to unlock the
combined atoms and replace them in their first positions.
But, to accomplish this, as much heat would be required
as was generated by their union. Such reversals occur
daily and hourly in Nature. By the solar waves, the oxy-
gen 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 hy-
drogen 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 every-
where throughout the air. In the leaves of plants the
sunbeams also wrench the atoms of carbonic acid asun-
der, and sacrifice themselves in the act; but when the
plants are burned, the amount of heat consumed in their
production is restored.

This, then, is the rhythniic play of Nature as regards

her forces. Throughout all her regions she oscillates from

tension to vis viva^ from vis viva to tension. We have the

same play in the planetary system. The earth's orbit is

an ellipse, one of the foci of which is occupied by the

sun. Imagine the earth at the most distant part of the

orbit. Her motion, and consequently her vis viva, is then

a minimum. The planet rounds the curve, and begins its

Science 2

£6 FRAGMENTS OF SCIENCE

approach to the sun. In front it has a store of tensions,
which are gradually consumed, an equivalent amount of
vis viva being generated. When nearest to the sun the
motion, and consequently the vis viva, reach a maximum.
But here the available tensions have been used up. The
earth rounds this portion of the curve and retreats from
the sun. Tensions are now stored up, but vis viva is lost,
to be again restored at the expense of the complementary
force on the opposite side of the curve. Thus beats the
heart of the universe, but without increase or diminution
of its total stock of force.

I have thus far tried to steer clear amid confusion, by
fixing the mind of the reader upon things rather than
upon names. But good names are essential; and here, as
yet, we are not provided with such. We have had the
force of gravity and living force — two utterly distinct
things. We have had pulls and tensions; and we might
have had the force of heat, the force of light, the force of
magnetism, or the force of electricity — all of which terms
have been employed more or less loosely by writers on
physics. This confusion is happily avoided by the intro-
duction of the term "energy," which embraces both ten-
sion and vis viva. Energy is possessed by bodies already
in motion; it is then actual, and we agree to call it actual
or dynamic energy. It is our old vis viva. On the other
hand, energy is possible to bodies not in motion, but
which, in virtue of attraction or repulsion, possess a power
of motion which would realize itself if all hindrances were
removed. Looking, for example, at gravity; a body on
the earth's surface in a position from which it cannot fall
to a lower one possesses no energy. It has neither mo-
tion nor power of motion. But the same body suspended

THE CONSTITUTION OF NATURE 27

at a "heiglit above the eartli 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 con-
sists of our old tensions. We, moreover, speak of the
conservation of energy, instead of the conservation of
force; and say that the sum of the potential and dynamic
energies of the material universe is a constant quantity.

A body cast upward consumes the actual energy of
projection, and lays up potential energy. When it reaches
its utmost height all its actual energy is consumed, its po-
tential energy being then a maximum. When it returns,
there is a reconversion of the potential into the actual. A
pendulum at the limit of its swing possesses potential en-
ergy; at the lowest point of its arc its energy is all actual.
A patch of snow resting on a mountain slope has poten-
tial energy; loosened, and shooting down as an avalanche,
it possesses dynamic energy. The pine-trees growing on
the Alps have potential energy; but rushing down the
Hohrinne 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 ac-
tual when the frost ruptures their cohesion and hands
them over to the action of gravity. The stone avalanches
of the Matterhorn and Weisshorn are illustrations in point
The hammer of the great bell of Westminster, when raised
before striking, possesses potential energy; when it falls,
the energy becomes dynamic; and, after the stroke, we
have the rhythmic play of potential and dynamic in the
vibrations of the bell. The same holds good for the mo-
lecular oscillations of a heated body. An atom is driven
against its neighbor, and recoils. The ultimate amplitude

28 FRAGMENTS OF SCIENCE

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 toward its neighbor
with accelerated speed; thus, by attraction, converting its
potential into dynamic energy. Its motion in this direc-
tion is also finally checked, and again, for an instant, its
energy is all potential. It once more retreats, converting,
by repulsion, its potential into dynamic energy, till the
latter attains a maximum, after which it is again changed
into potential energy. Thus, what is true of the earth, as
she swings to and fro in her yearly journey round the
sun, is also true of her minutest atom. "We have wheels
within wheels, and rhythm within rhythm.

When a body is heated, a change of molecular arrange-
ment always occurs, and to produce this change heat is
consumed. Hence, a portion only of the heat communi-
cated 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 ex-
ample, 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 dy-
namic energy destroyed. The true equivalent is this heat,
plus the potential energy conferred upon the molecules
by the placing of greater distances between them. This
molecular potential energy is afterward, on the cooling
of the body, converted into heat.

Wherever two atoms capable of uniting together by
their mutual attractions exist separately, they form a store
of potential energy. Thus our woods, forests, and coal-
fields on the one hand, and our atmospheric oxygen on

THE CONSTITUTION OF NATURE ^ 29

the other, constitute a vast store of energy of this kind —
vast, but far from infinite. We have, besides our coal-
fields, metallic bodies more or less sparsely distributed
through the earth's crust. These bodies can be oxi-
dized; and hence they are, so far as they go, stores of
energy. But the attractions of the great mass of the
earth's crust are already satisfied, and from them no
further energy can possibly be obtained. Ages ago the
elementary constituents of our rocks clashed together and
produced the motion of heat, which was taken up by the
ether and carried away through stellar space. It is lost
forever 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 constit-
uents of the earth's crust. They took their present form
in obedience to molecular force; they turned their poten-
tial energy into dynamic, and yielded it as radiant heat
to the universe, ages before man appeared upon this
planet. For him a residue of potential energy remains,
vast, truly, in relation to the life and wants of an individ-
ual, but exceedingly minute in comparison with the earth's
primitive store.

To sum up. The whole stock of energy or working-
power in the world consists of attractions, repulsions, and
motions. If the attractions and repulsions be so circum-
stanced as to be able to produce motion, they are sources
of working-power, but not otherwise. As stated a mo-
ment 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

30 FRAGMENTS OF SCIENCE

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 further. But though it has
ceased to be a source of energy, the attraction of gravity
still acts as a force, which holds the earth and weight
together.

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

If this be understood, it will be at once seen that
gravity, as before insisted on, may, in this sense, be said
to be convertible into heat; that it is in reality no more
an outstanding and inconvertible agent, as it is sometimes
stated to be, than is chemical affinity. By the exertion of
a certain pull through a certain space, a body is caused

THE CONSTITUTION OF NATURE 31

to clasli with a certain definite velocity against tlie earth.
Heat is thereby developed, and this is the only sense in
which gravity can be said to be converted into heat. In
no case is the force which produces the motion annihilated
or changed into anything else. The mutual attraction of
the earth and weight exists when they are in contact, as
when they were separate; but the ability of that attrac-
tion 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 con-
sumed. 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 attrac-
tion has conferred upon it a power of exerting a certain
definite pull, between the starting-point of the falling
weight and the earth.

When, therefore, writers on the conservation of energy
speak of tensions being *' consumed" and ''generated,'*
they do not mean thereby that old attractions have been
annihilated, and new ones brought into existence, but that,
in the one case, the powei of the attraction to produce
motion has been diminished by the shortening of the dis-

S2 FRAGMENTS OF SCIENCE

tance between the attracting bodies, wbile, in tbe otber
case, the power of producing motion has been augmented
by the increase of the distance. These remarks apply to
all bodies, whether they be sensible masses or molecules.
Of the inner quality that enables matter to attract mat-
ter we know nothing; and the law of conservation makes
no statement regarding that quality. It takes the facts of
attraction as they stand, and affirms only the constancy
of working- power. That power may exist in the form of
motion; or it may exist in the form of force, with dis-
tance to act through. The former is dynamic energy, the
latter is potential energy, the constancy of the sum of both
being affirmed by the law of conservation. The converti-
bility of natural forces consists solely in transformations
of dynamic into potential, and of potential into dynamic,
energy. In no other sense has the convertibility of force
any scientific meaning.

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

n

RADIATION*

1. Visible and Invisible Radiation

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

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

* The Rede Lecture delivered in the Senate House before the Universitj of
Cambridge, May 16, 1865.

(88)

84 FRAGMENTS OF SCIENCE

Kor does the optic nerve embrace tlie entire range even
of radiation. Some rays, when they reach it, are incom-
petent to evoke its power, while others never reach it at
all, being absorbed by the humors 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
afl'ect the optic nerve a certain temperature is necessary.
A cool poker thrust into a fire remains dark for a time,
but when its temperature has become equal to that of the
surrounding coals, it glows like them. In like manner, if
a current of electricity, of gradually increasing strength,
be sent through a wire of the refractory metal platinum,
the wire first becomes sensibly warm to the touch; for a
time its heat augments, still, however, remaining obscure;
at length we can no longer touch the metal with impu-
nity; and at a certain definite temperature it emits a fee-
ble red light. As the current augments in power the
light augments in brilliancy, until finally the wire ap-
pears of a dazzling white. The light which it now emits
is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the
texture of solar light, and by the same simple instrument
we can investigate the luminous changes of our platinum
wire. In passing through the prism all its rays (and they
are infinite in variety) are bent or refracted from their
straight course; and, as different rays are differently re-
fracted by the prism, we are by it enabled to separate one
class of rays from another. By such prismatic analysis
Dr. Draper has shown, that when the platinum wire first

RADIATION 35

begins to glow the light emitted is sensibly red. As tbe
glow augments tlie red becomes more brilliant, but at the
same time orange rajs are added to tbe emission. Aug-
menting 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 colors at the same time
the platinum wire must be white-hot: the impression of
whiteness being in fact produced by the simultaneous
action of all these colors on the optic nerve.

In the experiment just described we began with a pla-
tinum 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 be-
comes 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 incan-
descence, 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-lumin ^iis
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

86 FRAGMENTS OF SCIENCE

which the dark earth pours nightly into space. In fact,
the various kinds of obscure rays emitted by all the plan-
ets of our system are included in the present radiation of
the sun.

The great pioneer in this domain of science was Sir
William Herschel. Causing a beam of solar light to pass
through a prism, he resolved it into its colored constitu-
ents; he formed what is technically called the solar spec-
trum. Exposing thermometers to the successive colors, he
determined their heating power, and found it to augment
from the violet or most refracted end, to the red or least
refracted end of the spectrum. But he did not stop here.
Pushing his thermometers into the dark space beyond the
red he found that, though the light had disappeared, the
radiant heat falling on the instruments was more intense
than that at any visible part of the spectrum. In fact, Sir
William Herschel showed, and his results have been veri-
fied 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 refran-
gible end. Ritter discovered the extension of the spectrum
into the invisible region beyond the violet; and, in recent
times, this ultra-violet emission has had peculiar interest
conferred upon it by the admirable researches of Professor
Stokes. The complete spectrum of the sun consists, there-
fore, of three distinct parts: first, of ultra-red rays of high
heating power, but unsuited to the purposes of vision;
secondly, of luminous rays which display the succession

RADIATION m

of colors, red, orange, yellow, green, blue, indigo, violet;
thirdly, of ultra-violet rays which, like the ultra- red ones,
are incompetent to excite vision, but which, unlike the
ultra- red rays, possess a very feeble heating power. In
consequence, however, of their chemical energy these ultra-
violet rays are of the utmost importance to the organic
world.

2. Origin and Character of Radiation, The Ether

When we see a platinum wire raised gradually to a
white heat, and emitting in succession all the colors 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 colors originate, but the mind
irresistibly infers that the appearance of the colors corre-
sponds to certain contemporaneous changes in the wire.
What is the nature of these changes? In virtue of what
condition does the wire radiate at all? We must now
look from the wire, as a whole, to its constituent atoms.
Could we see those atoms, even before the electric current
has begun to act upon them, we should find them in a
state of vibration. In this vibration, indeed, consists such
warmth as the wire then possesses. Locke enunciated this
idea with great precision, and it has been placed beyond
the pale of doubt by the excellent quantitative researches
of Mr. Joule. *'Heat," says Locke, '*is a very brisk agi-
tation 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 noth-
ing 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

88 FRAGMENTS OF SCIENCE

swing tliroTigli wider ranges. Teclinically speaking, tlie
amplitudes of tlie oscillations are increased. The current
does this, however, without altering the periods of the old
vibrations, or the times in which thej were executed. But
besides intensifying the old vibrations the current gener-
ates new and more rapid ones, and when a certain definite
rapidity has been attained, the wire begins to glow. The
color first exhibited is red, which corresponds to the low-
est rate of vibration of which the eye is able to take cog-
nizance. By augmenting the strength of the electric cur-
rent 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 rapid-
ity, we pass through blue, indigo, and violet, to the ex-
treme ultra-violet rays.

Such are the changes recognized by the mind in the
wire itself, as concurrent with the visual changes taking
place in the eye. But what connects the wire with this
organ ? By what means does it send such intelligence of
its varying condition to the optic nerve? Heat being, as
defined by Locke, **a very brisk agitation of the 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 the
hot platinum, and hence the pertinence of the question,
By what agency are the vibrations of the wire transmitted
to the eye ?

The answer to this question involves one of the most
important physical conceptions that the mind of man has
yet achieved : the conception of a medium filling space and
fitted mechanically for the transmissioa of the vibrationa

RADIATION 89

of light and "heat, as air is fitted for the transmission of
fiound. 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 a second. The ether suffers no rupture
of continuity at the surface of the eye, the inter-molecular
spaces of the various humors are filled with it; hence the
waves generated by the glowing platinum can cross these
humors and impinge on the optic nerve at the back of the
eye. * Thus the sensation of light reduces itself to the ac-
ceptance 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 sys-
tems of waves, which, speeding from the centre of dis-
turbance, 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 proc-
ess which the science of mechanics does not even tend to
unravel, the tremor of the nervous matter is converted into
the conscious impression of light.

Darkness might then be defined as 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

* The action here described is analogous to the passage of sound-waves
through thick feh whose interstices are occupied bj i^.

40 FRAGMENTS OF SCIENCE

ultimate extinction. Every star is seen across the entan-
glement of wave -motions produced by all other stars. It
is the ceaseless thrill caused by those distant orbs collec-
tively in the ether that constitutes what we call the "tem-
perature of space.'' As the air of a room accommodates
itseK to the requirements of an orchestra, transmitting each
vibration of every pipe and string, so does the interstellar
ether accommodate itseK to the requirements of light and
heat. Its waves mingle in space without disorder, each
being endowed with an individuality as indestructible as
if it alone had disturbed the universal repose.

AIL vagueness with regard to the use of the terms
**radiation" and "absorption" will now disappear. Kadi-
ation is the communication of vibratory motion to the
ether; and when a body is said to be chilled by radia-
tion, as for example the grass of a meadow on a starlight
night, the meaning is, that the molecules of the grass have
lost a portion of their motion, by imparting it to the me-
dium in which they vibrate. On the other hand, the waves
of ether may so strike against the molecules of a body ex-
posed 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 phenomena of heat are in this way reducible to
interchanges of motion; and it is purely as the recipients
or the donors of this motion that we ourselves become
conscious of the action of heat and cold.

8. The Atomic Theory in reference to the Ether

The word "atoms" has been more than once employed
in this discourse. Chemists have taught us that all matter
is reducible to certain elementary forms to which they give

RADIATION il

t"his name. These atoms are endowed with powers of mu-
tual attraction, and under suitable circumstances they coa-
lesce to form compounds. Thus oxygen and hydrogen are
elements when separate, or merely mixed, but they may be
made to combine so as to form molecules, each consisting
of two atoms of hydrogen and one of oxygen. In this con-
dition 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. Pict-
uring in imagination the atoms of elementary bodies as
little spheres, the molecules of compound bodies must be
pictured as groups of such spheres. This is the atomic
theory as Dalton conceived it. Now, if this theory have
any foundation in fact, and if the theory of an ether per-
vading space, and constituting the vehicle of atomic mo-
tion, be founded in fact, it is surely of interest to examine
whether the vibrations of elementary bodies are modified
by the act of combination — whether as regards radiation
and absorption, or, in other words, whether as regards the
communication of motion to the ether, and the acceptance
of motion from it, the deportment of the uncombined atoms
will be different from that of the combined.

4. Absorption of Radiant Heat by Oases

We have now to submit these considerations to the
only test by which they can be tried, namely, that of
experiment. An experiment is well defined as a question
put to Nature; but, to avoid the risk of asking amiss, we
ought to purify the question from all adjuncts which do

42 FRAGMENTS OF SCIENCE

not necessarily belong to it. Matter has been shown to be
composed of elementary constituents, by the compounding
of which all its varieties are produced. But, besides the
chemical unions which they form, both elementary and
compound bodies can unite in another and less intimate
way. Gases and vapors 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 lib-
erating the atoms and molecules entirely from the bonds
of cohesion, and employing them in the gaseous or vapor-
ous form.

Let us endeavor to obtain a perfectly clear mental im-
age of the problem now before us. Limiting in the first
place our inquiries to the phenomena of absorption, we
have to picture a succession of waves issuing from a radi-
ant source and passing through a gas; some of them strik-
ing 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 ap-
parent 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 mole-
cules 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 emerg-
ing from the copper, the waves, in the first instance, pass
through a space devoid of air, and then enter a hollow

RADIATION 43

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 sensi-
ble obstacle to the passage of the calorific waves. After
passing through the tube, the radiant heat falls upon the
anterior face of a thermo-electric pile,* which instantly
converts the heat into an electric current. This current
conducted round a magnetic needle deflects it, and the
magnitude of the deflection is a measure of the heat fall-
ing upon the pile. This famous instrument, and not an
ordinary thermometer, is what we shall use in these in-
quiries, but we shall use it in a somewhat novel way. As
long as the two opposite faces of the thermo-electric pile
are kept at the same temperature, no matter how high that
may be, there is no current generated. The current is a
consequence of a difference of temperature between the
two opposite faces of the pile. Hence, if after the ante-
rior 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 neutralize the former.
"When the neutralization 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 act-
ing at the same time on the two opposite faces of the pile.
When, by means of an adjusting screen, perfectly equal

* In the Appendix to the first chapter of "Heat as a Mode of Motion," the
construction of the thermo-electric pile is fully explained.

44

FRAGMENTS OF SCIENCE

qnantities 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 in-
tercepting the calorific waves, the equilibrium previously
existing will be destroyed, the compensating source will
triumph, and a deflection of the magnetic needle will be
the immediate consequence. From the deflections thus
produced by different gases we can readily deduce the
relative amounts of wave-motion which their molecules
intercept.

In this way the substances mentioned in the following
table were examined, a small portion only of each being
admitted into the glass tube. The quantity admitted in
each case was just sufficient to depress a column of mer-
cury 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 respec-
tive 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

Relatire
absorptioL

1

1

1

1
■750
97S
1,005
1,590
1,860
2,100
5,460
6,030
6,480

RADIATION 45

Every gas in this table is perfectly transparent to light,
that is to say, all waves within the limits of the visible
spectrum pass through it without obstruction; but for the
waves of slower period, emanating from our heated plate
of copper, enormous differences of absorptive power are
manifested. These differences illustrate in the most un-
expected manner the influence of chemical combination.
Thus the elementary gases, oxygen, hydrogen, and nitro-
gen, and the mixture atmospheric air, prove to be prac-
tical vacua to the rays of heat; for every ray, or, more
strictly speaking, for every unit of wave -motion, which
any one of them intercepts, perfectly transparent ammonia
intercepts 5,460 units, olefiant gas 6,030 units, while sul-
phurous 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, hy-
drogen, and nitrogen, the waves of ether pass without ab-
sorption, and these gases are not sensibly changed in tem-
perature by the most powerful calorific rays. The position
of nitrous oxide in the foregoing table is worthy of par-
ticular notice. In this gas we have the same atoms in a
state of chemical union that exist uncombined in the at-
mosphere; but the absorption of the compound is 1,800
times that of air.

5. Formation of Invisible Foci

This extraordinary deportment of the elementary gases
naturally directed attention to elementary bodies in other
states of aggregation. Some of Melloni's results now at-
tained 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

46 FRAGMENTS OF SCIENCE

black glass (which owes its blackness to the element car-
bon), were to a considerable extent transparent to calorific
rays of low refrangibility. These facts, harmonizing so
strikingly with the deportment of the simple gases, sug-
gested further inquiry. Sulphur dissolved in bisulphide
of carbon was found almost perfectly diathermic. The
dense and deeply- colored element bromine was examined,
and found competent to cut off the light of our most bril-
liant 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 impracti-
cable to examine the substance in its usual solid condi-
tion. It, however, dissolves freely in bisulphide of car-
bon. There is no chemical union between the liquid and
the iodine; it is simply a case of solution, in which the
uncombined atoms of the element can act upon the radi-
ant 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 con-
centration. Intercepting the luminous portion of his spec-
trum, 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

' Professor Dewar lias recently succeeded in producing a medium highly-
opaque to light, and highly transparent to obscure heat, by fusing together
sulphur and iodine.

RADIATION 47

renders attainable far more powerful foci of invisible rays
than could possibly be obtained by the method of Sir Wil-
liam 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, lumi-
nous and non-luminous, we can intercept the former by the
iodine, and do what we please with the latter. Experi-
ments of this character, not only with the iodine solution,
but also with black glass and layers of lamp-black, were
publicly performed at the Eoyal Institution in the early
part of 1862, and the effects at the foci of invisible rays,
then obtained, were such as had never been witnessed
previously.

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

% FRAGMENTS OF SCIENCE

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 Hays of the Electric Light

We have next to examine what proportion the non-
luminous rays of the electric light bear to the luminous
ones. This the opaque solution of iodine enables us to
do with an extremely close approximation to the truth.
The pure bisulphide of carbon, which is the solvent of
the iodine, is perfectly transparent to the luminous, and
almost perfectly transparent to the dark, rays of the elec-
tric 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 ob-
tain the value of the purely luminous emission. Experi-
ments, performed in this way, prove that if all the visible
rays of the electric light were converged to a focus of daz-
zling brilliancy, its heat would only be one-eighth of that
produced at the unseen focus of the invisible rays.

Exposing his thermometers to the successive colors 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 repre-
sent 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 per-
pendiculars, he obtained a curve which showed at a glance
the manner in which the heat was distributed in the solar
spectrum. Professor Muller of Freiburg, with improved
instruments, afterward made similar experiments, and con-

RADIATION 49

structed a more accurate diagram of the same kind. We
have now to examine the distribution of heat in the spec-
trum of the electric light; and for this purpose we shall
employ a particular form of the thermo-electric pile, de-
vised by Melloni. Its face is a rectangle, which by means
of movable side-pieces can be rendered as narrow 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 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 toward the red, heat soon manifests itself, aug-
menting as we approach the red. Of all the colors of the
visible spectrum the red possesses the highest heating
power. On pushing the pile into the dark region be-
yond the red, the heat, instead of vanishing, rises sud-
denly and enormously in intensity, until at some dis-
tance beyond the red it attains a maximum. Moving
the pile still forward, the thermal power falls somewhat
more rapidly than it rose. It then gradually shades
away, but, for a distance beyond the red greater than
the length of the whole visible spectrum, signs of heat
may be detected.

Drawing a datum line, and erecting along it perpen-
diculars, proportional in length to the thermal intensity at
the respective points, we obtain the extraordinary curve,

SciENOiJ — —-3

50 FRAGMENTS OF SCIENCE

shown on the opposite page, which exhibits the distribu-
tion of heat in the spectrum of the electric light. In the
region of dark rays, beyond the red, the curve shoots up
to B, in a steep and massive peak — a kind of Matterhorn
of heat, which dwarfs the portion of the diagram c D E,
representing the luminous radiation. Indeed the idea
forced upon the mind by this diagram is that the light
rays are a mere insignificant appendage to the heat rays
represented by the area A B c D, thrown in, as it were, by
nature for the purpose of vision.

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

7. Combustion hy Invisible Rays

The sun's invisible rays far transcend the visible ones
in heating power, so that if the alleged performances of
Archimedes during the siege of Syracuse had any founda-
tion in fact, the dark solar rays would have been the phi-
losopher's chief agents of combustion. On a small scale
we can readily produce, with the purely invisible rays of
the electric light, all that Archimedes is said to have
performed with the sun's total radiation. Placing behind
the electric light a small concave mirror, the rays are

RADIATION

61

52 FRAGMENTS OF SCIENCE

converged, the cone of reflected rays and their point of
convergence being rendered clearly visible by the dust al-
ways floating in the air. Placing between the luminous
focus and the source of rays our solution of iodine, the
light of the cone is entirely cut away; but the intolerable
heat experienced when the hand is placed, even for a mo-
ment, at the dark focus, shows that the calorific rays pass
unimpeded through the opaque solution.

Almost anything that ordinary fire can effect may be
accomplished at the focus of invisible rays; the air at the
focus remaining at the same time perfectly cold, on account
of its transparency to the heat-rays. An air thermometer,
with a hollow rock-salt bulb, would be unaffected by the
heat of the focus: there would be no expansion, and in
the open air there is no convection. The ether at the
focus, and not the air, is the substance in which the heat
is embodied. A block of wood, placed at the focus, ab-
sorbs the heat, and dense volumes of smoke rise swiftly
upward, 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 burned 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 other-
wise tarnish them, so as to diminish their reflective power.
Blackened zinc foil, when brought into the focus of invis-
ible rays, is instantly caused to blaze, and burns with its
peculiar purple light. Magnesium wire flattened, or tar-

RADIATION 53

nislied magnesiiim ribbon, also bursts into flame. Pieces
of charcoal suspended in a receiver full of oxygen are also
set on fire when the invisible focus falls upon tliem; the
dark rays, after having passed through the receiver, still
possessing sufficient power to ignite the charcoal, and thus
initiate the attack of the oxygen. If, instead of being
plunged in oxygen, the charcoal be suspended in vacuo,
it immediately glows at the place where the focus falls.

8. Transmutation of Hays: ^ Calorescence

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

• I borrow this term from Professor Challis, "Philosophical Magazine,**
vol. xii. p. 521.

54 FRAGMENTS OF SCIENCE

upon botTi tlie carbon and tlie metal. Results similar to
tliose obtained with tbe electric light have also been
obtained with the invisible rajs of the lime-light and of
the sun.

Before a Cambridge audience it is hardly necessary
to refer to the excellent researches of Professor Stokes at
the opposite end of the spectrum. The above results con-
stitute a kind of complement to his discoveries. Professor
Stokes named the phenomena which he has discovered
and investigated Fluorescence ; for the new phenomena
here described I have proposed the term Galorescence.
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 refrangibility of the ultra-red rays
is so exalted as to render them visible. Looking through
a prism at the incandescent image of the carbon points,
the light of the image is decomposed, and a complete
spectrum is obtained. The invisible rays of the electric
light, remolded by the atoms of the platinum, shine thus
visibly forth; ultra-red rays being converted into red,
orange, yellow, green, blue, indigo, violet, and ultra-
violet ones. Could we, moreover, raise the original
source of rays to a sufficiently high temperature, we might
not only obtain from the dark rays of such a source a
single incandescent image, but from the dark rays of this
image we might obtain a second one, from the dark rays
of the second a third, and so on — a series of complete
images and spectra being thus extracted from the invisi-
ble emission of the primitive source.*

* On investigating the calorescence produced by rays transmitted through
glasses of various colors, it was found that in the case of certain specimens of

RADIATION 55

9. Deadness of the Optic Nerve to the Calorific Rays

The layer of iodine used in the foregoing experiments
intercepted the rays of the noonday sun. No trace of
light from the electric lamp was visible in the darkest
room, even when a white screen was placed at the focus
of the mirror employed to concentrate the light. It was
thought, however, that if the retina itself were brought
into the focus the sensation of light might be experienced.
The danger of this experiment was twofold. If the dark
rays were absorbed in a high degree by the humors of the
eye the albumen of the humors might coagulate along the
line of the rays. If, on the contrary, no such high ab-
sorption took place, the rays might reach the retina with
a force sufficient to destroy it. To test the likelihood of
these results, experiments were made on water and on a
solution of alum, and they showed it to be very improb-
able that in the brief time requisite for an experiment any
serious damage could be done. The eye was therefore

blue glass, the platinum foil glowed ^yith a pink or purplish light. The effect
was not subjective, and considerations of obvious interest are suggested by it.
Different kinds of black glass differ notably as to their power of transmitting
radiant heat. When thin, some descriptions tint the sun with a greenish hue:
others make it appear a glowing red without any trace of green. The latter are
far more diathermic than the former. In fact, carbon when perfectly dissolved
and incorporated with a good white glass is highly transparent to the calorific
rays, and by employing it as an absorbent the phenomena of "calorescence"
may be obtained, though in a less striking form than with the iodine. The
black glass chosen for thermometers, and intended to absorb completely the
solar heat, may entirely fail in this object, if the glass in which the carbon is
incorporated be colorless. To render the bulb of a thermometer a perfect
absorbent, the glass ought in the first instance to be green. Soon after the
discovery of fluorescence the late Dr. "William Allen Miller pointed to the lime-
Kght 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.

56 FRAGMENTS OF SCIENCE

caused to approacli the dark focus, no defence, in tTie first
instance, being provided; but the beat, acting upon the
parts surrounding tbe 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 afterward by the retina.
Eemoving the eye, but permitting the plate of metal to
remain, a sheet of platinum foil was placed in the posi-
tion occupied by the retina a moment before. The plati-
num became red hot. No sensible damage was done to
the eye by this experiment; no impression of light was
produced; the optic nerve was not even conscious of heat.

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

This subject will repay a moment's further attention.
At a common distance of a foot the visible radiation of

RADIATION 67

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 8d;ooo,ooo of its light at
the distance of a foot. Hence, to make the candle-light
a mile off equal in power to the non-luminous radiation
received from the electric light at a foot distance, its in-
tensity would have to be multiplied by 1,500 X 20,000,000,
or by thirty thousand millions. Thus the thirty thousand
millionth part of the invisible radiation from the electric
light, received by the retina at the distance of a foot,
would, if slightly changed in character, be amply suffi-
cient 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 con-
sonance, 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

* It will be borne in mind that the heat which any ray, luminous or non-
huninous, is competent to generate is the true measure of the energy of
the ray.

58 FRAGMENTS OF SCIENCE

the intensity of the old ones was increased. Thus, in Dr.
Draper's experiments, the rise of temperature that gener-
ated the orange, yellow, green, and blue, augmented the
intensity of the red. What is true of the red is true of
every other ray of the spectrum, visible and invisible.
We cannot indeed see the augmentation of intensity in
the region beyond the red, but we can measure it and ex-
press it numerically.

With this view the following experiment was per-
formed: A spiral of platinum wire was surrounded by
a small glass globe to protect it from currents of air;
through an orifice in the globe the rays could pass from
the spiral and fall afterward upon a thermo-electric pile.
Placing in front of the orifice an opaque solution of
iodine, the platinum was gradually raised from a low,
dark heat to the fullest incandescence, with the follow-
ing results:

Appearance Energy of

of spiral obscure radiation

Dark 1

Dark, but hotter .3

Dark, but still hotter 5

Dark, but still hotter 10

Feeble red 19

Dull red 25

Red 37

FuUred 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 in-
tense white heat, exalts at the same time the energy of the

RADIATION 69

obscure radiation, until at the end it is fully 440 times
what it was at the beginning.

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

Appearance Energy of

of spiral obscure rays

Dark 1

Dark •••..6

Faint red 10

DuUred 13

Eed . . r 18

Full red 27

Orange ,, 60

Yellow 93

White 122

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

60 FRAGMENTS OF SCIENCE

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 oxyhydro-
gen jet, pours out invisible rays with augmented energy, as
its light is increased. The same is true of lime, bricks,
and other substances. It is true of all metals which are
capable of being heated to incandescence. It also holds
good for phosphorus burning in oxygen. Every gush of
dazzling light has associated with it a gush of invisible
radiant heat, which far transcends the light in energy.
This condition of things applies to all bodies capable of
being raised to a white heat, either in the solid or the
molten condition. It would doubtless also apply to the
luminous fogs formed by the condensation of incandescent
vapors. In such cases when the curve representing the
radiant energy of the body is constructed, the obscure ra-
diation towers upward like a mountain, the luminous radi-
ation resembling a mere "spur" at its base. From the
very brightness of the light of some of the fixed stars we
may infer the intensity of that dark radiation, which is
the precursor and inseparable associate of their luminous
rays.

We thus find the luminous radiation appearing when
the radiant body has attained a certain temperature ; or, in
other words, when the vibrating atoms of the body have
attained a certain width of swing. In solid and molten
bodies a certain amplitude cannot be surpassed without
the introduction of periods of vibration, which provoke
the sense of vision. How are we to figure this? If per-
mitted to speculate, we might ask, Are not these more rapid

EADTATION 61

vibrations the progeny of tlie slower ? Is it not really tlie
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, what-
ever be the agency by which the large swinging space is
obtained, we shall have light-giving vibrations associated
with it. It matters not whether the large amplitudes be
produced by the strokes of a hammer, or by the blows of
the molecules of a non-luminous gas, like air at some
height above a gas-flame; or by the shock of the 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, and the light- giving
waves follow as the necessary product of the heat-giv-
ing ones.

11. Absorption of Hadiant Heat hy Vapors and Odors

We commenced the demonstrations brought forward in
this lecture by experiments on permanent gases, and we
have now to turn our attention to the vapors of vola-
tile 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 intercept-
ing the calorific waves. While some vapors allow the
waves a comparatively free passage, the faintest mix-
ture of other vapors causes a deflection of the magnetic
needle. Assuming the absorption effected by air, at a
pressure of one atmosphere, to be unity, the following

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are the absorptions effected by a series of vapors at a
pressure of i^th of an atmosphere:

Name of vapor Absorption

Bisulphide of carbon •••••••47

Iodide of methyl ..•••••• 115

Benzol 136

Amylene ..••••••. 321

Sulphuric ether .••••••. 440

Formic ether 548

Acetic ether •••••••• 612

BisulpMde of carbon is the most transparent vapor in
this list; and acetic ether the most opaque; Truth of an at-
mosphere of the former, however, produces 47 times the
effect of a whole atmosphere of air, while B^th of an atmos-
phere 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 to-
gether, the quantity of wave-motion intercepted by the
ether would be many thousand times that intercepted by
the air.

Any one of these vapors discharged into the free
atmosphere, in front of a body emitting obscure rays,
intercepts more or less of the radiation. A similar effect
is produced by perfumes diffused in the air, though their
attenuation is known to be almost infinite. Carrying,
for example, a current of dry air over bibulous paper,
moistened by patchouli, the scent taken up by the cur-
rent absorbs 80 times the quantity of heat intercepted
by the air which carries it; and yet patchouli acts more
feebly on radiant heat than any other perfume yet ex-
amined. Here follow the results obtained with various
essential oils, the odor, in each case, being carried by

RADIATION

63

a current of dry air into the tube already employed for
gases and vapors:

Name of perfume
Patchouli .
Sandal wood
Geranium ,
Oil of cloves
Otto of roses
Bergamot
Neroli
Lavender
Lemon
Portugal
Thyme
Rosemary-
Oil of laurel
Camomile flowers
Cassia
Spikenard
Aniseed

Absorption
30
32
33
34
37
44
47
60
65
67
68
74
80
87
109
355
372

Thus the absorption by a tube full of dry air being 1,
that of the odor of patchouli diffused in it is 80, that of
lavender 60, that of rosemary 74, while that of aniseed
amounts to 372. It would be idle to speculate on the
quantities of matter concerned in these actions.

12. Aqueous Vapor in relation to the lerrestrial
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 vapor. It would be an
error to confound clouds or fog, or any visible mist, with
the vapor of water, which is a perfectly impalpable gas,

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diffused, even on the clearest days, throughout the atmos-
phere. Compared with the great body of the air, the aque-
ous vapor it contains is of almost infinitesimal amount, 993^
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 vary-
ing constituent any important influence on terrestrial radi-
ation; 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 vapor ex-
erts 100 times the action of the air itself, would certainly
be an understatement of the fact. Comparing a single
molecule of aqueous vapor 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 vapor seems vast, because that of the air
with which it is compared is infinitesimal. Absolutely
considered, however, this substance, notwithstanding its
small specific gravity, exercises a very potent action.
Probably from 10 to 15 per cent of the heat radiated
from the earth is absorbed within 10 or 20 feet of the
earth's surface. This must evidently be of the utmost
consequence to the life of the world. Imagine the super-
ficial molecules of the earth agitated with the motion of
heat, and imparting it to the surrounding ether; this mo-
tion would be carried rapidly away, and lost forever to
our planet, if the waves of ether had nothing but the air
to contend with in their outward course. But the aqueous
vapor takes up the motion, and becomes thereby heated,

RADIATION 65

thus wrapping the earth like a warm garment, and pro-
tecting its surface from the deadly chill which it would
otherwise sustain. Various philosophers have speculated
on the influence of an atmospheric envelope. De Saus-
sure, 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 experi-
ments be coiTcct, to be transferred to the aqueous vapor.
The observations of meteorologists furnish important,
though hitherto unconscious, evidence of the influence of
this agent. Wherever the air is dry we are liable to daily
extremes of temperature. By day, in such places, the sun's
heat reaches the earth unimpeded, and renders the maxi=
mum high; by night, on the other hand, the earth's heat
escapes unhindered into space, and renders the minimum
low. Hence the difference between the maximum and min-
imum 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 on the
burning soil, the temperature runs rapidly down to freez-
ing, because there is no vapor overhead to check the cal-
orific 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 refrigera-
tion, the aqueous vapor 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 vapor is especially opaque.

66 FRAGMENTS OF SCIENCE

Hence the very act of condensation, consequent on ter-
restrial cooling, becomes a safeguard to the earth, impart-
ing 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 de-
rive all our heat from the sun, the self -same 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 ab-
sorbs the one must necessarily absorb the other. Through
a layer of water, for example, one -tenth of an inch in
thickness, the sun's rays are transmitted with comparative
freedom; but through a layer half this thickness, as Mel-
loni has proved, no single ray from the warmed earth could
pass. In like manner, the sun's rays pass with compara-
tive freedom through the aqueous vapor of the air: the
absorbing power of this substance being mainly exerted
upon the invisible heat that endeavors to escape from the
earth. In consequence of this differential action upon solar
and terrestrial heat, the mean temperature of our planet is
higher than is due to its distance from the sun.

13. Liquids and their Vapors in relation to Radiant Heat

The deportment here assigned to atmospheric vapor 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 after-
ward rendered artificially humid by pure distilled water.
It has also been established in the following way: Ten
volatile liquids were taken at random, and the power of

RADIATION 67

these liquids, at a common thickness, to intercept the
waves of heat, was carefully determined. The vapors of
the liquids were next taken, in quantities proportional to
the quantities of liquid, and the power of the vapors to in-
tercept the waves of heat was also determined. Commenc-
ing with the substance which exerted the least absorptive
power, and proceeding onward to the most energetic, the
following order of absorption was observed:

Liquids Vapors

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 con-
dition; but this change in their state of aggregation does
not change their relative powers of absorption. Nothing
could more clearly prove that the act of absorption de-
pends 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 vapor is determined by that of its liquid. Now, at the
very foot of the list of liquids stands water, signalizing it-
seK above all others by its enormous power of absorption.
And from this fact, even if no direct experiment on the
vapor of water Had ever been made, we should be entitled

€8 FRAGMENTS OF SCIENCE

to rank that vapor as our most powerful absorber of radi-
ant heat. Its attenuation, however, diminishes its action,
I have proved that a shell of air two inches in thickness
surrounding our planet, and saturated with the vapor of
sulphuric ether, would intercept 86 per cent of the earth's
radiation. And, though the quantity of aqueous vapor
necessary to saturate air is much less than the amount of
sulphuric ether vapor which it can sustain, it is still ex-
tremely probable that the estimate already made of the
action of atmospheric vapor 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 de-
termined, but certainly far beyond what has hitherto beea
imagined, for the temperature now existing at the surface
of the globe.

14. Beciprocity of Radiation and Absorption

Throughout the reflections which have hitherto occupied
us, the image before the mind has been that of a radiant
source sending forth calorific waves, which, on passing
among the molecules of a gas or vapor, were intercepted
by those molecules in various degrees. In all cases it was
the transference of motion from the ether to the compara-
tively quiescent molecules of the gas or vapor that occu-
pied our thoughts. "We have now to change the form of
our conception, and to figure these molecules not as ab-
sorbers, but as radiators, not as the recipients, but as the
originators of wave-motion. That is to say, we must figure
them vibrating, and generating in the surrounding ether
undulations which speed through it with the velocity of
light. Our object now is to inquire whether the act of
chemical combination, which proves so potent as regards

BADIATIOir 69

the phenomena of absorption, does not also manifest its
power in the phenomena of radiation. For the exami-
nation of this question it is necessary, in the first place,
to heat our gases and vapors 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 containing
the various gases to be examined. By gentle pressure the
gases were forced through the orifices of the burner against
the copper ball, where each of them, being heated, rose in
an ascending column. A thermo-electrio pile, entirely
screened from the hot ball, was exposed to the radia-
tion 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 self-
same molecular arrangement, which renders a gas a power-
ful 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 them-
selves unable to emit any sensible amount of radiant heat,
the molecules of compound gases were shown to be capable
of powerfully disturbing the surrounding ether. By spe
cial modes of experiment the same was proved to hold
good for the vapors of volatile liquids, the radiative power
of every vapor being found proportional to its absorptive
power.

The method of experiment here pursued, though not of

70 FRAGMENTS OF SCIENCE

the simplest character, is still easy to grasp. When air is
permitted to rush into an exhausted tube, the temperature
of the air is raised to a degree equivalent to the vis viva
extinguished.' 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 vapor, the warmed air communicates its
heat by contact to the vapor, the molecules of which con-
vert 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 Provostaye
and Desains, and Balfour Stewart, the same reciprocity, as
regards solid bodies, has been variously illustrated; while
the labors, theoretical and experimental, of Kirchhoff have
given this subject a wonderful expansion, and enriched
it by applications of the highest kind. To their results
are now to be added the foregoing, whereby gases and
vapors, which have been hitherto thought inaccessible to
experiments with the thermo-electric pile, are proved by
it to exhibit the indissoluble duality of radiation and
absorption, the influence of chemical combination on
both being exhibited in the most decisive and extraor-
dinary way.

' See page 19 for a definition of vis viva.

2 When heated air imparts its motion to another gas or vapor, the trans-
ference of heat is accompanied by a change of vibrating period. The
Dynamic Kadiation of vapors is rendered possible by this transmutation of
vibrations.

RADIATION n

15. Influence of Vibrating Period and Molecular Form,

Physical Analysis of the Human Breath

In the foregoing experiments with gases and vapors we
have employed throughout invisible rays, and found some
of these bodies so impervious to radiant heat, that in
lengths of a few feet they intercept every ray as effect-
ually as a layer of pitch. The substances, however, 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 for-
mer 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 vapors depends upon the
periods of the waves which impinge upon them. What is
the nature of this dependence? The admirable researches
of Kirchhoff help us to an answer. The atoms and mole-
cules of every gas have certain definite rates of oscillation,
and those waves of ether are most copiously absorbed
whose periods of recurrence synchronize with those of the
atomic groups among 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.

