About the middle of the century Clerk-Maxwell advanced the idea that

light waves were really electro-magnetic waves. If this were true and

light proved to be simply one form of electrical energy, then the same

would be true of radiant heat. Maxwell advanced this theory, but failed

to substantiate it by experimental confirmation. But Dr. Heinrich

Hertz, a few years later, by a series of experiments, demonstrated the

correctness of Maxwell's surmises. What are now called "Hertzian waves"

are waves apparently identical with light waves, but of much lower

pitch or period. In his experiments Hertz showed that, under proper

conditions, electric sparks between polished balls were attended by

ether waves of the same nature as those of light, but of a pitch of

several millions of vibrations per second. These waves could be dealt

with as if they were light waves--reflected, refracted, and polarized.

These are the waves that are utilized in wireless telegraphy.

ROENTGEN RAYS, OR X-RAYS

In December of 1895 word came out of Germany of a scientific discovery

that startled the world. It came first as a rumor, little credited; then

as a pronounced report; at last as a demonstration. It told of a new

manifestation of energy, in virtue of which the interior of opaque

objects is made visible to human eyes. One had only to look into a tube

containing a screen of a certain composition, and directed towards

a peculiar electrical apparatus, to acquire clairvoyant vision more

wonderful than the discredited second-sight of the medium. Coins within

a purse, nails driven into wood, spectacles within a leather case,

became clearly visible when subjected to the influence of this magic

tube; and when a human hand was held before the tube, its bones stood

revealed in weird simplicity, as if the living, palpitating flesh about

them were but the shadowy substance of a ghost.

Not only could the human eye see these astounding revelations, but the

impartial evidence of inanimate chemicals could be brought forward to

prove that the mind harbored no illusion. The photographic film recorded

the things that the eye might see, and ghostly pictures galore soon gave

a quietus to the doubts of the most sceptical. Within a month of the

announcement of Professor Roentgen's experiments comment upon the

"X-ray" and the "new photography" had become a part of the current

gossip of all Christendom.

It is hardly necessary to say that such a revolutionary thing as the

discovery of a process whereby opaque objects became transparent, or

translucent, was not achieved at a single bound with no intermediate

discoveries. In 1859 the German physicist Julius Plucker (1801-1868)

noticed that when there was an electrical discharge through an exhausted

tube at a low pressure, on the surrounding walls of the tube near the

negative pole, or cathode, appeared a greenish phosphorescence. This

discovery was soon being investigated by a number of other scientists,

among others Hittorf, Goldstein, and Professor (now Sir William)

Crookes. The explanations given of this phenomenon by Professor Crookes

concern us here more particularly, inasmuch as his views did not

accord exactly with those held by the other two scientists, and as his

researches were more directly concerned in the discovery of the

Roentgen rays. He held that the heat and phosphorescence produced in a

low-pressure tube were caused by streams of particles, projected from

the cathode with great velocity, striking the sides of the glass tube.

The composition of the glass seemed to enter into this phosphorescence

also, for while lead glass produced blue phosphorescence, soda glass

produced a yellowish green. The composition of the glass seemed to

be changed by a long-continued pelting of these particles, the

phosphorescence after a time losing its initial brilliancy, caused by

the glass becoming "tired," as Professor Crookes said. Thus when some

opaque substance, such as iron, is placed between the cathode and the

sides of the glass tube so that it casts a shadow in a certain spot

on the glass for some little time, it is found on removing the opaque

substance or changing its position that the area of glass at first

covered by the shadow now responded to the rays in a different manner

from the surrounding glass.

The peculiar ray's, now known as the cathode rays, not only cast a

shadow, but are deflected by a magnet, so that the position of the

phosphorescence on the sides of the tube may be altered by the proximity

of a powerful magnet. From this it would seem that the rays are composed

of particles charged with negative electricity, and Professor J. J.

Thomson has modified the experiment of Perrin to show that negative

electricity is actually associated with the rays. There is reason for

believing, therefore, that the cathode rays are rapidly moving charges

of negative electricity. It is possible, also, to determine the velocity

at which these particles are moving by measuring the deflection produced

by the magnetic field.

From the fact that opaque substances cast a shadow in these rays it was

thought at first that all solids were absolutely opaque to them. Hertz,

however, discovered that a small amount of phosphorescence occurred on

the glass even when such opaque substances as gold-leaf or aluminium

foil were interposed between the cathode and the sides of the tube.