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

72 FRAGMENTS OF SCIENCE

strokes of tlie waves augment tlie vibration of tlie mole-
cules, 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 diffi-
cult to see that as every wave arrives just in time to repeat
the action of its predecessor, the molecules must finally be
caused to swing through wider spaces than if the arrivals
were not so timed. In fact, it is not difiicult to see that
an assemblage of molecules, operated upon by contending
waves, might remain practically quiescent. This is act-
ually the case when the waves of the visible spectrum
pass through a transparent gas or vapor. 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 emit-
ting 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 car-
bonic oxide, where hot carbonic acid constitutes the main
radiating body. Of the rays emitted by our heated plate
of copper, olefiant gas absorbs ten times the quantity ab-
sorbed by carbonic acid. Of the rays emitted by a car-
bonic oxide flame, carbonic acid absorbs twice as much
as olefiant gas. This wonderful change in the power of

RADIATION 73

the former, as an absorber, is simply due to tbe fact that
the periods of the hot and cold carbonic acid are identi-
cal, and that the waves from the flame freely transfer their
motion to the molecules which synchronize 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 atmosphere 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 60 per cent of the entire radiation. Radiant heat may
indeed be employed as a means of determining practically
the amount of carbonic acid expired from the lungs. My
late assistant, Mr. Barrett, while und^er my direction, made
this determination. The absorption produced by the breath
freed from its moisture, but retaining its carbonic acid, was
first determined. Carbonic acid, artificially prepared, was
then mixed with dry air in such proportions that the ac-
tion of the mixture upon the rays of heat was the same as
that of the dried breath. The percentage of the former
being known, immediately gave that of the latter. The
same breath, analyzed chemically by Dr. Frankland, and
physically by Mr. Barrett, gave the following results:

Percentage of Cwrhonic Acid in the Human Breath

Chemical analysis Physical analj-sis

4-66 4-56

6-33 , . . 5-22

Science — Y — 4

74 FRAGMENTS OF SCIENCE

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

Still this question of period, though of the utmost im-
portance, is not competent to account for the whole of the
ot)served 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 oxjgen are just as ac-
ceptable as those of the atoms in a molecule of olefiant
gas; that the vibrating oxygen then stands so far below
the olefiant gas in radiant power must be referred not to
period, but to some other peculiarity. The atomic group
which constitutes the molecule of olefiant gas produces
many thousand times the disturbance caused by the oxy-
gen, 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 dif-
ference may be, that the vibrations, being those of the con-
stituent atoms of the molecule,* are generated in highly
condensed ether, which acts like condensed air upon sound.
But, whatever may be the fate of these attempts to visual-
ize 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 con-
dition of the ether within the molecules, by which the ex-
^mal ether is disturbed.

' See "Physical Considerations," Art. iv. p. 102.

EADIATIOy 76

16. Summary and Conclusion

Let us now cast a momentary glance over tlie ground
that we have left behind. The general nature of light and
heat was first briefly described: the compounding of mat-
ter from elementary atoms, and the influence of the act of
combination on radiation and absorption, were considered
and experimentally illustrated. Through the transparent
elementary gases radiant heat was found to pass as through
a vacuum, while many of the compound gases presented
almost impassable obstacles to the calorific waves. This
deportment of the simple gases directed our attention to
other elementary bodies, the examination of which led to
the discovery that the element iodine, dissolved in bisul-
phide of carbon, possesses the power of detaching, with
extraordinary sharpness, the light of the spectrum from its
heat, intercepting all luminous rays up to the extreme red,
and permitting the calorific rays beyond the red to pass
freely through it. This substance was then employed to
filter the beams of the electric light, and to form foci of
invisible rays so intense as to produce almost all the effects
obtainable in an ordinary fire. Combustible bodies were
burned, and refractory ones were raised to a white heat, by
the concentrated invisible rays. Thus, by exalting their
refrangibility, the invisible rays of the electric light were
rendered visible, and all the colors of the solar spectrum
were extracted from utter darkness. The extreme richness
of the electric light in invisible rays of low refrangibility
was demonstrated, one- eighth only of its radiation consist-
ing of luminous rays. The deadness of the optic nerve to
those invisible rays was proved, and experiments were then

T6 FRAGMENTS OF SCIENCE

added to show that the bright and the dark rays of a solid
body, raised gradually to incandescence, are strengthened
together; intense dark heat being an invariable accompani-
ment of intense white heat. A sun could not be formed,
or a meteorite rendered luminous, on any other condition.
The light-giving rays constituting only a small fraction of
the total radiation, their unspeakable importance to us is
due to the fact that their periods are attuned to the special
requirements of the eye.

Among the vapors of volatile liquids vast differences
were also found to exist, as regards their powers of ab-
sorption. We followed various molecules from a state of
liquid to a state of gas, and found, in both states of aggre-
gation, the power of the individual molecules equally as-
serted. The position of a vapor as an absorber of radiant
heat was shown to be determined by that of the liquid
from which it is derived. Eeversing our conceptions, and
regarding the molecules of gases and vapors not as the re-
cipients, but as the originators of wave-motion; not as ab-
sorbers, but as radiators; it was proved that the powers
of absorption and radiation went hand in hand, the seK
same chemical act which rendered a body competent to in-
tercept the waves of ether rendering it competent, in the
same degree, to generate them. Perfumes were next sub-
jected to examination, and, notwithstanding their extraor-
dinary 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 exami-
nation of the most widely diffused and most important of
all vapors — ^the aqueous vapor of our atmosphere, and we
found in it a potent absorber of the purely calorific rays.
The power of this substance to influence climate, and its

RADIATION 77

general influence on tlie temperature of tlie earth, were
then briefly dwelt upon. A cobweb spread above a blos-
som is sufficient to protect it from nightly chill; and thus
the aqueous vapor 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 ac-
crue 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 inquiries in the domain of Eadiation,
and my aim throughout has been to raise in your minds
distinct physical images of the various processes involved
in our researches. It is thought by some that natural sci-
ence 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 experi-
ence of the last hour must, I think, have convinced you
that the study of natural science goes hand in hand with
the culture of the imagination. Throughout the greater
part of this discourse we have been sustained by this fac-
ulty. We have been picturing atoms, and molecules, and
vibrations, and waves, which eye has never seen nor ear
heard, and which can only be discerned by the exercise of
imagination. This, in fact, is the faculty which enables us
to transcend the boundaries of sense, and connect the phe-
nomena of our visible world with those of an invisible
one. Without imagination we never could have risen to
the conceptions which have occupied us here to-day; and

fS FRAGMENTS OF SCIENCE

in proportion to jour power of exercising this facultj
aright, and of associating definite mental images with the
terms employed, will be the pleasure and the profit which
you will derive from this lecture. The outward facts of
nature are insufficient to satisfy the mind. We cannot be
content with knowing that the light and heat of the sun
illuminate and warm the world. We are led irresistibly to
inquire, *'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 sup-
plement that which is felt and seen, but which is incom-
plete, by something nnfelt 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 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 calo-
rific 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

' This line of thought was pursued further five years subsequently. See
'•Scientific Use of the Imagination" in VoL IL

RADIATION 79

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 tem-
porary existence was derived. This connection is typical.
Nature is not an aggregate of independent parts, but an
organic whole. If you open a piano and sing into it, a
certain string will respond. Change the pitch of your
voice; the first string ceases to vibrate, but another re-
plies. Change again the pitch; the first two strings are
silent, while another resounds. Thus is sentient man acted
on by Nature, the optic, the auditory, and other nerves of
the human body being so many strings differently tuned,
and responsive to different forms of the universal power.

Ill

ON RADIANT HEAT IN RELATION TO THE COLOR AND
CHEMICAL CONSTITUTION OF BODIES*

ONE of the most important functions of physical
science, considered as a discipline of the mind, is
to enable us by means of the sensible processes
of Nature to apprehend the insensible. The sensible proc-
esses give direction to the line of thought; but this once
given, the length of the line is not limited by the bounda-
ries of the senses. Indeed, the domain of the senses, in
Nature, is almost infinitely small in comparison with the
vast region accessible to thought which lies beyond them.
From a few observations of a comet, when it comes within
the range of his telescope, an astronomer can calculate its
path in regions which no telescope can reach: and in like
manner, by means of data furnished in the narrow world
of the senses, we make ourselves at home in other and
wider worlds, which are traversed by the intellect alone.
From the earliest ages the questions, "What is light?'*
and "What is heat?" have occurred to the minds of men;
but these questions never would have been answered had
they not been preceded by the question, "What is sound?'*
Amid the grosser phenomena of acoustics the mind was
first disciplined, conceptions being thus obtained from direct

* A discourse delirered in the Royal Institution of Great Britain, January
19, 1866.

(80)

RADIANT HEAT AND ITS RELATIONS 81

observation, whicli were afterward 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, moids the air around it into undula-
tions or waves, which speed away on all sides with a cer-
tain measured velocity, impinge upon the drum of the ear,
shake the auditory nerve, and awake in the brain the sen-
sation of sound. When sufficiently near a sounding body
we can feel the vibrations of the air. A deaf man, for ex-
ample, 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 render-
ing those sonorous vibrations not only tangible, but visi-
ble ; and it was not until numberless experiments of this
kind had been executed that the scientific investigator
abandoned himself wholly, and without a shadow of mis-
giving, to the conviction that what is sound within us is,
outside of us, a motion of the air.

But once having established this fact — once having
proved, beyond all doubt, that the sensation of sound
is produced by an agitation of the auditory nerve — the
thought soon suggested itself that light might be due to
an agitation of the optic nerve. This was a great step in
advance of that ancient notion which regarded light as
something emitted by the eye, and not as anything im-
parted to it. But if light be produced by an agitation of
the retina, what is it that produces the agitation ? New-
ton, you know, supposed minute particles to be shot
tnrough the humors of the eye against the retina, which
he supposed to hang like a target at the back of the eye.
The impact of these particles against the target, Newton

82 FRAGMENTS OF SCIENCE

believed to be the cause of light. But ISTewton'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 propa-
gation; but we cannot see, or feel, or taste, or smell this
medium. How, then, has its existence been established?
By showing that, by the assumption of this wonderful in-
tangible etherj all the phenomena of optics are accounted
for, with a fulness, and clearness, and conclusiveness,
which leave no desire of the intellect unsatisfied. When
the law of gravitation first suggested itself to the mind
of Newton, what did he do? He set himself to examine
whether it accounted for all the facts. He determined the
courses of the planets; he calculated the rapidity of the
moon's fall toward 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 re-
garded 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

RADIANT HEAT AND ITS RELATIONS 83

by that force which holds in its grasp, not only our plan*
etary system, but the immeasurable heavens themselves.

There is no more wonderful instance than this of the
production of a line of thought, from the world of the
senses into the region of pure imagination. I mean by
imagination here, not that play of fancy which can give
to airy nothings a local habitation and a name, but that
power which enables the mind to conceive realities which
lie beyond the range of the senses — to present to itself dis-
tinct images of processes which, though mighty in the ag-
gregate 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,
however, compensates for their minuteness. Trillions of
them have entered your eyes, and hit the retina at the
backs of your eyes, in the time consumed in the utterance
of the shortest sentence of this discourse. This is the
steadfast result of modern research; but we never could
have reached it without previous discipline. We never
could have measured the waves of light, nor even imag-
ined them to exist, had we not previously exercised our-
selves among the waves of sound. Sound and light are
now mutually helpful, the conceptions of each being ex-
panded, 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

64 FRAG31ENTS OF SCIENCE

bodies are composed. It is tlie motion of these atoms, and
not tliat of any sensible parts of bodies, that the ether con-
veys. 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. Different bodies,
when heated to the same temperature, possess very differ-
ent 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 ma-
terially disturbing its repose. Eecent experiments have
proved that elementary bodies, except under certain anom-
alous conditions, belong to the class of bad radiators. An
atom, vibrating in the ether, resembles a naked tuning-
fork vibrating in the air. The amount of motion commu-
nicated to the air by the thin prongs is too small to evoke
at any distance the sensation of sound. But if we permit
the atoms to combine chemically and form molecules, the
result, in many cases, is an enormous change in the power
of radiation. The amount of 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 color of
light Taking a slice of white light from the sun, or from

RADIANT HEAT AND ITS RELATIONS 85

an electric lamp, and causing the liglit to pass througli an
arrangement of prisms, it is decomposed. We have the
effect obtained by Newton, who first unrolled the solar
beam into the splendors of the solar spectrum. At one
end of this spectrum we have red light, at the other, vio-
let; and between those extremes lie the other prismatic
colors. As we advance along the spectrum from the red
to the violet, the pitch of the light — if I may use the ex-
pression— 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 color changes into another by insensible gradations.
It is as if an infinite number of tuning-forks, of gradually
augmenting pitch, were vibrating at the same time. But
turning to another spectrum — that, namely, obtained from
the incandescent vapor of silver — you observe that it con-
sists of two narrow and intensely luminous green bands.
Here it is as if two forks only, of slightly different pitchy
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: while the num-
ber 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

86 FRAGMENTS OF SCIENCE

wMch these bands were derived. This luminous stream is
the incandescent vapor of silver. The rates of vibration
of the atoms of that vapor are as rigidly fixed as those of
two tuning-forks; and to whatever height the temperature
of the vapor may be raised, the rapidity of its vibrations,
and consequently its color, which wholly depends upon
that rapidity, remain unchanged.

The vapor of water, as well as the vapor of silver, has
its definite periods of vibration, and these are such as to
disqualify the vapor, when acting freely as such, from be-
ing raised to a white heat. The oxy hydrogen flame, for
example, consists of hot aqueous vapor. It is scarcely vis-
ible in the air of this room, and it would be still less visi-
ble if we could bum the gas in a clean atmosphere. But
the atmosphere, even at the summit of Mont Blanc, is
dirty; in London it is more than dirty; and the burning
dirt gives to this flame the greater portion of its present
light. But the heat of the flame is enormous. Cast iron
fuses at a temperature of 2,000° Fahr. ; while the tempera-
ture of the oxy hydrogen 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
vapor which produces incandescence is here absolutely
dark. In the flame itself the platinum is raised to daz-
zling whiteness, and is even pierced by the flame. When
this flame impinges on a piece of lime we have the daz-
zling Drummond light. But the light is here due to the
fact that when it impinges upon the solid body, the vibra-
tions 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

RADIANT HEAT AND ITS RELATIONS 87

medium whicli 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 mo-
tion to the molecules on which they impinge, or will they
glide round the molecules, through the intermolecular
spaces, and thus escape ?

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

That the motion should thus transfer itself through the
air it is necessary that the two forks should be in perfect

88 FRAGMENTS OF SCIENCE

tmison. If a morsel of wax, not larger than a pea, be
placed on one of the forks, it is rendered thereby power-
less to affect, or to be affected by, the other. It is easy
to understand this experiment. The pulses of the one fork
can affect the other, because they are perfectly timed. A
single pulse causes the prong of the silent fork to vibrate
through an infinitesimal space. But just as it has com-
pleted this small vibration, another pulse is ready to strike
it. Thus, the impulses add themselves together. In the
^ye seconds during which the forks were held near each
other, the vibrating fork sent 1,280 waves against its neigh-
bor, 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 syn-
chronism on musical vibrations, is this: Three small gas-
flames are inserted into three glass tubes of different
lengths. Each of these flames can be caused to emit a
musical note, the pitch of which is determined by the
length of the tube surrounding the flame. The shorter
the tube the higher is the pitch. The flames are now si-
lent 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 power-
ful musical note, of gradually increasing pitch, can be pro-
duced. Beginning with a low note, and ascending grad-
ually 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 notea the second flame is started, and the third

RADIANT HEAT AND ITS RELATIONS 89

alone remains. A still higher note starts it also. Thus,
as the sound of the syren rises gradually m pitch, it
awakens every flame in passing, by striking it with a
series of waves whose periods of recurrence are similar
to its own.

Now the wave- motion from the syren is in part taken
up by the flame which synchronizes 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-mo-
tion. Let us apply these facts to radiant heat. This blue
flame is the flame of carbonic oxide; this transparent gas
is carbonic acid gas. In the blue flame we have carbonic
acid intensely heated, or, in other words, in a state of in-
tense 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 inter-
cepted 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 sci-
entific men to determine the substances of which the sun,
the stars, and even the nebulae, are composed; the princi-
ple, namely, that a body which is competent to emit any
ray, whether of heat or light, is competent in the same de-
gree 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.

90 FRAGMENTS OF SCIENCE

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

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

RADIANT HEAT AND ITS RELATIONS 91

focus a piece of ice, the ice is not melted by the concen-
trated beam. Matches, at the same place, are ignited, and
wood is set on fire. The powerful heat, then, of this lu-
minous beam is incompetent to melt the ice. On with-
drawing the cell of water, the ice immediately liquefies,
and the water trickles from it in drops. Eeintroducing the
cell of water, the fusion is arrested, and the drops cease
to fall. The transparent water of the cell exerts no sen-
sible absorption on the luminous fays, still it withdraws
something from the beam, which, when permitted to act, is
competent to melt the ice. This something is the dark
radiation of the electric light. Again, I place a slab of
pure ice in front of the electric lamp; send a luminous
beam first through our cell of water and then through the
ice. By means of a lens an image of the slab is cast upon
a white screen. The beam, sifted by the water, has little
power upon the ice. But observe what occurs when the
water is removed; we have here a star and there a star,
each star resembling a flower of six petals, and growing
visibly larger before our eyes. As the leaves enlarge, their
edges become serrated, but there is no deviation from the
six-rayed type. We have here, in fact, the crystallization
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 ad-
duced in illustration of the action of these lightless rays.
These two powders, for example, are both white, and in-
distinguishable from each other by the eye. The luminous
rays of the sun are unabsorbed by both — ^from such rays

92 FRAGMENTS OF SCIENCE

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

A substance, as many of you know, has been discov-
ered, by which these dark rays may be detached from the
total emission of the electric lamp. This ray- filter is a
liquid, black as pitch to the luminous, but bright as a dia-
mond to the non- luminous, radiation. It mercilessly cuts
off the former, but allows the latter free transmission.
When these invisible rays are brought to a focus, at a
distance of several feet from the electric lamp, the dark
rays form an invisible image of their source. By proper
means, this image may be transformed into a visible one
of dazzling brightness. It might, moreover, be shown, if
time permitted, how, out of those perfectly dark rays,
could be extracted, by a process of transmutation, all the
colors 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 lumi-
nous impression.

The dark rays being thus collected, you see nothing at

RADIANT HEAT AND ITS RELATIONS 93

iheir place of convergence. With a proper thermometer
it could be proved that even the air at the focus is just as
cold as the surrounding air. And mark the conclusion to
which this leads. It proves the 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 white-
ness, and almost fuse that refractory metal. It actually
can fuse gold, silver, copper, and aluminium. The mo-
ment, moreover, that wood is placed at the focus it bursts
into a blaze.

It has been already affirmed that, whether as regards
radiation or absorption, the elementary atoms possess but
little power. This might be illustrated by a long array of
facts; and one of the most singular of these is furnished
by the deportment of that extremely combustible sub-
stance, phosphorus, when placed at the dark focus. It
is impossible to ignite there a fragment of amorphous
phosphor as. But ordinary phosphorus is a far quicker
combustible, and its deportment toward radiant heat is
still more impressive. It may be exposed to the intense
radiation of an ordinary fire without bursting into flame.
It may also be exposed for twenty or thirty seconds at
an obscure focus, of sufficient power to raise platinum to
a red heat, without ignition. Notwithstanding the energy
of the 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 powerful waves

94 FRAGMENTS OF SCIENCE

of low refrangibilitj, and consequently cannot be affected
by their beat.

The knowledge we now possess will enable us to ana-
lyze 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 colors upon snow, exposed them to di-
rect sunshine, and found that they sank to different depths
in the snow. The black cloth sank deepest, the white did
not sink at all. Franklin inferred from this experiment
that black bodies are the best absorbers, and white ones
the worst absorbers, of radiant heat. Let us test the gen-
erality of this conclusion. One of these two cards is coated
with a very dark powder, and the other with a perfectly
white one. I place the powdered surfaces before a fire,
and leave them there until they have acquired as high a
temperature as they can attain in this position. Which of
the cards is then most highly heated ? It requires no ther-
mometer to answer this question. Simply pressing the
back of the card, on which the white powder is strewn,
against the cheek or forehead, it is found intolerably hot.
Placing the dark card in the same position, it is found
cool. The white powder has absorbed far more heat than
the dark one. This simple result abolishes a hundred con-
clusions which have been hastily drawn from the experi-
ment of Franklin. Again, here are suspended two deli-
cate mercurial thermometers at the same distance from a
gas-flame. The bulb of one of them is covered by a dark
substance, the bulb of the other by a white one. Both
bulbs have received the radiation from the flame, but the
white bulb has absorbed most, and its mercury stands
much higher than that of the other thermometer. This

RADIANT HEAT AND ITS RELATIONS 95

experiment miglit 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 color gives us intelli-
gence of only one portion, and that the smallest one, of
the rays impinging on the colored body. Were the rays
all luminous, we might with certainty infer from the color
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 color 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 ac-
count 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 trans-
parency to the invisible, and its extreme opacity to the
visible, rays. In the case of the radiation from our fire,
about 98 per cent of the whole emission consists of invis-
ible rays; the body, therefore, which was most opaque to
these triumphed as an absorber, though that body was a
white one.

And here it is worth while to consider the manner in
which we obtain from natural facts what may be called
their intellectual value. Throughout the processes of Na-
ture we have interdependence and harmony; and the main
value of physics, considered as a mental discipline, con«
sists in the tracing out of this interdependence, and the
demonstration of this harmony. The outward and visible
phenomena are the counters of the intellect; and our sci-
ence would not be worthy of its name and fame if it

96 FRAGMENTS OF SCIENCE

halted at facts, however practically useful, and neglected
the laws which accompany and rule the phenomena. Let
us endeavor then to extract from the experiment of Frank-
lin all that it can yield, calling to our aid the knowledge
which our predecessors have already stored. Let us im-
agine 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 inquire 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 hol-
low; 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 power-
less; the one cannot be warmed, nor the other melted, by
such rays. The cloth is white and the snow is white,
because their confusedly mingled fibres and particles are
incompetent to absorb the luminous rays. Whether, then,
the cloth will sink or not depends entirely upon the dark
rays of the sun. Now, the substance which absorbs these
dark rays with the greatest avidity is ice — or snow, which
is merely ice in powder. Hence, a less 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.

RADIANT HEKV AND ITS RELATIONS 97

Cloth is very powerful iu. 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 color, 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 in-
flux of heat is far more than sufficient to turn the balance
in favor 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. With regard
to gases and vapors, and to the liquids from which these
vapors 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 ex-
tend 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 per-
formed a most elaborate series of experiments on chemi-

■ Science — Y — **'

98 FRAGMENTS OF SCIENCE

cal precipitates of various kinds, and found that they one
^nd all manifested the same power of radiation. They
concluded from their researches, that when bodies are re-
duced to an extremely fine state of division, the influence
of this state is so powerful as entirely to mask and over-
ride whatever influence may be due to chemical consti-
tution.

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. Fill-
ing a brightly polished metal cube with boiling water, 1
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 per-
fectly transparent to luminous rays, is as opaque as pitch,
or lamp-black, to non- luminous ones. It is a powerful
emitter of dark rays; it is also a powerful absorber.
While, therefore, at the present moment, it is copiously
pouring forth radiant heat itself, it does not allow a single
^%j 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, experi-
mented thus: they mixed their powders and precipitates
with gum- water, and laid them, by means of a brush, upon
the surfaces of a cube like this. True, they saw their red
powders red, their white ones white, and their black ones

RADIANT HEAT AND ITS RELATIONS 99

black, but tliey saw these colors through the coat of varnish
which surrounded every particle. When, therefore, it was
concluded that color 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, per-
haps thousands, of experiments on radiant heat have been
performed in this way, by various inquirers, but the work
will, I fear, have to be done over again. I am not, in-
deed, 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 en-
velope of the particles, and not the particles themselves,
was the real radiator in the experiments just referred to.
To reason thus, and deduce their more or less probable
consequences from experimental facts, is an incessant ex-
ercise of the student of physical science. But having thus
followed, for a time, the light of reason alone through a
series of phenomena, and emerged from them with a purely
intellectual conclusion, our duty is to bring that conclu-
sion to an experimental test. In this way we fortify our
science.

For the purpose of testing our conclusion regarding the
influence of the gum, I take two powders presenting the
same physical appearance; one of them is a compound of
mercury, and the other a compound of lead. On two sur-
faces of a cube are spread these bright red powders, with-
out varnish of any kind. Filling the cube with boiling

1^ FRAGMENTS OF SCIENCE

water, and determining tlie radiation from the two snrfaces,
one of them is found to emit thirty-nine units of heat, while
the other emits seventy-four. This, surely, is a great dif-
ference. Here, however, is a second cube, having two of
its surfaces coated with the same powders, the only differ-
ence being that the powders are laid on by means of a
transparent gum. Both surfaces are now absolutely alike
in radiative power. Both of them emit somewhat more
than was emitted by either of the unvarnished powders,
simply because the gum employed is a better radiator than
either of them. Excluding all varnish, and comparing
white with white, vast differences are found; comparing
black with black, they are also different; and when black
and white are compared, in some cases the black radiates
far more than the white, while in other cases the white
radiates far more than the black. Determining, moreover,
the absorptive power of those powders, it is found to go
hand-in-hand with their radiative power. The good radi-
ator is a good absorber, and the bad radiator is a bad ab-
sorber. From all this it is evident that, as regards the
radiation and absorption of non-luminous heat, color 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 color may be altogether
delusive. This is the strict scientific upshot of our re-
searches. But it is not the less true that in the case of
wearing apparel — and this for reasons which I have given
in analyzing the experiment of Franklin — black dresses are
more potent than white ones as absorbers of solar heat.

Thus, in brief outline, have been brought before you a
few of the results of recent inquiry. If you ask me what
is the use of them, I can hardly answer you, unless you

RADIANT HEAT AND ITS RELATIONS 101

define the term use. If you meant to ask whether those
dark rajs 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 ethe-
real race than we are may cook their victuals, and perform
their work, in this transcendental way. Bat is it neces-
sary that the student of science should have his labors
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 sci-
ence practical uses, forget, or do not know, that it also is
great as a means of culture — that the knowledge of this
wonderful universe is a thing profitable in itself, and
requiring no practical application to justify its pursuit.

But, while the student of Nature distinctly refuses to
have his labors judged by their practical issues, unless the
term practical be made to include mental as well as ma-
terial good, he knows full well that the greatest practical
triumphs have been episodes in the search after pure natu-
ral truth. The electric telegraph is the standing wonder
of this age, and the men whose scientific knowledge, and
mechanical skill, have made the telegraph what it is, are
deserving of all honor. In fact, they have had their re-
ward, both in reputation and in those more substantial
benefits which the direct service of the public always car-
ries in its train. But who, I would ask, put the soul into
this telegraphic body? Who snatched from heaven the

102 FRAGMENTS OF SCIENCE

fire that flashes along the line ? This, 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 inquiries
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 sci-
ence, 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 ap-
peal 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 com-
plete, in which the knowledge of Nature is neglected or
ignored.

» Volta. 8 Faraday.

IV

NEW CHEMICAL EEACTIONS PRODUCED BY LIGHT

1868-1869

MEASUKED by their power, not to excite vision, but
to produce heat — in other words, measured bj their
absolute energy — the ultra-red waves of the sun
and of the electric light, as shown in the preceding arti-
cles, far transcend the visible. In the domain of chem-
istry, however, there are numerous cases in which the
more powerful waves are ineffectual, while the more mi-
nute 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 vapors of volatile liquids to
the action of concentrated sunlight, or to the concentrated
beam of the electric light. Their investigation led up to
the discourse on "Dust and Disease," which follows in
this volume; and for this reason some account of them
is introduced here.

A glass tube three feet long and three inches wide,
which had been frequently employed in my researches on
radiant heat, was supported horizontally on two stands.
At one end of the tube was placed an electric lamp, the
height and position of both being so arranged that the axis
of the tube, and 'that of the beam issuing from the lamp,

ao3)

104

FRAGMENTS OF SCIENCE

were coincident. In the first experiments tlie two ends of
tlie tube were closed bj plates of rock-salt, and subse-
quently by plates of glass. For the sake of distinction,
I call this tube the experimental tube. It was connected
with an air-pump, and also with a series of drying and
other tubes used for the purification of the air.

A number of test-tubes, like F, Fig. 2 (I have used at
least fifty of them), were converted into "Woulf's flasks.

,» Each of them was stopped by a cork,
through which passed two glass tubes:
one of these tubes {a) ended immedi-
ately below the cork, while the other
{b) descended to the bottom of the
flask, being drawn out at its lower
end to an orifice about 0*03 of an
inch in diameter. It was found nec-
essary to coat the cork carefully with
cement. In the later experiments
corks of vulcanized India-rubber were
invariably employed.

The little flask, thus formed, being
partially filled with the liquid whose
vapor was to be examined, was intro-
duced into the path of the purified
current of air. The experimental tube
being exhausted, and the cock which
Fig. 2. cut off the Supply of purified air be-

ing cautiously turned on, the air entered the flask through
the tube J, and escaped by the small orifice at the lower
end of b into the liquid. Through this it bubbled, loading
itself with vapor, after which the mixed air and vapor,
passing from the flask by the tube a, entered the experi-

DECOMPOSITION BY LIGHT 105

mental tube, wliere they were subjected to tbe action of
light.

The whole arrangement is shown in Fig. 8, where L rep-
resents the electric lamp, s s' the experimental tube, p p'
the pipe leading to the air-pump, and F the test-tube con-
taining the volatile liquid. The tube ^ ^' is plugged with
cotton-wool intended to intercept the floating matter of the
air; the bent tube T- contains caustic potash, the tube T
sulphuric acid, the one intended to remove the carbonic
acid and the other the aqueous vapor of the air.

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

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

Nitrite of Amyt — The vapor 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 afterward whirled through the tube.

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

i06

FRAGMENTS OF SCIENCE

DECOMPOSITION BY LIGHT lOT

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 precipi-
tation upon the beam was so rapid and intense that the
cone, which a moment before was invisible, flashed sud-
denly forth like a solid luminous spear. The effect was
the same when the air and vapor 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 labora-
tory.

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

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

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

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

This was further demonstrated by the deportment of

108 FRAGMENTS OF SCIENCE

the vapor itself. "Wlien it was permitted to enter tlie ex-
perimental tube unmixed witli air or any otlier gas, tlie
effect was substantially tlie same. Hence the seat of the
observed action is the vapor.

This action is not to be ascribed to heat. As regards
the glass of the experimental tube, and the air within the
tube, the beam employed in these experiments was per-
fectly 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 calo-
rific 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 beau-
tiful series of reactions, I left to them the examination
of the products of decomposition. The group of atoms
forming the molecule of nitrite of amyl is obviously
shaken asunder by certain specific waves of the electric
beam, nitric oxide and other products, of which the ni'
trate of amyl is probably one, being the result of the
decomposition. The brown fumes of nitrous acid were
seen mingling with the cloud within the experimental
tube. The nitrate of amyl, being less volatile than the
nitrite, and not being able to maintain itself in the con-
dition of vapor, would be precipitated as a visible cloud
along the track of the beam.

In the anterior portions of the tube a powerful sifting
of the beam by the vapor occurs, which diminishes the
chemical action in the posterior portions. In some ex-
periments 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

DECOMPOSITION BY LIGHT 109

Other end of the tube, copious precipitation occurred there
also.

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

Physical Considerations

I sought to determine the particular portion of the
light which produced the foregoing effects. When, pre-
vious to entering the experimental tube, the beam was
caused to pass through a red glass, the effect was greatly
weakened, but not extinguished. This was also the case
with various samples of yellow glass. A blue glass being
introduced before the removal of the yellow or the red,
on taking the latter away prompt precipitation occurred
along the track of the blue beam. Hence, in this case,
the more refrangible rays are the most chemically active.
The color of the liquid nitrite of amyl indicates that this
must be the case; it is a feeble but distinct yellow: in

llO FRAGMENTS OF SCIENCE

other words, the yellow portion of the beam is most freely
transmitted. It is not, however, the transmitted portion
of any beam which produces chemical action, but the ab-
sorbed portion. Blue, as the complementary color to yel-
low, 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 vapor. The assump-
tion is worth testing. A solution of the yellow chromate
of potash, the color 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 arrest-
ing the rays which act upon its vapor. A layer one-
eighth of an inch in thickness, which scarcely perceptibly
affected the luminous intensity, absorbed the entire chem-
ical energy of the concentrated beam of the electric light.

The close relation subsisting between a liquid and its
vapor, 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 vapor, 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 absorp-
tion, and where is its seat?' I figure, as others do, a

* "Phil. Trans.** 1864; "Heat, a Mode of Motion,*' chap. xii. ; and p. 67
of this volume.

' My attention was very forcibly directed to this subject some years ago by
a conversation with my excellent friend Professor Clausius.

DECOMPOSITION BY LIGHT 111

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

The molecule, as a whole, can only vibrate in virtue
of the forces exerted between it and its neighbor 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 identical ab-
sorption of the liquid and of the vaporous nitrite of amyl
indicates an identical vibrating period on the part of
liquid and vapor, and this, to my mind, amounts to an
experimental proof that the absorption occurs in the main
within the molecule. For it can hardly be supposed, if
the absorption were the act of the molecule as a whole,
that it could continue to affect waves of the same period
after the substance had passed from the vaporous to the
liquid state.

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

112 FRAGMENTS OF 8CIENCB

"wMcli binds the parts of the molecule together is snapped
asunder.

I anticipate wide, if not entire, generality for the fact
that a liquid and its vapor absorb the same rays. A cell
of liquid chlorine would, I imagine, deprive light more
effectually of its power of causing chlorine and hydrogen
to combine than any other filter of the luminous rays.
The rays which give chlorine its color have nothing to do
with this combination, those that are absorbed by the
chlorine being the really effective rays. A highly sensi-
tive 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 favor the view that chlorine
itself is molecular and not monatomic.

Production of Shy-hlue hy the Decomposition of Nitrite

of Amyl

When the quantity of nitrite vapor 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 vapor fills the experimental tube. The
effect now to be described was first obtained when the
vapor of the nitrite was derived from a portion of its
liquid which had been accidentally introduced into the

DECOMPOSITION BY LIGHT 113

passage through which the dry air flowed into the experi-
mental tube.

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

But the development of color in the attenuated nitrite-
of-amyl vapor is doubtless more similar to what takes
place in our atmosphere. The blue, moreover, is far
purer and more sky-like than that obtained from 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 pre-
cipitated vapor.

Iodide of Allyl. — Among the liquids hitherto subjected
to the concentrated electric light, iodide of allyl, in point
of rapidity and intensity of action, comes next to the
nitrite of amyl. With the iodide I have employed both
oxygen and hydrogen, as well as air, as a vehicle, and
found the effect in all cases substantially the same. The
cloud-column here was exquisitely beautiful. It revolved

114 FRAGMENTS OF SCIENCE

round tlie axis of tlie decomposing beam; it was nipped
at certain places like an hour-glass, and round tlie two
bells of tbe glass delicate cloud-filaments twisted tbem-
selves 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- ally 1 vapor a similar lustre was
most exquisitely shown. "With a suitable disposition of
the light, the purple hue of iodine-vapor came out very
strongly in the tube.

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

Iodide of IsopropyL — The action of light upon the va-
por 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 com-
mencement. After some minutes' exposure, however,
clouds begin to form, which grow in density and in
beauty as the light continues to act. In every experiment
hitherto made with this substance the column of cloud
filling the experimental tube was divided into two dis-
tinct parts near the middle of the tube. In one experi-
ment a globe of cloud formed at the centre, from which,

DECOMPOSITION BY LIGHT 115

right and left, issued an axis uniting the globe with two
adjacent cylinders. Both globe and cylinders were ani«
mated by a common motion of rotation. As the action
continued, paroxysms of motion were manifested; the vari-
ous parts of the cloud would rush through each other with
sudden violence. During these motions beautiful and gro-
tesque cloud-forms were developed. At some places the
nebulous mass would become ribbed so as to resemble the
graining of wood; a longitudinal motion would at times
generate in it a series of curved transverse bands, the
retarding influence of the sides of the tube causing an ap-
pearance resembling, on a 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 materi-
ally from all others that I had seen. A gorgeous mauve
color was observed in the last twelve inches of the tube;
the vapor of iodine was present, and it may have been
the sky-blue scattered by the precipitated particles which,
mingling with the purple of the iodine, produced the
mauve. As in all other cases here adduced, the effects
were proved to be due to the light; they never occurred
in darkness.

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

The tube being filled with the sensitive mixture, the
beam was sent through it, the lens at the same time being

116 FRAGMENTS OF SCIENCE

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 afterward a second cloud was formed five inches
further down the experimental tube. Both clouds were
united by a slender cord of the same bluish tint as them-
selves.

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

Kendering the light along the connecting-cord more
intense, it diminished in thickness and became whiter;
this was a consequence of the enlargement of its particles.
The cord finally disappeared, while the funnels melted

ARTIFICIAL SKY 117

into two ghost-like films, shaped like parasols. Thej
were barely visible, being of an exceedingly delicate blue
tint. They seemed woven of blue air. To compare them
with cobweb or with gauze would be to liken them to
something infinitely grosser than themselves.

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

§ 2. On the Blue Colob op the Sky, and the
polaeization op skylight*

1869

After the communication to the Eoyal Society of tha
foregoing brief account of a new Series of Chemical Eeac-
tions produced by Light, the experiments upon this sub-
ject were 'continued, the number of substances thus acted
on being considerably increased.

I now, however, beg to direct attention to two ques-
tions glanced at incidentally in the preceding pages — the
blue color of the sky, and the polarization of skylight.
Eeserving 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 meteor-
ology. Indeed it was the interest manifested in them by
Sir John Herschel, in a letter of singular speculative
power, addressed to myself, that caused me to enter upon
the consideration of these questions so soon.

The apparatus with which I work consists, as already
stated, of a glass tube about a yard in length, and from

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

118 FRAGMENTS OF SCIENCE

2}^ to 3 inclies internal diameter. The vapor to he ex-
amined 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 ac-
tivity of the substance has been declared.

It has hitherto been my aim to render the chemical
action of light upon vapors visible. For this purpose
substances have been chosen, one at least of whose prod-
ucts 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
vapor, this precipitation may be rendered of any degree
of fineness, forming particles distinguishable by the naked
eye, or far beyond the reach of our highest microscopic
powers. I have no reason to doubt that particles may be
thus obtained, whose diameters constitute but a small
fraction of the length of a wave of violet light.

In all cases when the vapors of the liquids employed
are sufficiently attenuated, no matter what the liquid may
be, the visible action commences with the formation of a
blue cloud. But here I must guard myself against all mis-
conception as to the use of this term. The "cloud" here
referred to is totally invisible in ordinary daylight. To
be seen, it requires to be surrounded by darkness, it only
being illuminated by a powerful beam of light. This blue
cloud differs in many important particulars from the finest
ordinary clouds, and might justly have assigned to it an
intermediate position between such clouds and true vapor.
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 endeavoring to decompose carbonic acid

ARTIFICIAL SKY 119

gas by light. A faint bluish cloud, due it may be, or it
may not be, to the residue of some vapor previously em-
ployed, was formed in the experimental tube. On look-
ing across this cloud through a JSTicol's prism, the line
of vision being horizontal, it was found that when the
short diagonal of the prism was vertical the quantity of
light reaching the eye was greater than when the long
diagonal was vertical. When a plate of tourmaline was
held between the eye and the bluish cloud, the quantity
of light reaching the eye when the axis of the prism was
perpendicular to the axis of the illuminating beam was
greater than when the axes of the crystal and of the beam
were parallel to each other.

This was the result all round the experimental tube.
Causing the crystal of tourmaline to revolve round the
tube, with its axis perpendicular to the illuminating beam,
the quantity of light that reached the eye was in all its
positions a maximum. When the crystallographic axis
was parallel to the axis of the beam, the quantity of light
transmitted by the crystal was a minimum. From the
illuminated bluish cloud, therefore, polarized light was
discharged, the direction of maximum polarization being
at right angles to the illuminating beam; the plane of
vibration of the polarized light was perpendicular to the
beam. *

Thin plates of selenite or of quartz, placed between
the Nicol and the actinic cloud, displayed the colors of
polarized light, these colors being most vivid when the

^ This is still an undecided point; but the probabilities are so much in its
favor, 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 ia
the text.

120 FRAGMENTS OF SCIENCE

line of vision was at right angles to the experimental
tube. The plate of selenite usually employed was a cir-
cle, thinnest at the centre, and augmenting uniformly in
thickness from the centre outward. When placed in its
proper position between the JSTicol and the cloud, it ex-
hibited a system of splendidly- colored rings.

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

And here it may be mentioned that a vapor, 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 vapor, exhibit vigorous, if not violent action. The
case is similar to that of carbonic acid gas, which, diffused
in the atmosphere, resists the decomposing action of solar
light, but when placed in contiguity with chlorophyl in
the leaves of plants has its molecules shaken asunder.

Dry air was permitted to bubble through the liquid
nitrite of butyl, until the experimental tube, which had
been previously exhausted, was filled with the mixed air
and vapor. The visible action of light upon the mixture
after fifteen minutes' exposure was slight. The tube was
afterward filled with half an atmosphere of the mixed air
and vapor, and a second half-atmosphere of air which had

ARTIFICIAL SKY 121

been permitted to bubble through fresh commercial hy-
drochloric acid. On sending the beam through this mixt-
ure, 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 perpen-
dicularly to its axis, showed scarcely any signs of polar-
ization. Looked at obliquely, the polarization was strong.

The . experimental tube being again cleansed and ex-
hausted, the mixed air and nitrite-of-butyl vapor was per-
mitted to enter it until the associated mercury column was
depressed is of an inch. In other words, the air and
vapor, united, exercised a pressure not exceeding ish of
an atmosphere. Air, passed through a solution of hydro-
chloric acid, was then added, till the mercury column was
depressed three inches. The condensed beam of the elec-
tric light was passed for some time through this mixture
without revealing anything within the tube competent to
scatter the light. Soon, however, a superbly blue cloud
was formed along the track of the beam, and it continued
blue sufficiently long to permit of its thorough examina-
tion. The light discharged from the cloud, at right angles
to its own length, was at first perfectly polarized. It
could be totally quenched by the Nicol. By degrees the
cloud became of whitish blue, and for a time the selenite
colors, obtained by looking at it normally, were exceed-
ingly brilliant. The direction of maximum polarization
was distinctly at right angles to the illuminating beam.
This continued to be the case as long as the cloud main-

SOIKNOE— —6

122 FRAGMENTS OF SCIENCE

tained a decided blue color, and even for some time after
the blue had changed to whitish blue. But, as the light
continued to act, the cloud became coarser and whiter,
particularly at its centre, where it at length ceased to dis-
charge polarized light in the direction of the perpendicular,
while it continued to do so at both ends.