Shortly afterwards Lenard discovered that the cathode rays can be made

to pass from the inside of a discharge tube to the outside air. For

convenience these rays outside the tube have since been known as "Lenard

rays."

In the closing days of December, 1895, Professor Wilhelm Konrad

Roentgen, of Wurzburg, announced that he had made the discovery of the

remarkable effect arising from the cathode rays to which reference

was made above. He found that if a plate covered with a phosphorescent

substance is placed near a discharge tube exhausted so highly that the

cathode rays produced a green phosphorescence, this plate is made to

glow in a peculiar manner. The rays producing this glow were not the

cathode rays, although apparently arising from them, and are what have

since been called the Roentgen rays, or X-rays.

Roentgen found that a shadow is thrown upon the screen by substances

held between it and the exhausted tube, the character of the shadow

depending upon the density of the substance. Thus metals are almost

completely opaque to the rays; such substances as bone much less so, and

ordinary flesh hardly so at all. If a coin were held in the hand that

had been interposed between the tube and the screen the picture formed

showed the coin as a black shadow; and the bones of the hand, while

casting a distinct shadow, showed distinctly lighter; while the soft

tissues produced scarcely any shadow at all. The value of such a

discovery was obvious from the first; and was still further enhanced by

the discovery made shortly that, photographic plates are affected by the

rays, thus making it possible to make permanent photographic records of

pictures through what we know as opaque substances.

What adds materially to the practical value of Roentgen's discovery is

the fact that the apparatus for producing the X-rays is now so simple

and relatively inexpensive that it is within the reach even of amateur

scientists. It consists essentially of an induction coil attached either

to cells or a street-current plug for generating the electricity, a

focus tube, and a phosphorescence screen. These focus tubes are made in

various shapes, but perhaps the most popular are in the form of a glass

globe, not unlike an ordinary small-sized water-bottle, this tube being

closed and exhausted, and having the two poles (anode and cathode)

sealed into the glass walls, but protruding at either end for attachment

to the conducting wires from the induction coil. This tube may be

mounted on a stand at a height convenient for manipulation.

The phosphorescence screen is usually a plate covered with some

platino-cyanide and mounted in the end of a box of convenient size, the

opposite end of which is so shaped that it fits the contour of the face,

shutting out the light and allowing the eyes of the observer to focalize

on the screen at the end. For making observations the operator has

simply to turn on the current of electricity and apply the screen to

his eyes, pointing it towards the glowing tube, when the shadow of any

substance interposed between the tube and the screen will appear upon

the phosphorescence plate.

The wonderful shadow pictures produced on the phosphorescence screen,

or the photographic plate, would seem to come from some peculiar form

of light, but the exact nature of these rays is still an open question.

Whether the Roentgen rays are really a form of light--that is, a form

of "electro-magnetic disturbance propagated through ether," is not fully

determined. Numerous experiments have been undertaken to determine this,

but as yet no proof has been found that the rays are a form of light,

although there appears to be nothing in their properties inconsistent

with their being so. For the moment most investigators are content to

admit that the term X-ray virtually begs the question as to the intimate

nature of the form of energy involved.

VIII. THE CONSERVATION OF ENERGY

As we have seen, it was in 1831 that Faraday opened up the field of

magneto-electricity. Reversing the experiments of his predecessors, who

had found that electric currents may generate magnetism, he showed that

magnets have power under certain circumstances to generate electricity;

he proved, indeed, the interconvertibility of electricity and magnetism.

Then he showed that all bodies are more or less subject to the influence

of magnetism, and that even light may be affected by magnetism as to its

phenomena of polarization. He satisfied himself completely of the

true identity of all the various forms of electricity, and of the

convertibility of electricity and chemical action. Thus he linked

together light, chemical affinity, magnetism, and electricity. And,

moreover, he knew full well that no one of these can be produced in

indefinite supply from another. "Nowhere," he says, "is there a pure

creation or production of power without a corresponding exhaustion of

something to supply it."

When Faraday wrote those words in 1840 he was treading on the very heels

of a greater generalization than any which he actually formulated; nay,

he had it fairly within his reach. He saw a great truth without fully

realizing its import; it was left for others, approaching the same truth

along another path, to point out its full significance.

The great generalization which Faraday so narrowly missed is the truth

which since then has become familiar as the doctrine of the conservation

of energy--the law that in transforming energy from one condition to

another we can never secure more than an equivalent quantity; that, in

short, "to create or annihilate energy is as impossible as to create or

annihilate matter; and that all the phenomena of the material universe

consist in transformations of energy alone." Some philosophers think

this the greatest generalization ever conceived by the mind of man. Be

that as it may, it is surely one of the great intellectual landmarks

of the nineteenth century. It stands apart, so stupendous and so

far-reaching in its implications that the generation which first saw the

law developed could little appreciate it; only now, through the vista of

half a century, do we begin to see it in its true proportions.

A vast generalization such as this is never a mushroom growth, nor does

it usually spring full grown from the mind of any single man. Always a

number of minds are very near a truth before any one mind fully grasps

it. Pre-eminently true is this of the doctrine of the conservation of

energy. Not Faraday alone, but half a dozen different men had an inkling

of it before it gained full expression; indeed, every man who advocated

the undulatory theory of light and heat was verging towards the goal.

The doctrine of Young and Fresnel was as a highway leading surely on

to the wide plain of conservation. The phenomena of electro-magnetism

furnished another such highway. But there was yet another road which led

just as surely and even more readily to the same goal. This was the road

furnished by the phenomena of heat, and the men who travelled it were

destined to outstrip their fellow-workers; though, as we have seen,

wayfarers on other roads were within hailing distance when the leaders

passed the mark.

In order to do even approximate justice to the men who entered into

the great achievement, we must recall that just at the close of the

eighteenth century Count Rumford and Humphry Davy independently showed

that labor may be transformed into heat; and correctly interpreted this

fact as meaning the transformation of molar into molecular motion. We

can hardly doubt that each of these men of genius realized--vaguely, at

any rate--that there must be a close correspondence between the amount

of the molar and the molecular motions; hence that each of them was in

sight of the law of the mechanical equivalent of heat. But neither of

them quite grasped or explicitly stated what each must vaguely have

seen; and for just a quarter of a century no one else even came abreast

their line of thought, let alone passing it.

But then, in 1824, a French philosopher, Sadi Carnot, caught step with

the great Englishmen, and took a long leap ahead by explicitly stating

his belief that a definite quantity of work could be transformed into

a definite quantity of heat, no more, no less. Carnot did not, indeed,

reach the clear view of his predecessors as to the nature of heat, for

he still thought it a form of "imponderable" fluid; but he reasoned none

the less clearly as to its mutual convertibility with mechanical work.

But important as his conclusions seem now that we look back upon

them with clearer vision, they made no impression whatever upon his

contemporaries. Carnot's work in this line was an isolated phenomenon

of historical interest, but it did not enter into the scheme of the

completed narrative in any such way as did the work of Rumford and Davy.

The man who really took up the broken thread where Rumford and Davy had

dropped it, and wove it into a completed texture, came upon the scene

in 1840. His home was in Manchester, England; his occupation that of

a manufacturer. He was a friend and pupil of the great Dr. Dalton.

His name was James Prescott Joule. When posterity has done its final

juggling with the names of the nineteenth century, it is not unlikely

that the name of this Manchester philosopher will be a household word,

like the names of Aristotle, Copernicus, and Newton.

For Joule's work it was, done in the fifth decade of the century, which

demonstrated beyond all cavil that there is a precise and absolute

equivalence between mechanical work and heat; that whatever the form of

manifestation of molar motion, it can generate a definite and measurable

amount of heat, and no more. Joule found, for example, that at the

sea-level in Manchester a pound weight falling through seven hundred and

seventy-two feet could generate enough heat to raise the temperature

of a pound of water one degree Fahrenheit. There was nothing haphazard,

nothing accidental, about this; it bore the stamp of unalterable law.

And Joule himself saw, what others in time were made to see, that this

truth is merely a particular case within a more general law. If

heat cannot be in any sense created, but only made manifest as a

transformation of another kind of motion, then must not the same

thing be true of all those other forms of "force"--light, electricity,

magnetism--which had been shown to be so closely associated, so mutually

convertible, with heat? All analogy seemed to urge the truth of

this inference; all experiment tended to confirm it. The law of the

mechanical equivalent of heat then became the main corner-stone of the

greater law of the conservation of energy.

But while this citation is fresh in mind, we must turn our attention

with all haste to a country across the Channel--to Denmark, in

short--and learn that even as Joule experimented with the transformation

of heat, a philosopher of Copenhagen, Colding by name, had hit upon the

same idea, and carried it far towards a demonstration. And then, without

pausing, we must shift yet again, this time to Germany, and consider the

work of three other men, who independently were on the track of the same

truth, and two of whom, it must be admitted, reached it earlier than

either Joule or Colding, if neither brought it to quite so clear a

demonstration. The names of these three Germans are Mohr, Mayer,

and Helmholtz. Their share in establishing the great doctrine of

conservation must now claim our attention.