But the cloud which had thus ceased to polarize the
light emitted normally, showed vivid selenite colors when
looked at obliquely, proving that the direction of maxi-
mum polarization 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 vapor in a still more at-
tenuated condition. The instance here cited is represen-
tative. In all cases, and with all substances, the cloud
formed at the commencement, when the precipitated par-
ticles are sufficiently fine, is hlue^ and it can be made to
display a color rivalling that of the purest Italian sky. In
all cases, moreover, this fine blue cloud polarizes perfectly
the beam which illuminates it, the direction of polarization
enclosing an angle of 90° with the axis of the illuminating
beam.

It is exceedingly interesting to observe both the perfec-
tion and the decay of this polarization. For ten or fifteen
minutes after its first appearance the light from a vividly
illuminated actinic cloud, looked at perpendicularly, is ab-
solutely quenched by a NicoFs prism with its longer diag-
onal vertical. But as the sky-blue is gradually rendered
impure by the growth of the particles — ^in other words, as
real clouds begin to be formed — ^the polarization begins to
decay, a portion of the light passing through the prism
in all its positions. It is worthy of note, that, for some

ARTIFICIAL SKY 123

time after the cessation of perfect polarization, 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 polarization, in a direction per-
pendicular to the illuminating beam, is also illustrated 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 experimental tube. A few bub-
bles 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 polarized 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 emer-
gent beam being therefore also vertical. As the light
continued to act, a superb blue cloud, visible to both my
assistant and myself, was slowly formed. But this cloud,
so deep and rich when looked at from the positions men-
tioned, utterly disajjpeared when looked at vertically down-
loardj or vertically upward. Reflection from the cloud was
not possible in these directions. When the large Nicol

* This shows that particles too large to polarize the blue, polarize perfectly
light of lower refrangibilitj.

124 FRAGMENTS OF SCIENCE

was slowly turned round its axis, the eye of tlie observer
being on the level of the beam, and the line of vision
perpendicular to it, entire extinction of the light emitted
horizontally occurred when the longer diagonal of the
large Nicol was vertical. But now a vivid blue cloud
was seen when looked at downward or upward. This
truly fine experiment, which I contemplated making on
my own account, was first definitely suggested by a
remark in a letter addressed to me by Professor Stokes.
As regards the polarization of skylight, the greatest
stumbling-block has hitherto been, that, in accordance
with the law of Brewster, which makes the index of re-
fraction the tangent of the polarizing angle, the reflection
which produces perfect polarization would require to be
made in air upon air; and indeed this led many of our
most eminent men, Brewster himself among the number, to
entertain the idea of ae'rial molecular reflection.* I have,
however, operated upon substances of widely different re-

' **The cause of the polarization is evidently a reflection of the sun's light
upon something. The question is on what ? Were the angle of maximum
polarization 76**, we should look to water or ice as the reflecting body, how-
ever 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-
about, is the correct angle, and that therefore whatever be the body on which
the light has been reflected, if polarized by a single reflection, the polarizing
angle must be 45**, and the index of refraction, which is the tangent of that
angle, unity; in other words, the reflection would require to be made in air
upon airl" (Sir John Herschel, * 'Meteorology," par. 233.)

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

ARTIFICIAL SKY 125

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

Suppose our atmosphere surrounded by an envelope
impervious to light, but with an aperture on the sunward-
side through which a parallel beam of solar light could
enter and traverse the atmosphere. Surrounded by air
not directly illuminated, the track of such a beam would
resemble that of the parallel beam of the electric lamp
through an incipient cloud. The sunbeam would be blue,
and it would discharge laterally light in precisely the same
condition as that discharged by the incipient cloud. In
fact, the azure revealed by such a beam would be to all
intents and purposes that which I have called a "blue
cloud." Conversely our "blue cloud" is, to all intents
and purposes, an artificial sJcy.^

* The opinion of Sir John Herschel, connecting the polarization and the
blue color of the sky, is verified by the foregoing results. "The more the sub-
ject [the polarization of skylight] is considered," writes this eminent philoso-
pher, "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 color
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 polarization is developed in its highest degree,
and that where there is the slightest perceptible tendency to cirrus it is mate-
rially impaired." This applies word for word to our "incipient clouds."

126 FRAGMENTS OF SCIENCE

But, as regards tlie polarization of the sky, we know
that not only is the direction of maximum polarization at
right angles to the track of the solar beams, but that at
certain angular distances, probably variable ones, from the
sun, "neutral points," or points of no polarization, exist,
on both sides of which the planes of atmospheric polar-
ization are at right angles to each other. I have made
various observations upon this subject which are reserved
for the present; but, pending the more complete examina-
tion 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 mat-
ter 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 polarization obtained with
the incipient clouds. The light discharged laterally from
the track of the illuminating beam was polarized, though
not perfectly, the direction of maximum polarization 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 polarization. Keeping
the positions of the Nicol and the selenite constant, the
same colors were observed throughout the entire beam,
when the line of vision was perpendicular to its length.

The horizontal column of air, thus illuminated, was
eighteen feet long, and could therefore be looked at very
obliquely. I placed myself near the end of the beam, as
it issued from the electric lamp, and^ looking through the

ARTIFICIAL SKY 127

Nicol and selenite more and more obliquely at the beam,
observed the colors fading until they disappeared. Aug-
menting the obliquity the colors 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 polarized
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
fourteen or fifteen feet being attainable. This beam ex-
hibited all the effects observed with the beam in the labo-
ratory. Even the uncondensed electric light falling on
the floating matter showed, though faintly, the effects
of polarization.

When the air was so sifted as to entirely remove the
visible floating matter, it no longer exerted any sensible
action upon the light, but behaved like a vacuum. The
light is scattered and polarized by particles^ not by mole-
cules 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 experiments on
neutral points, when my attention was drawn by Sir
Charles Wheatstone to an important observation com-
municated to the Paris Academy in 1860 by Professor
Govi, of Turin.' M. Govi had been led to examine a
beam of light sent through a room in which were succes-

1 ♦'

Comptes Rendus/' tome li. pp. 360 and 669.

128 FRAGMENTS OF SCIENCE

sively diffused the smoke of incense, and tobacco-smoke.
His first brief communication stated the fact of polariza-
tion by such smoke; but in his second communication he
announced the discovery of a neutral point in the beam,
at the opposite sides of which the light was polarized in
planes at right angles to each other.

But unlike my observations on the laboratory air, and
unlike the action of the sky, the direction of maximum
polarization in M. Govi'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. Govi or any other investigator has pursued it
further.

I had noticed, as before stated, that as the clouds
formed in the experimental tube became denser, the po-
larization of the light discharged at right angles to the
beam became weaker, the direction of maximum polariza-
tion becoming oblique to the beam. Experiments on the
fumes of chloride of ammonium gave me also reason to
suspect that the position of the neutral point was not
constant, but that it varied with the density of the illumi-
nated 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 sufficient time
being allowed for their uniform diffusion, the electric
beam was sent through the smoke. From the track of
the beam polarized light was discharged; but the direc-
tion of maximum polarization, instead of being perpen-
dicular, now enclosed an angle of only 12° or 13° with the
axis of the beam.

A neutral point, with complementary effects at oppo-

ARTIFICIAL SKY 129

site sides of it, was also exhibited by tte 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 turn-
ing 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 suc-
cessively from 54° to 49°, 43° and 33''.

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

1st obse

2d

3d

4th

5th

6th

vation . • • , . 2 feet 2 inches

* 2 " 6 "

• 2 " 10 "

3 *• 2 "

3 " 7 "

4 " 6 "

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

The laboratory was next filled with the fumes of gun-
powder. In five successive experiments, corresponding to

130 FRAGMENTS OF SCIENCE

five different densities of the gunpowder- smoke, the angles
enclosed between the line of vision to the neutral point
and the axis of the beam were 63°, 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 polarization enclosed, in this
case, an angle of 12°, or thereabout, with the axis of the
beam. Looked at, as in the former instances, from a posi-
tion near the electric lamp, no neutral point was observed
throughout the entire extent of the beam.

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

With the fumes of chloride of ammonium substantially
the same results were obtained. Sufficient, however, has
been here stated to illustrate the variability of the position
of the neutral point. '

By a puff of tobacco-smoke, or of condensed steam.

> Brewster has proved the variability of the position of the neutral point for
skylight with the sun's altitude, a result obviously connected with the foregoing
experiments.

ABTIFICIAL SKY 131

blown into the illuminated beam, the brilliancy of the
selenite colors may be greatly enhanced. But with differ-
ent clouds two different effects are produced. Let the
ring-system observed in the common air be brought to its
maximum strength, and then let an attenuated cloud of
chloride of ammonium be thrown into the beam at the
point looked at ; the ring-system flashes out with aug-
mented brilliancy, but the character of the polarization
remains unchanged. This is also the case when phos-
phorus, 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 snlphur-fumes the bril-
liancy of the colors is exceedingly intensified; but in none
of these cases is there any change in the character of the
polarization.

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 polarization 90®, causing
the centre of the ring-system to change from black to
white, and the rings themselves to emit their comple-
mentary colors.*

Almost all liquids have motes in them sufficiently
numerous to polarize 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

' Sir John Herschel suggested to me that this change of the polarization from
positive to negative may indicate a change from polarization by reflection to
polarization by refraction. This thought repeatedly occurred to me while look-
ing at the effects ; but it will require much following up before it emerges into
clearness.

132 FRAGMENTS OF SCIENCE

permitted to pass through it, scarcely any polarized light
is discharged, and scarcely any color produced with a
plate of selenite. But if a bit of soap be agitated in the
water above the beam, the moment the infinitesimal par-
ticles reach the light the liquid sends forth laterally al«
most perfectly polarized light; and if the selenite be em-
ployed, vivid colors flash into existence. A still more
brilliant result is obtained with mastic dissolved in a great
excess of alcohol.

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

§ 8. The Sky op the Alps

The vision of an object always implies a differential
action on the retina of the observer. The object is dis-
tinguished from surrounding space by its excess or defect
of light in relation to that space. By altering the illu-
mination, either of the object itself or of its environment,
we alter the appearance of the object. Take the case of
clouds floating in the atmosphere with patches of blue
between them. Anything that changes the illumination
of either alters the appearance of both, that appearance
depending, as stated, upon differential action. Now the
light of the sky, being polarized, may, as the reader of

THE SKY OF THE ALPS 133

the foregoing pages knows, be in great part quenched by
a Nicol's prism, while the light of a common cloud, being
unpolarized, cannot be thus extinguished. Hence the pos-
sibility 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
indistinguishable from the residual dull tint which out-
lives the extinction of the brilliancy of the sky. A cloud
less deeply shaded, but still deep enough, when viewed
with the naked eye, to appear dark on a bright ground,
is suddenly changed to a white cloud on a dark ground
by the quenching of the light behind it. When a reddish
cloud at sunset chances to float in the region of maximum
polarization, the quenching of the surrounding light causes
it to flash with a brighter crimson. Last Easter eve the
Dartmoor sky, which had just been cleansed by a snow-
storm, wore a very wild appearance. Round the horizon
it was of steely brilliancy, while reddish cumuli and cirri
floated southward. When the sky was quenched behind
them these floating masses seemed like dull embers sud-
denly 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 re-
ferred 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 splendor. The side of the Weisshorn seen from the
Bel Alp, being turned from the sun, was tinted mauve;
but I wished to observe one of the rose -colored buttresses

134 FRAGMENTS OF SCIENCE

of the mountain. Such a one was visible from a point a
few hundred feet above the hotel. The Matterhorn also,
though for the most part in shade, had a crimson projec-
tion, while a deep ruddy red lingered along its western
shoulder. Four distinct peaks and buttresses of the Dom,
in addition to its dominant head — all covered with pure
snow — were reddened by the light of sunset. The shoul-
der of the Alphubel was similarly colored, while the great
mass of the Fletschorn was all aglow, and so was the
snowy spine of the Monte Leone.

Looking at the Weisshorn through the Nicol, the glow
of its protuberance was strong or weak according to the
position of the prism. The summit also underwent strik-
ing changes. In one position of the prism it exhibited a
pale white against a dark background; in the rectangular
position it was a dark mauve against a light background.
The red of the Matterhorn changed in a similar manner;
but the whole mountain also passed through wonderful
changes of definition. The air at the time was filled with
a silvery haze, in which the Matterhorn almost disap-
peared. This could be wholly quenched by the Nicol,
and then the mountain sprang forth with astonishing so-
lidity 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 polarization.
By a little practice with the Nicol it was easy to render
the extinction of the light, or its restoration, almost in-
stantaneous. When the sky was quenched, the four minor
peaks and buttresses, and the summit of the Dom, to-
gether with the shoulder of the Alphubel, glowed as if
set suddenly on fire. This was immediately dimmed by

THE SKY OF THE ALPS 135

turning the Nicol througli 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 be-
tween them and me was highly opalescent, and the
quenching of this intermediate glare augmented remark-
ably the distinctness of the mountains.

On the morning of August 24 similar effects were
finely shown. At 10 A.M. all three mountains, the Dom,
the Matterhorn, and the Weisshorn, were powerfully
affected by the Nicol. But in this instance also, the line
drawn to the Dom being very nearly perpendicular to the
solar beams, the effects on this mountain were most strik-
ing. The gray summit of the Matterhorn, at the same
time, could scarcely be distinguished from the opalescent
haze around it; but when the Nicol quenched the haze,
the summit became instantly isolated, and stood out in
bold definition. It is to be remembered that in the pro-
duction of these effects the only things changed are the
sky behind, and the luminous haze in front of the moun-
tains; that these are changed because the light emitted
from the sky and from the haze is plane polarized light,
and that the light from the snows and from the moun-
tains, being sensibly unpolarized, 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 ren-
ders 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

136 FRAGMENTS OF SCIENCE

colored — 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 polarized. 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 dark-
ened by pines. This ridge may be projected upon the
dark slopes at the opposite side of the Ehone valley, and
between both we have the blue haze referred to, throwing
the distant mountains far away. But at certain hours of
the day the haze may be quenched, and then the Massa
ridge and the mountains beyond the Ehone 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 atmos-
phere. At certain times and places it is almost as blue
as the sky itself; but to see its color, the attention must
be withdrawn from the mountains and from the trees
which cover them. In point of fact, the haze is a piece
of more or less perfect sky; it is produced in the same
manner, and is subject to the same laws, as the firmament
itself. We live in the sky, not under it.

These points were further elucidated by the 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
Bhone, as looked at from the Bel Alp, was very remark-

THE SKY OF THE ALPS 187

able. Toward evening the sky above the mountains op-
posite to my place of observation yielded a series of the
most splendidly-colored iris-rings; but on lowering the sel-
enite until it had the darkness of the pines at the oppo-
site side of the Rhone valley, instead of the darkness of
space, as a background, the colors were not much dimin-
ished 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 favorable circumstances, produce chromatic effects
of polarization almost as vivid as those produced by the
sky itself.

Again: the light of a landscape, as of most other things,
consists of two parts; the one, coming purely from super-
ficial reflection, is always of the same color as the light
which falls upon the landscape; the other part reaches us
from a certain depth within the objects which compose the
landscape, and it is this portion of the total light which
gives these objects their distinctive colors. The white
light of the sun enters all substances to a certain depth,
and is partly ejected by internal reflection; each distinct
substance absorbing and reflecting the light, in accordance
with the laws of its own molecular constitution. Thus the
solar light is sifted by the landscape, which appears in
such colors and variations of color as, after the sifting
process, reach the observer's eye. Thus the bright green
of grass, or the darker color of the pine, never comes to
us alone, but is always mingled with an amount of light
derived from superficial reflection. A certain hard bril-
liancy is conferred upon the woods and meadows by this
superficially-reflected light. Under certain circumstances,
it may be quenched by a Nicol's prism, and we then ob-

138 FRAGMENTS OF SCIENCE

tain the true color 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 emis-
sion. The needles of the pines show this effect very well,
large-leaved trees still better; while a glimmering field of
maize exhibits the most extraordinary variations when
looked at through the rotating Nicol.

Thoughts and questions like those here referred to took
me, in August, 1869, to the top of the Aletschhorn. The
effects described in the foregoing paragraphs were, for the
most part, reproduced on the summit of the mountain. I
scanned the whole of the sky with my Nicol. Both alone,
and in conjunction with the selenite, it pronounced the
perpendicular to the solar beams to be the direction of
maximum polarization. But at no portion of the firma-
ment was the polarization complete. The artificial sky
produced in the experiments recorded in the preceding
pages could, in this respect, be rendered far more per-
fect than the natural one; while the gorgeous *' residual
blue,** which makes its appearance when the polarization
of the artificial sky ceases to be perfect, was strongly con-
trasted with the lack-lustre hue which, in the case of the
firmament, outlived the extinction of the brilliancy. With
certain substances, however, artificially treated, this dull
residue may also be obtained.

All along the arc, from the Matterhorn to Mont Blanc,
the light of the sky immediately above the mountains was
powerfully acted upon by the Nicol. In some cases the
variations of intensity were astonishing. I have already
said that a little practice enables the observer to shift the
Nicol from one position to another so rapidly as to render

THE SKY OF THE ALPS 139

the alternative extinction and restoration of the light im-
mediate. When this was done along the arc to which I
have referred, the alternations of light and darkness re-
sembled the play of sheet lightning behind the mountains.
There was an element of awe connected with the sudden-
ness with which the mighty masses, ranged along the line
referred to, changed their aspect and definition under the
operation of the prism.

pn the 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 impor-
tant American magazine.]

THE SKY'

INVITED to write for tlie ** Forum*' an article that
would have brought me face to face with *' problems
of life and mind,'* for which I was at the moment
unprepared, and unwilling to decline a request so cour-
teously 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 glance 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, re-
cording from day to day the aspects of Nature or the indi-
cations 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 can-
not rest content with observation and experiment, whose
love of causal unity tempts them perpetually to break

> From "The Forum," February, 1888.

(140)

THE SKY 141

througli the limitations of the senses, and to seek beyond
them the roots and reasons of the phenomena which the
observer and experimenter record. To such spirits — ad-
venturous and firm— we are indebted for our deeper knowl-
edge 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 knowl-
edge. 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— of patiently *4ntending the mind,** as
Newton phrased it; and as Copernicus, Newton, and Dar-
win practiced 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, never-
theless, 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
subsequently 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 reflec-
tion and transmission,** in virtue of which they were some-
times 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.

142 FRAGMENTS OF SCIENCE

The theorist takes his conceptions from the world 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, and 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
sft'ect. By the illustrious astronomer last named the con-
ception of waves was definitely transplanted from its ter-
restrial birthplace to a universal medium whose undula-
tions could only be intellectually discerned. Huyghens
did not establish the undulatory theory, but he took the
first firm step toward establishing it. Laying this theory
at the root of the phenomena of light, he went a good way
toward showing that these phenomena are the necessary
outgrowth of the conception.

By analysis and synthesis Newton proved the white
light of the sun to be a skein of many colors. The cause
of color was a question which immediately occupied his
thoughts; and here, as in other cases, he freely resorted
to hypothesis. He saw, with his mind's eye, his luminif-
erous corpuscles crossing the bodily eye, and imparting
successive shocks to the retina behind. To differences of
*' bigness'* in the light-awakening molecules Newton as-
cribed the different color-sensations. In the undulatory
theory we are also confronted with the question of color;
and here again, to inform and guide us, we have the anal-
ogy of sound. Aerial waves of different lengths, or peri-
ods, produce notes of different pitch; and to differences
of wave-length in that mysterious medium, the all-pervad-
ing ether, differences of color are ascribed. Hooke had

THE SKV 14S

already discoursed of "a very quick motion that causes
light, as well as a more robust that causes heat.*' New-
ton had ascribed the sensation of red to the shock of his
grossest, and that of violet to the shock of his finest, lumi-
niferous projectiles. Defining the one, and displacing the
other of these notions, the wave-theory affirms red to be
produced 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 grew in strength and favor, until it finally sup-
planted 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 ** colors 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 colors.
Hooke showed that all transparent films, if only thin
enough, displayed such colors; and he proved that the
particular color 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 colored lines.'*
Newton, bent on knowing the exact relation between the

144 FRAGMENTS OF SCIENCE

thickness of tlie film and the color 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 of air of gradually in-
creasing thickness from the place of contact outward.
As he expected, he found the place of contact surrounded
by a series of colored circles, still known all over the
world as *'!N"ewton's rings." The colors of his first cir-
cle, which immediately surrounded a black central spot,
Newton called "colors of the first order"; the colors of
the second circle, "colors 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
labors are his facts and measurements; his theory has
disappeared. It was reserved for the illustrious Thomas
Young, a man of intellectual 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 enclosed between the plane and convex glasses.
The colors of thin plates were "residual colors"- — surviv*
als 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 be-
ing 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 color 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 thick-

THE SKY 145

ness requisite to produce this color, were broken into bits
and scattered in the air, Newton inferred that the tiny
fragments would display the blue color. Tantamount to
this, he considered, was the action of minute water-parti-
cles in the incipient stage of their condensation from aque-
ous vapor. Such particles suspended in our atmosphere
ought, he supposed, to generate the serenes fc skies. New-
ton 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
England. 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 observation 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
colors of the sky. The assumption 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 endeavored to deduce from
them the blue of the firmament and the morning and
evening red.

It is not, however, necessary to invoke the blue of the
first order to explain the color of the sky; nor is it neces-
sary to impose upon condensing vapor the difficult, if not
impossible, task of forming bladders, when it passes into
the liquid condition. Let us examine the subject. JSau-
de- Cologne is prepared by dissolving aromatic gums or res-

SCIENCE — Y — 7

146 FRAGMENTS OF SCIENCE

ins in alcohol. Dropped into water, the scented Jdquid
immediately produces a white cloudiness, due to the pre-
cipitation of the substances previously held in solution.
The solid particles are, however, comparatively gross ; but,
by diminishing the quantity of the dissolved gum, the pre-
cipitate may be made to consist of extremely minute parti-
cles. Briicke, for example, dissolved gum-mastic, in cer-
tain proportions, in alcohol, and carefully dropping his
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 con-
tains them shows a distinctly blue color. The bluish color
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 pulverized 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
ijake of Geneva, 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 produc-
tion of the color. 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

THE SKY 147

thus generated, wliicli prove their brotherhood with the
natural 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 cer-
tainly, 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 num-
ber of substances might be mentioned whose vapors, when
mixed with air and subjected 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 generates them. And here I must appeal to
the inner vision already spoken of. Remembering the dif-
ferent sizes of the waves of light, it is not difficult to see
that our minute particles 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 im-
agine light-undulations of different dimensions, but all ex-
ceedingly 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 con-
spicuous action upon the smallest ones; and that the color-
sensation answering to the smallest waves — in other words,
the color line — will be predominant in the scattered light.
This harmonizes perfectly 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 colors of the spectrum; blue is merely the predom-
inant color. By means of our artificial skies we can take,
as it were, the firmament in our hands and examine it at

148 FRAGMENTS OF SCIENCB

our leisure. Like the natural sky, the artificial one shows
all the colors of the spectrum, but blue in excess. Mixing
very small quantities of vapor with air, and bringing the
decomposing luminous beam into action, we produce parti-
cles too small to shed any sensible light, but which may,
and doubtless do, exert an action on the ultra-violet waves
of the spectrum. We can watch these particles, or rather
the space they occupy, till they grow to a size able to
yield the firmamental azure. As the particles grow larger
under the continued action of the light, the azure becomes
less deep; while later on a milkiness, such as we often ob-
serve 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" diifuses
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 sky bends toward the horizon, the purer blue is
impaired. To measure the intensity of the color De Saus-
sure invented a cyanometer, and Humboldt has given us
a mathematical formula to express the diminution of the
blue, in arcs drawn east and west from the zenith down-
ward. This diminution is a natural consequence 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 atmos-

THE SKY 149

phere. Now, the higher we ascend, the more do we leave
behind us the particles which scatter the light; the nearer,
in fact, do we approach 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 mi-
nute 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 looking through
the prism at a snow slope, or a white wall. Turning the
prism round its axis, the light coming from these objects
does not undergo any sensible change. But when the
prism is directed toward the sky the great probability is
that, on turning it, variations in the amount of light
reaching the eye will be observed. Testing various por-
tions of the sky with due diligence, we at length discover
one particular direction where the difference of illumina-
tion becomes a maximum. Here the Nicol, in one posi-
tion, seems to offer no impediment to the passage of the
skylight; 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 skylight because that light is polar-
ized, while the light from the white wall or the white
snow, being unpolarized, is not affected by the rotation
of the prism.

In the case of our manufactured sky not only is the

150 FRAGMENTS OF SCIENCE

azure of the firmament reproduced, but these phenomena
of polarization 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 polarized
at right angles to the illuminating beam. The artificial
sky may, in fact, be employed as a second Nicol, between
which and a prism held in the hand many of the beautiful
chromatic phenomena observed in an ordinary polariscope
may be reproduced.

Let us now complete our thesis by following the larger
light-waves, which have been able to pass among the
aerial particles with comparatively little fractional loss.
"Without going beyond inferential considerations, we can
state what must occur. The action of the particles upon
the solar light increases with the atmospheric distances
traversed by the sun's rays. The lower the sun, there-
fore, the greater the action. The shorter waves of the
spectrum being more and more withdrawn, the teadency
is to give the longer waves an enhanced predominance
in the transmitted light. The tendency, in other words,
of this light, as the rays traverse ever- increasing dis-
tances, is more and more toward red. This, I say, might
be stated as an inference, but it is borne out in the most
impressive manner by facts. When the Alpine sun is
getting, or, better still, some time after he has set, leav-
ing 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
mountain heads, under favorable atmospheric conditions,

THE SKY 151

shine like rubies. And all this splendor is evoked by the
simple mechanism of minute particles, themselves without
color, suspended in the air. Those who referred the ex-
traordinary succession of atmospheric glows, witnessed
some years ago, to a vast and violent discharge of vol-
canic 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 mech-
anism necessary to produce the morning and the evening
red, though of variable efficiency, 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 re-
ferred. It was the long-continued repetition of the glows
which rendered the volcanic theory highly probable.

VI

VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE

1870

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

At three p.m. on Tuesday, December 6, we got away,
gliding .successively past Whitecliff Bay, Bembridge, San-
down, Shanklin, Yentnor, and St. Catherine's Lighthouse.
On Wednesday morning we sighted the Isle of Ushant,
on the French side of the Channel. The northern end of
the island has been fretted by the waves into detached
tower-like masses of rock of very remarkable appearance.
In the Channel the sea was green, and opposite Ushant
it was a brighter green. On Wednesday evening we com-
mitted ourselves to the Bay of Biscay. The roll of the
Atlantic was full, but not violent. There had been
(152)

VOYAGE TO ALGERIA 163

scarcely a gleam of sunshine throughout the day, but the
cloud-forms were fine, and their apparent solidity impres-
sive. On Thursday morning the green of the sea was
displaced by a deep indigo blue. The whole of Thurs-
day we steamed across the bay. We had little blue sky,
but the clouds were again grand and varied — cirrus, stra-
tus, cumulus, and nimbus, we had them all. Dusky hair-
like trails were sometimes dropped from the distant clouds
to the sea. These were falling showers, and they some-
times occupied the whole horizon, while we steamed
across the rainless circle which was thus surrounded.
Sometimes we plunged into the rain, and once or twice,
by slightly changing our course, avoided a heavy shower.
From time to time perfect rainbows spanned the heavens
from side to side. At times a bow would appear in 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 Kicol's prism, a dark aperture
being thus opened into stellar space.

At sunset on Thursday the denser clouds were fiercely
fringed, while through the lighter ones seemed to issue
the glow of a conflagration. On Friday morning we
sighted C^pe 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. Toward evening the wind strength-

154 FRAGMENTS OF SCIENCE

ened to a gale, and at dinner it was difficult to preserve
the plates and dishes from destruction. Our thinned
company hinted that the rolling had other consequences.
It was very wild when we went to bed. I slumbered
and slept, but after some time was rendered anxiously
conscious that my body had become a kind of projectile,
with the ship's side for a target. I gripped the edge of
my berth to save myself from being thrown out. Out-
side, 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 labored 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.
*'!N'o man knows the force of water," said one of the offi-
cers, ''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 sa-
loon to chaos. Furniture crashed, glasses rang, and
alarmed inquiries immediately followed. Amid the noises
I heard one note of forced laughter; it sounded very
ghastly. Men tramped through the saloon, and busy
voices were heard aft, as if something there had gone
wrong.

I rose, and not without difficulty got into my clothes.
In the after- cabin, under the superintendence of the able
and energetic navigating lieutenant, Mr. Brown, a group
of blue-jackets were working at the tiller-ropes. These
had become loose, and the helm refused to answer the

VOYAGE TO ALGERIA 155

wheel High moral lessons might be gained on ship-
board, by observing what steadfast adherence to an object
can accomplish, and what large effects are heaped up by
the addition of infinitesimals. The tiller-rope, as the
blue- jackets strained in concert, seemed hardly to move;
still it did move a little, until finally, by timing the pull
to the lurching of the ship, the mastery of the rudder
was obtained. I had previously gone on deck. Eound
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 toward a cleat not far from
the wheel.' Eound it I coiled my arms. With the ex-
ception 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 nar-
row, and during our expedition she lacked the steadying
influence of sufiicient ballast. She was for a time prac-
tically rudderless, and lay in the trough of the sea. I
could see the long ridges, with some hundreds of feet be-
tween their crests, rolling upoQ the ship perfectly parallel
to her sides. As they approached, they so grew upon the
eye as to render the expression "mountains high" intel-
ligible. At all events, there was no mistaking their me-
chanical might, as they took the ship upon their shoulders
and swung her like a pendulum. The deck sloped some-
times at an angle which I estimated at over forty-five de-

* The cleat is a J-shaped mass of metal employed for the fastening of ropeg.

166 FRAGMENTS OF SCIENCE

grees ; wanting my previous Alpine practice, I should liave
felt less confidence in my grip of the cleat. Here and
there the long rollers were tossed by interference into
heaps of greater height. The wind caught their crests,
and scattered them over the sea, the whole surface of
which was seething white. The aspect of the clouds was
a fit accompaniment to the fury of the ocean. The moon
was almost full — at times concealed, at times revealed, as
the scud flew wildly over it. These things appealed to
the eye, while the ear was filled by the groaning of the
screw and the whistle and boom of the storm.

Nor was the outward agitation the only object of inter-
est 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 for-
eign 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 excel-
lent 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 thamping 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 roll-
ing diminished, a certain amount of pitching taking its

VOYAGE TO ALGERIA 157

place. Our speed had fallen from eleven knots to two.
I went again to bed. After a space of calm, wlien we
seemed crossing the vortex of a storm, heavy tossing re-
commenced. 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 Fri-
day at noon to Saturday at noon we accomplished sixty-
six miles, or an average of less than three miles an hour.
I overheard the sailors talking about this storm. The
"Urgent," according to those that knew her, had never
previously experienced anything like it.*

All through Saturday the wind, though somewhat so-
bered, blew dead against us. The atmospheric efl'ects
were exceedingly fine. The cumuli resembled mountains
in shape, and their peaked summits shone as white as
Alpine snows. At one place this resemblance was greatly
strengthened by a vast area of cloud, uniformly illumi-
nated, and lying like a neve 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. Eainbows had been frequent
throughout the day, and at night a perfectly continuous
lunar bow spanned the heavens from side to side. Its
colors were feeble; but, contrasted with the black ground
against which it rested, its luminousness was extraor-
dinary.

Sunday morning found us opposite to Lisbon, and at
midnight we rounded Cape St. Vincent, where the lurch-
ing seemed disposed to recommence. Through the kind-

^ There is, it will be seen, a fair agreement between these impressions and
those so vigorously described by a scientific correspondent of the "Times."

158 FRAGMENTS OF SCIENCE

ness of Lieutenant Walton, a cot liad 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 boun-
daries. But the wind was now behind us, and we went
along at a speed of eleven knots. We felt certain of
reaching Cadiz by three. But a new lighthouse came in
sight, which some affirmed to be Cadiz Lighthouse, while
the surrounding houses were declared to be those of Cadiz
itself. Out of deference to these statements, the navigat-
ing lieutenant changed his course, and steered for the
place. A pilot came on board, and he informed us that
we were before the mouth of the Guadalquivir, and that
the lighthouse was that of Cipiona. Cadiz was still some
eighteen miles distant.

We steered toward the city, hoping to get into the har-
bor 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 ko be
small turrets. A pilot was taken on board; for there is
a dangerous shoal in the harbor. The appearance of the
town as the sun shone upon its white and lofty walls was
singularly beautiful. We cast anchor; some officials ar-
rived and demanded a clean bill of health. We had none.
They would have nothing to do with us; so the yellow
quarantine flag was hoisted, and we waited for permission
to land the Cadiz party. After some hours' delay, the
English consul and vice-consul came on board, and with.

VOYAGE TO ALGERIA 159

tiiem a Spanisli officer ablaze with gold lace and decora-
tions. Under slight pressure the requisite permission had
been granted. We landed our party, and in the afternoon
weighed anchor. Thanks to the kindness of our excellent
paymaster, I was here transferred to a more roomy berth.
Cadiz soon sank beneath the sea, and we sighted in
succession Cape Trafalgar, Tarifa, and the revolving light
of Ceuta. The water was very calm, and the moon rose
in a quiet heaven. She swung with her convex surface
downward, the common boundary between light and shadow
being almost horizontal. A pillar of reflected light shim-
mered up to us from the slightly rippled sea. I had pre-
viously noticed the phosphorescence of the water, but to-
night it was stronger than usual, especially among the
foam at the bows. A bucket let down into the sea
brought up a number of the little sparkling organisms
which caused the phosphorescence. I caught some of
them in my hand. And here an appearance was observed
which was new to most of us, and strikingly beautiful to
all. Standing at the bow and looking forward, at a dis-
tance of forty or fifty yards from the ship, a number of
luminous streamers were seen rushing toward 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 oth-
ers. Sometimes as many as six at a time would rush at
us, bend with extraordinary rapidity round a sharp curve,
and afterward 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

160 FRAGMENTS OF SCIENCE

rush of the creatures through the water had started the
phosphorescence, every spark of which was converted by
the motion of the retina into a line of light. Each por-
poise was thus wrapped in a luminous sheath. The phos-
phorescence did not cease at the creature's tail, but was
carried many porpoise-lengths behind it.

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

At six o'clock next morning, the gun at the Signal
Station on the summit of the rock boomed. At eight
the band on board the "Trafalgar" training-ship, which
was in the harbor, struck up the national anthem; and im-
mediately afterward a crowd of mite- like cadets swarmed
up the rigging. After the removal of the apparatus be-

VOYAGE TO ALGERIA 161

longing to tlie Gribraltar party we went on sliore. Winter
was in England when we left, but here we had the warmth
of summer. The vegetation was luxuriant — palm-trees,
cactuses, and aloes, all ablaze with scarlet flowers. A
visit to the Governor was proposed, as an act of neces-
sary courtesy, and I accompanied Admiral Ommaney and
Mr. Huggins to "the Convent," or Government House.
We sent in our cards, waited for a time, and were then
conducted by an orderly to his Excellency. He is a fine
old man, over six feet high, and of frank military bearing.
He received us and conversed with us in a very genial
manner. He took us to see his garden, his palms, his
shaded promenades, and his orange-trees loaded with fruit,
in all of which he took manifest delight. Evidently "the
hero of Kars" had fallen upon quarters after his own
heart. He appeared full of good nature, and engaged
us on the spot to dine with him that day.

We sought the town-major for a pass to visit the lines.
While awaiting his arrival I purchased a stock of white
glass bottles, with a view to experiments on the color of
the sea. Mr. Huggins and myself, who wished to see the
rock, were taken by Captain Salmon d to the library, where
a model of Gibraltar is kept, and where we had a useful
preliminary lesson. At the library we met Colonel Ma-
berly, 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 U3
over to an intelligent Irishman, who was directed to show
us everything that we desired to see, and to hide nothing
from us. We took the "upper line," traversed the gal-
leries hewn through the limestone; looked through the
embrasures, which opened like doors in the precipice, to-

162 FRAGMENTS OF SCIENCE

ward the hills of Spain; reached St. George *8 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 neu-
tral ground. Behind the Spanish lines rose the conical
hill called the Queen of Spain's Chair. The general as-
pect of the mainland from the rock is bold and rugged.
Doubling back from the galleries, we struck upward
toward the crest, reached the Signal Station, where we
indulged in *' shandy -gaff*' and bread and cheese. Thence
to O'Hara's Tower, the highest point of the rock. It
was built by a former governor, who, forgetful of the
laws of terrestrial curvature, thought he might look from
the tower into the port of Cadiz. The tower is riven, and
it may be climbed along the edges of the crack. We got
to the top of it; thence descended the curious Mediter-
ranean Stair — a zigzag, mostly of steps down a steeply
falling slope, amid palmetto brush, aloes, and prickly
pear.

Passing over the Windmill Hill, we were joined at the
**Governor's Cottage'* by a car, and drove afterward to
the lighthouse at Europa Point. The tower was built,
I believe, by Queen Adelaide, and it contains a fine diop-
tric 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 conspic-
uous. The freshness of the Governor's nature showed
itself best when he spoke of his old antagonist in arms,
Mouravieff. Chivalry in war is consistent with its stern

VOYAGE TO ALGERIA 163

prosecution. These two men were chivalrous, and after
striking the last blow became friends forever. Our kind
and courteous reception at Gibraltar is a thing to be re-
membered with pleasure.

On December 15 we committed ourselves to the Medi-
terranean. The views of Gibraltar with which we are
most acquainted represent it as a huge ridge; but its as-
pect, 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 adjacent neutral
ground. There are theories afloat as to its having been
blown from Sahara. The Mediterranean throughout this
first day, and indeed throughout the entire voyage to
Oran, was of a less deep blue than the Atlantic. Pos-
sibly the quantity of organisms may have modified the
color. At night the phosphoresence was startling, break-
ing suddenly out along the crests of the waves formed by
the port and starboard bows. Its strength was not uni-
form. Having flashed brilliantly for a time, it would
in part subside, and afterward regain its vigor. Several
large phosphorescent masses of weird appearance also
floated past.

On the morning of the 16th we sighted the fort and
lighthouse of Marsa el Kibir, and beyond them the white
walls of Oran lying in the bight of a bay, sheltered by
dominant hills. The sun was shining brightly; during
our whole voyage we had not had so fine a day. The
wisdom which had led us to choose Oran as our place

164 FRAGMENTS OF SCIENCE

of observation seemed demonstrated. A ratlier excitable
pilot came on board, and lie guided us in behind the
Mole, wbicli 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 labors.

M. Janssen, who had rendered his name celebrated by
his observations of the eclipse in India in 1868, when
he showed the solar flames to be eruptions of incandes-
cent hydrogen, was already encamped in the open country
about eight miles from Oran. On December 2 he had
quitted Paris in a balloon, with a strong young sailor as
his assistant, had descended near the mouth of the Loire,
seen M. Gambetta, and received from him encouragement
and aid. On the day of our arrival his encampment was
visited by Mr. Huggins, and the kind and courteous
Engineer of the Port drove me subsequently, in his own
phaeton, to the place. It bore the best repute as regards
freedom from haze and fog, and commanded an open out-
look; 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 disposition of the party. Here the ' tents were

^ Esparto is a kind of grass now much used in the manufacture of paper.

VOYAGE TO ALGERIA 165

T)itclied, on tlie Saturday, by Captain Salmond and his
intelligent corps of sappers, tlie 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
southward, about twenty yards to tlie right is a small
bastionet, intended to carry a gun or two. Its roof I
thought would form an admirable basis for my telescope,
while the view of the surrounding country was unimpeded
in all directions. The authorities kindly allowed me the
use of this bastionet. Two men, one a blue-jacket named
Elliot, and the other a marine named Hill, were placed
at my disposal by Lieutenant "Walton; and, thus aided,
on Monday morning I mounted my telescope. The in-
strument 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 loiter-
ers, and the thresholds were occupied by picturesque
groups. Some of the men were very fine. We saw many
straight, manly fellows who must have been six feet four
in height. They passed us with perfect indifference, evinc-
ing 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.
Fc», was a tall, good-humored fellow, who came smiling
up to me, and muttered something about "les Anglais.'*
The mixed population of Oran is picturesque in the high-
est degree: the Jews, rich and poor, varying in their
costumes as their wealth varies; the Arabs more pict-

166 FRAGMENTS OF SCIENCE

uresque still, and of all shades of complexion — tlie ne-
groes, the Spaniards, the French, all grouped together,
each race preserving its own individuality, formed a pict-
ure 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 kej^ and on Tuesday
morning Elliot went for it. He brought back the intel-
ligence 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 star-gazer, and not with the inten-
tion of devoting myself to the observation of any par-
ticular phenomenon. I wished to see the whole — the first
contact, the advance of the moon, the successive swallow-
ing up of the solar spots, the breaking of the last line
of crescent by the lunar mountains into Bailey's beads,
the advance of the shadow through the air, the appear-
ance 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.

VOYAGE TO ALGERIA ^ 16T

I was provided with a telescope of admirable defini-
tion, mounted, adjusted, packed, and most liberally placed
at my disposal by Mr. Warren De La Rue. The telescope
grasped the whole of the sun, and a considerable portion
of the space surrounding it. But it would not take in
the extreme limits of the corona. For this I had lashed
on to the large telescope a light but powerful instrument,
constructed by Ross, and lent to me by Mr. Huggins.
I was also furnished with an excellent binocular by Mr,
Dallmeyer. In fact, no man could have been more effi-
ciently supported. It required a strict parcelling out of
the interval of totality to embrace in it the entire series
of observations. These, while the sun remained visible,
were to be made with an unsilvered diagonal eye-piece,
which reflected but a small fraction of the sun's light, this
fraction being still further toned down by a dark glass.
At the moment of totality the dark glass was to be re-
moved, and a silver reflector pushed in, so as to get the
maximum of light from the corona and prominences. The
time of totality was distributed as follows:

1. Observe approach of shadow through the air: totality

2. Telescope 30 seconds

3. Finder ..,.,.. 30 seconds

4. Double image prism , . 15 seconds

5. Naked eye 10 seconds

6. Finder or binocular 20 seconds

7. Telescope 20 seconds

8. Observe retreat of shadow

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
CO finder, from finder to polariscope, from polariscope to

168 FRAGMENTS OF SCIENCE

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

But the opportunity did not occur. For several days
the weather had been ill-natured. We had wind so strong
as to render the hawsers at the stern of the ' ' Urgent' ' as
rigid as iron, and to destroy the navigating lieutenant's
sleep. We had clouds, a thunder-storm, and some rain.
Still the hope was held out' that the atmosphere would
cleanse itself, and if it did we were promised air of ex-
traordinary limpidity. Early on the 22d 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 instru-
ment 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 practiced men
fastened the sail at the top, and loaded it with bowlders
at the bottom. It was tried severely, but it stood firm.