As to Karl Friedrich Mohr, it may be said that his statement of the

doctrine preceded that of any of his fellows, yet that otherwise it was

perhaps least important. In 1837 this thoughtful German had grasped

the main truth, and given it expression in an article published in the

Zeitschrift fur Physik, etc. But the article attracted no attention

whatever, even from Mohr's own countrymen. Still, Mohr's title to

rank as one who independently conceived the great truth, and perhaps

conceived it before any other man in the world saw it as clearly, even

though he did not demonstrate its validity, is not to be disputed.

It was just five years later, in 1842, that Dr. Julius Robert Mayer,

practising physician in the little German town of Heilbronn, published a

paper in Liebig's Annalen on "The Forces of Inorganic Nature," in which

not merely the mechanical theory of heat, but the entire doctrine of

the conservation of energy, is explicitly if briefly stated. Two years

earlier Dr. Mayer, while surgeon to a Dutch India vessel cruising in the

tropics, had observed that the venous blood of a patient seemed redder

than venous blood usually is observed to be in temperate climates. He

pondered over this seemingly insignificant fact, and at last reached

the conclusion that the cause must be the lesser amount of oxidation

required to keep up the body temperature in the tropics. Led by this

reflection to consider the body as a machine dependent on outside forces

for its capacity to act, he passed on into a novel realm of thought,

which brought him at last to independent discovery of the mechanical

theory of heat, and to the first full and comprehensive appreciation

of the great law of conservation. Blood-letting, the modern physician

holds, was a practice of very doubtful benefit, as a rule, to the

subject; but once, at least, it led to marvellous results. No straw is

go small that it may not point the receptive mind of genius to new and

wonderful truths.

MAYER'S PAPER OF 1842

The paper in which Mayer first gave expression to his revolutionary

ideas bore the title of "The Forces of Inorganic Nature," and was

published in 1842. It is one of the gems of scientific literature, and

fortunately it is not too long to be quoted in its entirety. Seldom if

ever was a great revolutionary doctrine expounded in briefer compass:

"What are we to understand by 'forces'? and how are different forces

related to each other? The term force conveys for the most part the idea

of something unknown, unsearchable, and hypothetical; while the term

matter, on the other hand, implies the possession, by the object in

question, of such definite properties as weight and extension. An

attempt, therefore, to render the idea of force equally exact with that

of matter is one which should be welcomed by all those who desire to

have their views of nature clear and unencumbered by hypothesis.

"Forces are causes; and accordingly we may make full application in

relation to them of the principle causa aequat effectum. If the cause

c has the effect e, then c = e; if, in its turn, e is the cause of a

second effect of f, we have e = f, and so on: c = e = f... = c. In a

series of causes and effects, a term or a part of a term can never, as

is apparent from the nature of an equation, become equal to nothing.

This first property of all causes we call their indestructibility.

"If the given cause c has produced an effect e equal to itself, it has

in that very act ceased to be--c has become e. If, after the production

of e, c still remained in the whole or in part, there must be still

further effects corresponding to this remaining cause: the total effect

of c would thus be > e, which would be contrary to the supposition c =

e. Accordingly, since c becomes e, and e becomes f, etc., we must regard

these various magnitudes as different forms under which one and the same

object makes its appearance. This capability of assuming various forms

is the second essential property of all causes. Taking both properties

together, we may say, causes an INDESTRUCTIBLE quantitatively, and

quantitatively CONVERTIBLE objects.

"There occur in nature two causes which apparently never pass one into

the other," said Mayer. "The first class consists of such causes as

possess the properties of weight and impenetrability. These are kinds of

matter. The other class is composed of causes which are wanting in the

properties just mentioned--namely, forces, called also imponderables,

from the negative property that has been indicated. Forces are therefore

INDESTRUCTIBLE, CONVERTIBLE, IMPONDERABLE OBJECTS.

"As an example of causes and effects, take matter: explosive gas, H + O,

and water, HO, are related to each other as cause and effect; therefore

H + O = HO. But if H + O becomes HO, heat, cal., makes its appearance

as well as water; this heat must likewise have a cause, x, and we have

therefore H + O + X = HO + cal. It might be asked, however, whether H

+ O is really = HO, and x = cal., and not perhaps H + O = cal., and x =

HO, whence the above equation could equally be deduced; and so in many

other cases. The phlogistic chemists recognized the equation between

cal. and x, or phlogiston as they called it, and in so doing made a

great step in advance; but they involved themselves again in a system of

mistakes by putting x in place of O. In this way they obtained H = HO +

x.

"Chemistry teaches us that matter, as a cause, has matter for its

effect; but we may say with equal justification that to force as a cause

corresponds force as effect. Since c = e, and e = c, it is natural to

call one term of an equation a force, and the other an effect of force,

or phenomenon, and to attach different notions to the expression force

and phenomenon. In brief, then, if the cause is matter, the effect is

matter; if the cause is a force, the effect is also a force.

"The cause that brings about the raising of a weight is a force. The

effect of the raised weight is, therefore, also a force; or, expressed

in a more general form, SEPARATION IN SPACE OF PONDERABLE OBJECTS IS

A FORCE; and since this force causes the fall of bodies, we call it

FALLING FORCE. Falling force and fall, or, still more generally,

falling force and motion, are forces related to each other as cause and

effect--forces convertible into each other--two different forms of one

and the same object. For example, a weight resting on the ground is not

a force: it is neither the cause of motion nor of the lifting of another

weight. It becomes so, however, in proportion as it is raised above the

ground. The cause--that is, the distance between a weight and the earth,

and the effect, or the quantity of motion produced, bear to each other,

as shown by mechanics, a constant relation.

"Gravity being regarded as the cause of the falling of bodies, a

gravitating force is spoken of; and thus the ideas of PROPERTY and

of FORCE are confounded with each other. Precisely that which is

the essential attribute of every force--that is, the UNION of

indestructibility with convertibility--is wanting in every property:

between a property and a force, between gravity and motion, it is

therefore impossible to establish the equation required for a rightly

conceived causal relation. If gravity be called a force, a cause

is supposed which produces effects without itself diminishing, and

incorrect conceptions of the causal connections of things are thereby

fostered. In order that a body may fall, it is just as necessary that it

be lifted up as that it should be heavy or possess gravity. The fall of

bodies, therefore, ought not to be ascribed to their gravity alone. The

problem of mechanics is to develop the equations which subsist between

falling force and motion, motion and falling force, and between

different motions. Here is a case in point: The magnitude of the falling

force v is directly proportional (the earth's radius being assumed--oo)

to the magnitude of the mass m, and the height d, to which it is

raised--that is, v = md. If the height d = l, to which the mass m is

raised, is transformed into the final velocity c = l of this mass, we

have also v = mc; but from the known relations existing between d and c,

it results that, for other values of d or of c, the measure of the

force v is mc squared; accordingly v = md = mcsquared. The law of the

conservation of vis viva is thus found to be based on the general law of

the indestructibility of causes.

"In many cases we see motion cease without having caused another motion

or the lifting of a weight. But a force once in existence cannot be

annihilated--it can only change its form. And the question therefore

arises, what other forms is force, which we have become acquainted with

as falling force and motion, capable of assuming? Experience alone

can lead us to a conclusion on this point. That we may experiment to

advantage, we must select implements which, besides causing a real

cessation of motion, are as little as possible altered by the objects

to be examined. For example, if we rub together two metal plates, we see

motion disappear, and heat, on the other hand, make its appearance, and

there remains to be determined only whether MOTION is the cause of heat.

In order to reach a decision on this point, we must discuss the question

whether, in the numberless cases in which the expenditure of motion is

accompanied by the appearance of heat, the motion has not some other

effect than the production of heat, and the heat some other cause than

the motion.

"A serious attempt to ascertain the effects of ceasing motion has never

been made. Without wishing to exclude a priori the hypothesis which

it may be possible to establish, therefore, we observe only that, as a

rule, this effect cannot be supposed to be an alteration in the state of

aggregation of the moved (that is, rubbing, etc.) bodies. If we assume

that a certain quantity of motion v is expended in the conversion of a

rubbing substance m into n, we must then have m + v - n, and n = m + v;

and when n is reconverted into m, v must appear again in some form or

other.

"By the friction of two metallic plates continued for a very long time,

we can gradually cause the cessation of an immense quantity of movement;

but would it ever occur to us to look for even the smallest trace of the

force which has disappeared in the metallic dust that we could collect,

and to try to regain it thence? We repeat, the motion cannot have been

annihilated; and contrary, or positive and negative, motions cannot be

regarded as = o any more than contrary motions can come out of nothing,

or a weight can raise itself.