The clouds and blue spaces fought for a time with
varying success. The sun was hidden and revealed at
intervals, hope oscillating in synchronism with the changes

VOYAGE TO ALGERIA 169

of the sky. At tlie moment of first contact a dense cloud
intervened; but a minute or two afterward the cloud had
passed, and the encroachment of the black body of the
moon was evident upon the solar disk. The moon marched
onward, and I saw it at frequent intervals; a large group
of spots were approached and swallowed up. Subse-
quently I caught sight of the lunar limb as it cut through
the middle of a large spot. The spot was not to be dis-
tinguished from the moon, but rose like a mountain above
it. The clouds, when thin, could be seen as gray scud
drifting across the black surface of the moon; but they
thickened more and more, and made the intervals of clear-
ness scantier. During these moments I watched with an
interest bordering upon fascination the march of the silver
sickle of the sun across the field of the telescope. It was
so sharp and so beautiful. No trace of the lunar limb
could be observed beyond the sun's boundary. Here, in-
deed, 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 Koyal Engineers, had the kind-
ness to take charge of my note-book. 1 mentioned, and
he wrote rapidly down, such things as seemed worthy of
remembrance. Thus my hands and mind were entirely
free; but it was all to no purpose. A patch of sunlight
fell and rested upon the landscape some miles away. It
was the only illuminated spot within view. But to the
northwest there was still a space of blue which might
reach us in- time. Within seven minutes of totality an-
other space toward the zenith became very dark. The

Science — V — a

170 FRAGMENTS OF SCIENCE

atmospliere was, as it were, on tlie brink of a precipice,
being charged with humidity, which required but a slight
chill to bring it down in clouds. This was furnished by
the withdrawal of the solar beams: the clouds did come
down, covering up the space of blue on which our hopes
had so long rested. 1 abandoned the telescope and walked
to and fro in despair. As the moment of totality ap-
proached, the descent toward darkness was as obvious as.
a falling stone. I looked toward 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 per-
spective they appear divergent, having the sun, in fact,
for their point of convergence. The darkness took posses-
sion of the ridge referred to, lowered upon M. Janssen's
observatory, passed over the southern heavens, blotting
out the beams as if a sponge had been drawn across them.
It then took successive possession of three spaces of blue
sky in the southeastern atmosphere. I again looked to-
ward the ridge. A glimmer as of day- dawn was behind
it, and immediately afterward the fan of beams, which had
been for more than two minutes absent, revived. The
eclipse of 1870 had ended, and, as far as the corona and
flames were concerned, we had been defeated.

Even in the heart of the eclipse the darkness was by
no means perfect. Small print could be read. In fact,
the clouds which rendered the day a dark one, by scatter-
ing light into the shadow, rendered the darkness less in-
tense than it would have been had the atmosphere been

VOYAGE TO ALGERIA 171

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 bastion et, hop-
ing 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 afterward, 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 exceed-
ingly fine.

The able and well- instructed medical officer of the
* 'Urgent," Mr. Goodman, observed the following tem-
peratures during the progress of the eclipse:

Hour

Deg.

Hour

Deg

11.45

56

12.43

51

11.55

55

1.05

52

12.10

54

1.27

53

12.37

53

1.44

56

12.39

52

2.10

57

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

The wind was so strong on the 23d that Captain Hen-
derson would not venture out. Gruided by Mr. Goodman,
I visited a cave in a remarkable stratum of shell-breccia,
and, thanks to my guide, secured specimens. Mr. Busk
informs me that a precisely similar breccia is found at
Gibraltar, at approximately the same level. During the
afternoon. Admiral Ommaney and myself drove to the fort
of Marsa el Kibir The fortification is of ancient origin,,

172 FRAGMENTS OF SCIENCE

the Moorisli arches being still there in decay, but the fort
is now very strong. About four or five hundred fine-
looking dragoons were looking after their horses, waiting
for a lull to enable them to embark for France. One of
their officers was wandering in a very solitary fashion over
the fort. We had some conversation with him. He had
been at Sedan, had been taken prisoner, but had effected
his escape. He shook his head when we spoke of the 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 promontory on which
the fort stands, smiting the rocks, breaking into foam, and
jumping, after impact, to a height of a hundred feet and
more into the air. As we returned our vehicle broke
down through the loss of a wheel. The Admiral went on
board, while I remained long watching the agitated sea.
The little horses of Oran well merit a passing word. Their
speed and endurance, both of which are heavily drawn
upon by their drivers, are extraordinary.

The wind sinking, we lifted anchor on the 24th. For
some hours we went pleasantly along; but during the
afternoon the storm revived, and it blew heavily against
us all the night. When we came opposite the Bay of
Almeria, on the 25th, the captain turned the ship, and
steered into the bay, where, under the shadow of the
Sierra Nevada, we passed Christmas night in peace. Next
morning "a rose of dawn" rested on the snows of the ad-
jacent 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

VOYAGE TO ALGERIA 173

maximum of grandeur out of tlie height. We weighed
anchor at eight A.M., passing for a time through shoal
water, the bottom having been evidently stirred up. • The
adjacent land seemed eroded in a remarkable manner. It
has its floods, which excavate these valleys and ravines,
and leave those singular ridges behind. Toward evening
I climbed the mainmast, and, standing on the cross-trees,
saw the sun set amid a blaze of fiery clouds. The wind
was strong and bitterly cold, and I was glad to slide back
to the deck along a rope, which stretched from the mast-
head to the ship's side. That night we cast anchor beside
the Mole of Gibraltar.

On the morning of the 27th, in company with two
friends, I drove to the Spanish lines, with the view of
seeing the rock from that side. It is an exceedingly noble
mass. The Peninsular and Oriental mail-boat had been
signalled and had come. Heavy duties called me home-
ward, and by transferring myself from the "Urgent" to
the mail- steamer I should gain three days. I hired a
boat, rowed to the steamer, learned that she was to start
at one, and returned with all speed to the "Urgent.'*
Making known to Captain Henderson my wish to get
away, he expressed doubts as to the possibility of reach-
ing the mail- steamer in time. With his accustomed kind-
ness, 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 oar 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 no4; yet hauled in. The men

174 FRAGMENTS OF SCIENCE

put forth all their strength, animated by the exhortations
of the officer at the helm. The roughness of the sea ren-
dered their efforts to some extent nugatory: still we were
rapidly approaching the steamer. At length she moved,
punctual almost to the minute, at first slowly, but soon
with quickened pace. We turned to the left, so as to cut
across her bows. Five minutes' pull would have brought
us up to her. The officer waved his cap and I my hat.
*'If they could only see us, they might back to us in a
moment." But they diol not see us, or if they did, they
paid us no attention. I returned to the *' Urgent/' dis-
comfited, but grateful to the fine fellows who had wrought
so hard to carry out my wishes.

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

The water drips from the roof charged with bicarbonate
of lime. Exposed to the air, the carbonic acid partially
escapes, and the simple carbonate of lime, which is hardly
at all soluble in water, deposits itself as a solid, forming
stalactites and stalagmites Even the exposure of chalk

VOYAGE TO ALGERIA 175

or limestone water to the open air partially softens it. A
specimen of the Redboume water exposed bj Professors
Graham, Miller, and Hofmann, in a shallow basin, fell
from eighteen degrees to nine degrees of hardness. The
softening process of Clark is virtually a hastening of the
natural process. Here, however, instead of being per-
mitted to evaporate, half the carbonic acid is appropriated
by lime, the half thus taken up, as well as the remaining
half, being precipitated. The solid precipitate is per-
mitted to sink, and the clear supernatant liquid is limpid
soft water.

We returned to the real mouth of St. MichaeFs 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 col-
umns sprang complete from floor to roof, while incipient
columns were growing to meet each other, upward and
downward. The water which trickles from the stalactite,
after having in part yielded up its carbonate of lime, falls
upon the floor vertically underneath, and there builds the
stalagmite. Consequently, the pillars grow from above
and below simultaneously, along the same vertical. It
is easy to distinguish the stalagmitic from the stalactitic
portion of the pillars. The former is always divided into
short segments by protuberant rings, as if deposited peri-
odically, while the latter presents a uniform surface. In
some cases the points of inverted cones of stalactite rested
on the centres of pillars of stalagmite. The process of
solidification and the consequent architecture were alike
beautiful.

We followed our guide through various branches and
arms of the cave, climbed and descended steps, halted at

176 FRAGMENTS OF SCIENCE

the edges of dark shafts and apertures, and squeezed our-
selves through narrow passages. From time to time we
halted, while Mr. Crookes illuminated, with ignited mag-
nesium wire, the roof, columns, dependent spears, and
graceful drapery of the stalactites. Once, coming to a
magnificent cluster of icicle-like spears, we helped our-
selves to specimens. There was some difiicultj in detach-
ing the more delicate ones, their fragility was so great.
A consciousness of vandalism, which smote me at the
time, haunts me still; for, though our requisitions were
moderate, this beauty ought not to be at all invaded.
Pendent from the roof, in their natural habitat, nothing
can exceed their delicate beauty; they live^ as it were, sur-
rounded 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 Gibraltar is so burrowed with cav-
erns 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

VOYAGE TO ALGERIA 177

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 further to the south they actually turn
over and dip to the - east.

The rock is thus divided into three sections, separated
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 exclu-
sively found. Based on the observations of Dr. Falconer
and himself, an excellent and most interesting account of
these caves, and of the human remains and works of art
which they contain, was communicated by Mr. Busk to
the meeting of the Congress of Prehistoric Archaeology
at Norwich, and afterward printed in the "Transactions"
of the Congress.^ Long subsequent to the operation of
the twisting force just referred to, the promontory under-
went 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 African side of the

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

178 FRAGMENTS OF SCIENCE

strait, Mr. Busk informs me, has undergone similar
disturbances. *

In the harbor of Gibraltar, on the morning of our de-
parture, I resumed a series of observations on the color
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 Gibraltar, 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- 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 offi-
cers 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, afterward, through the dirty London air.

The mode of examination applied to these bottles has
been already described.* The liquid is illuminated by a

^ No one can rise from the perusal of Mr. Busk's paper without a feeling
of admiration for the principal discoverer and indefatigable explorer of the
Gibraltar caves, the late Captain Frederick Brome.

3 "Moating Matter of the Air," Art. "Dust and Disease."

VOYAGE TO ALGERIA

179

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 ex-
amined by the method alluded to, produce not only sen-
sible, 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:

No.
1

Locality

Color of Sea

Appearance in Luminous Beam

Gibraltar Harbor

Green

Thick with fine particles

2

Two miles from Gibraltar

Clearer green

Thick with very fine particles

3

Oflf Cabreta Point

Bright green

Still thick, but less so

4

Off Cabreta Point

Black-indigo

Much less thick, very pure

5

Off Tarifa

Undecided

Thicker than No. 4

6

Beyond Tarifa

Cobalt-blue

Much purer than No. 5
Very tnick

7

Twelve miles from Cadiz

Yellow-green

8

Cadiz Harbor

Yellow-green

Exceedingly thick
Thick, but less so

9

Fourteen miles from Cadiz

Yellow-green

10

Fourteen miles from Cadiz

Bright green

Much less thick

11

Between Capes St. Mary and

Vincent

Deep indigo

Very little matter, very pure

12

Off the Burlings

Strong green

Thick, with fine matter

13

Beyond the Burlings

Indigo

Very little matter, pure

14

Off Cape Finisterre

Undecided

Less pure

Very little matter, very pm^

15

Bay of Biscay

Black-indigo

16

Bay of Biscay
Off Ushant

Indigo

Very fine matter. Iridescent

17

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 Gibral-
tar Harbor, at a point two miles from the harbor, and off

180 FRAGMENTS OF SCIENCE

Cabreta Point. The home examination 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 nav-
igating lieutenant, Mr. Brown, steered along the coast,
thus avoiding the adverse current which sets in, through
the Strait, from the Atlantic to the Mediterranean. He
was at length forced to cross the boundary of the Atlantic
current, which was defined with extraordinary sharpness.
On the one side of it the water was a vivid 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. Two bottles were secured, one on each side
of this remarkable boundary. In the distance the At-
lantic had the hue called ultramarine; but looked fairly
down upon, it was of almost inky blackness — black quali-
fied by a trace of indigo.

What change does the home examination here reveal?
In passing to indigo, the water becomes suddenly aug-
mented in purity, the suspended matter becoming sud-
denly less. Off Tarifa, the deep indigo disappears, and
the sea is undecided in color. Accompanying this change,
we have a rise in the quantity of suspended matter. Be-
yond 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 Har-
bor, and also of a point fourteen miles from Cadiz in the

VOYAGE TO ALGERIA 181

homeward direction. Here there is a sudden change from
yellow-green to a bright emerald-green, and accompany-
ing 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 dimi-
nution of the suspended matter being the concomitant
phenomenon.

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 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-colored, according to the di-
rection of the line of vision. Finally, we come to our last
two bottles, the one taken opposite St. Catherine's light-
house, 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 with suspended matter.

Two distinct series of observations are here referred
to — the one consisting of direct observations of the color
of the sea,' conducted during the voyage from Gibraltar to

182 FRAGMENTS OF SCIENCE

Portsmoutli : the other carrisd out in the laboratory of the
Royal Institution. And here it 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 re-
garding 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-color corresponding to
the various specimens ascertained. The home observa*
tions, therefore, must have been perfectly unbiased, and
they clearly establish the association of the green color with
fine suspended matter, and of the ultramarine color, 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 preliminary re-
mark or two will clear our way toward an explanation.
Color resides in white light, appearing when any constit-
uent of the white light is withdrawn. The hue of a pur-
ple 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 colors produces the purple. But
while such a liquid attacks with special energy the yellow
and green, it enfeebles the whole spectrum. By increas-

* A note, written to me on October 22, by my friend Canon Kingsley, con-
tains the following reference to this point: **! have never seen the Lake of
Cteneva, 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 sponson on to it without fear; this was to me the most wonderful
thing whicli I saw on my voyages to and from the West Indies."

VOYAGE TO ALGERIA 183

ing the thickness of the stratum we may absorb the whole
of the light. The color of a blue liquid is similarly ac-
counted for. It first extinguishes the red; then, as the
thickness 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 ther-
mal rays are in part visual, the visual rays in part chem-
ical, 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 evap-
oration. At the same time the whole spectrum suffers en-
feeblement; water attacks all its rays, but with different
degrees of energy. Of the visual rays, the red are first
extinguished. As the solar beam plunges deeper into the
sea, orange follows red, yellow follows orange, green fol-
lows 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 color, would reach us from the body of the
water.

In very clear and deep sea- water this condition is ap-

184 FRAGMENTS OF SCIENCE

proximately fulfilled, and hence the extraordinary dark-
ness 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 densities. A modi-
cum of light is thus thrown back to the eye, before the
depth necessary to absolute extinction has been attained.
An effect precisely similar occurs under the moraines of
glaciers. The ice here is exceptionally 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 color 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 securely fastened to it. Fifty or sixty yards of
strong hempen line were attached to the plate. My assist-
ant, Thorogood, occupied a boat, fastened as usual to the
davits of the *' Urgent," while I occupied a second boat
nearer the stem of the ship. He cast the plate as a mari-
ner 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 color as the
plate sank, but at its greatest depth, even in indigo water,
the color was still a blue green."

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

• In no case, of course is the green pure, but a mixture of green and blu«.

VOYAGE TO ALGERIA 185

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 pbop down upon the screw. The sur-
face-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, lieu-
tenant Walton had a sail and tarpaulin thrown over the
mouth of the well. Underneath this I perched myself on
the plank and watched the screw. In an indigo sea the
play of color 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
extraordinary. 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 ruffied. Liquid
lenses were thus formed, by which the colored light was
withdrawn from some places and concentrated upon 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 color to be connected with the sus-

186 FBAGMENTS OF SCIENCE

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 sufficiently
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 examination re-
veals act in all essential particulars like the plate, or like
the screw-blades, or like tho foam, or like the bellies of
the porpoises. Thus I think the greenness of tne 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
realized, has left behind it pleasant memories, both of the
aspects of nature and the kindliness of men.

VII

NIAGARA '

IT is one of the disadvantages of reading books about nat-
ural 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 impres-
sions. 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 cur-
rency to notions which have often led to disappointment.
A record of a voyage, in 1535, 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 Rageneau, in a letter to his superior at
Paris, mentions Niagara as "a cataract of frightful
height.*' ' 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 shows that serious changes
have taken place since his time. He describes it as *'a

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

' Prom 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.

(187)

188 FRAGMENTS OF SCIENCE

great and prodigious cadence of water, to which the uni-
verse 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 remark-
ably close estimate. At that time, viz., a hundred and
fifty years ago, it had the shape of a horseshoe, and rea-
sons 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 re-
peated to the present hour, to be altogether extravagant.
He is perfectly right. The thunders of Niagara are for-
midable 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 reso-
nance; 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 Eeuss at the Devil's Bridge, when full,
to thunder more loudly than the Niagara.

NIAGARA 189

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. Immedi-
ately 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 disap-
pointment, knowing, from old experience, that time and
close acquaintanceship, the gradual interweaving of mind
and nature, must powerfully 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 preci-
pice, like the swell of a muscle unbroken. The ledge
here overhangs, 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 con-
nects the American and Horseshoe Falls. Midway be-
tween 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

190 FRAGMENTS OF SCIENCE

cataract was immediately observed. A thick layer of
limestone formed the upper portion of 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, under-
mining the ledge above, which, unsupported, eventually
breaks off, and produces the observed recession.

At the southern extremity of the Horseshoe is a prom-
ontory, formed by the doubling back of the gorge exca-
vated 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 kind-
ness 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 evi-
dently much deeper than the American branch; and in-
stead 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 alternating with bands of brighter color.
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 im-
mediately drawn into long white striae. ' Lower down, the
surface, shaken by the reaction from below, incessantly

^ 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.

NIAGARA 191

rustles into whiteness. Thie descent finally resolves itself
into a rhythm, the water reaching the bottom of the fall
in periodic gushes. Nor is the 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 whicli 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 be intelli-
gible.

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

192 FRAGMENTS OF SCIENCE

of Biddle's Stair, the guide to the Cave of the Winds.
He was in the prime of manhood — large, well built, firm
and pleasant in mouth and eye. My interest in the scene
stirred up his, and made him communicative. Turning
to a photograph, he described, by reference to it, a feat
which he had accomplished some time previously, and
which had brought him almost under the green water of
the Horseshoe Fall. "Can you lead me there to-mor-
row?" I asked. He eyed me inquiringly, weighing, per-
haps, the chances of a man of light build, and with gray
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 endeavor to follow." His scrutiny
relaxed into a smile, and he said, "Yery 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 be-
ing chilled; and he' was right. A suit and hood of yel-
low oilcloth covered all. Most laudable precautions were
taken by the young assistant who helped to dress me to
keep the water out; but his devices broke down imme-
diately when severely tested.

We descended the stair; the handle of a pitchfork do-
ing, in my case, the duty of an alpenstock. At the bot-
tom, the guide inquired whether we should go first to the
Cave of the Winds, or to the Horseshoe, remarking that
the latter would try us most. I decided on getting the
roughest done first, and he turned to the left over the

NIAGARA 193

stones. They were sharp and trying. The base of the first
portion of the cataract is covered with huge bowlders, ob-
viously the ruins of the limestone ledge above. The water
does not distribute itself uniformly among these, but seeks
out channels through which it pours torrentially. We
passed some of these with wetted feet, but without diffi-
culty. At length we came to the side of a more formi-
dable 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 toward 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 bowlders, against which the current rose violently.
He struggled and swayed, but he struggled successfully,
and finally reached the shallow water at the other side.
Stretching out his arm, he said to me, "ITow come on."
I looked down the torrent, as it rushed to the river be-
low, which was seething with the tumult of the cataract.
De Saussure recommended the inspection of Alpine dan-
gers, with the view of making them familiar to the eye
before they are encountered; 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.

Science — Y — 9

194 FRAGMENTS OF SCIENCE

Fnrtlier struggle was impossible; and. feeling my balance
hopelessly gone, I turned, flung myself toward the bank
just quitted, and was instantly, as expected, swept into
shallower water.

The oilcloth covering was a great encumbrance; 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 try again.
Prudence was at my elbow, whispering dissuasion; but,
taking everything into account, it appeared more immoral
to retreat than to proceed. Instructed by the first mis-
adventure, 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 wa-
vered; but, by keeping the left hip well against it, I re-
mained 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 travel-
ler," he said, **was ever here before.'* Soon afterward,
by trusting to a piece of driftwood which seemed firm,
I was again taken off my feet, but was immediately caught
by a protruding rock.

We clambered over the bowlders toward 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 upward; but the de-

NIAGARA 195

fence was useless. The guide continued to move on, but
at a certain place lie 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 eyes could be protected from the blinding shock of
the spray, while the line of vision to the upper ledges
remained to some extent clear. On looking upward over
the guide's shoulder I could see the water bending over
the ledge, while the Terrapin Tower loomed fitfully
through the intermittent spray- gusts. We were right un-
der the tower. A little further 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 com-
motion, along the arm of the Horseshoe, until the bowl-
ders failed us, and the cataract fell into the profound
gorge of the Niagara Kiver.

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 emi-
nent 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 emo-
tions, he says, nervous currents are liberated which stimu-
late 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 eJ3Eect of the

196 FBAQMENTS OF SCIENCE

game order 1 experienced amid the spray and tlmnder of
Niagara. Quickened by the emotions there aroused, the
blood sped exultingly through the arteries, abolishing in-
trospection, clearing the heart of all bitterness, and ena-
bling 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 describe all this.*' He
rightly thought it indescribable. The name of this gal-
lant 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 toward the bank, and was swept into the shal-
lows. Standing in the stream near its edge, he stretched
his arm toward me. I retained the pitchfork handle, for
it had been useful among the bowlders. By wading some
way in, the staff could be made to reach him, and I pro-

NIAGARA 197

posed his seizing it. "If you are sure," lie 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 afterward
roamed sociably among the torrents and bowlders 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 bowlders, 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. Bake-
well, Jr. : " 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 «a

198 FRAGMENTS OF SCIENCE

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 neces-
sary 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 intervention of Mr. Town-
send, was overcome. The main one was to secure oarsmen
sufiiciently 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 boat which strength and daring could
accomplish. He came. His figure and expression of face
certainly indicated extraordinary firmness and power. On
Tuesday, 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 immediately 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 outward, not downward. 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 dis-
appeared 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

1 <'

Mag. of Nat. Hist.," 1830, pp. 121, 122.

mAGARA 199

from it in huge protuberant masses of foam and spray.
We passed Goat Island, came to the Horseshoe, and
worked for a time along its base, the bowlders 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 vio-
lent 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
bow came obliquely to the surge; the boat was turned
suddenly round and shot with astonishing rapidity down
the river. The men returned to the charge, now trying to
get up between the half -concealed rock and the bowlders
to the left. But the torrent set in strongly through this
channel. The tugging was quick and violent, but we
made little way. At length, seizing a rope, the princi-
pal oarsman made a desperate attempt to get upon one of
the bowlders, 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 Fall, run-
ning 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 neighbor 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

200 FRAGMENTS OF SCIENCE

sliining above the smoke of spray appeared lifted to an
extraordinary elevation. Had Hennepin and La Hontan
seen tlie 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 rail-
way suspension bridge. Between ferry and bridge the
river Niagara flows unruffled; but at the suspension
bridge the bed steepens and the river quickens its mo-
tion. Lower down the gorge narrows, and the rapidity
and turbulence increase. At the place called the "Whirl-
pool Eapids," 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 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 rap-
ids; for this, and for his agreeable company to the spot,
I have to thank him. From the edge of the clrff 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 mo-
tion of translation and a motion of undulation — the race
of the river through its gorge, and the great waves gen-
erated by its collision with, and rebound from, the obsta-
cles in its way. In the middle of the river the rush and
tossing are most violent; at all events, the impetuous force

NIAGARA 201

of the individual waves is here most strikingly displayed.
Yast pyramidal heaps leap incessantly 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, carrying away the lighter drops and leaving the heav-
ier 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 exquisitely beau-
tiful. The complexity of the action was still further illus-
trated 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 explana-
tion, of these rapids is, that the central bed of the river
is cumbered with large bowlders, and that the 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 sufficient reason to be taken
into account. Bowlders derived 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

202 FRAGMENTS OF SCIENCE

have tlie coalescence of waves witli waves. Where crest
and furrow cross eacli otlier, tlie motion is annulled; where
furrow and furrow cross, the river is plowed to a greater
depth; and where crest and crest aid each other, we have
that astonishing leap of the water which breaks the cohe-
sion 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 cen-
tre, are perfectly obvious. If this explanation be correct,
the phenomena observed at the Whirlpool Kapids form one
of the grandest illustrations of the principle of interference.
The Nile "cataract," Mr. Huxley informs me, offers more
moderate examples of the same action.

At some distance below the Whirlpool Eapids we have
the celebrated whirlpool itself. Here the river makes a
sudden bend to the northeast, forming nearly a right angle
with its previous direction. The water strikes the concave
bank with great force, and scoops it 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 without 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 northeast, 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 increase. The basin

NIAGARA 203

is enclosed by higli and almost precipitous banks — covered,
at the time, with russet woods. A kind of mystery at-
taches 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 whirl-
pool, pine-trees are sucked down, to be ejected mysteri-
ously 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 color is, I think, correctly accounted for
in the last Fragment. While crossing the Atlantic, in
1872-1873, I had frequent opportunities of testing the
explanation 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 free-
dom from mechanically suspended matter. In small thick-
nesses water is sensibly transparent to all kinds of light;
but, as the thickness increases, the rays of low refrangi-
bility are first absorbed, and after them the other rays.
"Where, therefore, the water is very deep and very pure,
all the colors are absorbed, and such water ought to ap-
pear 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.

204 FRAGMENTS OF SCIENCE

Throw a white pebble into such water; as it sinks it
becomes greener and greener, and, before it disappears,
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 around
you. You rise in the morning, find it a vivid green, and
correctly infer that you are crossing the bank of New-
foundland. 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 neces-
sary. A storm can render the water muddy, by render-
ing the particles too numerous and gross. Such a case
occurred toward 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 green of the Horseshoe.

Nothing can be more superb than the green of the
Atlantic waves, when the circumstances are favorable to
the exhibition of the color. As long as a wave remains
unbroken no color appears; but when the foam just
doubles over the crest, like an Alpine snow-cornice, un-
der the cornice we often see a display of the most exqui-
site green. It is metallic in its brilliancy. But the foam
is necessary to its production. The foam is first illumi-
nated, and it scatters the light in all directions; the light
which passes through the higher portion of the wave alone
reaches the eye, and gives to that portion its matchless
color. The folding of the wave, producing as it does a

NIAGARA 205

series of longitudinal protuberances and furrows whicli
act like cylindrical lenses, introduces variations in tlie
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,
and they are in a certain sense convertible. 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
Ly ell's Bay, near Wellington, in New Zealand. They
were described by Mr. Travers in the "Transactions of
the New Zealand Institute." Unacquainted with their
origin, you would certainly ascribe their forms to human
workmanship. They resemble knives and spear-heads,
being apparently chiselled oS. into facets, with as much
attention to symmetry as if a tool, guided by human
intelligence, had passed over them. But no human in-
strument has been brought to bear upon these stones.
They have been wrought into their present shape by the
wind-blown sand of Ly ell's Bay. Two winds are domi-
nant 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. *

* "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.

206 FRAGMENTS OF SCIENCE

The Sphinx of Egypt is nearly covered up by the
sand of the desert. The neck of the Sphinx 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 helpful friend, Mr. Josiah
Quincy, to see the action of the sand-blast. A kind of
hopper containing fine siliceous 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 grind-
ing. 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.

which are those of wedges, knives, arrow-heads, etc., 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 bowlder- 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 forms are so
peculiar, and the edges, in a few of them, so perfect, that if they were discov-
ered associated with human works there is no doubt that they would have been
referred to the so-called 'stone period.' " — Extracted from the Minutes of the
Wellington Philosophical Society, February 9, 1869.

NIAGARA 207

But this was not all. By protecting certain portions
of tlie surface, and exposing others, figures and tracery of
any required form could be etched upon the glass. The
figures of open iron- work could be thus copied; while
wire-gauze placed oyer the glass produced a reticulated
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 diffusing the shock of the par-
ticle, such substances practically destroy the local erosive
power. The hand can bear, without inconvenience, a
sand- shower which would pulverize glass. Etchings exe-
cuted on glass with suitable kinds of ink are accurately
worked out by the sand-blast. In fact, within certain
limits, the harder the surface, the greater is the concen-
tration of the shock, and the more effectual is the erosion.
It is not necessary that the sand should be the harder
substance of the two; corundum, for example, is much
harder than quartz; still, quartz-sand can not only depol-
ish, but actually blow a hole 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
from a cistern at the top of the house, the column of

208 FRAGMENTS OF SCIENCE

water, from the cistern downward, 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 may be entirely
avoided. We have here an example of the concentration
of energy in time. The sand-blast illustrates the concen-
tration of energy in space. The action of flint and steel
is an illustration of the same principle. The heat re-
quired to generate the spark is intense; and the mechan-
ical 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 collision. 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 localized.

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 express
my acknowledgments to General Tilghman,* who is the
inventor of the sand-blast. To his spontaneous kindness
I am indebted for some beautiful illustrations of his proc-

* The absorbent power, if I may use the phrase, exerted by the industrial
arts in the United States, is forcibly illustrated by the 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 honor 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 astonishingly
short time, from military to civil life.

NIAGARA 209

ess. In one thick plate of glass a figure has been worked
out to a depth of fths of an inch. A second plate, Jths
of an inch thick, is entirely perforated. In a circular
plate of marble, nearly half an inch thick, open work of
most intricate and elaborate description has been exe-
cuted. It would probably take many days to perform this
work by any ordinary process; with the sand-blast it was
accomplished in an hour. So much for the strength of
the blast; its delicacy is illustrated by this beautiful ex-
ample 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 vor-
tex, can wear away the hardest rock; "potholes'* and
deep cylindrical shafts being thus produced. An ex-
traordinary instance of this kind of erosion is to be seen
in the Yal 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 trans-
verse 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; 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

210 FRAGMENTS OF SCIENCE

dissolved; but added to this we had the action of the
sand and gravel carried along by the water, which, on
striking the rock, chipped it away like the particles of the
sand-blast. Thus, by solution and mechanical erosion,
the great chasm of the Finsteraarschlucht was formed. It
is demonstrable 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
barriers. Almost every valley in Switzerland furnishes
examples of this kind; the untenable hypothesis of earth-
quakes, once so readily resorted to in accounting for these
gorges, being now for the most part abandoned. To pro-
duce the canons of Western America, no other cause is
needed than the integration of effects individually infini-
tesimal.

And now we come to Niagara. Soon after Europeans
had taken possession of the country, the conviction ap-
pears to have arisen that the deep channel of the river
Niagara below the falls had been excavated by the cata-
ract. In Mr. Bakewell's *' Introduction to Geology," the
prevalence of this belief has been referred to. It is ex-
pressed thus by Professor Joseph Henry in the "Transac-
tions of the Albany Institute" : * "In viewing 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 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 Pro-

* Quoted b7 Bakewell.

NIAGARA 211

fessor Kamsay, indeed by most of those wlio have inspected
the place.

A connected image of the origin and progress of the
cataract is easily obtained. Walking northward from the
village of !N"iagara 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 cUEs of
this gorge are from 800 to 360 feet high. We reach the
whirlpool, trend to the northeast, 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. At some hundreds of feet below
us is a comparatively level plain, which stretches to Lake
Ontario. The declivity 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 mem-
ory 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 backward be-
gin its retrograde course? To minds disciplined in such
researches the answer has been, and will be — At the pre-
cipitous declivity which crossed the Niagara from Lewis-
ton on the American to Queenston on the Canadian side.
Over this transverse barrier the united affluents of all the
upper lakes once poured their waters, and here the work
of erosion began. The dam, moreover, was demonstrably
of sufficient height to cause the river above it to submerge
Goat Island ; and this would perfectly account for the find-
ing, by Sir Charles Lyell, Mr. Hall, and others, in the

212 FRAGMENTS OF SCIENCE

sand and gravel of the island, tlie 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
Eamsay to reduce to demonstration the popular belief that
the Niagara once flowed through a shallow valley.

The phj^sics of the problem of excavation, which I made
clear to my mind before quitting Niagara, are revealed 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 his excellent
chapter on Niagara Falls, Mr. Hall alludes to this fact.
Here we have the most copious and the most violent whirl-
ing of the shattered liquid; here the most powerful eddies
recoil against the shale. From this portion of the fall, in-
deed, the spray sometimes rises without solution of conti-
nuity to the region of clouds, becoming gradually more at-
tenuated, and passing finally through the, condition of true
cloud into invisible vapor, which is sometimes reprecipitated
higher up. All the phenomena point distinctly to the cen-
tre of the river as the place of greatest mechanical energy,
and from the centre the vigor of the fall gradually dies
away toward the sides. The Horseshoe form, with the
concavity facing downward, is an obvious and necessary
consequence of this action. Bight along the middle of the
river the apex of the curve pushes its way backward, cut-
ting along the centre a deep and comparatively narrow
groove, and draining the sides as it passes them.' Hence
the remarkable discrepancy between the widths of the

* In the discourse the excavation of the centre and drainage of the sides
action was iUustrated by a model devised by my assistant, Mr. John CottrelL

NIAGARA 1215

Niagara above and below the Horseshoe. All along its
course, from Lewiston Heights to its present position, the
form of the fall was probably that of a horseshoe ; for this
is merely the expression of the greater depth, and conse-
quently 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 Horseshoe
Fall comes strikingly into view when it and the American
Fall are compared together. The American 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 Amer-
ican 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 islando This point,
which impressed me forcibly, has not, I have just learned,
escaped the acute observation of Professor Eamsay * The
river bends; the Horseshoe immediately accommodates it-
self to the bending, and will follow implicitly the direction
of the deepest water in the upper stream. The 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

» 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
title Canadian Fall has receded from the north corner of Goat Island to the
innermost curve of the Horseshoe Fall." — Quarterly Journal of Geological
Society, May, 1859.

214 FRAGMENTS OF SCIENCE

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 tolerably uniform text-
ure, 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 River cut the gorge; it
has carried away the chips of its own workshop. The
shale, being probably crumbled, is easily carried away.
But at the base of the fall we find the huge bowlders
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 bowlders, 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 bowlders, thus withdrawing their
support and urging them gradually down the river. Solu-
tion also does its portion of the work. That solid matter
is carried down is proved by the difference of depth be-
tween the Niagara River and Lake Ontario, where the river
enters it. The depth falls from 72 feet to 20 feet, m con-
sequence of the deposition of solid matter caused by the
diminished motion of the river. '

The annexed highly instructive map has been reduced

* Near the mouth of the gorge at Queenston, the depth, according to the
Admiralty Chart, is 180 feet; well within the gorge it is 132 fe»t.

NIAGARA

215

from one published in Mr. Hall's *' Geology of New York.**
It is based on surveys executed, in 1842, by Messrs. Gib*

Fig. 4.

8on and Evershed. The ragged edge of tbe American Fall
north of Goat Island marks the amount of erosion which

216 FRAGMENTS OF SCIENCE

it lias been able to accomplisli, wbile tbe 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 conducted by Conroy. Probably since 1842
the Horseshoe has worked back beyond the position here
assigned to it.

In conclusion, we may say a word regarding the proxi-
mate future of Niagara. At the rate of excavation as-
^gned 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 fall at this moment
we shall have the gorge enclosing a right angle, a second
whirlpool being the consequence. To those who visit Ni-
agara a few millenniums hence I leave the verification of
this prediction. All that can be said is, that if the causes
now in action continue to act, it will prove itself literally
true.

NIAGARA 217

Postscript

A year or so after I liad quitted tlie United States, a
man sixty years of age, while engaged in painting one of
the bridges which connect Goat Island with the Three Sis-
ters, slipped through the rails of the bridge into the rapids,
and was carried impetuously toward the Horseshoe Fall.
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 bystand-
ers 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 vio-
lently downward, 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
lay guide Conroy.

SOIENCE^- -V — 10

VIII

THE PAEALLEL ROADS OF GLEN" ROY*

THE first published allusion to the Parallel Eoads of
Grlen Roy occurs in the appendix to the third vol-
ume 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 com-
plete 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 if drawn with a line of rule and compass."

The correct heights of the three roads of Glen Roy
are respectively 1,150, 1,070, 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 ex-

^ A discourse delivered at the Royal Institution of Great Britain on June 9,
1816.

(218)

THE PARALLEL ROADS OF GLEN ROY 219

planation of them given by the country people in his
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 roused, 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 inquiry, this impulse did not make itself more
rapidly and energetically felt. Their remoteness may per-
haps account for the fact that until the year 1817 no sys-
tematic description of them, and no scientific attempt at
an explanation of them, appeared. In that year Dr. Mac-
Culloch, who was then President of the Geological So-
ciety, presented to that society a memoir, in which the
roads were discussed, and pronounced to be the margins
of lakes once embosomed in Glen Roy. Why there
should be three roads, or why the lakes should stand at
these particular levels, was left unexplained.

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 pre-
sented to the Royal Society of Edinburgh, on the 2d 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 differentia-

220 FRAGMENTS OF SCIENCE

tion of minds, and to single out those who went by a
kind of instinct to the core of the question, from those
who erred in it, or who learnedly occupied themselves
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 conspicu-
ously than Sir Thomas Dick-Lauder. Two distinct men-
tal processes are involved in the treatment of such a ques-
tion. First, the faithful and sufiicient observation of the
data; and secondly, that higher mental process in which
the constructive 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 Glen Roy 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 Glen Eoy. 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 1,170 feet above the
sea; that is to say, 20 feet above the highest road in Glen
Ecy.

From this col a lateral branch-valley— Glen Turrit —
iod down to Glen Roy. Our explorer descended from

THE PARALLEL ROADS OF GLEN ROY

221

the col to the highest road of the latter glen, and pursued
it exactly as he had pursued the road in Glen G-luoy.
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 highest shelf,
until finally he reached a col, or water- shed, looking into

PARALLEL ROADS OF OLEN ROY.

After a Sketch by Sir Thomas Dick-Lauder.

Glen Spey, and of precisely the same elevation as the
highest road of Glen Roy.

He then dropped down to the lowest of these roads,
and followed it toward the mouth of the glen. Its eleva-
tion above the bottom of the valley gradually increased;
not because the shelf rose, but because it remained level
while the valley sloped downward. He found this low-
est road doubling round the hills at the mouth of Glen
Roy, and running along the sides of the mountains which

222 FRAGMENTS OF SCIENCE

flank Glen Spean. He followed it eastward. The bottom
of the Spean Valley, like the others, gradually rose, and
therefore gradually approached the road on the adjacent
mountain- side. He came to Loch Laggan, the surface of
which rose almost to the level of the road, and beyond
the head of this lake he found, as in the other two cases,
a col, or watershed, at Makul, of exactly the same level
as the single road in Glen Spean, which, it will be remem-
bered, 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 indis-
tinct. 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 w<is
this: Taking all their features into account, he was con-
vinced 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 high, 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 eol 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

THE PARALLEL ROADS OF GLEN ROY 223

its shelf, which lake, acting upon the loose drift of the
flanking mountains, would form the shelf revealed by
observation.

So much for Glen Gluoy. But suppose the mouth of
Glen Eoy 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 gradually rise,
until it had reached the level of the col which divides
Glen Koy from 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 re-
mained at the mouth of Glen Roy. 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 Roy.

And now let us suppose the barrier to be so far re-
moved from the mouth of Glen Roy as to establish a con-
nection 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 Roy 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 Roy 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 ques-

224 FRAGMENTS OF SCIENCE

tion next occurs, What was the character of the assumed
barrier which stopped the glens? There are at the pres-
ent 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. Bj some un-
known convulsion, this detritus had been heaped up.
But, once given, and once granted that it was subse-
quently removed in the manner indicated, the single road
of Grlen Griuoj and the highest and lowest roads of Glen
Eoj would be explained in a satisfactory manner.

To account for the second or middle road of Glen Roy,
Sir Thomas Dick-Lauder invoked a new agency. He sup-
posed 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 detected by Mr. Dar-
win, 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 im-
portance, 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 Glen 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

THE PARALLEL ROADS OF GLEN ROY 225

which the middle and lowest roads are fairly shown. The
principal stream running through the glen turns at a cer-
tain point northward and loses itself among hills too high
to offer any outlet. But another branch of the glen turns
to the southeast; and, following up this branch, Mr.
Milne-Home reached a col, or watershed, 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 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 unexpected 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 itseK — ^that the par-
allel roads were formed by the sea; that this whole region
was once submerged and subsequently upheaved; that
there were pauses in the process of upheaval, 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 Roy roads from Glen Gluoy, where the moun-
tain flanks are quite as impressionable as in Glen Roy.
It would not account for the absence of the shelves from

226 FRAGMENTS OF SCIENCE

the other mountains in the neighborhood, 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 unassail-
able moral strength.

But, granting tbe 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 detritus
as he assumed could have existed without leaving traces
behind them ; but there is no trace left. There is detritus
enough in Glen Spean, but not where it is wanted. The
two highest parallel roads stop abruptly at different points
near the mouth of Glen Boy, but no remnant of the bar-
rier against which they abutted is to be seen. It might
be urged that the subsequent 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 Geikie has favored me with a drawing
of the Glen 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 cir-
cumstances 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 transcending
experience, are in reality experience modified by scien-
tific thought and pushed into an ultra experiential region.
At the time that he wrote, Sir Thomas Dick-Lauder could

THE PARALLEL ROADS OF GLEN ROY 227

not possibly have discerned the cause subsequently as-
signed for the blockage of these glens. A knowledge of
the action of ancient glaciers was the necessary ante-
cedent 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 Yenetz, 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 subsequently followed up with distin-
guished success by Charpentier, Studer, and others. With
characteristic vigor Agassiz grappled with it, extending
his observations far beyond the domain of Switzerland.
He came to this country in 1840, and found in various
places indubitable marks of ancient glacier action. Eng-
land, Scotland, Wales, and Ireland he proved to have
once given birth to glaciers. He visited Glen Roy, sur-
veyed the surrounding neighborhood, 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 con-
firmation of this theory.

And let me here say that Agassiz is only too likely to
be misrated and misjudged ly those who, though accurate
within a limited sphere, fail to grasp in their totality the
motive powers invoked in scientific investigation. 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 all sciences,
are essential to the creator of knowledge. To Agassiz was

228 FRAGMENTS OF SCIENCE'

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 ther-
mometer is immersed, is placed a Bunsen's lamp. The wa-
ter is heated, reaches a temperature of 212°, and then begins
to boil. The rise of the thermometer then ceases, although
heat continues to be poured by the lamp into the water.
"What becomes of that heat? We know that it is con-
sumed in the molecular work of vaporization. In the ex-
periment here arranged, the steam passes from the flask
through a tube into a second vessel kept at a low tem-
perature. 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 surrounding the sec-
ond vessel would not produce ice. The cold must have
the proper material to work upon; and this material —
aqueous vapor — 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 j)hilosopher
regarding that glacial epoch which the researches of Agas-
siz and others have revealed. This profoundly thoughtful

THE PARALLEL ROADS OF GLEN ROY 229

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 tem-
perature of space" is a familiar expression with scientific
men. It was considered probable by Poisson that our sys-
tem, 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 incomplete
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 vapor raised
mainly from the tropical ocean by the sun. The solar fire
is as necessary a factor in the process as our lamp in the
experiment 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

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

280 FRAGMENTS OF SCIENCE

times the weight of the glacier not only to a white he«,t>
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 distil-
lation, in which the fire of the sun, which generates the
vapor, plays as essential a part as the cold of the moun-
tains which condenses it.*

It was their ascription to glacier action that first gave
the parallel roads of Grlen Koy an interest in my eyes; and
in 1867, with a view to self -instruction, I 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 regarding the common level of the shelves at oppo-
site 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 Eoy 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 por-
tions of the rock being perfectly distinct to this hour.
My knowledge of the region was, however, far from com*
plete, and nine years had dimmed the memory even of the

' 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 temperature 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 ROADS OF GLEN ROY 231

portion whicli had been thoroughly examined. Hence mj
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 tele-
gram from Roy Bridge informed me that the roads were
snowed up. Finding books and memories poor substitutes
for the flavor of facts, I resolved subsequently to make
another effort to see the roads. Accordingly last Thurs-
day fortnight, after lecturing here, I packed up, and started
(not this time alone) for the Korth. 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 ex-
cellent hostlery 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 days
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 Glen Roy and Glen Fintaig we bore northward,
struck the ridge above Glen Gluoy, and 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 account to raise wavelets of any
strength to act upon the mountain drift. A second rea-
son is that they were land-locked in the higher portions
and protected from the southwesterly winds, the stillness

232 FRAGMENTS OF SCIENCE

of their waters causing tliem to produce but a feeble im-
pression upon the mountain sides. From Glen Gluoy we
passed down Glen Turrit to Glen Eoy, and through it
homeward, thus accomplishing two or three and twenty
miles of rough and honest work.

Next day we thoroughly explored Glen Glaster, follow-
ing 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 Eoy. Thence we
crossed southward over the mountain Creag Dhuhh^ 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 observations of Mr. Jamie-
son upon this region, including the mouth of Glen Trieg,
are in the highest degree interesting. We entered Glen
Spean, and continued a search begun on the evening of
our arrival at Eoy Bridge — the search, namely, for glacier
polishings and markings. We did not find them copious,
but they are indubitable. One of the proofs most con-
venient 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. Further 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 ob-
server more readily and thoroughly convince himself of
this 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 mon-

THE PARALLEL ROADS OF GLEN ROY 233

arch of these is Ben Nevis, 4,370 feet high. The position
of Ben Nevis and his colleagues, in reference to the vapor-
laden winds of the Atlantic, is a point of the first impor-
tance. It is exactly similar to that of Carrantual and the
Macgillicuddy Eeeks in the southwest of Ireland. These
mountains are, and were, the first to encounter the south-
western Atlantic winds, and the precipitation, even at
present, in the neighborhood of Killarney, is enormous.
The winds, robbed of their vapor, and charged with the
heat set free by its precipitation, pursue their direction
obliquely across Ireland; and the efiect 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 vapor fell as snow, and the consequence
was a system of glaciers which have left traces and evi-
dences 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 Eeeks, moved
through the Black Valley, took possession of the lake-
basins, and left its traces on every rock and island emer-
gent from the waters of the upper lake. They are all
conspicuously glaciated. Not in Switzerland itself do we
find clearer traces of ancient glacier action.

"What the Macgillicuddy Reeks did in Ireland, Ben
Nevis and the adjacent mountains did, and continue to
do, in Scotland. We had an example of this on the
morning we quitted Roy Bridge. From the bridge west-
ward 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

234 FRAGMENTS OF SCIENCE

at Fort William is 86 inches, while at Laggan it is only
46 inches, annually. The difference between west and east
is forcibly brought out by observations at the two ends of
the Caledonian Canal. Fort William at the southwestern
end has, as just stated, 86 inches, while Culloden, at its
northeastern end, has only 24. To the researches of that
able and accomplished meteorologist, Mr. Buchan, we are
indebted for these and other data of the most interesting
and valuable kind.

Adhering to the facts now presented to us, it is not
difficult to restore in idea the process by which the gla-
ciers of Lochaber were produced and the glens dammed
by ice. When the cold of the glacial epoch began to in-
vade the Scottish hills, the sun at the same time acting
with sufficient power upon the tropical ocean, the vapors
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 by the frozen material above, and their consump-
tion in the milder air below. For a period supply ex-
ceeded consumption, and the ice extended, filling Glen
Spean to an ever-increasing height, and abutting against
the mountains to the north of that glen. But why, it may
be asked, should the valleys south of Glen Spean be re-
ceptacles 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 received the loads of moisture
carried by the Atlantic winds, and not until they had been
in part dried, and also warmed by the liberation of their

THE PARALLEL ROADS OF GLEN ROT 235

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 the winter we
should have found all the mountains covered: had it been
in the summer we should have found the snow all gone.
But happily it was at a season when the 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 Grlen
Spean to be filled with glaciers from the south, while the
hills and valleys on the north, 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 consump-
tion 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 milder
weather set in, consumption would be in excess, a lower-
ing of the barriers and a retreat of the ice being the conse-
quence. But for a long time the conflict between supply
and consumption would continue, retarding 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 sufiiciently far to block up Glen Glaster, would
gradually retreat. Glen Glaster and its col being opened,

236 FRAGMENTS OF SCIENCE

the subsidence of the lake eighty feet, from the level of
the highest to that of the second parallel road, would fol-
low 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 Eoy 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 pursue
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 atmospheric conditions or to the charac-
ter of the dam itself, or through local softness in the drift,
small pseudo -terraces would be formed, which, to the per-
plexity 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
above 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 un-

THE PARALLEL ROADS OF GLEN ROY 21^7

doubtedly have been able to maintain themselves 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 roadg
is materially augmented by differences of vegetation. The
grass upon the terraces is not always of the same char-
acter as that above and below them, while on heather-
covered hills the absence of the dark shrub from the roads
greatly enhances their conspicuousness.

The annexed sketch of a model (p. 238) will enable the
reader to grasp the essential features of the problem and
its solution. Glen Gluoy and Glen Roy are lateral val-
leys which open into Glen Spean. Let us suppose Glen
Spean filled from V 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.

238

FRAGMENTS OF SCIENCE

Vn.6,

THE PARALLEL ROADS OF GLEN ROY 239

The streams from the mountains 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, 1,170 feet above the sea. Over
this col tbe water would flow into Glen Roy. But in Glen
Roy it could not rise higher than 1,150 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 succession, 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 connection was
established between Glen Roy and Glen Glaster, a com-
mon 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 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 Glen
Glaster and Glen Spean, would be formed. Finally, on
the disappearance of the ice from the lower part of Glen

240 FRAGMENTS OF SCIENCE

Spean, the waters would flow down their respective val-
leys as they do to-day.

Reviewing our work, we find three considerable steps
to have marked the solution of the problem of the Par-
allel Roads of Grlen 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 conception
by the very thorough researches of Mr. Jamieson. Ko
circumstance or incident connected with this discourse
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 ob-
servation, 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 ISTevis and his colleagues
to the vapor-laden winds of the Atlantic had not escaped
Mr. Jamieson. To him obviously the exploration of Loch-
aber, and the development of the theory of the Parallel
Roads, has been a labor 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 widening of the intellectual horizon and the reaction
of expanding knowledge upon the intellectual organ itself.
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 contemplation of these
facts so strengthens and expands the intellectual powers

THE PARALLEL ROADS OF GLEN ROY 241

that where truth once could not find an entrance it event-
ually finds a home.*

LITERATURE OF THE SUBJECT

Thomas Pennant. — ^A Tour in Scotland. Vol. iii. 1116, p. 394.

John MacOullooh. — On the Parallel Roads of Glen Roy. Greol. See. Trans,
vol. iv. 1817, p. 314.

Thomas Lauder Dick (afterward Sie Thomas Diok-Laudbb, Bart.). — On tho
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. 39.

Sib Charles Ltell. — Elements of Geology. Second edition, 1841.

Louis Agassiz. — ^The Glacial Theory and its Recent Progress — ^Paralld Ter-
races. Edin. New Phil. Journal, 1842, vol xzziii. p. 236.

David Milne (afterward David Milne-Home). — On the Parallel Roads of
Lochaber; with Remarks on the Change of Relative Levels of Sea and
Land in Scotland, and on the Detrital Deposits in that Country. Edin.
Roy. Soc. Trans. 184T, voL xvi. p. 396.

Robert Chambers. — ^Ancient Sea Margins. Edinburgh, 1848.

H. D. Rogers. — On the Parallel Roads of Glen Roy. Royal Inst. Proceedings,
1861, voL iii. 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, vdL
xiz. p. 235.

Sib Charles Ltell. — ^Antiquity of Man. 1863, p. 253.

Rev. R. B. Watson. — On the Marine Origin of the Parallel Roads of Glen Roy.
Quart. Joum. G«ol. Soc. 1865, vol. xxii. p. 9.

* The formation, connection, successive subsidence, and final disappearance
of the glacial lakes of Lochaber were illustrated 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 succes-
sive shiftings 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.

Science 11

242 FRAGMENTS OF SCIENCE

Sir John Lubbock. — On the Parallel Eoads of Glen Roy. Quart. Journ. GeoL

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

Journ. GeoL Soc. 1868, vol. xxiv. p. 273.
James Nicol. — On the Origin of the Parallel Roads of Glen Roy, 1869. G^ol.

Soc. Journal, vol. xxv. p. 282.
James Nicol. — How the Parallel Roads of Glen Roy were formed, 1872. Geoi.

Soc. Journal, vol. xxviii. p. 237.
Major-General Sir Henry James, R.E. — Notes on the Parallel Roada of

Lochaber. 4to. 1874.

IX

ALPINE SCULPTURl
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 main-
tains 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 further
up, and just before reaching the first bridge which spans
the chasm, I found more rolled stones, associated with

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244 FRAGMENTS OF SCIENCE

sand and gravel. Through this mass of detritus, fortu-
nately, 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 Yia 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 existence
of such floods is not well authenticated, and that, if the
supposition were true, it would be an additional argument
in favor 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 upward, 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 oleft 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 inspection 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 downward from a point near

ALPINE SCULPTURE 245

the bridge; but looking upward from the bridge itself, the
evidence of aqueous erosion was equally convincing.

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 itself with carbonate of
lime without damage to its transparency; the rock is dis-
solved in the water; and the gorges cut by water in such
rocks often resemble 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 which
interpenetrate limestone formations. A rock capable of
being thus dissolved will expose a smooth surface after
the water has quitted 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 which 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 Yia 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 neighborhood. This man conversed 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 per-
fectly. In fact, in former times, and subsequent to the
retreat of the great glaciers, a rocky barrier crossed the

f46 FRAGMENTS OF SCIENCE

valley at this place, damming the river which came from
the mountains 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 Julier Pass to Pontresina.
There are three or four ancient lake-beds between Tie-
fenkasten 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 neighborhood 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 con-
vince any person possessing an educated eye. Where,
moreover, the interior of the mound is exposed, it exhib-
its moraine-matter — detritus pulverized by the ice, with
bowlders 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-

ALPINE SCULPTURS 247

fourth of the moraine to its right, and the remaining three-
fourths to its left. Other moraines of a more resisting
character hold their ground as barriers to the present
day. In the Yal di Campo, for example, about three-
quarters of an hour from Pisciadello, there is a moraine
composed of large bowlders, 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. Behind the moraine is a lake-bed, now
converted into a level meadow, which rests on a deep
layer of mould.

At Pontresina a very fine and instructive gorge is to
be 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 char-
acter 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 Pontresina. The
hollowing out of the rock by the eddies of the water is
here quite manifest. A few minutes' walk upward brings
us to the end of the gorge; and behind it we hare the
usual indications of an ancient lake, and terraces of dis-
tinct 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 resist-
ing 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 Eosegg glacier. On this incline the water

248 FRAGMENTS OF SCIENCE

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 exami-
nation of the bed of the river ever proved the existence
of this fissure; and it is certain that water, particularly
when charged with solid matter in suspension, can cut a
channel through unfissured rock. Cases of deep cutting
can be pointed out where the clean bed 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 Grasthaus, 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
80 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 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 ot those gorges by the agent
which produced them.

Numerous cases might be pointed out, varying in mag-
nitude, but all identical in kind, of barriers which 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 Finsteraarschlucht
in the valley of Hasli. Here the ridge called the Kirchet

ALPINE SCULPTURE 249

seems split across, and the river Aar rushes throtigli 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 appar-
ent show of reason, conclude that the Finsteraarschlucht
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 — when probably a hundred
cases of the same kind, though different 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 bar-
rier 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 barrier the Aar tumbled toward Mey-
ringen, cutting, as the centuries passed, its bed ever deeper,
until finally it became deep enough to drain the lake, leav-
ing 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 pol-
ished, so as to render palpable to the most careless eye
that it is a gorge of erosion. But it was regarding the

250 FRAGMENTS OF SCIENCE

sides of the great cliasm that instruction was needed, and
from its edge nothing to satisfy me could be seen. 1
therefore stripped and waded into the river until a point
was reached which commanded 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
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 fract-
ure theory is, that the valleys themselves follow the tracks
of primeval fissures produced by the u|)heaval 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 for-
mation 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

ALPINE SCULPTURE 251

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 con-ceive what may be effected
in geologic periods. Thus the modern portion of the Yia
Mala throws light upon the whole. Near Bergiin, in the
valley of the Albula, there is also a little Via Mala, which
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. But
the most striking illustration of water-action upon limestone
rock that I have ever seen is the gorge at Pfaffers. Here
the traveller passes along the side of the chasm midway
between top and bottom. Whichever way he looks, back-
ward or forward, upward or downward, toward the sky or
toward 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 through 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 formation
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

252 FRAGMENTS OF SCIENCE

of difficulty. Special localities might be found whicli would
eeem to contradict every solution wHcli refers the confor-
mation of the Alps to the operation of a single cause.

Still the Alps present features of a character sufficiently
definite to bring the question of their origin within 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 re-
quired 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
throughout geologic time, in some cases depressing the
land, and in others causing the sea-bottom to protrude be-
yond 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 consider
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 dislocation. Fissures would be
produced by these changes; and such fissures, the advo-
cates 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, I
think, be doubted, but that the valleys of the Alps are
thus formed is a conclusion not at all involved in the ad-

ALPINE SCULPTURE 253

mission of dislocations. I never met witli a precise state-
ment of the manner in which the advocates of the fissure
theory suppose the forces to have acted — whether they as-
sume 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 upheaval 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 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 would 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 inquire what propor-
tion 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 required; for the problem is of such a
character as to render minute precision unnecessary.

No one, I think, would affirm that the area of the fis-
sures 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 corresponding 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

254 FRAGMENTS OF SCIENCE

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 comparing the width of the 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 realize what a small ratio the area
of the open fissures must bear to the unfissured crust.
They 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 unthinkable. If we
suppose the elevation to be due to the shrinking or subsi*
dence 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 ele-
vation, it is needless to say that the Alps themselves bear
no sort of resemblance to the picture which this theory pre-
sents 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 agitated by a storm. The val-
leys, instead of being much narrower than the ridges, oc-
cupy the greater space. A plaster cast of the Alps turned
upside down, so as to invert the elevations and depres-
sions, would exhibit blunter and broader mountains, with
narrower valleys between them, than the present ones.

ALPINE SCULPTURE 255

The valleys that exist cannot, I think, with any correct-
ness of language, be called fissures. It may be urged that
they originated 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 re-
garded 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 through-
out the entire region, is, in my opinion, utterly incompe-
tent to account for the conformation of the country. If,
on the other hand, we are compelled to resort to local dis-
turbances, 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 of the region from many of the
more accessible eminences — from the Gralenstock, the Grrau-
haupt, the Pitz Languard, the Monte Confinale — or, better
still, from Mont Blanc, Monte Rosa, the Jungfrau, the
Finsteraarhorn, 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, convince
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 unexagger-
ated 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 con-
formation, are, with reference to our customary standards,
vast and impressive. In a true model those local peculi-
arities disappear; for on the scale of a model they are too

256 FRAGMENTS OF SCIENCE

email 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 ^:s. upon
the point at which fracture begins to play a material part.

In ascending one of the larger valleys, we enter it wher«
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 upward, 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. Cross-
ing 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 pro-
longing 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 man-
ner indicated. The cols are simply depressions; in many

ALPINE SCULPTURE 257

of whicli tlie 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 pla-
teaus— in the region of cols — the power is not fully devel-
oped; but lower down tributaries unite, erosion is carried
on with increased vigor, 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 pro-
duced in the earlier portions of its course entirely dis-
appear.

I have hitherto confined myself to the consideration
of the broad question of the erosion theory as compared
with 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 atmosphere, including
frost and rain. Water and ice, however, are the principal
agents, and which of these two has produced the greatest
effect it is perhaps impossible 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,
unassailable; but whether the conclusion then announced
fairly follows from the facts is, I confess, an open ques-
tion.

The arguments which have been thus far urged against

» Phil. Mag., vol. xxiv. p. 169.

258 FRAGMENTS OF SCIENCE

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. It is,^
however, to be remembered that a precisely similar posi-
tion was taken up by many excellent 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 who so wisely took the side of "boldness" in
that discussion beware lest they place themselves, with ref-
erence to the question of glacier erosion, in the position
formerly occupied by their opponents.

Looking at the little glaciers of the present day — mere
pygmies as compared to the giants of the glacial epoch —
we find that from every one of them issues a 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 pulverized 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 Rhone is charged with
it, and tens of thousands of acres of cultivable land are
formed by the silt above the Lake of Geneva.

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 tearing it bodily from the rock to which
it belongs; while the water which everywhere circulates
upon the bed of the glacier continually washes the detritus

ALPINE SCULPTURE 269

away and leaves the rock clean for further abrasion. Con-
fining 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.
Kocks 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 priori
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 pounds, and
on every square yard of its bed with a weight of 486,000
pounds. With a vertical pressure of this amount the gla-
cier is urged down its valley by the pressure from behind.
We can hardly, I think, deny to 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 gla-
cier exercises against its bed from the fact that the resist-
ance, 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 Ehone at Martigny

260 FRAGMENTS OF SCIENCE

has also "been regarded as conclusive evidence against the
theory of erosion. "Why," it has been asked, "did not
the glacier of the Khone go straight forward instead of
making this awkward bend?" But if the valley be a
crack, why did the crack make this bend? The craclv,
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 perfectly ap-
plicable 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 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 in-
teresting paper ' ' On the Lakes of Switzerland, ' ' M. Studer
also refers to the bend of the Rhine at Sargans in proof
that the river must there follow a pre-existing fissure. I
made a special expedition to the place in 1864; and, though
it was plain that M. Studer had good grounds for the se-
lection 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

ALPINE SCULPTURE

261

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 dis-
posed of so easily. Let me record here my experience of
the Morteratsch glacier. I took with me, in 1864, a theod-
olite 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 trib-
utaries. 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 extended over several days. On
each line eleven stakes were fixed, which are designated
by the figures 1, 2, 8, etc., in the Tables.

Morteratsch Glacier^ Line A

No. of Stake

1

2

3

4

5

6

1

8

9
10
11

Hourly MotioM

0-35 inch

0-49 '*

0-53 "

0-64 '*

0-56 "

0-54 "

0-52 "

0-49
0-40
0-29
0-20

262

FRAGMENTS OF SCIENCE

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

Morteratsch Glacier, Line B

No. of Stake
1

2 .

3 .

4 .

5 .

6 .
T .

8 .

9 .

10 .

11 .

Hourly Motion
inch

•05
•14
•24
•32
•41
•44
•44
•45
•43
•44
•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 from carrying the
line sufficiently far across to render the retardation of the
further side of the glacier fully evident.

Morteratsch Glacier, Line C

No. of stake Hourly Motion

1 0^05 inch

2 0-09 "

3 0^18 "

4 0-20 "

5 025 "

6 0^27 "

7 0^27 "

8 030 "

9 021 "

10 0-20 "

11 016 "

Comparing the three lines together, it will be observed
that the velocity diminishes as we descend the glacier. In

ALPINE SCULPTURE 263

100 hours the maximum motion of the three lines respeo-
lively is as follows:

Maximum Motion in 100 hours

Line A 56 inches

*• B 45 **

" C 30 "

This deportment explains an appearance which must
strike every observer who looks upon the Morteratsch from
the Piz Languard, or from the new Bernina Eoad. A me-
dial moraine runs along the glacier, commencing as a nar-
row streak, but toward 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
measurements, 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 ma-
terials are more and more squeezed together, and they must
consequently move laterally and render 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 run 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

* The snout of the Aletsch glacier has a diuraal motion of Jess 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 striking.

264 . FRAGMENTS OF SCIENCE

glacier. The opinion appears to be prevalent tliat it is tlie
snout of a glacier tliat must act the part of plowshare; 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, notwith-
standing 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 my-
self 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 plowed
away.

The question of Alpine conformation stands, I think,
thus: We have, in the first place, great valleys, such as
those of the Ehine and the Khone, which we might con-
veniently call valleys of the first order. The mountains
which flank these main valleys are also cut by lateral val-
leys running into the main ones, and which may be called
valleys of the second order. When these latter are exam-
ined, 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,

ALPINE SCULPTURE 265

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,
grinding down and tearing away the rocks over which they
passed. We have, moreover, on the plains at the foot 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 val-
leys we have also suggestions as to the magnitude of the
erosion which has taken place. This moraine-matter, more-
over, 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 plowing-out of the gla-
cier itself. This accounts for the magnitude of many of the
ancient moraines, which date from a period when, almost
all the mountains were covered with ice and snow, and
when, consequently, the quantity of moraine-matter derived
from the naked crests cannot have been considerable.

The erosion theory ascribes the formation of Alpine
valleys to the agencies here briefly referred to. It in-
vokes 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 demonstrable that they are com-
petent 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

Science — Y — 12

266 FRAGMENTS OF SCIENCE

must in the long run be invoked, and its power therefore
conceded. The fracture theory infers from the disturb-
ances 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
natural 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
efiiects 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. Tliey
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 en-
hanced by our partial ability to conceive it. In the fall-
ing 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 propor-
tional to the magnitude of the phenomena.

I will here record a few other measurements executed
on the Rosegg glacier: the line was staked out across the

ALPINE SCULPTURE 267

trunk formed by the junction of the Eosegg proper with
the Tschierva glacier, a short distance below the rocky
promontory called Agaliogs.

Rosegg

Glacier

No. of Stake

Hourly Motion

1 .

. 0-01 inch

2 .

. 0-05 '

3 .

. 0-07 '

4 .

. 0-10 '

5 .

. 0-11 *

6 .

. 0-13 '

■7 .

, o-u '

8 .

. 0-18 *

9 .

. 0-24 '

10 .

. 0-23 '

11 .

. 0-24 '

This is an extremely slowly moving glacier; the maxi-
mum motion hardly amounts to seven inches a day. Cre-
vasses prevented us from continuing the line quite across
the glacier.

RECENT EXPEEIMENTS ON FOG-SIGNALS*

THE care of its sailors is one of the first duties of a
maritime people, and one of tlie 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
sometimes 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 parabolio 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 apparatus 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 illustrious 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 lumi-
nous sheet which grazes the sea-horizon. In revolving

> A discourse delivered in the Royal Institution, March 22, 18t8.

(268)

RECENT EXPERIMENTS ON FOG-SIGNALS 269

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 strengthen-
ing of the light is intended, for here it is not needed. Nor
is it for densely foggy weather, for here it is ineffectual.
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 in-
debted 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 max-
imum number of jets being 824. 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 inven-
tion.

The last great agent employed in lighthouse illumination
is electricity. It was in this Institution, beginning in 1881,
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 prophetic ring. *' I have rather, ' *

270 FRAGMENTS OF SCIENCE

he writes, in 1831, "been desirous of discovering new lacts
and new relations dependent 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 labors of Holmes, of the Paris Alliance
Company, of Wilde, and of Gramme, constitute a brilliant
^Ifilment of this prediction.

But, as regards the augmentation of power, the greatest
itep hitherto made was independently taken a few years
ago by Dr. "Werner Siemens and Sir Charles Wheatstone.
Through the application of their discovery a machine en-
dowed with an infinitesimal charge of magnetism may, by
a process of accumulation at compound interest, be caused
so to enrich itself magnetically as to cast by its perform-
ance 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 1,000 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 lighthouses 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 illumination. Hence the necessity

RECENT EXPERIMENTS ON FOG-SIGNALS 271

of employing sound-signals in dense fogs. Bells, gongs,
horns, whistles, guns, and 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 em-
ployed with useful effect at the North Stack, near Holy-
head, on the Kish Bank near Dublin, at Lundy Island,
and at other points on our coasts. During the long, labo-
rious, and I venture to think memorable, series of observa-
tions 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, firing
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 accident-
ally upon the best possible shape, arrangements were made
with the War Office for the construction of a gun specially
calculated to produce the loudest sound attainable from
the combustion of 3 lbs. of powder. To prevent the un-
necessary landward waste of the sound, the gun was fur-
nished with a parabolic muzzle, intended to project the
sound over the sea, where it was most needed. The con-
struction of this gun was based on a searching series of
experiments executed at Woolwich with small models, pro-
vided with muzzles of various kinds. A drawing of the
gun is annexed (p. 272). It was constructed on the prin-
ciple of the revolver, its various chambers being loaded
and brought in rapid succession into the firing position.
The performance of the gun proved the correctness of the
principles on which its construction was hased.

An incidental point of some interest was decided by

272

FRAGMENTS OF SCIENCE

tlie earliest Woolwich experiments. It liad been a widely
spread opinion among artillerists, tliat a bronze gun pro-
duces a specially loud report. 1 doubted from the outset
whether this would help us; and, in a letter dated 22d
April, 1874, I ventured to express myself 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^

Fig. 6.— Breech-loading Fog-signal Gun, with Bell Mouth, ^ proposed by
Major Maitland, R. A,, Assistant Superintendent.

rings like a bell. This latter, I apprehend, will disappear
at considerable distances." 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 gun has been modified in construction since this
drawing was made.

* General Campbell assigns a true cause for this difference. The ring of the

RECENT EXPERIMENTS ON FOG-SIONALS 273

Coincident with these trials of guns at Woolwich, gun-
cotton was thought of as a probably effective sound-pro-
ducer. From the first, indeed, theoretic considerations
caused me to fix my attention persistently on this sub-
stance; for the remarkable experiments of Mr. Abel,
whereby its rapidity of combustion and violently explo-
sive energy are demonstrated, seemed to single it out as
a substance eminently calculated to fulfil the conditions
necessary to the production of an intense wave of sound.
What those conditions are we shall now more particularly
inquire, 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 depend
on the sudden conversion of a solid body into an intensely
heated gas. Now the work which the artillerist requires
the expanding gas to perform is the displacement 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 perform. 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 dis-
tances through the atmosphere, and this requires a certain
choice and management of the explosive material.

A sound-wave consists essentially of two parts — a con-
densation and a rarefaction. Now, air is a very mobile
fluid, and if the shock imparted to it lack due promptness,

bronze gun represents so much energy withdrawn from the explosive force of
the gunpowder. Further experiments would, however, be needed to place the
superiority of the cast-iron gun at a distance beyond question.

274 FRAGMENTS OF SCIENCE

the wave is not produced. Consider tlie case of a common
clock pendulum, wliich oscillates to and fro, and wliicli
might be expected to generate corresponding 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 formation 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 pen-
etration, refers the damping power, first described by Sir
John Leslie, of hydrogen upon sound.

A tuning-fork which executes 256 complete vibrations
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 rarefactions, whereby the
formation of sound-pulses is forestalled. 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 j)art of the energy of the shock
converted into wave-motion. And as different kinds of
gunpowder vary considerably in their rapidity of combus-

RECENT EXPERIMENTS ON FOG-SIGNALS 275

tion, it may be expected that they will also vary as pro-
ducers of sound. This theoretic inference is completely
verified by experiment. In a series of preliminary 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. G.),
Large-grain (L. G.), Rifle Large-grain (R. L. G.), and
Pebble-grain (P.) (See annexed figures.) The charge in
each case amounted to 4^4 lbs.; four 24-lb. howitzers

F.G. L.G. B.L.G. p.

Pio. 7.

being employed to fire the respective charges. There
were eleven observers, all of whom, without a single dis-
sentient, 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 pronouncing the pebble-powder
the worst sound -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 surprised 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

276 FRAGMENTS OF SCIENCE

tube of the gun these lumps of solid matter gradually re-
solve themselves into gas, which on issuing from the muz-
zle imparts a kind of push to the air, instead of 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 ra-
pidity sufficient to forestall the formation of the wave. On
d priori grounds, then, we are entitled to infer the effective-
ness 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 accom-
plished help in the use of it, the resources of Woolwich
Arsenal have been freely placed at the disposal of the
Elder Brethren. General Campbell, General Younghus-
band, Colonel Fraser, Colonel Maitland, and other officers,
have taken an active personal part in ths investigation,
and in most cases have incurred the labor of reducing and
reporting on the observations. Guns 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 22d 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 against 4 oz. of gun-cotton
detonated both in the open, and in the focus of a parabolic

RECENT EXPERIMENTS ON FOG-SIGNALS 277

reflector.* The sound produced by the gun-cotton, rein-
forced bj the reflector, was unanimously pronounced loud-
est of all. With equal unanimity, the gun-cotton deto-
nated in free air was 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 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 manipulation of the gun-
cotton required some preliminary discipline — ^promptness,
certainty, and effectiveness of firing, augmenting as expe-
rience increased. As 1 lb. of gun-cotton costs as much as
8 lbs. of gunpowder, these quantities were compared to-
gether on the 22d of February. The guns employed to
discharge the gunpowder were a 12-lb. brass howitzer, a
24-lb. cast-iron howitzer, and the long 18-pounder em-
ployed at the South Foreland. 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. howitzer and the 18-pounder were both
beaten by the gun-cotton. On the 2d of May, on the other
hand, the gun-cotton is reported as having been beaten
by all the guns.

* For charges of this weight the reflector is of moderate size, and may be
employed without fear of fracture.

278 FRAGMENTS OF SCIENCE

Meanwhile, the parabolic-muzzle gun, expressly in-
tended for fog-signalling, was pushed rapidly forward,
and on March 22 and 23, 1876, its power was tested at
Shoebiuyness. Pitted against it were a 16-pounder, a
53^ -inch howitzer, 1^ lb. of gun-cotton detonated in the
focus of a reflector (see annexed figure), and IJ^ lb. ol
gun-cotton detonated in free air. On this occasion nine*

Fra. 8.— Gun-cotton Slab (1^ lb.) Detonated in the Focus of a Cast-iron Reflector

teen different series of experiments were made, when the
new experimental gun, firing a 3-lb. charge, demonstrated
its superiority over all guns previously employed to fire
the same charge. As regards the comparative merits of
the gun-cotton fired in the open, and the gunpowder fired
from the new gun, the mean values of their sounds were
the same. Fired in the focus of the reflector, the gun-

RECENT EXPERIMENTS ON FOG-SIGNALS 279

cotton clearly dominated over all the other sound-pro-
ducers.*

The whole of the observations here referred to were
embraced by an angle of about 70°, of which 50° lay oa
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 133^ miles from the firing-point. In all these observa-
tions, the reinforcing action of the reflector, and of the
parabolic muzzle of the gun, came into play. But the re-
inforcement of the sound in one direction implies its with-
drawal from some other direction, and accordingly it was
found that at a distance of 5^ 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 83^ miles and ISJ^
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 13J^ miles,
against a light wind which was blowing at the time.

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
upward or downward, 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 bellhangers must have been aware of the fact that

' The reflector was fractured bj the explosion, but it did good servioe
afterward.

280 FRAGMENTS OF SCIENCE

the sides of tlie bell, and not it;^ mouth, emitted the strong-
est sound, their practice being probixbly determined by this
knowledge. Our slabs of gun-cotton also emit waves of
different densities in different parts. It has occurred in
the experiments at Shoeburyness that when the broad side
of a slab was turned toward the suspending wire of a sec-
ond slab six feet distant, the wire was cut by the explo-
sion, while when the edge of the slab was turned to the
wire this never occurred. To the circumstance that the
broad sides 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 con-
stitution of the wave of sound. I did not think large rect-
angular slabs the most favorable 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 so great as the dif-
ferences in the quantities of explosive material might lead
one to expect. The mean values of eighteen series of ob-
servations made on board the ** Galatea," at distances vary-
ing from If mile to 4*8 miles, were as follows:

Weighta .

4 oz.

6 oz.

9 oz.

12 oz.

Value of sound .

3-12

3-34

40

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;

RECENT EXPERIMENTS ON FOG-SIGNALS 281

6 oz., 2 inclies 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 taken
as a ready means of expressing the approximate relative
intensity of the sounds as estimated by the ear. When we
find a 9-oz. charge marked 4, and a 12 -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, the values of the sounds at
the distances recorded would not, in my opinion, show a
greater advance with the increase of material than that
indicated by the foregoing numbers. Subsequent experi-
ments 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 parabolic
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 sound all round them as from
central foci. As a signal in rock lighthouses, where nei-
ther syren, steam- whistle, nor gun could be mounted; and
as a handy fleet-signal, dispensing with the lumber of spe-
cial signal-guns, the gun-cotton will prove invaluable.
But in most of these cases we have the drawback that

282 FRAGMENTS OF SCIENQB

local damage may be done by the explosion. The lantern
of the rock lighthouse might sufEer 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 such
a signal, instead of the ubiquitous but ineffectual 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 ascen-
sional force of which should be employed to carry the
disk to an elevation of 1,000 feet or thereabout, where,
by the ignition of a fuse associated with a detonator, the
gun-cotton should be fired, sending its sound in all direc-
tions vertically and obliquely down upon earth and sea.
The first attempt to realize this idea was made on July
18, 1876, at the firework manufactory of the Messrs. Brock,
at Nunhead. Eight rockets were then fired, four being
charged with 5 oz. and four with 1]4 oz. of gun-cotton.
They ascended to a great height, and exploded with a
very loud report in the air. On July 27, the rockets were

RECENT EXPERIMENTS ON FOG-SIGNALS 283

tried at Shoeburyness. The most noteworthy result on
this occasion was the hearing of the sounds at the Mouse
Lighthouse, 83^ miles E. by S., and at the Chapman Light-
house, 8^ miles W. by N. ; that is to say, at opposite
Bides of the firing -point. It is worthy of remark that, in
the case of the Chapman Lighthouse, land and trees inter-
vened between the firing-point and the place of observa-
tion. "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 pro-
duced near the surface of the earth with others 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.
The quantity of gun-cotton employed was 73^ 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
J^-lb. disks, suspended from a horizontal iron bar about
4J^ 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

284 FRAGMENTS OF SCIENCE

gqualls of snow and hail, the direction of the sound being
at right angles to that of the wind. Five series of obser-
vations were made on board the "Vestal," at distances
varying from 3 to 6 miles. The mean value of the explo-
sions in the air exceeded that of the explosions near the
ground by a small but sensible quantity. At Windmill
Hill, Q-ravesend, 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 loud-
est; while in the remaining five the rocket had the
advantage.

Toward the close of the day the atmosphere became
very serene. A few distant cumuli sailed near the hori-
zon, but the zenith and a vast angular space all round it
were absolutely free from cloud. From the deck of the
*' Galatea" a rocket was discharged, which reached a great
elevation, and exploded with a loud report. Following
this solid nucleus of sound was a continuous train of
echoes, which retreated to a continually greater distance,
dpng gradually off into silence after seven seconds' dura-
tion. 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 23d of March the experiments were resumed,
the most noteworthy results of that day's observations
being that the sounds were heard at Tillingham, 10 miles
to the N.E.; at West Mersea, 15i miles to the N.E. by
E. ; at Brightlingsea, 17}i miles to the N.E. ; and at Clac-
ton Wash, 203^ miles to the N.E. by K E. The wind
was blowing at the time from the S.E. Some of these
sounds were produced by rockets, some by a 24-lb. how-
itzer, and some by an 8-inch Maroon.

RECENT EXPERIMENTS ON FOG-SIGNALS 285

In December, 1876, Mr. Gardiner, 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 explosive material being
thus compressed into the same volume, Mr. Gardiner
thought that a greater sonorous effect must be produced
by the powder. At the instance of Mr. Mackie, who had
previously gone very thoroughly into the subject, a com-
mittee 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 desire on this occasion
was to get as near to windward as possible, but the Swale
and 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 -con tinned.

On the 17th of October, 1877, another series of experi-
ments 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.,

286 FRAGMENTS OF SCIENCE

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 "G-al-
atea," positions being successively assumed which per-
mitted the sound to reach the observers with the wind,
against the wind, and across the wind. The distance of
the "Galatea" varied from 3 to 7 miles, that of the
*' Vestal," which was more restricted in her movements,
being 2 to 3 miles. Briefly summed up, the result is that
the howitzer, firing a 3-lb. charge, which, it will be re-
membered, was our best gun at the South Foreland, was
beaten by the 12-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 referred to:

1.

Leigh .

6i miles W.N.W.

24 out of 40 sounds heard

2.

Girdler Light-vessel

. 12 *•

S.E. by E.

5

3.

Reculvers

. iH *'

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 driz-
zling rain; the wind about N.W. by N. all day; at times
squally, rising to a force of 6 or 7, and sometimes drop-
ping to a force of 2 or 3. The station at Leigh excepted,
all these places were to leeward of Shoeburyness. At
four other stations to leeward, varying in distance from
15J^ to 243^ miles, nothing was heard, while at eleven sta-
tions to windward, varying from 8 to 26 miles, the sounds

RECENT EXPERIMENTS ON FOG-SIGNALS 287

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 pro-
ceeded to Dungeness with the view of making a series of
strict comparative experiments with gun-cotton and cotton-
powder. Eockets containing 8 oz., 4 oz., and 2 oz. of
gun-cotton had been prepared at the Koyal Arsenal; while
others, containing similar quantities of cotton-powder, had
been supplied by the Cotton-powder Company at Faver-
sham. "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.

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

The effectiveness 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 the 2-oz.
rocket. The former was recorded as 6*9 and the latter as
6*9, the value of the 4-oz. rocket being intermediate be-
tween them. These results were recorded by a number
of very practiced observers on board the *' Galatea."
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 result, possibly, to
be in part ascribed to the imperfection of the powder.
The performance of the syren was, on the whole, less sat-
isfactory 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.

288 FRAGMENTS OF SCIENCE

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 between
the gun-cotton rocket and other fog-signals; but they are
not the only ones. On the 2d of August, 1877, for ex-
ample, experiments were made at Lundy Island with the
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 com-
manded 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 to that instrument. Proceeding along the sea
margin at Flamboro' Head, Mr. Edwards states that at a
distance of IJ 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 powerful
sound in the face of an opposing wind.

On the evening of February 9, 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 partic-
ulars. The first column in the annexed statement eon-

RECENT EXPERIMENTS ON FOG-SIGNALS

289

tains tlie name of the place of observation, the second its
distance from the firing- point, and the third the result
observed :

Stoke Hill, Ipswich . 10 miles
Melton . . . 15 '*

Framlingham

18

Stratford. St. Andrews 19

Tuddenham.

St. Martin 10

Christ Church Park

. 11

Nettlestead Hall .

. 6

Bildestone .

. 6

Nacton

. 14

Aldboro' .

. 25

Capel Mills .

. 11

Lawford

. 15i

Rockets clearly seen and sounds distinctly
heard 53 seconds after the flash.

Signals distinctly heard. Thought at first
that sounds were reverberated from the
sea.

Signals very distinctly heard, both in the
open air and in a closed room. Wind in
favor of sound.

Reports loud ; startled pheasants in a cover
close by.

Reports very loud ; rolled away like thunder.

Report arrived a little more than a minute
after flash.

Distinct in every part of observer's house.
Very loud in the open air.

Explosion very loud, wind against sound.

Reports quite distinct — mistaken by inhabi-
tants for claps of thunder.

Rockets seen through a very hazy atmos-
phere ; a rumbling detonation heard.

Reports heard within and without the ob-
server's house. Wind opposed to sound.

Reports distinct : attributed to distant thun-
der.

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 command-
ing 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 light-and-sound rocket as a signal of dis-

SCIENCE — Y — 13

290 FRAGMENTS OF SCIENCE

tress, which proposal was subsequently realized, but in a
form too elaborate and expensive for practical use. The
idea of a gun-cotton roc^ket fit for signalling in fogs is, I
believe, wholly due to Sir Eichard 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 sig-
nal 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 light-
ships, but in the Navy also it may be turned to important
account. Soon after the loss of the * 'Vanguard** I vent-
ured to urge upon an eminent naval officer the desirabil-
ity 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 difficulty. It is manip-
ulated 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 desir-
able, a fog-signal station might be extemporized 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 these echoes I attached a fundamental signifi-

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

RECENT EXPERIMENTS ON FOG-SIGNALS 291

cance. 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 duration, 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 perfectly continuous as long
as the sea was clear of ships, "tapering" by imperceptible
gradations into absolute silence. But when a ship hap-
pened to throw itself athwart the course of the sound, the
echo from the broad side of the vessel was returned as
a shock which rudely interrupted the continuity of the
dying atmospheric music.

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 inten-
sity 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. Causing 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 toward which
the trumpet was directed. They could not, under the cir-
cumstances, come from the glassy sea; while both their
variation of direction and their perfectly continuous fall

:i92 FRAGMENTS OF SCIENCE

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 Dungeness, the
smoothness of the sea and the serenity of the air caused
me to test the echoing power of the atmosphere. A single
ship lay about half a mile distant between us and the
land. The result of the proposed experiment was clearly
foreseen. It was this. The rocket being sent up, it ex-
ploded at a great height; the echoes retreated in their
usual fashion, becoming less and less intense as the dis-
tances of the invisible surfaces of reflection from the ob-
servers increased. About five seconds after the explo-
sion, a single loud shock was sent back to us from the
side of the vessel lying between us and the land. Obliter-
ated for a moment by this more intense echo, the aerial
reverberation continued its retreat, dying away into silence
in two or three seconds afterward.*

I have referred to the firing of an 8-oz. rocket from the
deck of the *' Galatea'* on March 8, 1877, stating the dura-
tion of its echoes to be seven seconds. Mr. Prentice, who
was present at the time, assured me that in his experi-
ments 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.

» The echoes of the gun fired on shore this day were very bnef ; those of
the 12- oz. gun-cotton rocket were 12" and those of the 8-oz. cottoa-powdar
rocket 11" in duration.

RECENT EXPERIMENTS ON FOG-SIGNALS 293

To attempt to interpret an experiment wliicli 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 Jo-
seph Henry, which he considers adverse to the notion of
aerial echoes. He took the trouble to point the trumpet
of a syren toward the zenith, and found that when the
syren was sounded no echo was returned. Now the re-
flecting surfaces which give rise to these echoes are for
the most part due to differences of temperature between
sea and air. If, through any cause, the air above be
chilled, we have descending streams — if 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 reflect-
ing 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 enun-
ciate. But, as I have indicated, not only to see but to
vary such an experiment is a necessary prelude to grasp-
ing its full significance.