"Without the recognition of a causal relation between motion and heat,

it is just as difficult to explain the production of heat as it is

to give any account of the motion that disappears. The heat cannot be

derived from the diminution of the volume of the rubbing substances.

It is well known that two pieces of ice may be melted by rubbing them

together in vacuo; but let any one try to convert ice into water by

pressure, however enormous. The author has found that water undergoes

a rise of temperature when shaken violently. The water so heated (from

twelve to thirteen degrees centigrade) has a greater bulk after being

shaken than it had before. Whence now comes this quantity of heat, which

by repeated shaking may be called into existence in the same apparatus

as often as we please? The vibratory hypothesis of heat is an approach

towards the doctrine of heat being the effect of motion, but it does not

favor the admission of this causal relation in its full generality. It

rather lays the chief stress on restless oscillations.

"If it be considered as now established that in many cases no other

effect of motion can be traced except heat, and that no other cause

than motion can be found for the heat that is produced, we prefer the

assumption that heat proceeds from motion to the assumption of a cause

without effect and of an effect without a cause. Just as the chemist,

instead of allowing oxygen and hydrogen to disappear without further

investigation, and water to be produced in some inexplicable manner,

establishes a connection between oxygen and hydrogen on the one hand,

and water on the other.

"We may conceive the natural connection existing between falling force,

motion, and heat as follows: We know that heat makes its appearance

when the separate particles of a body approach nearer to each other;

condensation produces heat. And what applies to the smallest particles

of matter, and the smallest intervals between them, must also apply to

large masses and to measurable distances. The falling of a weight is a

diminution of the bulk of the earth, and must therefore without doubt be

related to the quantity of heat thereby developed; this quantity of heat

must be proportional to the greatness of the weight and its distance

from the ground. From this point of view we are easily led to the

equations between falling force, motion, and heat that have already been

discussed.

"But just as little as the connection between falling force and motion

authorizes the conclusion that the essence of falling force is motion,

can such a conclusion be adopted in the case of heat. We are, on the

contrary, rather inclined to infer that, before it can become heat,

motion must cease to exist as motion, whether simple, or vibratory, as

in the case of light and radiant heat, etc.

"If falling force and motion are equivalent to heat, heat must also

naturally be equivalent to motion and falling force. Just as heat

appears as an EFFECT of the diminution of bulk and of the cessation

of motion, so also does heat disappear as a CAUSE when its effects are

produced in the shape of motion, expansion, or raising of weight.

"In water-mills the continual diminution in bulk which the earth

undergoes, owing to the fall of the water, gives rise to motion, which

afterwards disappears again, calling forth unceasingly a great quantity

of heat; and, inversely, the steam-engine serves to decompose heat again

into motion or the raising of weights. A locomotive with its train may

be compared to a distilling apparatus; the heat applied under the boiler

passes off as motion, and this is deposited again as heat at the axles

of the wheels."

Mayer then closes his paper with the following deduction: "The solution

of the equations subsisting between falling force and motion requires

that the space fallen through in a given time--e. g., the first

second--should be experimentally determined. In like manner, the

solution of the equations subsisting between falling force and motion on

the one hand and heat on the other requires an answer to the question,

How great is the quantity of heat which corresponds to a given quantity

of motion or falling force? For instance, we must ascertain how high a

given weight requires to be raised above the ground in order that its

falling force maybe equivalent to the raising of the temperature of

an equal weight of water from 0 degrees to 1 degrees centigrade. The

attempt to show that such an equation is the expression of a physical

truth may be regarded as the substance of the foregoing remarks.

"By applying the principles that have been set forth to the relations

subsisting between the temperature and the volume of gases, we find

that the sinking of a mercury column by which a gas is compressed is

equivalent to the quantity of heat set free by the compression; and

hence it follows, the ratio between the capacity for heat of air under

constant pressure and its capacity under constant volume being taken as

= 1.421, that the warming of a given weight of water from 0 degrees to

equal weight from the height of about three hundred and sixty-five

metres. If we compare with this result the working of our best

steam-engines, we see how small a part only of the heat applied under

the boiler is really transformed into motion or the raising of weights;

and this may serve as justification for the attempts at the profitable

production of motion by some other method than the expenditure of the

chemical difference between carbon and oxygen--more particularly by

the transformation into motion of electricity obtained by chemical

means."(1)

MAYER AND HELMHOLTZ

Here, then, was this obscure German physician, leading the humdrum life

of a village practitioner, yet seeing such visions as no human being in

the world had ever seen before.