In a paper published in the ** Philosophical Transac-
tions'* for 1876, Professor Osborne Eeynolds refers to
these echoes in the following terms: "Without attempt-
mg 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

* These carried 12 oz. of gunpowder, which has been found by Colonel Fraser
to require an iron case to produce an effective explosion.

294 FRAGMENTS OF SOIENCE

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 aSrial 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. 295), 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 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 heterogeneous 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

^ Fully described in my "Lectures on Sound," 3d edition, p. 227.

RECEN7 EXPERIMENTS ON FOG-SIGNALS

295

front of the South Foreland strikingly imitated.^ Turning
ofE the gas, and removing the sensitive flame to f^ some
distance behind the reed, it burned there tranquilly,
though the reed was sounding. Again lighting the gas
as it issued from the brass tubes, the sound reflected from

Pig. 9.

the heterogeneous air threw the sensitive flame into vio-
lent 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 dynamite marks it as a sub-
stance specially suitable for the production of sound. At

1 <

'Lectures on Sound." 3d edition, p. 268.

296 FRAGMENTS OF SCIENCE

tlie suggestion of Professor Dewar, Mr. McRoberts has
carried out a series of experiments on dynamite, witli ex-
tremely promising results. Immediately after tlie delivery
of tlie foregoing lecture I was informed tliat Mr. Brock
proposed the employment of dynamite in the Collinson
rocket.

XI

ON THE STUDY OF PHYSICS'

I HOLD in my liand 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 Impor-
tance of the Study of Physics as a Means oi 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 con-
sists, 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 windpipe which throw into vibration
the air forced between them from the lungs, thus pro-
ducing 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

* From a Lecture delivered in the Royal Institution of Great Britain in the
spring of 1854.

298 FRAGMENTS OF SCIENCE

we call the education of tlie voice. We may choose for
our 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 development of the mental
faculties, I shall consider the study of Physics as a means
toward 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 Lect-
ure, refers to that portion of natural science which lies
midway between astronomy and chemistry. The former,
indeed, is Physics applied to "masses of enormous
weight," while the latter is Physics applied to atoms
and molecules. The subjects of Physics proper are there-
fore those which lie nearest to human perception: light
and heat, color, sound, motion, the loadstone, electrical
attractions and repulsions, thunder and lightning, rain,
snow, dew, and so forth. Oar senses stand between these
phenomena and the reasoning mind. We observe the fact,
but are not satisfied with the mere act of observation : th«
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 thought : look at them,
compare them, observe their mutual relations and connec-
tions, 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 direc-
tion— in its progress from the multiplicity of facts to the

ON THE STUDY OF PHYSICS 299

central cause on which they depend. But, having guessed
the cause, we are not yet contented. We set 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 reasoning 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-opera-
tion of every muscle, and thus confer upon the whole
frame the benefits of healthy action.

The first experiment a child makes is a physical exper-
iment: the suction-pump is but an imitation of the first
act of every new-born infant. Kor do I think it calcu-
lated 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 extracting 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 lit-
tle fingers acquire sufiicient 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 re-
ceives his first lessons upon sound and gravitation. There
are pains and penalties, however, in the path of the in-
quirer: he is sure to go wrong, and Nature is just as sure

800 FRAGMENTS OF SCIENCE

to inform him of the fact. He falls downstairs, burns his
fingers, cuts his hand, scalds his tongue, and in this way
learns the conditions of his physical 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 confectioner's shop occupies the fore-
ground of human happiness; but the blossoms of a finer
life are already beginning to unfold themselves, and the
relation of cause and effect dawns upon the boy. He
begins to see that the present condition of things is not
final, but depends upon one that has gone before, and will
be succeeded by another. He becomes a puzzle to him-
self; and, to satisfy his newly- awakened curiosity, asks all
manner of inconvenient questions. The needs and ten-
dencies of human nature express themselves through these
early yearnings of the child. As thought ripens, he de-
sires 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 re-
pressed, 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 fairly 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 modern edu-
cation, 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

ON THE STUDY OF PHYSICS 301

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 utterance; for such are prolific through-
out 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 universal. We
have conquered and possessed ourselves of continents of
land, concerning which antiquity knew nothing; and if
new continents of thought reveal themselves to the ex-
ploring human spirit, shall we not possess them also?
In these latter days, the study of Physics has given us
glimpses of the methods of Kature which were quite hid-
den 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 op-
portunity of checking my assumptions and conclusions by
experience. In the present case, it is true, your own con-
sciousness might be appealed to in proof of the tendency
of the human mind to inquire into the phenomena pre-
sented to it by the senses; but I trust you will excuse me
if, instead of doing this, I take advantage of the facts
which have fallen in my way through life, referring to

802 FRAGMENTS OF SCIENCE

your judgment to decide whetlier such facts are truly
representative and general, and not merely individual
and local.

At an agricultural college in HampsMre, 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
for the purpose of reading reports and papers upon vari-
ous subjects. The society had its president and treasurer;
and abstracts of its proceedings were published 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 ques-
tions were either written out previously in a book, or, if
a question happened to suggest itself during the meeting,
it was written upon a slip of paper and handed in to the
secretary, who afterward read all the questions aloud. A
number of teachers were usually present, and they and
the boys made a common stock of their wisdom in fur-
nishing replies. As might be expected from an assem-
blage 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 questions
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 ?

ON THE STUDY OF PHYSICS 303

Why are thunder and lightning more frequent in sum-
mer 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 he wetted with water, why does the wet por-
tion become darker than before ?

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 sta-
tionary ?

What is the cause of perspiration ?

Is it true that men were once monkeys ?

What is the difference between the soul and the mindf

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 sug-
gested directly by natural objects, and are not such as
had an interest conferred on them by previous culture.
!N"ow, the fact is beyond the boy's control, and so cer-
tainly is the desire to know its cause. The sole question

804 FRAGMENTS OF SCIENCE

then is, whether this desire is to be gratified or not.
Who created the fact? Who implanted the desire? Cer-
tainly not man. Who then will undertake to place him-
self between the desire and its fulfilment, and proclaim 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 wis-
dom to rescue the boy from the consequences of a wish
which acts to his prejudice? Or, recognizing the propri-
ety of the question, how shall we answer it? It is im-
possible 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 me-
dium and entering another, a portion of light is always
reflected, but on this condition — 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

ON THE STUDY OF PHYSICS 805

in a Kquid of tlie same refractive index as itself, it im-
mediately disappears. I remember once dropping the
eyeball of an ox into water; it vanisbed as if by magic,
with the exception of the crystalline lens, and the sur-
prise was so great as to cause a bystander to suppose that
the vitreous humor had been instantly dissolved. This,
however, was not the case, and a comparison of the re-
fractive index of the humcr 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 have
a transparent solid mixed with air. At every transition
from solid to air, or from air to solid, a portion of light
is reflected, and this takes place so often that the light is
wholly intercepted. Thus from the mixture of two trans-
parent 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 the 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
deportment of various minerals, such as hydrophane and
tabasheer, the transparency of tracing paper used by engi-
neers, and many other considerations of the highest scien-
tific interest, are involved in the simple inquiry of this
unsuspecting little boy.

Again, take the question regarding the rising or falling
of the dew — a question long agitated, and finally set at

806 FRAGMENTS OF SCIENCE

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 opin-
ion of Father Laurus, that a goose egg, filled in the morn-
ing vnth 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 phenomenon to which his
question refers without first making him acquainted with
the radiation and conduction of heat. Take, for example,
a blade of grass, from which one of these orient pearls is
depending. During the day the grass, and the earth be-
neath 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 con-
duction. Now, in the case before us, the power of radia-
tion 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
vapor floating around the surface so cooled is condensed
upon it, and there accumulates 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 measure, sanctified
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

ON THE STUDY OF PHYSICS 307

brotherhood in Nature are organically united, and finds
the detection of such analogies a source of perpetual de-
light. To enlist pleasure on the side of intellectual per-
formance is a point of the utmost importance; for the ex-
ercise of the mind, like that of the body, depends for its
value upon the spirit in which it is accomplished. Every
physician knows that something more than mere mechan-
ical motion is comprehended under the idea of healthful
exercise — that, indeed, being most healthful which makes
Tis 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 unconsciously,
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 muscle,
but you cannot create the joy and gladness of the game,
and where these are absent, the charm and the health of
the exercise are gone. The case is similar with the edu-
cation 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 ad
vance 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 hon
est receptivity, and a willingness to abandon all precon
ceived 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 ex-

808 FRAGMENTS OF SCIENCE

perience of the true votary of science. And if a man
be not capable of this self-renunciation — this loyal sur-
render of himself to Nature and to fact — he lacks, in my
opinion, the first mark of a true philosopher. 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's 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 Nat-
ure's organ — ^his elevation is the instant consequence of
his humility. I should not wonder if my remarks pro-
voked a smile, for they seem to indicate that I regard the
man of science as a heroic, if not indeed an angelic, char-
acter; and cases may occur to you which indicate the re-
verse. You may point to the quarrels of scientific men,
to their struggles 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 weak-
ness— or, if you will — ^to the selfishness of Man, but not
to the charge of Physical Science.

The second process in physical investigation is deduc-
tion^ or the advance of the mind from fixed principles to
the conclusions which flow from them. The rules of logic
are the formal statement of this process, which, however,

ON THE STUDY OF PHYSICS 809

was practiced by every healtliy 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 ob-
servation. 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 treasuries 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 fur-
nishes a screen against 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, ab-
stracts, and generalizes, and provides a mental scenery
appropriate to these processes. The strictest precision of
thought is everywhere enforced, and prudence, foresight,
and sagacity are demanded. By its appeals to experiment,
it continually checks itself, and thus walks on a founda-
tion of facts. Hence the exercise it invokes does not end

310 FRAGMENTS OF SCIENCE

in a mere game of intellectual gymnastics, sucli as tlie
ancients deliglited in, but tends to the mastery of Nature.
This gradual conquest of the external 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
that faculty. Take for example the idea of an all-per-
vading 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 meas-
ured with the same ease and certainty as that with which
an engineer measures a base and two angles, and from
these finds the 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 of size which ap-
peal to the poetic sense. It is a mistake to suppose, with
Dr. Young, that

An undevout astronomer is mad;

there being no necessary connection 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 arithmetical processes of
science be too exclusively pursued, they may impair the

ON THE STUDY OF PHYSICS 311

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 investigator himself: it does not
reach the mass of mankind. Indeed, the conceptions fur-
nished by his cold unimaginative reckonings may furnish
themes for the poet, and excite in the highest degree that
sentiment 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 colors 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 ac-
complish, not only intellectual ends, but minister, at the
same time, to our material necessities. And so it is with
scientific research. While the love of science is a suffi-
cient incentive to the pursuit of science, and the investi-
gator, in the prosecution of his inquiries, is raised above
all material considerations, the results of his labors 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

312 FRAGMENTS OF SCIENCE

Mgliest embodiment of human genius and tlie only legiti-
mate object of scientific researcli — beware of prescribing
conditions to the investigator. Let him beware of at-
tempting 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 utili-
tarian is unfortunately, in most cases, the very last man
to see the occult sources from which useful results are de-
rived. He admires the flower, but is ignorant of the con-
ditions of its growth. The scientific man must approach
Nature in his own way; for, if you invade his freedom
^J 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 inquiries by the cal-
culations 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 favorite pur-
suits 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 star-
vation of the late G-raff; but compare what is 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."

ON THE STUDY OF PHYSICS 313

But wHle tlie scientific investigator, standing upon tlie
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 exclusive object for the
time, he cannot but feel the deepest interest in the prac-
tical application of the truth discovered. There is some-
thing 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 direc-
tions 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 ren-
der: 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, but rather as an ear-
nest of what she is yet to do. They mark our first great
advances upon the dominion of Nature. Animal strength
fails, but here are the forces which hold the world to-
gether, and the instincts and successes of Man assure him
that these forces are his when he is wise enough to com-
mand them.

As an instrument of intellectual culture, the study of
Physics is profitable to all: as bearing upon special func-
tions, its value, though not so great, is still more tangible.
Why, for example, should Members of Parliament be
ignorant of the subjects concerning which they are called

upon to legislate? In this land of practical physics, why

Science — V — 14

814 FRAGMENTS OF SCIENCE

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 inter-
ested disputants when a scientific question is discussed,
until 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 peo-
ple, in the ordinary sense of the term working, the study
of Physics would, I imagine, be profitable, not only as a
means of intellectual cultiire, 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 ex-
erted in the choice of employment 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 permanently 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 tempta-
tion. Besides this, our factories and our foundries pre-
sent an extensive field of observation, and were those who
work in them rendered capable, by previous culture, of
observing what they see^ the results might be incalculable.
Who can say what intellectual Samsons are at the present
moment toiling with closed eyes in the mills and forges

ON THE STUDY OF PHYSICS 815

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
multitudinous technical operations we are constantly play-
ing with forces our ignorance of which is often the cause
of our destruction. There are agencies at work in a loco-
motive 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 prob-
ably emanate a system by which these shocking accidents
might be avoided. Possessed of the knowledge, their per-
sonal 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 dimi-
nution 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 en-
acted in my own limited practice. There 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

816 FRAGMENTS OF SCIENCE

already cited, which presented themselves to my notice
during 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 in-
struction of a class in mathematics, and I usually found
that Euclid and the ancient geometry generally, when
properly and sympathetically addressed to the understand-
ing, formed a most attractive study for youth. But it
was my habitual practice to withdraw the boys from the
routine of the book, and to appeal to their self-power in
the treatment of questions not comprehended in that rou-
tine. At first, the change from the beaten track usually
excited aversion: the youth felt like a child amid stran-
gers; but in no single instance did this feeling continue.
When utterly disheartened, I have encouraged the boy by
the anecdote of Newton, where he attributes the differ-
ence between him and other men mainly to his own pa-
tience; or of Mirabeau, when he ordered his servant, who
had stated something to be impossible, never again to use
that blockhead of a word. Thus cheered, the boy has re-
turned to his task with a smile, which perhaps had some-
thing of doubt in it, but which, nevertheless, evinced a
resolution to try again. I have seen his eye brighten,
and, at length, with a pleasure of which the ecstasy of
Archimedes was but a simple expansion, heard him ex-
claim, *'I have it, sir." The consciousness of self -power,
thus awakened, was of immense value; and, animated by
it, the progress of the class was astonishing. It was often
my custom to give the boys the choice of pursuing their

ON THE STUDY OF PHYSICS 317

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 habit-
ually declined. The boys had tasted the sweets of intel-
lectual conquest and demanded victories of their own.
Their diagrams were scratched on the walls, cut into the
beams upon the playground, and numberless other illus-
trations 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 discourse, and endeavoring 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 math-
ematical 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, particularly in slip-
pery 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 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 notwithstanding. It was also pleasant

318 FRAGMENTS OF SCIENCE

to prove by mathematics, 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 upward
as downward. 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, our 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 apologize to you for dwelling so long upon
this subject; but the days spent among these young phi-
losophers 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 impor-
tance, I believe it to be that of the schoolmaster; and
if there be a position where selfishness and incompetence
do most serious mischief, by lowering the moral tone and
exciting irreverence and cunning where reverence and
noble truthfulness ought to be the feelings evoked, it
is that of the principal of a school. When a man of

ON THE STUDY OF PHYSICS 319

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 would be idle to talk of the position of such
a man being honorable. 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 behooves those who busy themselves with the mechan-
ics 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 culture,
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 phe-
nomena; on the other side, mind, which requires law for
its equilibrium, and through its own indestructible in-
stincts, 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 surrounded with objects which
provoke inquiry. Descending for a moment from this
high plea to considerations which lie closer to us as a
nation — as a land of gas and furnaces, of steam and elec-

320 FRAGMENTS OF SCIENCE

tricity: as a land wliicli science, practically applied, lias
made great in peace and mighty in war — I ask you
wlietlier tliis "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 ex-
clusively 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 — lift-
ing 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.

xn

ON CRYSTALLINE AND SLATY CLEAVAGE'

WHEN" the student of physical science has to in-
vestigate 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 downward
and elongates it. If he would examine the problem in
its purity, he must do as Plateau has done — detach 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
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

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

(321)

822 FRAGMENTS OF SCIENCE

music, still the mixture of all would produce mere noise.
Thus it is with the processes of nature, where mechanical
and molecular laws intermingle and create apparent con-
fusion. 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 components, 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 crystallization. Here, for example, is a
solution of common sulphate of soda or Grlauber salt.
Looking into it mentally, we see the molecules of that
liquid, like disciplined squadrons under a governing eye,
arranging themselves into battalions, gathering round dis-
tinct 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 expres-
sion of the crystalline forces; the molecules rush together
with the confusion of an unorganized mob, and not with
the steady accuracy of a disciplined host. In this mass
of bismuth also we have an example of confused crystal-
lization; 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 beautiful masses which we see upon the table. Bj
permitting alum to crystallize in this slow way we obtain
these perfect octahedrons; by allowing carbonate of lime
to crystallize, nature produces these beautiful rhomboids;

ON CRYSTALLINE AND SLATY CLEAVAGE 823

when silica crystallizes, we have formed these hexagonal
prisms capped at the ends by pyramids; by allowing salt-
petre to crystallize we have these prismatic masses, and
when carbon crystallizes we have the diamond. If we
wish to obtain a perfect crystal we must allow the molec-
ular forces free play; if the crystallizing mass be per-
mitted 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 themselves into layers which are perfectly parallel
to each other, and which can be separated by mechanical
means; this is called the cleavage of the crystal. The
crystal of sugar I hold in my hand has, thus far, es-
caped the solvent and abrading forces which sooner or
later determine the fate of sugar-candy. I readily dis-
cover that it cleaves with peculiar facility in one direc-
tion. 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.
Rocksalt cleaves in three directions, and the resulting
solid is this perfect cube, which may be broken up into
any number of smaller cubes. Iceland 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 differ-

324 FRAGMENTS OF SCIENCE

ent directions: heavy spar presents an example of tliis
kind of cleavage.

Turn we now to the consideration of some other phe-
nomena 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 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 re-
gard 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 pro-
duce 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 me-
chanical suspension by water. The powder was composed
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 suspen-
sion: how will it sink? The rounded grains of sand will
reach the bottom first, because they encounter least re-
sistance, the mica afterward, 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

ON CRYSTALLINE AND SLATY CLEAVAGE 325

subside; thej will arrange themselves in the manner indi-
cated, and by repeating the process you can actually build
up a mass which shall be the exact counterpart of that
presented by Kature. 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 Over Dar-
wen 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 imag-
ined, 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 phenome-
non to which I refer. We have long drawn our supply
of roofing-slates from such quarries; schoolboys ciphered
on these slates, they were used for tombstones in church-
yards, and for billiard-tables in the metropolis; but not
until a comparatively late period did men begin to in-
quire how their wonderful structure was produced. What
is the agency which enables us to split Honister Crag, or
the cliffs of Snowdon, into laminsB from crown to base?
This question is at the present moment one of the great
difficulties of geologists, and occupies their attention per-
haps more than any other. You may wonder at this.

826 FRAGMENTS OF SOIENOE

Lookiiig into the quarry of Penrhyn, you may be dis-
posed 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 strati-
fication 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. Thanks to Sir Eoderick 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 mak-
ing 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, Pro-
fessor 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 dimensions
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: '* Crystalline 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

> "Transactions of the Geological Societj," ser. ii. vol. iii. p. 477.

ON CRYSTALLINE AND SLATY CLEAVAGE 827

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 the action of polar forces and
geologic magnetism, which rather astonish those who know
something about the subject. According to this theory-
whole districts of North Wales and Cumberland, moun-
tains 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 mixture became consolidated, and the theory be-
fore us assumes that a process of crystallization afterward
rearranged the particles and developed in it a single plane
of cleavage. Though a bold, and I think inadmissable,
stretch of analogies, this hypothesis has done good ser-
vice. Eight or wrong, a thoughtfully uttered theory has
a dynamic power which operates against intellectual stag-
nation; and even by provoking opposition is eventually
of service to the cause of truth. It would, however, have
been remarkable if, among the ranks of geologists them-
selves, men were not found to seek an explanation of
slate-cleavage involving a less hardy assumption.

The first step in an inquiry of this kind is to seek

» In a letter to Sir Charles Lyell, dated from the Cape of Good Hope, Febru-
ary 20, 1836, Sir John Herschel writes as follows: "If rocks have been so
heated as to allow of a commencement of crystallization, that is to say, if they
have been heated to a point at which the particles can begin to move among
themselves, or at least on their own axes, some general law must then deter-
mine 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."

328 FRAGMENTS OF SCIENCE

facts. This has been done, and the labors of Daniel
Sharpe (the late President of the Geological Society, who,
to the loss of science and the sorrow of all who knew
him, has so suddenly been taken away from ns), 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 have
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 enor-
mous pressure in a direction at right angles to the planes
of cleavage. The shells are all flattened and spread out
in these planes. Compare this fossil trilobite of normal
proportions with these others which have suffered dis-
tortion. 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 trilobites 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 ma-
terial, 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 sinu-
osities like a contorted ribbon. Mr. Sorby has described
a striking case of this kind. This crumpling can be ex-
perimentally imitated; the amount of compression might,

ON CRYSTALLINE AND SLATY CLEAVAGE 329

moreover, be roughly estimated by supposing the con-
torted 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.

Let me now direct your attention to another proof of
pressure; you see the varying colors which indicate the
bedding on this mass of slate. The dark portion is gritty,
being composed of comparatively coarse particles, which,
owing to their size, shape and gravity, sink first and con-
stitute the bottom of each layer. Gradually, from bottom
to top the coarseness diminishes, and near the upper sur-
face we have a layer of exceedingly fine grain. It is the
fine mud thus consolidated from which are derived the
German razor- stones, so much prized for the sharpening
of surgical instruments. When 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 depos-
ited is, as might be expected, often rolled up into nodular
masses, carried forward, and deposited among coarser ma-
terial by the rivers from which the slate- mud has sub-
sided. Here are such nodules enclosed in standstone.
Everybody, moreover, who has ciphered upon a school-
slate must remember the whitish-green spots which some-
times 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, hite the pencil like the sur-
rounding gritty portions of the slate. Here is a beautiful
example of th^se spots: yet observe them, on the cleav-

830 FRAGMENTS OF SCIENCE

age surface, in broad round patches. But turn the slate
edgewise 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 Cum-
berland, and to many of the slate yards of London, and
found the fact general. Thus we elevate a common ex-
perience of our boyhood into evidence of the highest sig-
nificance 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:

Analysis op Slate
Dark Slate, two analyses

1. Percentage of iron , , 5*86

2. '• •* 6-13

Mean . 5-99

Whitish Green Slate

1. Percentage of iron . 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 magnetic experi-
ments 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 trilobites squeezed, beds

ON ORYSTALLINE AND SLATY CLEAVAGE 331

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 slate with
its bedding marked upon it; the lower portions of each
layer being composed of a comparatively coarse gritty ma-
terial something like what you may suppose the contorted
bed to be composed of. Kow, in crossing these gritty por-
tions, the cleavage turns, as if tending to cross the bed
ding 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 be-
came gradually more transverse, and the direction of its
cleavage is exactly such as you would infer from an ac-
tion 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. Sup-
posing 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 phe-
nomena of cleavage and pressure — that they accompany
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.

332 FRAGMENTS OF SCIENCE

This geologist is Sorby, wlio lias attacked the question
in the true spirit of a physical investigator. Call to
mind the cleavage of the flags of Halifax and Over Dar-
wen, 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 pow-
der, and on squeezing the mass finds that the tendency
of the scales is to set 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 by
Mr. Sorby, it might be shown how true his conclusion
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 production 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 self- same 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 sur-
passing tenuity, and proves at a single stroke that press-

ON CRYSTALLINE AND SLATY CLEAVAGE 333

•ore is sufficient to produce cleavage, and that this cleav-
age is independent of intermixed 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 re-
semblance of its cleavage to that of the wax. Compare
the surface of the wax with the surface of this slate from
Borrodale in Cumberland. You have precisely 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.*

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, and
it 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

* I have usually softened the wax by warming it, kneaded it with the fingers,
and pressed it between thick plates of glass previously wetted. At the ordinary
summer 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.

334 FRAGMENTS OF SCIENCE

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 compara-
tively 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 sub-
jected to pressure — ^it yields and spreads out in the direc-
tion of least resistance ; * the little polyhedra become con-
verted into laminae, separated from each other by surfaces
of weak cohesion, and the infallible result will be a ten-
dency 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 develop 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 bis-
cuit during this period without remarking the cleavage

* 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 wished
to obtain white-lead in a fine granular state, and to accomplish this he first com-
pressed it. The mold was conical, and permitted the lead to spread out a little
laterally. The lamination was as perfect as that o^ alate, and it quite defeated
him in his effort to obtain a granular powder.

ON CRYSTALLINE AND SLATY CLEAVAGE 835

developed by the rolling-pin. You have only to break a
biscuit across, and to look at the fracture, to see the lam-
inated structure. We have here the means of pushing the
analogy further. I invite you to compare the structure
of this slate, which was subjected to a high temperature
during the conflagration of Mr. Scott Eussell's premises,
with that of a biscuit. Air or vapor within the slate has
caused it to swell, and the mechanical structure it reveals
is precisely that of a biscuit. During these inquiries 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 accidental
cleavage, but this is cleavage with intention. The voli-
tion of the pastry-cook has entered into its formation. It
has been his aim to preserve a series of surfaces of struct-
ural 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 pre-
vent the butter from melting, and diffusing itself, thus
rendering the paste more homogeneous and less liable to
split. Puff-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 aggregate in fact of small nodules: it
is at a welding heat, and at this temperature is submitted
io the process of rolling. Bright, smooth bars are the
result. But notwithstanding the high heat the nodules
do not perfectly blend together. The process of rolling

336 FRAGMENTS OF SCIENCE

draws tliem 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 slidden;
the granules have yielded and become plates. They ex-
foliate 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 crystallization, 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 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 trust-
worthy 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

* For some further observations on this subject, by Mr. Sorby and myself,
see "Philosophical Magazine" for August, 1856.

ON CRYSTALLINE AND SLATY CLEAVAGE 837

brought before jou — bow tbe pressure is sufficient to pro-
duce the cleavage. Expanding our field of view, we find
tbe self- same law, wbose footsteps we trace amid tbe crags
of Wales and Cumberland, extending into tbe domain of
tbe pastry-cook and iron-founder; naj, a wbeel cannot roll
over tbe balf- dried mud of our streets witbout revealing
to us more or less of tbe features of tbis law. Let me
say, in conclusion, tbat tbe spirit in wbicb tbis problem
bas been attacked by geologists indicates tbe dawning of
a new day for tbeir science. Tbe great intellects wbo
bave labored at geology, and wbo bave raised it to its
present pitcb of grandeur, were compelled to deal witb
tbe subject in mass; tbey bad no time to look after de-
tails. But tbe desire for more exact knowledge is increas-
ing; facts are flowing in wbicb, wbile tbey leave un-
toucbed tbe intrinsic wonders of geology, are gradually'
supplanting by solid trutbs tbe uncertain speculations
wbicb beset tbe subject in its infancy. Greologists now
aim to imitate, as far as possible, tbe conditions of
nature, and to produce ber results; tbey are approacbing
more and more to tbe domain of pbysics, and I trust tbe
day will soon come wben we sball interlace our friendly
arms across tbe common boundary of our sciences, and
pursue our respective tasks in a spirit of mutual helpful-
ness, encouragement and goodwill.

[I would now lay more stress on tbe lateral yielding,
referred to in tbe note at tbe bottom of page 334, accom-
panied as it is by tangential sliding, tban I was prepared
to do wben tbis Lecture was given. Tbis sliding is, I
tbink, tbe principal cause of tbe planes of weakness, both

in pressed wax and slate rock. J. T., 1871.]

Science — V — 15

XIII

ON PABAMAGNETIO AND DIAMAGNETIC FORCES*

THE notion of an attractive force, which draws bodies
toward the centre of the earth, was entertained by
Anaxagoras and his pupils, by Democritus, Pythag-
oras, 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 affirmed the extension of this
force to the most distant stars. Thus it would appear
that vn the attraction of iron by a magnet originated the
conception of the force of gravitation. Nevertheless, if
we look closely at the matter, it will be seen that the
magnetic force possesses characters strikingly distinct from
those ol the force which holds the universe together. The
theory of gravitation is, that every particle of matter at-
tracts every other particle; in magnetism also we have
attraction, but we have always, at the same time, repul-
sion, the final effect being due to the difference 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 mag-

* Abstract of a discourse delivered in the Royal Institution of Great Britain,
Itebruary 1, 1866.

(338)

PARAMAGNETIC AND DIAMAGNETIC FORCES 339

netization, a dipping needle, when its centre of gravity is
supported, stands accurately level; but, after magnetiza-
tion, one end of it, in our latitude, is pulled toward the
north pole of the earth. The needle, however, being sus-
pended from the arm of a fine balance, its weight is found
unaltered by its magnetization. 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 toward that pole.
The reason is known to be, that although the marked
end of the needle is attracted by the north pole, the un-
marked 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 attracted — 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 re-
pelled; and because the attracted portions are nearer to
the magnet than the repelled ones, we have a balance in.
favor of attraction. Here then is the special characteris-
tic of the magnetic force, which distinguishes it from that
of gravitation. The latter is a simple unpolar 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 toward 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

S40 FRAGMENTS OF SCIENCE

like that of magnetism would not be able to transact the
business of the universe.

The object of this discourse is to inquire whether the
force of diamagnetism, which manifests itself as a repul-
sion of certain bodies by the poles of a magnet, is to be
ranged as a polar force, beside that of magnetism; or as
an unpolar force, beside that of gravitation. When a
cylinder of soft iron is placed within a wire helix, and
surrounded by an electric current, the antithesis of its
two ends, or, in other words, its polar excitation, 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 simi-
larly examined. The reason is, 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 diamagnetic body that
the pure action of the body upon a magnetic needle may
be observed, unmixed 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 repul-
sion. Such a helix is caused to stand between the two
poles N' s' of an astatic system.* The two magnets s N'
and s' N are united by a rigid cross piece at their centres,

* 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.

PARAMAGNETIC AND VIAMAGNETIO FORCES 341

ffjid are suspended from the point a, so that both magnets
Bwing 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 ex-
amine the action of a body placed within the helix and
excited by it, undisturbed by the influence of the latter.
The helix being 12 inches high, 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,

^

NC

1^

3S'

nu

Fio. 10.

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 deflecting 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 immediate 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 mo-

342 FRAGMENTS OF SCIENCE

tion double tliat of the latter, a very slight motion of
the magnet is sufficient to produce a displacement of the
image through several yards.

This then is the principle of the beautiful apparatus*
by which the investigation was conducted. It is manifest
that if a second helix be placed between the poles s N
with a cylinder within it, the action upon the astatic mag-
net may be exalted. This was the arrangement made use
of in the actual inquiry. Thus to intensify the feeble ac-
tion, 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 astatically. Secondly, by mak-
ing 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 ap-
paratus was enclosed 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 telescope and scale placed at a considerable distance
from the instrument.

A pair of bismuth cylinders was first examined. Send-
ing a current through the helices, and observing that the
magnets swung perfectly free, it was first arranged 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 simultaneously upon the mag-

* Devised by Professor W. "Weber, and constructed by M. Leyser, of Leipzig.

PARAMAGNETIC AND DIAMAGNETIC FORCES 343

netic poles: the magnet moved promptly, and after some
oscillations' came to rest at tlie 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 deflection was the consequence, and the final
position of equilibrium was 526: the movement being from
a larger to a smaller number. We thus observe a mani-
fest polar action of the bismuth cylinders upon the mag-
net; one pair of ends deflecting it in one direction, and
the other 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 potas-
sium, 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 deflection 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 de-
flection is permanent, and cannot therefore be due to in-
duced currents, which are only of momentary duration.
It has also been urged 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

* To lessen these a copper damper was made use ol

344 FRAGMENTS OF SCIENCE

the conductor the more exalted will be the effect. This
requirement was complied with.

Cylinders of antimony were substituted for those of
bismuth. This metal is a better conductor of electricity,
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 antimony than with bis-
muth; but if it springs from a true diamagnetic polarity,
the action of the bismuth ought to exceed that of the anti-
mony. 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. Cop-
per cylinders were next examined: here we have a metal
which conducts electricity fifty times better than bismuth,
but its diamagnetic 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 diamag-
netic polarity to coat fragments of bismuth with some in-
sulating substance, so as to render the formation of induced
currents impossible, and to test the question with cylin-
ders of these fragments. This requirement was also ful-
filled. It is only necessary to reduce the bismuth to pow-
der and expose it for a short time to the air to cause the
particles to become so far oxidized as to render them per-
fectly insulating. The insulating power of the powder was
exhibited experimentally; nevertheless, this powder, en-
closed in glass tubes, exhibited an action scarcely less
powerful than that of the massive bismuth cylinders.

But the most rigid proof, a proof admitted to be con-
clusive by those who have denied the antithesis of mag-

PABAMAONETIC AND DIAMAGNETIC FORCES 345

netism and diamagnetism, remains to be stated. Prisms
of the same heavy glass as that with which the diamag-
netic force was discovered were substituted for the metal-
lic cylinders, and their action upon the magnet was proved
to be precisely the same in kind as that of the cylinders
of bismuth. The inquiry was also extended to other in-
sulators: to phosphorus, sulphur, nitre, calcareous spar,
statuary marble, with the same invariable result: each of
these substances was proved 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 contrary, 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 pur-
pose of proving experimentally some of the statements
made in reference to this subject. A cube of bismuth was
suspended by a twisted string between 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 trian-
gular pieces of looking-glass. A beam of light was suf-
fered to fall upon this reflector, and as the reflector fol-
lowed the motion of the cube the images cast from its
sides followed each other in succession, each describing

346 FRAGMENTS OF SCIENCE

a circle about tliirty feet in diameter. As tlie velocity of
rotation augmented, these images blended into a continu-
ous 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 in-
stantly motionless when the circuit was established.

XIY

PHYSICAL BASIS OF 60LAR CHEMISTRY*

OMITTING all preface, attention was first drawn to
an experimental arrangement intended to prove
that gaseous bodies radiate heat in different de-
grees. 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 raj from the
ball could reach the instrument, was an excellent thermo-
electric pile, connected bj wires with a» very delicate gal-
vanometer. The pile was known to be an instrument
whereby heat is applied to the generation of electric cur-
rents; the strength of the current being an accurate meas-
ure of the quantity of the heat. As long as both faces
of the pile are at the same temperature, no current is pro-
duced; 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 equilibrium being
shown by the needle of the galvanometer standing at zero.

' From a discourse delivered at the Royal Institution of Great Britain, June
1, 1861.

(347)

848 FRAGMENTS OF SCIENCE

The rajs emitted bj the current of hot air already referred
to were permitted to fall upon one of the faces of the pile;
and an extremely slight movement of the needle showed
that the radiation from the hot air, though sensible, was
extremely feeble. Connected 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 at-
mospheric air.

A second holder containing olefiant gas was then con-
nected with the ring-burner. Oxygen and air had already
flowed over the ball and cooled it in some degree. Hence
the olefiant gas labored under a disadvantage. But on
permitting the gas to rise from the ball, it cast 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 ra-
diant 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 ebulli-
tion; 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 neutralizing each otheh The
needle of the galvanometer being at zero, a sheet of oxy-
gen 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 ppssessed any sensible power of intercepting the ther-
mal rays from the cube, one face of the pile being de-

PHYSICAL BASIS OF SOLAR CHEMISTRY 349

prived of the lieat thus intercepted, a difference of tem-
perature between its two faces would instantly set in, and
the result would be declared by the galvanometer. The
quantity absorbed by the oxygen under those circum-
stances 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 quit-
ting the zero line, moved with energy to its sto^DS. Thus
the olefiant gas, so light and clear and pervious to lumi-
nous 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. Groing through the entire list of gases
and vapors in this way, we find radiation and absorption
to be as rigidly associated as positive and negative in elec-
tricity, or as north and south polarity in magnetism. So
that if we make the number which expresses the absorp-
tive power the numerator of a fraction, and that which
expresses its radiative power the denominator, the result

350 FRAGMENTS OF SCIENCE

would be, that on account of the numerator and denom-
inator varying in the same proportion, the value of that
fraction would always remain the same, whatever might
be the gas or vapor experimented with.

But why should this reciprocity exist? What 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 ham-
mer 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 philosophy reduces
light and heat to the same mechanical category. A lumi-
nous 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 undula-
tions strike, the sound being, in technical language, ab-
sorbed; so also with regard to light and heat, absorption
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

PHYSICAL BASIS OF SOLAR CHEMISTRY 351

general rule, is the radiation and absorption. Let us get
definite ideas here, however gross, and purify them after-
ward 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 oar runs freely edgewise
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 ver-
tical, is a good radiator; broad 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 radi-
ate a certain amount of heat. Cause these gases to com-
bine chemically and form nitrous oxide, both the absorp-
tion and radiation are thereby augmented hundreds of
times !

In this way we look with the telescope of the intellect
into atomic systems, and obtain a conception of processes
which the eye of sense can never reach. But gases and
vapors 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 experiment of Sir
David Brewster's, modified to suit present requirements.
Into a glass cylinder, with its ends stopped by disks of
plate-glass, a small quantity of nitrous acid gas is intro-

852 FRAGMENTS OF SCIENCE

duced; the presence of tlie gas being indicated bj its rich
brown color. 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 dark bands, the rays answering to
which are intercepted by the nitric gas, while the light
which falls upon the intervening spaces is permitted to
pass with comparative impunity.

Here also the principle of reciprocity, as regards radi-
ation and absorption, holds good; and could we, without
otherwise altering its physical character, render that ni-
trous 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 elec-
tric spark, the spectra which we obtain from them consist
of a series of bright bands. But such spectra are pro-
duced with the greatest brilliancy when, instead of ordi-
nary gases, we make use of metals heated so highly as to
volatilize them. This is easily done by the voltaic cur-
rent. 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 volatilized condition passes between them. The spec-
trum of this arc is not continuous like that of the solid
carbon points, but consists of a series of vivid bands,
each corresponding in color to that particular portion of
the spectrum to which its rays belong. Copper gives its

PHYSICAL BASIS OF SOLAR CHEMISTRY 853

system of bands; zinc gives its system; and brass, wbicli
is an alloy of copper and zinc, gives a spectrum made up
0,f tbe bands belonging to both metals.

Kot 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 dis-
guise 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 belong-
ing to the. metal reveal themselves with perfect definition.
Into holes drilled in a cylinder of retort carbon, pure cul-
inary 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 stron-
tium, 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 obtained 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 audi-
bly calling out, "I am here!" Nor -is this language af-
fected by distance. If we find that the sun or the stars

* The vividness of the colors of the lithium spectrum is extraordinary; the
spectrum, moreover, contained a blue band of indescribable splendor. 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 times since; and my friend Dr. Miller, having kindly analyzed the
substance made use of, pronounces it pure chloride of lithium. — J. T.

854 FRAGMENTS OF SCIENCE

give us the bands of our terrestrial metals, it is a decla-
ration 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 ter-
restrial metals, and prove either their identity or differ-
ence? 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. WoUaston, but were
multiplied and investigated with profound skill by Fraun-
hofer, and named, after him, Fraunhofer's lines. They
had been long a standing puzzle to philosophers. The
bright lines yielded by metallic vapors had been also
known to us for years; but the connection between both
classes of phenomena was wholly unknown, until Kirch-
hoff, with admirable acuteness, revealed the secret, and
placed it at the same time in our power to chemically
analyze the sun.

We have now some difficult work before us. Hitherto
we have been delighted by objects which addressed them-
selves as much to our aesthetic taste as to our scientific
iaculty; 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 components are refracted in differ-
ent degrees, and thus its colors are drawn apart. Now,
the color depends solely upon the rate of oscillation of the
atoms of the luminous body; red light being produced by
one rate, blue light by a much quicker rate, and the col-

PHYSICAL BASIS OF SOLAR CHEMISTRY 855

ars between red and blue by the intermediate rates. The
solid incandescent coal-points give us a continuous spec-
trum; or, in other words, they emit rays of all possible
periods between the two extremes of the spectrum. Color,
as many of you know, is to light what pitch 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, short-
ening 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 colors in-
sensibly blend together without gap or interruption, from
the red of the lowest pitch to the violet of the highest.
But suppose the player, instead of gradually shortening
his string, to press his finger on a certain point, and to
sound the corresponding note; then to pass on to another
point more or less distant, and sound its note ; then to an
other, 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 composed of separate bright bands
with intervals of darkness between them. But this,
though a perfectly true and intelligible analogy, is not
sufficient for our purpose; we must look with the mind's
eye at the oscillating atoms of the volatilized metal. Fig-
Tire 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 com-
ing to rest, to quiver for a certain time at a certain defi-

856 FRAGMENTS OF SCIENCE

nite rate determined by tlie strengtli of the spring. Now,
the volatilized metal wMcli 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 different 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. Hav-
ing thus far cleared our way, let us make another effort
to advance.

A heavy ivory ball is here suspended from a string.
1 blow against this ball; a single puff of my breath moves
it a little way from its position of rest; it swings back
toward 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 communicated 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 m the same intervals as my puffs; it
is obvious that these waves would communicate their mo-
tion 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

PHYSICAL BASIS OF SOLAR CHEMISTRV 357

then tlie motion imparted to the pendulum by one wave
would be neutralized by another, and there could not be
the accumulation 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 pen-
dulum. But if such a pendulum set oscillating in air could
produce 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 completely, if
they struck against it.

Perhaps the most curious effect of these timed impulses
ever described was that observed by a watchmaker, 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 afterward
he found, 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 curious effects were at the same time ob-
served. 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 brotherhood with
the vibrations of our pendulum. Suppose ethereal waves
striking upon atoms which oscillate in the same periods

858 FRAGMENTS OF SCIENCE

as the waves, tlie motion of tlie 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 undulations which are synchro-
nous with their own periods of vibration. There will be
on the part of those particular rays a transference of mo-
tion from the agitated ether to the atoms of the volatilized
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 colors upon the screen. Between the lamp and the
prism I interpose a snap-dragon light. Alcohol and water
are here mixed with common salt, and the metal dish
that holds them is heated by a spirit-lamp. The vapor
from the mixture ignites and we have a monochromatic
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 everybody present.

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 containing a bit of sodium less than a
pea in magnitude is plunged into the flame. The sodium
soon volatilizes and burns with brilliant incandescence.
The beam crosses the flame, and at the same time the. yel-
low band of the spectrum is clearly and sharply cut out,
a band of intense darkness occupying its place. On with-
drawing 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 color of the

PHYSICAL BASIS OF SOLAR CHEMISTRY 359

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 indefinite.
This vagueness may be entirely removed. By dipping the
carbon-point used for the positive electrode into a solu-
tion of common salt, and replacing it in the lamp, the
bright yellow band produced by the sodium vapor 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 conclusion
that if, instead of the flame of sodium alone, we were to
introduce into the path of the beam a flame in which lith-
ium, strontium, magnesium, calcium, etc., are in a state
of volatilization, each metallic vapor would cut out a sys-
tem of bands, corresponding exactly in position with the
bright bands of the same metallic vapor. 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 with
the most conclusive experiment we reach the solution of
one of the grandest of scientific problems — the constitu-
tion 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 photo-
sphere, as our beam had to pass through the flame, those
rays 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

360 FRAGMENTS OF SCIENCE

the solar nucleus, and we should have a spectrum show-
ing a bright line in the place of every dark line of Fraun-
hofer. These lines are therefore not absolutely dark, but
dark by an amount corresponding to the difference be-
tween the light of the nucleus intercepted by the photo-
sphere, and the light which issues from the latter.

The man to whom we owe this noble generalization is
Kirchhoff, Professor of Natural Philosophy in the Uni-
versity of Heidelberg;' but, like every other great discov-
ery, it is compounded of various elements. Mr. Talbot
observed the bright lines in the spectra of colored flames.
Sixteen years ago Dr. Miller gave drawings and descrip-
tions of the spectra of various colored flames. Wheat-
stone, with his accustomed ingenuity, analyzed 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 es-
say on these bands; Van der Willigen, and more recently
Pliicker, have given us beautiful drawings of the spectra,
obtained from the discharge of Ruhmkorff's coil. But
none of these distinguished men betrayed the least knowl-
edge of the connection between the bright bands of the
metals and the dark lines of the solar spectrum. The
man who came nearest to the philosophy of the subject
was Angostrom. In a paper translated from Poggendorff's
*'Annalen" by myself, and published in the "Philosoph-
ical 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 impression of a reversal

* Now Professor in the University of Berlin.

PHYSICAL BASIS OF SOLAR CHEMISTRY 361

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 vapors, some of the results of which
I placed before you at the commencement of this dis-
course, would have led me in 1859 to the law on which
all Kirchhoff's speculations are founded, had not an acci-
dent withdrawn me from the investigation. But Kirch-
hojff's claims are unaffected by these circumstances. True,
much that 1 have referred to formed the necessary basis
of his discovery; so did the laws of Kepler furnish to
Newton the basis of the theory of gravitation. But what
Kirchhoff has done carries us far beyond all that had
before been accomplished. He has introduced the order
of law amid a vast assemblage of empirical observations,
and has ennobled our previous knowledge by showing its
relationship to some of the most sublime of natural phe-
nomena.

Science— V — 16

XT

ELEMENTARY MAGlfETISafi

A LECTURE TO SCHOOLMASTERS

WE have no reason to believe that the sheep or the
dog, or indeed any of the lower animals, feel
an interest in the laws by which natural phe-
nomena are regulated. A herd may be terrified by a
thunderstorm; 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 inquiring 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 ap-
peal to an inner power of which the senses are the mere
instruments and excitants. No fact is to him either orig
inal or final. He cannot limit himself to the contempla-
tion of it alone, but endeavors 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 effiux 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
(362)

ELEMENTARY MAGNETISM 863

of physical powers become more and more manifest: until
lie is finally led to regard Nature as an organic whole — as
a body each of whose members sympathizes with the rest,
changing, it is true, from age to age, but changing with-
out 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 vari-
ous parts are taken up by different minds, and thus re-
ceive a greater amount of attention than could possibly
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 individ-
uals, each of whom directs his efforts, more or less, along
a single line. Accepting, in many respects, his culture
from his fellow-men —taking it from spoken words or from
written books — ^in some one direction, the student of Nat-
ure ought actually to tottch 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
judgment which direct and habitual contact with natural
truth can alone impart.

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 generally, 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 phe-

364 FRAGMENTS OF SCIENCE

nomena and laws of magnetism and 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 nat-
ural philosophy, though at present so large as to need the
exclusive attention of him who would cultivate it pro-
foundly. Astronomy is the application 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 vegetable world, and by it the animal world. The
touch of the seK-same beams causes hydrogen and chlo-
rine to unite with sudden explosion, and to form by their
combination a powerful acid. Thus physics and chem-
istry intermingle. Physical agents are, however, employed
by the chemist as a means to an end; while in physios
proper the laws and phenomena of the agents themselves,
both qualitative and quantitative, are the primary 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, I se-
lect as a sample the subject of magnetism. I might read-
ily 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 mis-

ELEMENTARY MAGNETISM 865

taken for science itself. I might, of course, ring changes
on the steam-engine and the telegraph, the electrotype and
the photograph, the medical applications 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 exam-
ple, to get a knowledge of magnetism; well, provide your-
self 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. HaK of our book writers describe
experiments which they never made, and their descrip-
tions 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 radiations, 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

366 FRAGMENTS OF SCIENCE

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 somebody like my-
self to magnetize it. Procure some darning needles, and
also a little unspun silk, which will give you a suspend-
ing 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. Sus-
pend 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 apon by your magnet. A rod of the metal nickel,
or of the metal cobalt, from which the blue color 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, how-
ever, 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. Ex-
periment, as I have said, is the language by which we

ELEMENTARY MAGNETISM 367

address Nature, and through which she sends her replies ,-
in the use of this language a lack of straightforwardness
is as possible, and as prejudicial, as in the spoken lan-
guage of the tongue. If, therefore, you wish to become
acquainted with the truth of Nature, jou must from the
first resolve to deal with her sincerely.

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 at-
traction observed in the first instance is now replaced
by a dual force. Eepeat 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 disturb-
ances, 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 agaiB
come to the same position. If you have obtained your
magnet from a philosophical instrument maker, you will
see a mark on one of its ends. Supposing, then, that
you drew your needle along the end thus marked, and
that 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 toward the north. Make

368 FRAGMENTS OF SCIENCE

sure of this, and do not take the statement on my
authority.

Now, take a second darning-needle like the first, and
magnetize it in precisely the same manner: freely sus-
pended 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 magnetized, upon
each other.

Take one of them in your hand, and leave the other
suspended; bring the eye- end of the former near the eye-
end of the latter; the suspended needle retreats: it is re-
pelled. Make the same experiment with the two points;
you obtain the same result, the suspended needle is re-
pelled. Kow 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 confined to what I have uttered
here.

I have eaid that one end of your bar-magnet has a
mark upon it; lay several silk fibres together, so as to get
sufficient strength, or employ a thin silk ribbon, and form
a ioop large enough to hold your magnet. Suspend it; it
turns its marked end toward 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 conven-

ELEMENTARY MAGNETISM 369

lent to determine its north pole yourself, and to mark
it with a file. Vary your experiments by causing your
magnetized darning-needle to attract and repel your large
magnet; it is quite competent to do so. In magnetizing
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; and remember, under-
standing it is not sufficient: you must obtain a manual
aptitude in addressing Nature. If you speak to your
fellow-man you are not entitled to use jargon. Bad ex-
periments are jargon addressed to Nature, and just as
much to be deprecated. Manual dexterity in illustrating
the interaction of magnetic poles is of the utmost impor-
tance 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
which you will endeavor to follow out: questions will arise
which you will try to answer. The same experiment may
be twenty different things to twenty people. Having wit-
nessed the action 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, paste-
board, 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

870 FRAGMENTS OF SCIENCE

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 yourself
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 colors, 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, ia
fact, almost as brittle as glass. Six inches of this, mag-
netized in the manner of the darning-needle, will be bet-
ter able to carry your paper indexes. Having secured
such a strip, you proceed thus:

Magnetize a small sewing-needle and determine its
poles; or, break half an inch, or an inch, off your mag-
netized 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 distribu-
tion 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. Eaise jrour
needle along the strip; its oscillations, 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 thoroughly: you thus learn that the entire lower
half of the strip attracts one end of the needle, while the

ELEMENTARY MAGNETISM 871

entire upper half attracts the opposite end. Supposing
the north end of your little needle to be that attracted
below, you infer that the entire lower half of your mag-
netized 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 sugges-
tiveness 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 experi-
ment; break your strip of steel, and test each half as you
tested the whole. The mere presentation of its two ends
in succession to your test-needle suffices to show that you
have not sl magnet with a single pole — ^that each half pos-
sesses 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 mat-
ter 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 mechanical impossibility; but does the mind stop there?
No: you follow the breaking process in idea when you
can no longer realize 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

872 FRAGMENTS OF SCIENCE

call forth at pleasure or cause to disappear. We magne-
tize our strip of steel by drawing it along the pole of a
magnet; we can demagnetize it, or reverse its magnetism,
by properly drawing it along the same pole in the oppo-
site 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 magnetized? 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 creations from the air, but one informed
and inspired by facts; capable of seizing firmly on a
physical image as a principle, of discerning its conse-
quences, 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 phe-
nomena— ^if, from an assumed cause, the observed acts
necessarily 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 phys-
ical science, our powers of observation, memory, imagina-
tion, and inference, are all drawn upon. We observe
facts and store them up; the constructive imagination
broods upon these memories, tries to discern their inter-
dependence and weave them to an organic 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 assump-
tion which can only partially stand the test of a compar-
ison with facta may be of eminent use in enabling us to

ELEMENTARY MAGNETISM 873

connect and classify groups of phenomena. The theory
of magnetic fluids is of this latter character, and with it
we must now make ourselves 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 complementary colors;
their mixture produces white. Now I ask you to imagine
each of these colors to possess a self -repulsive power; that
red repels red, that green repels green; but that red at-
tracts green and green attracts red, the attraction of the
dissimilar colors being equal to the repulsion of the sim-
ilar ones. Imagine the two colors mixed so as to produce
white, and suppose two strips of wood painted with this
white; what will be their action upon each other? Sus-
pend 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 then it will attract the green, and, the forces being
equal, they neutralize each other. In fact, the least re-
flection shows you that the strips will be as indifferent
to each other as two unmagnetized darning-needles would
be under the same circumstances.

But suppose, instead of mixing the colors, 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 toward each other exactly as
our two magnetized darning-needles — the red end would
repel the red and attract the green, the green would repel
the green and attract the red; so that, assuming two col-
ors thus related to each other, we could by their mixture
produce the neutrality of an unmagnetized body, while.

374 FRAGMENTS OF SCIENCE

by their separation we could produce the duality of action
of magnetized bodies.

But you have already anticipated a defect in my con-
ception; for if we break one of our strips of wood in the
middle we have one half entirely red, and the other en-
tirely green, and with these it would be impossible to
imitate the action of our broken magnet. How, then,
must we modify our conception? We must evidently sup-
pose 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 colors of each
atom could be caused to mix so as to produce white, we
should have, as before, perfect neutrality.

For these two self-repellent and mutually attractive
colors substitute in your minds two invisible self-repel-
lent and mutually attractive fluids, which in ordinary
steel are mixed to form a neutral compound, but which
the act of magnetization separates from each other, plac-
ing the opposite fluids on the opposite face of each mole-
cule. 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 decomposed. Accord-
ing to this theory nothing is actually transferred from the
exciting magnet to the excited steel. The act of mag-
netization consists in the forcible separation of two fluids
which existed in the steel before it was magnetized, but
which then neutralized each other by their coalescence.
And if you test your magnet, after it has excited a hun-
dred pieces of steel, you will find that it has lost no force
—no more, indeed, than I should lose, had my words such

ELEMENTARY MAGNETISM 875

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 fervor. The magnet also is the gainer by the reac-
tion of the body which it magnetizes.

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 permanent? 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 neutralize 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 offer most re-
sistance to being magnetized — which require the greatest
amount of * 'coercion" to tear their fluids asunder — are the
very ones which offer the greatest resistance to the re-
union of the fluids after they have been once separated.
Such kinds of steel are most suited to the formation of
permanent magnets. It is manifest, indeed, that without
coercive force a permanent magnet would not be at all
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 what I have in-
dicated. You are almost sure to have caused a bit of
iron to hang from the end of jour magnet, and you have

376 FRAGMENTS OF SCIENCE

probably succeeded in causing a second bit to attach itseH
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 together with the greatest
tenacity ; and that a bit of iron clings more firmly to the
corner of your magnet than to one of its flat surfaces. In
short, you will in all likelihood have enriched your expe-
rience 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 contact 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 of glass or paper, or a space
of air, may exist between the magnet and the nail; the
latter is still magnetized, 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
toward the magnetic pole has the opposite magnetism of
the pole which excites it; the end most remote from the
pole has the same magnetism as the pole itself, and be-
tween the two poles the nail, like the magnet, possesses
a magnetic equator.

Conversant as you now are with the theory of magnetic
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 pos-
sessing 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 attract-
ing the unlike fluid; thus exciting in the parts of the

ELEMENTARY MAGNETISM 377

iron nearest to itself the opposite polarity. But the iron
ia 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 to-
gether again, and neutrality is restored. Imagination must
be quite nimble in picturing 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 vari-
ous 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 de-
pend. Still, while you use this 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 regarded
as a symbol merely — a symbol, moreover, which is incom-
petent to cover all the facts,* but which does good prac-
tical service while we are waiting for the actual truth.

The state of excitement into which iron is thrown by
the influence of a magnet is sometimes called *'magneti-
igation by influence." More commonly, however, the mag-

The theory breaks down when applied to diamagnetic bodies which are
repelled by magnets. Like soft iron, such bodies are thrown into a state of
temporary excitement, in virtue of which they are repelled ; but any attempt
to explain such a repulsion by the decomposition of a fluid will demonstrate
Its own futility.

878 FRAGMENTS OF SCIENCE

netism is said to be "induced" in the iron, and henco
this mode of magnetizing is called "magnetic 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 magnetic induction. The
quality of steel is in some 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 unmagnetized
darning-needle was attracted in your first experiment;
and from this you may at once deduce the consequence
that, after the steel has been magnetized, 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
the 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 van-
ishes, and the repulsion of like poles is sensibly equal
to the attraction of unlike ones.

I dwell thus long on elementary principles, because
they are of the first importance, and it is the tempta-
tion of this age of unhealthy cramming to neglect them^
Now follow me a little further. In examining the distri-
bution 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 mag^

ELEMENTARY MAGNETISM 879

set really exert no influence on the pole presented w) its
centre ? Let us see.

Let s N, Fig. 11, 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 imaginary
case, as you can never in reality thus detach your north
magnetism from its neighbor. But, supposing us to have
done so, what would be the action of the two poles of
the magnet on w? Your reply will, of course, be that
the pole s attracts n while the pole N repels it. Let the
magnitude and direction of the attraction be expressed by

Fia. 11.

the line n m, and the magnitude and direction of the
repulsion by the line n o. Now, the particle n being
equally distant from 8 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 direction n p, exactly mid-
way between m n and n o. Hence you see that, although
there is no tendency of the particle n to move toward 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 op-
posite to the magnetic equator, it would evidently be
urged along the line n q; and if, instead of two separate

880 FRAGMENTS OF SCIENCE

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 g-,
and its north along n p, the little needle will be com-
pelled to set itself parallel to the magnet s N. Make
the experiment, and satisfy yourselves 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 ex-
actly 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 toward p^ and south pole
toward q, just like the needle.

But supposing you shift the position of your particle
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 of the mag-
net 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 its new position. It is
repelled by K, 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 par-
allelogram m 71 0 Pf and drawing its diagonal n p. Along
n Pf then, a particle of north magnetism would be urged
oy 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 w a short magnetic nee-
dle, its north pole will be urged along n p, its south pole
along n q^ the only position possible to the needle, thus

ELEMENTARY MAGNETISM 381

acted on, being along the line p q^ which is no longer
parallel to the magnet. Verify this deduction by actu«4
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 direction to the
magnetic needle at every particular place. And substi-
tuting, 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

Fio. 12.

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. For the present experiment, 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 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 ar-

882 FRAGMENTS OF SCIENCE

ranges themf They embrace the magnet in a series of
beautiful curves, which are technically called **magnetio
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 direction of tho
needle, or of the wire, to be exactly that of the particle
of iron, or of the magnetic curve, at that point. Go
round and round the magnet; the direction of your nee-
dle always coincides with the direction of the curve on
which \t is placed. These, then, are the lines along
which a particle ot south magnetism, if you could detach
it, would move to the north pole, and a bit of north mag-
netism 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 direc-
tion, the needle must necessarily set itself as a tangent
to the curve.

1 will not seek to simplify this subject further. U,
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
confusion to order, and to supply what is needed to ren-
der 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.

In this thorough manner we must master our materials,

ELEMENTARY MAGNETISM

383

¥tB* 13.— Magnetic Lines of Force (from a Plaotograph by Prof. Mayei|i

884 FRAGMENTS OF SCIENCE

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 generalization. We soon recognize a brother-
hood between the larger phenomena of Kature and the
minute efiects which we have observed in our private
chambers. Why, we inquire, does the magnetic needle
set north and south? Evidently 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 your 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, below 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 toward 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 ex-
actly vertical. Move it back to the centre, it resumes its
horizontality; pass it on toward the south pole, its north
end now dips, and directly over the south pole the needle
becomes vertical, its north end being now turned down-
ward. Thus we learn that on the one side of the mag-
netic equator 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

ELEMENTARY MAGNETISM 885

dips; and over the north or south terrestrial magnetic
pole the needle sets vertical. The south magnetic pole
has not yet been found, but Sir James Ross discovered
the north magnetic pole on June 1, 1831. In this manner
we establish a complete parallelism between the action of
the earth and that of an ordinary magnet.

The terrestrial magnetic poles do not coincide with the
geographical ones; nor does the earth's magnetic equator
quite coincide with the geographical equator. The direc-
tion of the magnetic needle in London, which is called the
magnetic meridian, encloses an angle of 24° with the as-
tronomical meridian, this angle being called the Declina-
tion of the needle for London. The north pole of the
needle now lies to the west of the true meridian; the dec-
lination is westerly. In the year 1660, however, the decli-
nation was nothing, while before that time it was easterly.
All this proves that the earth's magnetic constituents are
gradually changing their distribution. This change is
very slow: it is therefore called the secular change^
and the observation cf it has not yet extended over a
sufficient period to enable us to guess, even approxi-
mately, 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
constraining the two fluids of the poker to separate, mak-
ing the lower end of the poker a north pole, and the
upper end a south pole. Mark the experiment: When
t^^e knob is uppermost, it attracts the north end of a mag-
netic needle; when undermost it attracts the south end of
a magnetic needle. With such a poker repeat this ex-

SOIENCE 17

386 FRAGMENTS OF SCIENCTE '

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
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 molecules and enable the earth
to pull them apart. But 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 permanent magnet. By reversing its position
and tapping it again we reverse its magnetism. A thought-
ful and competent teacher will know how to place these
remarkable facts before his pupils in a manner which will
excite their interest. By the use of sensible images, more
or less gross, he will first give those whom he teaches defi-
nite conceptions, purifying these conceptions afterward, as
the minds of his pupils become more capable of abstrac-
tion. 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,

ELEMENTARY MAGNETISM 887

or the appeal to the bodily senses and the power of
memory alone, could never inspire.

As an expansion of the note at p. 3*1*1, 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 exhibited 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 neutralized each other, on account of their
confused grouping. The act of ma,gnetization 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 oper-
ated 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? Accord-
ing to Coulomb's and Poisson's theory, the act of magnetization consists in the
decomposition of a neutral magnetic fluid ; the north pole of a magnet, for ex-
ample, possesses an attraction for the south fluid of a piece of soft iron sub-
mitted to its influence, draws the said fluid toward it, and with it the material
particles with which the fluid is associated. To account for diamagnetic phe-
nomena 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, ' in-
volves a contradiction. For if the north fluid be supposed to be attracted
toward the influencing north pole, it is absurd to suppose that its presence there
could produce repulsion. 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 direction
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 molec-
ular currents are not the mechanism by which diamagnetic induction is effected.
The consciousness of this, I doubt not, drove M. "Weber to the assumption that
the phenomena of diamagnetism are produced by molecular currents, not directed,
but actually excited in the bismuth by the magnet. Such induced currents

388 FRAGMENTS OF SCIENCE

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 in-
duced molecular currents, once excited, continue to flow without resistance." »
«'Diamagnetism and Magne-crystaUic Action," pp. 136-137.

1 In assuming these non-resisting channels M. Weber, it must be admitted,
did not go beyond the assumptions of Ampere.

XVI

ON FORCE'

ASPHEKE of lead was suspended at a height of 16
feet above the theatre floor of the Royal Institu-
tion. It was liberated, and fell by gravity. That
weight required a second to fall to the floor from that
elevation; 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 weight
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 16 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

* A discourse delivered in the Royal Institution, June 6, 1862.

(889)

S90 FRAGMENTS OF SCIENCE

of work done in all these cases, as far as the raising of
the weight is concerned, would be absolutely 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 matter m, and the height through
which it is lifted hj then the product of m into h, or m A,
expresses, or is proportional to, the amount of work done. .

Supposing, instead of imparting a velocity of 82 feet
a second we impart at starting twice this velocity. To
what height will the weight rise? You might be disposed
to answer, **To twice the height"; but this would be
quite incorrect. Instead of twice 16, or 32 feet, it would
reach a height of four times 16, 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 start-
ing, we attain sixteen times the height. Thus, with a
fourfold velocity of 128 feet a second at starting, the
weight would attain an elevation of 256 feet. With a
sevenfold 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 be-
fore 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 pro-
portional to or represented by m v^. In the case consid-
ered, I have supposed the weight to be cast upward, being

ON FORCE 391

opposed in its flight by the resistance of gravity; but the
same holds true if the projectile be sent into water, mud,
earth, timber, or other resisting material. If, for example,
we double the velocity of a cannon-ball, we quadruple its
mechanical effect. Hence the importance of augmenting
the velocity of a projectile, and hence the philosophy of
Sir William Armstrong in using a large charge of powder
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. Mr. Fairbairn informs
me that in the experiments at Shoeburyness it is a com-
mon thing to see a flash, even in broad daylight, when the
ball strikes the target. And if our lead weight be exam-
ined 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 square of the velocity. Double your
mass, other things being equal, and you double your
amount of heat; double your velocity, other things re-
maining equal, and you quadruple your amount of heat.
Here then we have common mechanical motion destroyed
and heat produced. When a violin bow is drawn across
a string, the sound produced is due to motion imparted
to the air, and to produce that motion muscular force has
been expended. We may here correctly say that the me-
chanical force of the arm is converted into music. In a
similar way we say that the arrested motion of our de-
scending weight, or of the cannon-ball, is converted into
heat. The mode of motion changes, but motion still con-

392 FRAGMENTS OF SCIENCE

tinues; tlie motion of the mass is converted into a motion
of tlie atoms of the mass; and these small motions, com-
municated to the nerves, produce the sensation we call
heat.

We know the amount of heat which a given amount of
mechanical force can develop. Our lead ball, for exam-
ple, 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 velocity would be
small for a rifle bullet; multiplying jths by the square
of 40, we find that the amount of heat developed by col-
lision 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, how-
ever, 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 as-
certain whether rifle bullets do not, under some circum-
stances, show signs of fusion.'

From the motion of sensible masses, by gravity and
other means, we now pass to the motion of atoms toward
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 concentrated light fell upon the
balloon, the gases within it exploded, hydrochloric acid

* Eight years subsequently this surmise was proved correct. In the Franco-
Grerman War signs of fusion were observed in the case of bullets impinging on
bones.

ON FORCE 893

being the result. Here the atoms virtually fell together,
the amount of heat produced showing the enormous force
of the collision. The burning of charcoal in oxygen is
an old experiment, but it has now a significance beyond
what it used to have ; we now regard the act of combina-
tion on the part of the atoms of oxygen and coal as we
regard the clashing of a falling weight against the earth.
The heat produced in both cases is referrible to a common
cause. A diamond, which burns in oxygen as a star of
white light, glows and burns in consequence of the falling
of the atoms of oxygen against it. And could we meas-
ure the velocity of the atoms when they clash, and could-
we find their number and weights, multiplying 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 heat developed 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 ex-
pended 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 enor-
mous store of mechanical power. A pound of coal produces
by its combination with oxygen an amount of heat which,
if mechanically applied, would suffice to raise a weight
of 100 lbs. to a height of 20 miles above the earth's sur-
face. 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 combus
tion of a pound of coal. Wherever work is done by heat,
heat disappears. A gun which fires a ball is less heated

894: FRAGMENTS OF SCIENCE

than one wliicli fires a 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 recondensation 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 in-
formed us, in his interesting discourse, that we dig an-
nually 84 millions of tons of coal from our pits. The
amount of 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 800 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 afiinity seems
almost infinite. But let us give gravity fair play by per-
mitting it to act throughout its entire range. Place a
body at such a distance from the earth that the attraction
of our planet is barely sensible, and let it fall to the earth
from this distance. It would reach the earth with a final
velocity of 86,747 feet a second; and on collision with the
earth the body would generate about twice the amount 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 from an infinite distance has
already used up 1,299,999 parts out of 1,800,000 of the
earth's pulling power, when it ha^ arrived within 16 feet

ON FORCE 396

of the surface; on this space only i,3oi,oooths 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 Herschel
and M. Pouillet have informed us of the annual expendi-
ture of the sun as regards heat; and by an easy calcula-
tion we ascertain the precise amount of the expenditure
which falls to the share of our planet. Out of 2,300 million
parts of light and heat the earth receives one. The whole
heat 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 tempera-
ture of the sun's surface. Besides, were the sun a burn-
ing 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 4,600 years of expenditure.
In this short time it would burn itself out. What agency
then can produce the temperature and maintain the out-
lay? We have already regarded the case of a body fall-
ing from a great distance toward the earth, and foand
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! The maximum velocity
with which a body can strike the earth is about 7 miles
in a second; the maximum velocity 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

896 FRAGMENTS OF SCIENCE

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 fric-
tion against the air they are raised to incandescence and
caused to emit light and heat. At certain 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 Zodi-
acal 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 develop an amount of heat suffi-
cient 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 sur-
face of the sun, would utterly vanish from perception.
Indeed, the quantity of matter competent to produce the
required effect would, during the range of history, cause

ON FORCE 397

no appreciable augmentation in the sun^s magnitude. The
augmentation of tlie sun's attractive force would be more
sensible. However this hypothesis may fare as a repre-
sentant of what is going on in Nature, it certainly shows
how a sun might be formed and maintained 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 tempera-
ture of a globe of lead of the same size as the earth
884,000 degrees of the centigrade thermometer. It has
been prophesied that **the elements shall melt with fer-
vent heat.'* The earth's own motion embraces the con-
ditions of fulfilment; stop that motion, and the greater
part, if not the whole, of our planet would be reduced
to vapor. If the earth fell into the sun, the amount of
heat developed by the shock would be equal to that de-
veloped by the combustion of a mass of solid coal 6,435
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 re-
versed, 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 gen-
erated 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 turn-
ing like a wheel from west to east in its diurnal rotation.
Suppose a high mountain on the earth's surface approach-

398 FRAGMENTS OF SCIENCE

ing the earth's meridian; that mountain is, as it were, laid
hold of b J the moon ; it forms a kind of handle by which
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 di-
minish the velocity of rotation as much as it previously
augmented it; thus the action of all fixed bodies on the
earth's surface is neutralized. 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 position — 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 brake they must diminish the velocity of the earth's
rotation.* Supposing then that we turn a mill by the ac-
tion of the tide, and produce heat by the friction of the
millstones; that heat has an origin totally different from
the heat produced by another mill which is turned by a
mountain stream. The former is produced at the expense
of the earth's rotation, the latter at the expense of the
sun's radiation.

The sun, by the act of vaporization, lifts mechanically
all the moisture of our air, which, when it condenses, 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 inter-
poses, liberates the solidified liquid, and permits it to roll
by gravity to the sea. The mechanical force of every

> Kant surmised an action of this kind.

ON FORCE 899

river in the world as it rolls toward tlie ocean is drawn
from the lieat of tlie 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 performs,
and its connection with which is not so obvious. Trees
and vegetables grow upon the earth, and when burned
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 transparent carbonic acid
gas, formed by the falling together 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 de-
rived; 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 can-
not be effected, and an amount of sunlight is consumed
exactly equivalent to the molecular work done. Thus
trees are formed; thus the cotton on which Mr. Bazley
discoursed last Friday is produced. I ignite this cotton.

400 FRAGMENTS OF SCIENCE

and it flames; the oxygen again unites with the carbon j
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 veg-
etable; the animal consumes the vegetable thus formed,
a reunion of the several 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 di-
rectly to the sun. 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
mechanical energy drawn from the sun. A man weighing
160 pounds has 64 pounds of muscle; but these, when
dried, reduce themselves to 16 pounds. Doing an ordi-
nary day's work, for eighty days, this mass of muscle
would be wholly oxidized. Special organs which do more
work would be more quickly consumed: the heart, for ex-
ample, if entirely unsustained, would be oxidized in about
a week. Take the amount of heat due to the direct oxi-
dation of a given weight of food; less heat is developed
by the oxidation of the same amount of food in the work-
ing animal frame, and the missing quantity is the equiva-
lent of the mechanical work accomplished by the muscles.

I might extend these considerations; the work, indeed,
is done to my hand — but I am warned that you have been
already kept too long. To whom then are we indebted
for the most striking generalizations of this evening's

ON FORCE 401

discourse? They are tlie work of a man ot wliom you
have scarcely ever heard — the published labors of a Ger-
man doctor, named May en Without external stimulus,
and pursuing his profession as town physician in Heil-
bronn, this man was the first to raise the conception of
the interaction of heat and other natural forces to clear-
ness 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 Yalue of Heat**; but in
1842 Mayer had actually calculated the mechanical equiva-
lent of heat from data which only a man of the rarest
penetration could turn to account. In 1845 he published
his memoir on ** Organic Motion,** and applied the me-
chanical 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.
Waterston proposed, independently, the meteoric theory
of the sun*s heat, and in 1854 Professor William Thom-
son applied his admirable mathematical powers to the
development of the theory; but six years previously 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 circumstances 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 arriving at the most
important results in advance of those whose lives were
entirely devoted to Natural Philosophy. It was the acci-
dent of bleeding a feverish patient at Java in 1840 that

402 FRAGMENTS OF SCIENCE

led Mayer to speculate on tliese subjects. He noticed tliat
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 de-
sire to know what has become of this man. His mind,
it is alleged, gave way; it is said he became insane, and
he was certainly sent to a lunatic asylum. In a biograph-
ical dictionary of his country it is stated that he died
there, but this is incorrect. He recovered; and, I believe,
is at this moment a cultivator of vineyards in Heilbronn.

June 20, 1862.
While preparing for publication my last course of lec-
tures on Heat, I wished to make myself acquainted with
all that Dr. Mayer had done in connection with this sub-
ject. I accordingly wrote to two gentlemen who above all
others seemed likely to give me the information which
I needed.* Both of them are Germans, and both par-
ticularly distinguished in connection with the Dynamical
Theory of Heat. Each of them kindly furnished me with
the list of Mayer's publications, and one of them (Clau-
sius) was so friendly as to order them from a bookseller,
and to send them to me. This friend, in his reply to my
first letter regarding Mayer, stated his belief that I should
not find anything very important in Mayer's writings;
but before forwarding the memoirs to me he read them
himself. His letter accompanying them contains the fol-
lowing words; **I must here retract the statement in my

1 Helmlioltz and Clausius.

ON FORCE 403

last letter, tliat you would not find much matter of impor-
tance in Mayer's writings: I am astonished at the multi-
tude of beautiful and correct thoughts which they con-
tain'*; 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 Himmels,"
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 **Macmillan's Maga-
zine" I infer that he is now aware of it. Mayer's physio-
logical writings have been referred to by physiologists —
by Dr. Carpenter, for example — in terms of honoring
recognition. We have hitherto, indeed, obtained frag-
mentary glimpses of the man, partly from physicists and
partly from physiologists; but his total merit has never
yet been recognized as it assuredly would have been had
he chosen a happier 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 disadvantage 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 honorable position 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

404 FRAGMENTS OF SCIENCE

wMcli I may Lave fallen regarding tlieir author. **Bemer-
kungen iiber die Krafte der unbelebten Natur,'' Liebig's
*'Annalen," 1842, vol. 42, p. 231; "Die Organiscbe Be-
wegung in ibrem Zusammenbange mit dem Stoffwecbsel,'*
Heilbronn, 1845; "Beitrage zur Dynamik des Himmels,"
Heilbronn, 1848; **Bemerkungen iiber das Mecbanische
Equivalent der Warme,** Heilbronn, 1851.

In Memoriam. — Dr. Julius Robert Mayer died at Heil-
bronn on Marcb 20, 1878, aged 63 years. It gives me
pleasure to reflect tbat tbe great position wbicb he will
forever occupy in tbe annals of science was first virtually
assigned to bim in tbe foregoing discourse. He was sub-
sequently chosen by acclamation a member of the French
Academy of Sciences; and he received from the Royal
Society the Copley medal — its highest reward.*

November, 1878.
At the meeting of the British Association at Glasgow
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 uncourteous vent dates
from 1863," when it fell to my lot to maintain, in opposi-
tion 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, 1 regret to say, never missed an opportunity,

> See "The Copley Medalist for 1871,'* p. 479.

* See "Philosophical Magazine" for this and the succeeding years.

ON FORCE 405

however small, of carping at Mayer's claims. Tlie action
of the Academy of Sciences and of the Eoyal Society sum-
marily disposes of this detraction, to which its object,
during his lifetime, never vouchsafed either remonstrance
or reply.

Some time ago Professor Tait published a volume of
lectures entitled "Eecent 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 por-
tions of this book. In March last it was subjected to a
brief but pungent critique by Du Bois-Eeymond, the cele-
brated Perpetual Secretary of the Academy of Sciences in
Berlin. Du Bois-Eeymond's address was on "National
Feeling," and his critique is thus wound up: "The au-
thor of the 'Lectures' is not, perhaps, 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, unhappily, is not weak-
ened by his other writings — that the fiery Celtic blood of
his country occasionally runs away with him, converting
him for the time into a scientific Chauvin. Scientific
Chauvinism," adds the learned secretary, "from which
German investigators have hitherto kept free, is more
reprehensible (gehassig) than political Chauvinism, inas-
much as self-control (sittUche Haltung) is more to be
expected from men of science than from the politically
excited mass." *

' Festrede, delivered before the Academy of Sciences of Berlin, in celebra-
tion of the birthday of the Emperor and King, March 28, 1878,

406 FRAGMENTS OF SCIENCE

In tlie case before, this "expectation" would, I fear,
be doomed to disappointment. But Du Bois-Reymond
and his countrymen must not accept the writings of Pro-
fessor 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 the Copley medal to Arago, scorn-
fully brushed aside that spurious patriotism which would
run national boundaries through the free domain of
science, chivalry toward 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.

XYII

CONTEIBUTIONS TO MOLECULAR PHYSIC8 *

HAYING- on previous occasions dwelt upon the
enormous differences which exist among gaseous
bodies both as regards their power of absorbing
and emitting radiant heat, I have now to consider the
effect of a change of aggregation. When a gas is con-
densed to a liquid, or a liquid congealed to a solid, the
molecules coalesce and grapple with each other by forces
which are insensible as long as the gaseous state is main*
tained. But, even in the solid and liquid conditions, the
luminiferous ether still surrounds the molecules: 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 ma-
terially 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 exhibit a deportment
toward radiant heat altogether different from that of the
vapors from which they are derived.

The first part of an inquiry conducted in 1863-64 was
devoted to an exhaustive examination of this question.
Twelve different liquids were employed, and five different

' A discourse delivered at the Royal Institution, March 18, 1864 — supple-
menthig, though of prior date, the Rede Lecture on Radiation.

(407)

408 FRAGMENTS OF SCIENCE

layers of each, var3dng 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 modified the
incident heat, but between plates of transparent rock-salt,
which only slightly affected the radiation. The source of
heat throughout these comparative experiments consisted
of a platinum wire, raised to incandescence 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
vapors of these liquids were subsequently examined, the
quantities of vapor employed being rendered proportional
to the quantities of liquid previously traversed by the ra-
diant heat. The result was that, for heat from the same
source, the order of absorption of liquids and of their
vapors proved absolutely the same. There is no known
exception to this law; so that, to determine the position
of a vapor as an absorber or a radiator, it is only nec-
essary to determine the position of its liquid.

This result proves that the state of aggregation, as far,
at all events, as the liquid stage is concerned, is of alto-
gether subordinate moment — a conclusion which will prob-
ably 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 ab-
sorber and radiator determine that of its vapor, the posi-
tion of water fixes that of aqueous vapor. Water has
been compared with other liquids in a multitude of ex-
periments, and it has been found, both as a radiant and
as an absorbent, to transcend them all. Thus, for exam-
ple, a layer of bisulphide of carbon 0*02 of an inch in
thickness absorbs 6 per cent and allows 94 per cent of

CONTRIBUTIONS TO MOLECULAR PHYSICS 409

the radiation from tlie red-hot platinum spiral to pass
through it; benzol absorbs 43 and transmits 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 excep-
tion is water. A layer of this substance, 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 vapor of water, its
vigorous action upon radiant heat might be inferred from
the deportment of the liquid.

The relation of absorption and radiation to the chemi-
cal 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 the compound
molecule augments. Thus, 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 carbon being 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 ele-
ment. Benzol contains carbon and hydrogen, while alco-
hol 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 toward radiant heat being perfectly anomalous,

Science— V— 18

410 FRAGMENTS OF SCIENCE

if tlie cliemical formula at present ascribed to it be
correct.

Sir William Herscliel made tlie important discovery
that, beyond tbe limits of tbe red end of tbe solar spec-
trum, rays of bigli beating power exist which are incom-
petent to excite vision. The discovery is capable of ex-
tension. Dissolving iodine in the bisulphide of carbon, 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 perfectly dia-
thermic. The transparent bisulphide, which is highly
pervious to invisible heat, exercises on it the same ab-
sorption as the perfectly opaque solution. A hollow prism
filled with the opaque liquid being placed in the path of
the beam from an electric lamp, the light-spectrum is com-
pletely intercepted, but the heat-spectrum may be received
upon a screen and there examined. Falling upon a
thermo-electric pile, its invisible 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 radiant heat? The vis-
ible rays of the spectrum differ from the invisible ones
simply in period. The sensation of light 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
recurrence coincide with the periods of oscillation possible
to the atoms of the dissolved iodine. The elastic forces
which keep these atoms apart compel them to vibrate in
definite periods, and, when these periods synchronize with

CONTRIBUTIONS TO MOLECULAR PHYSICS 411

those of the ethereal waves, the latter are absorbed.
Briefly defined, then, transparency in liquids, as well
as in gases, is synonymous with discord, while opacity
is synonymous with accord, between the periods of the
waves of ether and those of the molecules on which they
impinge.

According to this view transparent and colorless sub-
stances owe their transparency to the dissonance existing
between the oscillating periods of their atoms and those
of the waves of the whole visible spectrum. From the
prevalence of transparency in compound bodies, the gen-
eral discord of the vibrating periods of their atoms with
the light-giving waves of the spectrum may be inferred;
while their synchronism 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
transparent to the luminous rays, which proves that its
atoms do not readily oscillate in the periods which excite
vision. It is highly opaque to the ultra-red undulations,
which proves the synchronism of its vibrating periods
with those of the longer waves.

If, then, to the radiation from any source water shows
itself eminently or perfectly opaque, 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 incan-
descent aqueous vapor, the temperature of which, as cal-
culated by Bunsen, is 3,259° C, so that, if the penetrative
power of radiant heat, as generally supposed, augment
with the temperature of its source, we may expect the
radiation from this flame to be copiously transmitted by
water. While, however, a layer of the bisulphide of car-

412 FRAGMENTS OF SCIENCE

bon 0*07 of an incli in thickness transmits 72 per cent of
tlie incident radiation, and while every other liquid exam-
ined 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 accord be-
tween the periods of the atoms of cold water and those
of aqueous vapor at a temperature of 3,259° C. But the
periods of water have already 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 incandescence by
electricity is insensible, while that by the ordinary undried
air is 6 per cent. Substituting for the platinum spiral a
hydrogen flame, the absorption by dry air still remains
insensible, while that of the undried air rises to 20 per
cent of the entire radiation. The temperature of the hy-
drogen flame is, as stated, 3,259° C. ; that of the aqueous
vapor of the air 20° C. Suppose, then, the temperature of
aqueous vapor to rise from 20° C. to 3,259° C, we must
conclude that the augmentation of temperature is applied
to an increase of amplitude or width of swing, and not to
the introduction of quicker periods into the radiation.

The part played by aqueous vapor in the economy of
nature is far more wonderful than has been hitherto sup-
posed. To nourish the vegetation of the earth the actinic
and luminous rays of the sun must penetrate our atmos-
phere; and to such rays aqueous vapor is eminently trans-
parent. The violet and the ultra-violet rays pass through
it with freedom. To protect vegetation from destructive
chills the terrestrial rays must be checked in their transit
toward stellar space; and this is accomplished by the
aqueous vapor diffused through the air. This substance

CONTRIBUTIONS TO MOLECULAR PHYSIOS 413

is the great moderator of tlie earth's temperature, bring-
ing its extremes into proximity, and obviating contrasts
between day and nigbt which would render life insupport-
able. But we can advance beyond this general statement,
now that we know the radiation from aqueous vapor is
intercepted, in a special degree, by water, and, recipro-
cally, the radiation from water by aqueous vapor; for it
follows from this that the very act of nocturnal refrigera-
tion which produces the condensation of aqueous vapor 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 al-
lowing the violet and ultra-violet rays of the spectrum to
fall upon sulphate of quinine and other substances, Pro-
fessor Stokes has changed the periods of those rays. At-
tempts 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 with-
out 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 breaking up 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; first, by its being in the opposite di-
rection— that is, from a lower refrangibility to a higher;
and, secondly, in the circumstance that the lime is heated
by the collision of the molecules of aqueous vapor, before
their heat has assumed the radiant form. But it cannot

414 FRAGMENTS OF SCIENCE

be doubted that the same effect would be produced by
radiant heat of the same periods, provided the motion
of the ether could be rendered sufficiently intense.* The
effect in principle is the same, whether we consider the
lime to be struck by a particle of aqueous vapor oscillat-
ing 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 colorless glass,
moreover, transmits 68 per cent of the radiation from the
hydrogen flame; but when the flame and spiral are em-
ployed, 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 afterward
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

* This was soon afterward accomplished. See pp. 63-66.

CONTRIBUTIONS TO MOLECULAR PHYSICS 415

50 per cent of the radiation from a hydrogen flamo
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 vapor, 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 opaque. The platinum wire, therefore,
which augmented the radiation through the pure glass,
augmented the absorption of the black glass and mica.

No more striking or instructive illustration of the in-
fluence of coincidence could be adduced than that fur-
nished 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 quan-
tity of the gas, only one-thirtieth of an atmosphere in
density, contained in a polished brass tube four feet long,
intercepts 60 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 com-
pound gases. Moreover, for the radiation from a hydro-
gen flame 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 mercury,
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 temperature of 20° C.
and carbonic acid at a temperature of over 3,000° 0.,
the periods of oscillation of both the incandescent and

416 FRAGMENTS OF SCIENCE

the cold gas belonging to the ultra-red portion of the

spectrum.