The great principle he had discovered became the dominating thought of

his life, and filled all his leisure hours. He applied it far and wide,

amid all the phenomena of the inorganic and organic worlds. It taught

him that both vegetables and animals are machines, bound by the same

laws that hold sway over inorganic matter, transforming energy, but

creating nothing. Then his mind reached out into space and met a

universe made up of questions. Each star that blinked down at him as he

rode in answer to a night-call seemed an interrogation-point asking,

How do I exist? Why have I not long since burned out if your theory

of conservation be true? No one had hitherto even tried to answer that

question; few had so much as realized that it demanded an answer. But

the Heilbronn physician understood the question and found an answer.

His meteoric hypothesis, published in 1848, gave for the first time a

tenable explanation of the persistent light and heat of our sun and the

myriad other suns--an explanation to which we shall recur in another

connection.

All this time our isolated philosopher, his brain aflame with the glow

of creative thought, was quite unaware that any one else in the world

was working along the same lines. And the outside world was equally

heedless of the work of the Heilbronn physician. There was no friend to

inspire enthusiasm and give courage, no kindred spirit to react on this

masterful but lonely mind. And this is the more remarkable because there

are few other cases where a master-originator in science has come upon

the scene except as the pupil or friend of some other master-originator.

Of the men we have noticed in the present connection, Young was the

friend and confrere of Davy; Davy, the protege of Rumford; Faraday, the

pupil of Davy; Fresnel, the co-worker with Arago; Colding, the confrere

of Oersted; Joule, the pupil of Dalton. But Mayer is an isolated

phenomenon--one of the lone mountain-peak intellects of the century.

That estimate may be exaggerated which has called him the Galileo of the

nineteenth century, but surely no lukewarm praise can do him justice.

Yet for a long time his work attracted no attention whatever. In 1847,

when another German physician, Hermann von Helmholtz, one of the most

massive and towering intellects of any age, had been independently

led to comprehension of the doctrine of the conservation of energy

and published his treatise on the subject, he had hardly heard of his

countryman Mayer. When he did hear of him, however, he hastened to

renounce all claim to the doctrine of conservation, though the world at

large gives him credit of independent even though subsequent discovery.

JOULE'S PAPER OF 1843

Meantime, in England, Joule was going on from one experimental

demonstration to another, oblivious of his German competitors and almost

as little noticed by his own countrymen. He read his first paper before

the chemical section of the British Association for the Advancement of

Science in 1843, and no one heeded it in the least. It is well worth our

while, however, to consider it at length. It bears the title, "On the

Calorific Effects of Magneto-Electricity, and the Mechanical Value

of Heat." The full text, as published in the Report of the British

Association, is as follows:

"Although it has been long known that fine platinum wire can be ignited

by magneto-electricity, it still remained a matter of doubt whether heat

was evolved by the COILS in which the magneto-electricity was generated;

and it seemed indeed not unreasonable to suppose that COLD was produced

there in order to make up for the heat evolved by the other part of

the circuit. The author therefore has endeavored to clear up this

uncertainty by experiment. His apparatus consisted of a small compound

electro-magnet, immersed in water, revolving between the poles of a

powerful stationary magnet. The magneto-electricity developed in the

coils of the revolving electro-magnet was measured by an accurate

galvanometer; and the temperature of the water was taken before and

after each experiment by a very delicate thermometer. The influence of

the temperature of the surrounding atmospheric air was guarded against

by covering the revolving tube with flannel, etc., and by the adoption

of a system of interpolation. By an extensive series of experiments with

the above apparatus the author succeeded in proving that heat is evolved

by the coils of the magneto-electrical machine, as well as by any other

part of the circuit, in proportion to the resistance to conduction

of the wire and the square of the current; the magneto having, under

comparable circumstances, the same calorific power as the voltaic

electricity.

"Professor Jacobi, of St. Petersburg, bad shown that the motion of an

electro-magnetic machine generates magneto-electricity in opposition

to the voltaic current of the battery. The author had observed the same

phenomenon on arranging his apparatus as an electro-magnetic machine;

but had found that no additional heat was evolved on account of the

conflict of forces in the coil of the electro-magnet, and that the heat

evolved by the coil remained, as before, proportional to the square of

the current. Again, by turning the machine contrary to the direction of

the attractive forces, so as to increase the intensity of the voltaic

current by the assistance of the magneto-electricity, he found that the

evolution of heat was still proportional to the square of the current.