It will be seen from the foregoing remarks and experi-
ments how impossible it is to determine the effect of tem-
perature pure and simple on the transmission of radiant
heat if different sources of heat be employed. Through-
out 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 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 vapors referred to at the com-
mencement of this lecture with those of the ultra-red un-
dulations. Hence, by gradually heating a platinum wire
from darkness up to whiteness, we ought gradually to
augment the discord between it and these vapors, and
thus augment the transmission. Experiment entirely con-
firms this conclusion. Formic ether, for example, absorbs
45 per cent of the radiation from a platinum spiral heated
to barely visible redness; 82 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. Ee-
markable cases of inversion as to transparency also occur.
For barely visible redness 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 relation-
ship of the two substances to the luminiferous ether. As
we introduce waves of shorter period the sulphuric ether
augments most rapidly in opacity; that is to say, its accord

CONTRIBUTIONS TO MOLECULAR PHYSICS 417

with the shorter waves is greater than that of the formic.
Hence we may infer that the atoms of formic ether oscil-
late, 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 ether in comparison with sulphuric is very de-
cided. With this source also the positions of chloroform
and iodide of methyl are inverted. For a white-hot spiral,
the absorption of chloroform vapor being 10 per cent,
that of iodide of methyl is 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 in-
version is not the result of temperature merely; for when
a platinum wire, heated to the temperature of boiling
water, is employed 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 synchronizes 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 ex-
cess of that by bisulphide of carbon; while, for the flame
of a Bunsen's burner, from which the incandescent carbon
particles are removed by the free admixture of air, the
absorption by bisulphide of carbon is nearly twice that by
chloroform. The removal of the carbon particles more than
doubles the relative transparency of the chloroform. Test-
ing, moreover, the radiation from various parts of the
same flame, it was found that for the blue base of the

418 FRAGMENTS OF SCIENCE

flame the bisulphide of carbon was most opaque, wbile
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 bisul-
phide was also most opaque, and its opacity very decid-
edly exceeded that of the chloroform when the source of
heat was the flame of bisulphide of carbon. Comparing
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 vapors, all but one absorbed the radiation
from the isinglass most powerfully; the single exception
was chloroform.

It is worthy of remark that whenever, through a change
of source, the position of a vapor as an absorber of radiant
heat was altered, the position of the liquid from which the
vapor was derived underwent a similar change.

It is still a point of difference between eminent investi-
gators whether radiant heat, up to a temperature of 100**
C, is monochromatic or not. Some affirm this; some
deny it. A long series of experiments enables me to state
that probably no two substances at a temperature 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 employed. MM. de la Provostaye and
Deeains maintain the former view, Melloni and M. Knob-
lauch maintain the latter I tested this point without

CONTRIBUTIONS TO MOLECULAR PHYSICS 419

changing anything but the temperature of the source; its
size, distance, and surroundings remaining the same. The
experiments proved rock-salt to be colored 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 reasoning, reach
the conclusion that the 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 radia-
tors; they are also extremely bad conductors. The mo-
ment we pass from the metals 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
furnish a striking illustration of what 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.

XVIII

LIFE AND LETTERS OF FARADAY
1870

UNDERTAKEN" and executed in a reverent and lov-
ing spirit, tlie work of Dr. Bence Jones makes
Faraday the virtual writer of his own life. Every-
body now knows the story of the philosopher's birth;
that his father was a smith; that he was bom at New-
ington Butts in 1791 ; that he ran along the London pave-
ments, 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 book-
binder— a kindly man, who became attached to the little
fellow, and in due time made him his apprentice without
fee; that during his apprenticeship 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 correspondent'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 thoughts
(420)

LIFE AND LETTERS OF FARADAY 421

and memoranda. He made his notes in the laboratory, in
the theatre, and in the streets. This distrust of his mem-
ory reveals itself in his first letter to Abbott. To a propo-
sition that no new inquiry should be started between them
before the old one had been exhaustively discussed, Fara-
day objects. "Your notion," he says, "I can hardly al-
low, 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 experience cause not pain, but, I hope, pleas-
ure." Faraday notes his own impetuosity, and inces-
santly 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 Eoche
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 grimness, too, per-
vaded 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, whatever repugnance
it might give rise to. Being in this state of mind, I
should have refrained from writing to you, did I not

422 FRAGMENTS OF SCIENCE

conceive from the general tenor of yonr letters that your
mind is, at proper times, occupied upon 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 har-
bors it, but is often a cause of disturbed digestion to
his friends.

About three months after his engagement with De la
Roche, 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 recognized
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 an-
other 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 ex-
hausted, it stood for a moment, and then exploded with

LIFE AND LETTERS OF FARADAY 423

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 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 in-
tellectual power, made Faraday a great experimental phi-
losopher. 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 su-
periority of the organ of vision. Late in life I have heard
him say that he could never fully understand an experi-
ment 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 behavior
should evince respect for his audience." These recom-
mendations were afterward, 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 bewildered followers, he would dash alone
through the jungle into which he had unwittingly led
them; thus saving them from ennui by the exhibition of
a vigor which, for the time being, they could neither share
nor comprehend.

424 FRAGMENTS OF SCIENCE

In October, 1813, lie quitted England with Sir Hnmpliry
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, pre-
ferring at times to be his own servant rather than impose
on Faraday duties which he disliked. But Lady 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 Q-eneva. De la Rive,
the elder, had known Davy in 1799, and, by his writings
in the "Biblioth^que Britannique, " had been the first to
make the English chemist's labors known abroad. He
welcomed Davy to his country residence in 1814. Both
were sportsmen, and they often went out shooting to-
gether.

On these occasions Faraday charged Davy's gun while
De la Rive charged his own. Once the Oenevese phi-
losopher found himself by the side of Faraday, and, in
his frank and genial way, entered into conversation 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 inquiry De la Rive was some-
what shocked to find that the soi-disant domestique was
really preparateur in the laboratory of the Royal Institu-
tion; and he immediately proposed that Faraday thence-
forth should join the masters instead of the servants at
their meals. To this Davy, probably out of weak defer-
ence to his wife, objected; but an arrangement was come
to that Faraday thenceforward should have his food in
his own room. Rumor states that a dinner in honor of
Faraday was given by De la Rive. This is a delusion;
there was no such banquet; but Faraday never forgot the

LIFE AND LETTERS OF FARADAY 425

kindness of the friend who saw his merit when he was a
mere gargon de labor atoire.^

He returned, in 1815, to the Eojal 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 opinioa
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 ia
a state of transition." And again: *' Nothing is more dif-
ficult and requires more caution than philosophical deduc-
tion, 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 contrast with this intel-
lectual expansiveness was his fixity in religion, but this
is a subject which cannot be discussed here.

Of all the letters published in these volumes none pos-
sess 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 earnestness." Abbott

' 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 then 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 rumor
of a banquet at Geneva illustrates thejtendency to subititute for the youth of
1814 the Faradaly of later years.

426 FRAGMENTS OF SCIENCE

and he sometimes swerved into word-play about love;
but up to 1820, or thereabout, the passion was potential
merely. Faraday's journal, indeed, contains entries which
show that he took pleasure in the assertion of his con-
tempt for love; but these very entries became links in his
destiny. It was through 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 varying moods which preceded his
acceptance. They reveal more than the common alterna-
tions of light and gloom; at one moment he wishes that
his flesh might melt and that he might become nothing;
at another he is intoxicated 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,
suggesting, as it did, to the conscientious mind of Miss
Barnard, doubts of her capability to return it with ade-
quate 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 delightful
to the last moment of my stay with my companion, be-
cause 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 sub-
sequent letter, '*to convey most perfectly my affection and
love for you? Can I or can truth say more than that for
this world I am yours?" Assuredly he made his pro-
fession good, and no fairer light falls upon his character
than that which reveals his relations to his wife. Never,
1 believe, existed a manlier, purer, steadier love. Like a

LIFE AND LETTERS OF FARADAY 427

burning diamond, it continued to sTied, 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 after-
ward, 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 appearing
in these volumes, which justify his opinion that Davy in
those days had become jealous of Faraday. This, which
is the prevalent belief, is also reproduced in an excellent
article in the March number of "Fraser's 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 celebrated
discovery which connects electricity with magnetism, and
immediately afterward the acute mind of Wollaston per-
ceived 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 realize this result
in the laboratory of the Eoyal Institution. Faraday was
not present at the moment, but he came in immediately
afterward and heard the conversation of Wollaston and
Davy about the experiment. He had also heard a rumor
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 re-
peated 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

428 FRAGMENTS OF SCIENCE

round a magnetic pole. This was not the result sought
by WoUaston, 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 his
greatest discoveries; and we, who know this, should be
justified in concluding that, even had WoUaston not pre-
ceded 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 impos-
sible 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. Eumor and fact had connected the
name of Wollaston with these supposed interactions be-
tween magnets and currents. When, therefore, Faraday
in October published his successful experiment, without
any allusion to Wollaston, general, though really un-
grounded, criticism followed. I say ungrounded because,
first, Faraday's experiment was not that of Wollaston,
and secondly, Faraday, before he published it, had ac-
tually called upon Wollaston, and not finding him at
home, did not feel himself authorized to mention his
name.

In December, Faraday published a second paper on
the same subject, from which, through a misapprehension,
the name of Wollaston was also omitted. Warburton and
others thereupon affirmed that Wollaston's ideas had been
appropriated without acknowledgment, 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 his friend Stodart, "every day

LIFE AND LETTERS OF FARADAY 429

iQore and more of these sounds, whicb, thongh only whis-
pers to me, are, I suspect, spoken aloud among scientifio
men." He might have written explanations and defences,
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 candor and truth of character which these
vivd voce defences revealed, as much as the defences them-
selves, that disarmed resentment at the time.

As regards Davy, another cause of dissension arose in
1823. In the spring of that year Faraday analyzed the
hydrate of chlorine, a substance once believed to be the
element chlorine, but proved by Davy to be a compound
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 discov-
ery 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 inquiry,
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 al-
ways 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

430 FRAGMENTS OF SCIENCE

now hard to avoid magnifying this error. But liad Fara-
day died or ceased to work at this time, or had his sub-
sequent 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 without 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 language to him was this: **It
gives me great pleasure to hear that you are comfortable
at the Royal Institution, and I trust that you will not
only do something good and honorable 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 opposition to the election of Faraday to
the Royal Society is, I am persuaded, to be ascribed.

These matters are touched upon with perfect candor
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 sur-
rounded by injurious rumors which I would willingly
scatter forever. The pupil's magnitude, and the splen-
dor 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 inner consciousness that he had fairly won renown.
They were both proud men. But with Davy pride pro-
jected itself into the outer world; while with Faraday it
became a steadying and dignifying inward force. In one

LIFE AND LETTERS OF FARADAY 431

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 discovery, 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 will 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 knowledge and
in power. Consciously or unconsciously, the relation of
Action to Eeaction was ever present to Faraday's mind.
It had been fostered by his discovery of Magnetic Eota-
tions, 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 for-
tified by previous trials, which, though failures, had be-
gotten instincts directing him toward the truth. He, like
every strong worker, might at times miss the outward ob-
ject, but he always gained the inner light, education, and
expansion. Of this Faraday's life was a constant illustra-
tion. By November he had discovered and colligated a
multitude of the most wonderful and unexpected phe-
nomena. He had generated currents by currents; cur-
rents by magnets, permanent and transitory ; and he after-
ward generated currents by the earth itself. Arago's
"Magnetism of Eotation," which had for years offered

432 FRAGMENTS OF SCIENCE

itself as a challenge to the best scientific intellects of
Europe, now fell into his hands. It proved to be a beau-
tiful, but still special, illustration of the great principle
of Magneto- electric Induction. Nothing equal to this
latter, in the way of pure experimental inquiry, had pre-
viously been achieved.

Electricities from various sources were next examined,
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. Eenounc-
ing professional work, from which at this 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 emerged from his researches with the
great principle of Definite Electro-chemical Decomposition
in his hands. If his discovery of Magneto- electricity may
be ranked with that of the pile by Volta, this new discov-
ery may almost stand beside that of Definite Combining
Proportions in Chemistry. He passed on to Static Elec-
tricity— its Conduction, Induction, and Mode of Propaga-
tion. He discovered and illustrated the principle of In-
ductive 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 comer. Faraday held — and his
views are gaining ground — that his experiments proved
the fact of curvilinear propagation, and hence the opera-

LIFE AND LETTERS OF FARADAY 433

tion of a medium. Others dened this; but none can
deny the profound and philosophic character of his lead-
ing thought. * The first volume of the Besearches contains
all the papers here referred to.

Faraday had heard it stated that henceforth physical
discoveries would be made solely by the aid of mathemat-
ics; that we had our data, and needed only to work de-
ductively. Statements of a similar character crop out
from time to time in our day. They arise from an im-
perfect acquaintance with the nature, present condition,
and prospective vastness of the field of physical inquiry.
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 expres-
sion. But our approach to this result is asymptotic; and
for ages to come — possibly for all the ages of the human
race — Nature will find room for both the philosophical ex-
perimenter and the mathematician. Faraday entered his
protest against the foregoing statement by labelling his
investigations "Experimental Besearches in Electricity."
They were completed in 1854, and three volumes of them
have been published. For the sake of reference, he num-
bered every paragraph, the last number being 8,862. In
1859 he collected and published [a. fourth volume of pa-
pers, under the title, ** Experimental Besearches in Chem-
istry and Physics." Thus did this apostle of experiment
illustrate its power and magnify his office.

The second volume of the Besearches embraces mem-

* In a very remarkable paper published in Poggendorff*s **Annalen" for
1857, Werner Siemens accepts and develops Faraday's theory of Molecular
Induction.

Science— V — 19

434 FRAGMENTS OF SCIENCE

oirs on the Electricity of the Gjmnotus; on the Source
of Power in the Voltaic Pile; on the Electricity evolved
by the Friction of Water and Steam, in which the phe-
nomena and principles of Sir William Armstrong's Hydro-
electric machine are described and developed; a paper on
Magnetic Eotations, and Faraday's letters in relation to
the controversy it aroused. The contribution of most per-
manent value here is that on the Source of Power in the
Voltaic Pile. By it the Contact Theory, pure and simple,
was totally overthrown, 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 Magnetization 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 polarization,
which It announced, seems pregnant with great results.
The writings of William Thomson on the theoretic aspects
of the discovery; the excellent electro-dynamic measure-
ments of Wilhelm Weber, which are models of experi-
mental completeness and skill; Weber's labors in con-
junction with his lamented friend Kohlrausch — 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 phenom-
ena. The hope of such a connection was first raised by
the discovery here referred to.* Faraday himself seemed

* A letter addressed to me by Professor Weber on March 18 last, contaiiui
the following reterence to the connection here mentioned: "Die Hoffnung emtf
solchen Combination ist durch Faraday's Entdeckung der Drehung der Polar-
Isationsebene durch magnetische Directionskraft zuerst, und sodann durch die

LIFE AND LETTERS OF FARADAY 435

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.

Rapidly following it is the discovery of Diamagnetism,
or the repulsion of matter by a magnet. Brugmans had
shown that bismuth repelled a magnetic needle. Here he
stopped. Le Bailliff 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 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 ardor and patience — the one prompting the at-
tack, 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 incommunicable; it depends on

tJebereinstimmung derjenigen Geschwindigkeit, welche das Yerhaltniss der
electro-dynaruischen Einheit zur electro-statischen ausdruckt, mit der Geschwin-
digkeit des Lichts angeregt worden; und mir scheint von alien Yersuchen^
welche zur Yerwirklichung dieser Hoffnung gemacht worden sind, das von
Herrn Maxwell demachte am erfolgreichsten.*'

436 FRAGMENTS OP SCIENCE

the individual rather than on the systero, and the mark
is missed when Faraday's researches are pointed to as
merely illustrative of the power of the inductive philoso-
phy. 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 philo-
sophic method; the energy of a strong soul expressing
itself after its own fashion, and acknowledging no medi-
ator 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 relations. It con-
tains letters from Humboldt, Herschel, Hachette, De la
Rive, Dumas, Liebig, Melloni, Becquerel, Oersted, Pliicker,
Du Bois-Eeymond, Lord Melbourne, Prince Louis Napo-
leon, and many other distinguished men. I notice, with
particular pleasure, a letter from Sir John Herschel, ux
reply to a sealed packet addressed to him by Faraday,
but which he had permission to open if he pleased. The
packet referred to one of the many unfulfilled hopes which
spring up in the minds of fertile investigators:

**G-o on and prosper, *from strength to strength,* like
a victor marching with assured step to further conquests;
and be 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 your-
self, than that of J. F. W. Herschel."

Faraday's behavior to Melloni, in 1835, merits a word
of notice. The young man was a political exile in Paris.
He had newly fashioned and applied the thermo-electric

LIFE AND LETTERS OF FARADAY 437

pile, and had obtained with it results of the greatest im-
portance. But they were not appreciated. With the sick-
ness of disappointed hope, Melloni waited for the report
of the Commissioners, appointed by the Academy of Sci-
ences 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 Eoyal Society. A sum of money
always accompanies this medal; and the pecuniary help
was, at this time, even more essential than the mark
of honor to the young refugee. Melloni 's gratitude was
boundless:

"Et vous, monsieur," he writes to Faraday, '*qui ap-
partenez a une socidte ^ laquelle je n'avais rien offert,
vous qui me connaissiez k peine de nom; vous n*avez pas
demande si j' avals des ennemis faibles ou puissants, ni
calculd quel en etait le nombre; mais vous avez parle
pour I'opprime Stranger, pour celui qui n*avait pas le
moindre droit a tant de bienveillance, et vos paroles ont
^te accueillies favorablement par des collogues conscien-
cieux! Je reconnais bien la des hommes dignes de leur
noble mission, les veritable representants de la science
d'un pays libre et g^nereux. "

Within the prescribed limits of this article it would be
impossible to give even the slenderest summary of Fara-
day'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, regarding his pen-
sion, illustrate his uncompromising independence. The
Prime Minister had offended him, but assuredly the apol-
ogy demanded and given was complete. I think it cer-

438 FRAGMENTS OF SCIENCE

tain that, notwithstanding the very full account of this
transaction given by Dr. Bence Jones, motives and influ-
ences were at work which even now are not entirely re-
vealed. The minister was bitterly 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 manifestly thought that Faraday ought
to have come forward in Lord Melbourne's defence, and
there is a flavor of resentment in one of her letters to
him on the subject. 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 vigor 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 else-
where quoted: *' Clout-nail making goes on here rather
considerably, and is a very neat and pretty operation to
observe. I love a smith's shop and anything relating to
smithery. My father was a smith." This is from his
journal; but he is unconsciously speaking 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

LIFE AND LETTERS OF FARADAY 439

sympathetic. But amid it all, and in reference to it all,
he tells his sister that "true enjoyment is from within,
not from without." In those days Agassiz was living
under a slab of gneiss on the glacier of the Aar. Fara-
day met Forbes at the Grimsel, and arranged with him
an excursion to the "Hotel des Neuch^telois" ; but indis-
position put the project out.

From the Fort of Ham, in 1843, Faraday received a
letter addressed to him by Prince Louis Napoleon Bona-
parte. He read this letter to me many years ago, and the
desire, shown in various ways by the French Emparor, 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?" Should the necessity arise, the French Em
peror 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.*

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 distinctly than

* 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 happy to think, have improved the time since the above words were
written [1878J.

440 FRAGMENTS OF SCIENCS

anywhere else. From the house of Dr. Percy, in Birming-
ham, lie 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 communion 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 sunset; for to me
who love scenery, of all that I have seen or can see, there
is none surpasses that of heaven. A glorious sunset 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 belonging
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 acted indirectly as the safeguard of his faith. For

LIFE AND LETTERS OF FARADAY 441

his investigations so filled his mind as to leave no room
for sceptical questionings, thus shielding from the assaults
of philosophy the creed of his youth. His religion was
constitutional and hereditary. It was implied in the ed-
dies 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 constitu-
ents— ^awe, reverence, truth, and love.

It is worth inquiring how so profoundly religious a
mind, and so great a teacher, would be likely to regard
our present discussions on the subject of education. Far-
aday would be a ** secularist'* were he now aHve. He had
no sympathy with those who contemn knowledge 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 re-
marks; 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. Kext, I have no inten-
tion 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, J^ will ob-

442 FRAGMENTS OF SCIENCE

serve tliat we have no right to judge religious 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 per-
haps be called — will accord with what I have before de-
scribed 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 1 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 religious 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 de-
scribed 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 habit of keep-
ing himself free from the distractions of society. He was
bright and joyful — boylike, 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 sweet-
ness, 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.

LIFE AND LETTERS OF FARADAY 443

I will not call tlie labors of the biographer final. So great
a character will challenge 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 thor-
oughly." 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 m 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 by Dr. Bence Jones's
labor of lovs.

XIX

THE COPLEY MEDALIST OF 1870

THIETY years ago Electro -magnetism was looked to
as a motive power wMcli miglit possibly compete
witli steam. In centres of industry, sucL. as Man-
chester, attempts to investigate and apply tliis 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 Sturgeon's "Annals of Electricity'*
between 1839 and 1841, described various attempts at the
construction and perfection of electro-magnetic engines.
The spirit in which Mr. Joule pursued these inquiries is
revealed in the following extract: "I am particularly anx-
ious,'* he says, *'to communicate any new arrangement in
order, if possible, to forestall the monopolizing designs
of those who seem to regard this most interesting subject
merely in the light of pecuniary speculation." He was
naturally led to investigate the laws of electro- magnetic
attractions, and in 1840 he announced the important prin-
ciple that the attractive force exerted by two electro-mag-
nets, or by an electro- magnet and a mass of annealed iron,
is directly proportional to the square of the strength of the
magnetizing current; while the attraction exerted between
an electro-magnet and the pole of a permanent steel mag-
net varies simply as the strength of the current. These
(444)

THE COPLEY MEDALIST OF 1870 445

investigations were conducted independently of, though
a little subsequently to, the celebrated inquiries of Henry,
Jacobi, and Lenz and Jacobi, on the same subject.

On December 17, 1840, Mr. Joule communicated to the
Royal Society a paper on the production of heat by Vol-
taic electricity. In it he announced the law that the calo-
rific effects of equal quantities of transmitted electricity are
proportional to the resistance overcome by the current,
whatever may be the length, thickness, shape, or char-
acter of the metal which closes the circuit; and also pro-
portional 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 Royal
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 electrolytes, and found, in all cases, that the heat
evolved by the proper action of any Voltaic current is pro-
portional to the square of the intensity of that current,
multiplied by the resistance to conduction which it expe-
riences. From this law he deduced a number of conclu-
sions of the highest importance to electro-chemistry.

It was during these inquiries, which are marked
throughout by rare sagacity and originality, that the great
idea of establishing quantitative relations between Me-
chanical 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 Calo-
rific 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 written it must have
appeared hopelessly entangled. This, I should think, was

446 FRAGMENTS OF SCIENCE

the reason why Faraday advised Mr. Joule not to submit
the paper to the Koyal 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
demonstrated, experimentally, that the mechanical power
exerted in turning a magneto-electric machine is converted
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 ob-
tained 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 deter-
mination, it is not surprising that the "mechanical 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 temperature.

One noteworthy result of his inquiries, which was
pointed out at the time by Mr. Joule, had reference to
the exceedingly small fraction of the heat actually con-
verted into useful effect in the steam-engine. The thoughts
of the celebrated Julius Robert Mayer, who was then en-
gaged in Germany upon the same question, had moved
independently in the same groove; but to his labors due
reference will be made on a future occasion.' In the
memoir now referred to, Mr. Joule also announced that
lie had proved heat to be evolved during the passage of
water through narrow tubes; and he deduced from these

* "Phil. Mag.," May, 1845. ' See the next Fragment.

i

THE COPLEY MEDALIST OF 1870 447

experiments an equivalent of 770 foot-pounds, a figure
remarkably near the one now accepted. A detached state-
ment regarding the origin and convertibility of animal heat
strikingly illustrates the penetration of Mr. Joule, and his
mastery of principles, at the period now referred to. A
friend had mentioned to him Haller's hypothesis, 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 must still be referred to the
chemical changes. But if the animal were engaged in
turning a piece of machinery, or in 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 expe-
rienced." 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 regard-
ing the nature of chemical and latent heat. "I had," he
writes, "endeavored to prove that when two atoms com-
bine 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 attraction of affinity, but rather the
mechanical force expended by the atoms in falling toward
one another, which determines the intensity of the cur-

448 FRAGMENTS OF SCIENCE

rent, 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
60,000,000 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 combination 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 commanding grasp of molecular physics, in
their relation to the mechanical theory of heat, implied
by this statement.

Perfectly assured of the importance of the principle
which his experiments aimed at establishing, Mr. Joule
did not rest content with results presenting such discrep-
ancies as those above referred to. He resorted in 1844 to
entirely new methods, and made elaborate experiments
on the thermal changes produced in air during its expan-
sion: first, against a pressure, and therefore performing
work; secondly, against no pressure, and therefore per-
forming 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 differ-
ent mechanical equivalents; the agreement between them
being far greater than that attained in his first experi-

THE COPLEY MEDALIST OF 1870 449

ments. 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 labors of the previous ten years had
made Mr. Joule completely master of the conditions essen-
tial to accuracy and success. Bringing his ripened experi-
ence to bear upon the subject, he executed, in 1849, a
series of 40 experiments on the friction of water, 50 ex-
periments on the friction of mercury, and 20 experimentg
on the friction of plates of cast iron. He deduced from
these experiments our present mechanical equivalent of
heat, justly recognized all over the world as "Joule's
equivalent. ' '

There are labors so great and so pregnant in conse-
quences that they are most highly praised when they are
most simply stated. Such are the labors of Mr. Joule.
They constitute the experimental foundation of a princi-
ple of incalculable moment, not only to the practice, but
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.

I have omitted all reference to the numerous minor
papers with which Mr. Joule has enriched scientific liter-
ature. Nor have I alluded to the important investigations

450 FRAGMENTS OF SCIENCE

which he has conducted jointly with Sir William Thom-
son. But sufficient, I think, has been here said to show
that, in conferring upon Mr. Joule the highest honor 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 honored himself and England by bestowing
?V32 annual pension of 200Z. on Dr. Joule.

XX

THE COPLEY MEDALIST OF 1871

DE. JULIUS EGBERT MAYEE 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 pa-
tients had a singularly bright red color. The observation
riveted his attention; he reasoned upon it, and came to
the conclusion that the brightness of the color 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 as-
cribed 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 combustion, the
same amount of heat; that this law holds good even for
vital processes; and that hence the living body, notwith-
standing all its enigmas and wonders, is incompetent to
generate heat out of nothing."

But beyond the power of generating internal heat, the

(451)

452 FRAGMENTS OF SCIENCE

animal organism can also generate lieat outside of itself.
A blacksmitli, for example, by hammering can heat a nail,
and a savage bj friction can warm wood to its point of
ignition. Now, unless we give up the physiological 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 oxidized
in the body."

From this, again, he inferred that the heat generated
externally must stand in a fixed relation to the work ex-
pended 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, quan-
tity 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 ex-
press 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 beginning 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

THE COPLEY MEDALIST OF 1871 453

purely scientific inquiry. He thouglit it wise, therefore,
to secure himself against accident, and in the spring of
1842 wrote to Liebig, asking him to publish in his **Aii-
nalen" a brief preliminary notice of the work then accom-
plished. 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 principles on
which his physiological deductions were to rest. He be-
gins, 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 con-
vertible, but not destructible. In the terminology of his
time, he then gives clear expression to the ideas of poten-
tial and dynamic energy, illustrating his point by a weight
resting upon the earth, suspended at a height above the
earth, and actually 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 re-
fers 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 de.
stroyed. **Our locomotives," he observes with extraor-
dinary sagacity, '*may be compared to distilling apparatus;
the heat beneath the boiler passes into the motion of the

454 FRAGMENTS OF SCIENCE

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 toward the end of
his first paper he makes the attempt. It was known that
a definite amount of air, in rising one degree in tempera-
ture, can take up two different amounts of heat. If its
volume be kept constant, it takes up one amount: if its
pressure be kept constant it takes up a different amount.
These two amounts are called the specific heat under con-
stant volume and under constant pressure. The ratio of
the first to the second is as 1 : 1421. No man, to my
knowledge, prior to Dr. Mayer, penetrated the significance
of these two numbers. He first saw that the excess 0421
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 determined the mechanical equivalent
of heat. Even in this first paper he is able to direct
attention to the enormous discrepancy between the theo-
retic power of the fuel consumed in steam-engines and
their useful effect.

Though this paper contains but the germ of his furthei
labors, 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 him-
self against what he calls " E ventualitaten, * ' he devoted
every hour of his spare time to his studies, and, in 1845,

THE COPLEY MEDALIST OF 1871 455

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,
*'Kemarks 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 illus-
trates the physical principles laid down in his first brief
paper. He goes fully through the calculation of the me-
chanical equivalent of heat. He calculates the perform-
ances of steam-engines, and finds that 100 lbs. of coal, in
a good working engine, produce only the same amount of
heat as 96 lbs. in an unworking one; the 5 missing lbs.
having been converted into work. He determines the use-
ful effect of gunpowder, and finds nine per cent of the
force of the consumed charcoal invested on the moving
ball. He records observations on the heat generated in
water agitated by the pulping- engine of a paper manufac-
tory, and calculates the equivalent of that heat in horse-
power. He compares chemical combination with mechan-
ical 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 gener-
ated by its collision would raise an equal weight of water
17,856° C. in temperature. He then determines the ther-
mal 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 developed with such extraordinary ability three
years afterward. He also points to the almost exclusive
efficacy of the sun's heat in producing mechanical motions
upon the earth, winding up with the profound remark,

456 FRAGMENTS OF SCIENCE

tliat tlie lieat developed hj friction in tlie wheels of our
wind and water mills comes from 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 princi-
ples to bear upon the phenomena of vegetable and animal
life. Wood and coal can burn; whence come their heat,
and the work producible by that heat? From the im-
measurable reservoir of the sun. Kature has proposed to
herself the task of storing up the light which streams
earthward from the sun, and of casting into a permanent
form the most fugitive of all powers. To this end she
has overspread the earth with organisms which, while liv-
ing, 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 where-
by the wave- motion of the sun is changed into the rigid
form of chemical tension, and thus prepared for future use.
With this prevision, as shall subsequently be shown, the
existence of the human race itself is inseparably con-
nected. 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

THE COPLEY MEDALIST OF 1871 457

bj tlie combustion of tbe tree. The beat and work poten-
tial in our coal strata are so mucb strengtb witbdrawn
from tbe sun of former ages. Mayer lays tbe axe to tbe
root of tbe notions regarding ** vital force,'* wbicb were
prevalent wben be wrote. Witb tbe plain fact before us
tbat in tbe absence of tbe solar rays plants cannot per-
form tbe work of reduction, or generate cbemical tensions,
it is, be contends, incredible tbat tbese tensions sbould be
caused by tbe mystic play of tbe vital force. Sucb a by-
potbesis would cut off all investigation ; it would land us in
a cbaos of unbridled fantasy. "I count,'* be says, ''tbere-
fore, upon your agreement witb me wben I state, as an
axiomatic trutb, tbat during vital processes tbe conversion
only, and, never tbe creation of matter or force occurs."
Having cleared bis way tbrougb tbe vegetable world,
as be bad previously done tbrougb inorganic nature,
Mayer passes on to tbe otber organic kingdom. Tbe
pbysical forces collected by plants become tbe property
of animals. Animals consume vegetables, and cause tbem
to reunite witb tbe atmospberic oxygen. Animal beat is
tbus produced; and not only animal beat, but animal mo-
tion. Tbere is no indistinctness about Mayer bere; be
grasps bis subject in all its details, and reduces to figures
tbe concomitants of muscular action. A bowler wbo im-
parts to an 8-lb. ball a velocity of 30 feet, consumes in
tbe act JO of a grain of carbon. A man weigbing 150
lbs., wbo lifts bis own body to a beigbt of 8 feet, con-
sumes in tbe act 1 grain of carbon. In climbing a moun-
tain 10,000 feet bigb, tbe consumption of tbe same man
would be 2 oz. 4 drs. 50 grs. of carbon. Boussingault
bad determined experimentally tbe addition to be made

to tbe food of borses wben actively working, and Liebig

Science— Y— 20

458 FRAGMENTS OF SCIENCE

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 in-
creased 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 cal-
culations. The muscles of a laborer 150 lbs. in weight
weigh 64 lbs.; but when perfectly desiccated they fall to
15 lbs. Were the oxidation corresponding to that labor-
er'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 33^
days. Here, in his own words, emphasized in his own
way, is Mayer's pregnant conclusion from these calcula-
tions: **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 mechanical
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 scientific con-
clusions of his time; but eminent investigators have since
amply verified it.

THE COPLEY MEDALIST OF 1871 459

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 compared
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 themselves.

As regards these questions of weightiest import to the
science of physiology, Dr. Mayer, in 1845, was assuredly
far in advance of all living men.

Mayer grasped the mechanical theory of heat with com-
manding power, illustrating it and applying it in the most
diverse domains. He began, as we have seen, with phys-
ical principles; he determined the numerical relation be-
tween heat and work; he revealed the source of the ener-
gies of the vegetable world, and showed the relationship
of the heat of our fires to solar heat. He followed the
energies which were potential in the vegetable 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 fall-
ing body in weight, 17,856° C. He also found, in 1845,
that the gravitating force between the earth and sun was

4:60 FRAGMENTS OF SCIENCE

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 difficul-
ties 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 as to 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 mechan-
ical theory of heat through all its applications. Whether
the meteoric theory be a matter of fact or not, with him
abides the honor of proving to demonstration that the
light and heat of suns and stars may be originated and
maintained by the collisions of cold planetary matter.

It is the man who with the scantiest data could ac-
complish all this in six short years, and in the hours
snatched from the duties of an arduous profession, that
the Eoyal Society, in 1871, crowned with its highest
honor.

Comparing this brief history with that of the Copley
Medalist of 1870, the differentiating influence of "envi-
ronment," on two minds of similar natural cast and en-

THE COPLEY MEDALIST OF 1871 461

dowment, comes out in an instructive manner. With-
drawn from mechanical appliances, Mayer fell back upon
reflection, selecting with marvellous sagacity, from exist-
ing 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 re-
sorted to experiment, and laid the broad and firm founda-
tion which has secured for the mechanical theory the ac-
ceptance 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 ab-
struse and impressive applications. With their places re-
versed, however, Joule might have become Mayer, and
Mayer might have become Joule.

It does not lie within the scope of these brief articles
to enter upon the developments of the Dynamical Theory
accomplished since Joule and Mayer executed their mem-
orable labors.

XXI

DEATH BY LIGHTNING

PEOPLE in general imagine, when they think at all
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 intelligence 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 in-
flicted 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 ner-
vous 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 in-
jury 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 arrangement of the brain

* A most admirable lecture on the velocity of nervous transmission has been
published by Dr. Du Bois-Reymond in the "Proceedings of the Royal Institu-
tion" for 1866, vol. iv. p. 675.

(462)

DEATH BY LIGHTNING 468

— that every thought or feeling has its physical correlative
in that organ; and nothing can be more certain than that
every physical change, whether molecular or mechanical,
requires time for its accomplishment. So that, besides the
interval of transmission, 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 com-
pletion 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 intel-
ligence through the sensor nerves to the head, one-tenth
of a second consumed by the brain in completing the ar-
rangements 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 conscious-
ness, its power of arrangement might be destroyed. 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 whatever.

The time reqiiired 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 painless. But

464 FRAGMENTS OF SCIENCE

there are other actions which far transcend in rapidity
that of the rifle-bullet. A flash of lightning 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 illu-
minated be in motion, it appears at rest at the place
where the flash falls upon it. When a color-top with
differently-colored sectors is caused to spin rapidly the
colors blend together. Such a top, rotating in a dark
room and illuminated by an electric spark, appears mo-
tionless, each distinct color being clearly seen. Professor
Dove has found that a flash of lightning produces the
same effect. During a thunderstorm he put a color-top
in exceedingly rapid motion, and found that every flash
revealed the top as a motionless object with its colors dis-
tinct. 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 example, 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 rifle-bullet move with sufficient rapidity to
destroy life without the interposition of sensation, much

DEATH BY LIGHTNING 4fi6

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, on re-
covery, had no memory of pain. The following circum-
stantial case is described by Hemmer;

On June 80, 1788, a soldier in the neighborhood of
Mannheim, being overtaken by rain, placed himself under
a tree, beneath which a woman had previously taken shel-
ter. He looked upward to see whether the branches were
thick enough to afford the required protection, and, in
doing so, was struck by lightning, and fell senseless to
the earth. The woman at his side experienced the shock
in her foot, but was not struck down. Some hours after-
ward 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 conscious to the unconscious condition
without pain. The visible marks of a lightning stroke
are usually insignificant: the hair is sometimes burned;
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. Some
time ago I happened to stand in the presence of a nu-
merous audience, with a battery of fifteen large Leyden
jars charged beside me. Through some awkwardness on
my part, I touched a wire leading from the battery, and
the discharge went through my body. Life was abso-
lutely blotted out for a very sensible interval, without a
trace of pain. In a second or so consciousness returned;
I vaguely discerned the audience and apparatus, and, by

466 FRAGMENTS OF SCIENCE

the help of these external appearances, immediately con-
cluded 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 often been my desire to receive accidentally
such a shock, and that my wish had at length been ful-
filled. But, while making this remark, the appearance
which my body presented to my eyes was that of a num-
ber of separate pieces. The arms, for example, were de-
tached 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 sensation, unaccompanied by a pang.

XXII

SCIENCE AND THE "SPIKITS

THEIR refusal to investigate "spiritual phenomena"
is often 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 inquiry 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 so
privileged, and he therefore kindly 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 prob-
able that some physical principle, not evident to the spir-
itualists themselves, might underlie their manifestations.
Extraordinary effects are produced by the accumulation
of small impulses. Galileo set a heavy pendulum in mo-
tion by the well-timed puffs of his breath. Ellicott set
one clock going by the ticks of another, even when the
two clocks were separated by a wall. Preconceived no-
tions can, moreover, vitiate, to an extraordinary degree,

(467)

468 FRAGMENTS OF SCIENCE

the testimony of even veracious persons. Hence my de-
sire to witness those extraordinary phenomena, the exist-
ence 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 neighborhood of London. My host, his intelligent
wife, and a gentleman 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 no 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 downward,
and impelled him to write oracular sentences. I listened
for a time, offering no observation. "And now," contin-
ued X., "this ^Dwer 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 dur-
ing the day." Here, I thought, is something that can be at

SCIENCE AND THE "SPIRITS'' 469

©nee tested. I said immediately to X. : *'If you wisli to wia
to your cause an apostle, who will proclaim your principles to
tlie world from the housetop, tell me what I am now think-
ing of.'* X. reddened, and did not tell me my thought.

Some time previously I had visited Baron Eeichenbach,
in Vienna, and I now asked the 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 the day following the stance.

Medium, — *'0h, yes; but I see light around all bodies.'*

/. — *'Even in perfect darkness?'*

Medium, — ** Yes; I see luminous atmospheres round all
people. The atmosphere which surrounds Mr. R. 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.*'

/. — "Am I to understand that, if this room were per*
fectly 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. "

J.— **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.*'

T, — *'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 stammered,
*'No; 1 am not en rap'port with you."

470 FRAGMENTS OF SCIENCE

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, compared
with those with which my own work had made me famil-
iar. I therefore began to match the wonders related to me
by other wonders. A lady present discoursed on spiritual
atmospheres, which she could see as beautiful colors when
she closed her eyes. I professed myself able to see similar
colors, 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 mu-
sical 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

SCIENCE AND THE ''SPIRITS** 471

meant "Not yet,*' and that three knocks meant **Yes/*
In answer to a question whether I was a medium, the
response was three brisk and vigorous knocks. 1 noticed
that the knocks issued from a particular locality, and
therefore requested the spirits to be good enough to an-
swer 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 disconcerted by
the act; they lost their playfulness, and did not recover
it for a considerable time.

Somewhat weary of the proceedings, I once threw my-
self 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 oscillat-
ing 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 ex-
press the conviction which I really entertained ? 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 can-
did 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.
I, however, informed him that it was the moving hair
acting on the glass. The explanation was not well re-
ceived; and X., in a tone of severe pleasantry, demanded

472 FRAGMENTS OF SCIENCE

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.

The 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, how-
ever, 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 em-
ployed to figure the changes and distribution of spiritual
power. The spirits, it was alleged, were provided with
atmospheres, which combined with and interpenetrated
each other, and considerable ingenuity was shown in dem-
onstrating the necessity of time in effecting the adjust-
ment of the atmospheres. A rearrangement of our posi-
tions was proposed and carried out; and soon afterward
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

SCIENCE AND THE ''SPIRITS'' 473

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 was promptly drawn to the motion; and a gen-
tleman beside me, whose value as a witness I was par-
ticularly 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 warm-hearted old gentleman A. *'Why,
sir," he continued, *'I feel them at this moment shaking
my chair." I stopped the motion of the leg. "Kow, sir,"
A. exclaimed, "they are gone." I began again, and A.
once more affirmed their presence. I could, however,
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 pro-
voke 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
toward its close. The spirits were requested to spell the
name by which I was known in the heavenly world. 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

474 FRAGMENTS OF SCIENCE

the occasion ought to have been superinduced by a pe-
rusal of the Bible immediately before the seance. The
spelling, however, went on, and sure enough I came out
a poet. But matters did not end here. Our host con-
tinued his repetition of the alphabet, and the next letter
of the name proved to be "O." Here was manifestly an
unfinished word; and the spirits were apparently in their
most communicative mood. The knocks came from under
the table, but no person present evinced the slightest de-
sire 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. exclaimed,
**He has a right to look into the very dregs of it, to con-
vince 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 un-
der that table for at least a quarter of an hour, after which,
with a feeling of despair as regards the prospects of hu-
manity never before 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 scien-
tific man to look into these spiritual phenomena. It is
not encouraging; and for this reason. The present pro-
moters 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 undeceived. Science
is perfectly powerless in the presence of this frame of

SCIENCE AND THE ''SPIRITS'' 475

mind. It is, moreover, a state perfectly 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
unfit for its cultivation. When science appeals to uni-
form experience, the spiritualist will retort, "How do
you know that a uniform experience will continue uni-
form? You tell me that the sun has risen for six thou-
sand years: that is no proof that it will rise to-morrow;
within the next twelve hours it may be pufEed 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 impostors are exposed, and the
special demon cast out. He has but slightly to change
his shape, return to his house, and find it "empty, swept,
and garnished."

Since the time when the foregoing remarks were writ-
ten I have been more than once among the spirits, at their
own invitation. They do not improve on acquaintance.
Surely no baser delusion ever obtained dominance over
the weak mind of man.

END OF VOL. I. OP ''FRAGMENTS OF SCIENCE"

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