The author discovered, therefore, that the heat evolved by the voltaic

current is invariably proportional to the square of the current, however

the intensity of the current may be varied by magnetic induction. But

Dr. Faraday has shown that the chemical effects of the current

are simply as its quantity. Therefore he concluded that in the

electro-magnetic engine a part of the heat due to the chemical actions

of the battery is lost by the circuit, and converted into mechanical

power; and that when the electro-magnetic engine is turned CONTRARY to

the direction of the attractive forces, a greater quantity of heat is

evolved by the circuit than is due to the chemical reactions of the

battery, the over-plus quantity being produced by the conversion of the

mechanical force exerted in turning the machine. By a dynamometrical

apparatus attached to his machine, the author has ascertained that,

in all the above cases, a quantity of heat, capable of increasing the

temperature of a pound of water by one degree of Fahrenheit's scale, is

equal to the mechanical force capable of raising a weight of about eight

hundred and thirty pounds to the height of one foot."(2)

JOULE OR MAYER?

Two years later Joule wished to read another paper, but the chairman

hinted that time was limited, and asked him to confine himself to

a brief verbal synopsis of the results of his experiments. Had the

chairman but known it, he was curtailing a paper vastly more important

than all the other papers of the meeting put together. However, the

synopsis was given, and one man was there to hear it who had the genius

to appreciate its importance. This was William Thomson, the present

Lord Kelvin, now known to all the world as among the greatest of natural

philosophers, but then only a novitiate in science. He came to

Joule's aid, started rolling the ball of controversy, and subsequently

associated himself with the Manchester experimenter in pursuing his

investigations.

But meantime the acknowledged leaders of British science viewed the

new doctrine askance. Faraday, Brewster, Herschel--those were the great

names in physics at that day, and no one of them could quite accept

the new views regarding energy. For several years no older physicist,

speaking with recognized authority, came forward in support of the

doctrine of conservation. This culminating thought of the first half

of the nineteenth century came silently into the world, unheralded and

unopposed. The fifth decade of the century had seen it elaborated and

substantially demonstrated in at least three different countries, yet

even the leaders of thought did not so much as know of its existence.

In 1853 Whewell, the historian of the inductive sciences, published a

second edition of his history, and, as Huxley has pointed out, he did

not so much as refer to the revolutionizing thought which even then was

a full decade old.

By this time, however, the battle was brewing. The rising generation

saw the importance of a law which their elders could not appreciate, and

soon it was noised abroad that there were more than one claimant to the

honor of discovery. Chiefly through the efforts of Professor Tyndall,

the work of Mayer became known to the British public, and a most

regrettable controversy ensued between the partisans of Mayer and those

of Joule--a bitter controversy, in which Davy's contention that science

knows no country was not always regarded, and which left its scars upon

the hearts and minds of the great men whose personal interests were

involved.

And so to this day the question who is the chief discoverer of the law

of the conservation of energy is not susceptible of a categorical answer

that would satisfy all philosophers. It is generally held that the first

choice lies between Joule and Mayer. Professor Tyndall has expressed the

belief that in future each of these men will be equally remembered in

connection with this work. But history gives us no warrant for such a

hope. Posterity in the long run demands always that its heroes shall

stand alone. Who remembers now that Robert Hooke contested with Newton

the discovery of the doctrine of universal gravitation? The judgment of

posterity is unjust, but it is inexorable. And so we can little doubt

that a century from now one name will be mentioned as that of the

originator of the great doctrine of the conservation of energy. The man

whose name is thus remembered will perhaps be spoken of as the Galileo,

the Newton, of the nineteenth century; but whether the name thus

dignified by the final verdict of history will be that of Colding, Mohr,

Mayer, Helmholtz, or Joule, is not as, yet decided.

LORD KELVIN AND THE DISSIPATION OF ENERGY

The gradual permeation of the field by the great doctrine of

conservation simply repeated the history of the introduction of every

novel and revolutionary thought. Necessarily the elder generation, to

whom all forms of energy were imponderable fluids, must pass away before

the new conception could claim the field. Even the word energy, though

Young had introduced it in 1807, did not come into general use till some

time after the middle of the century. To the generality of philosophers

(the word physicist was even less in favor at this time) the various

forms of energy were still subtile fluids, and never was idea

relinquished with greater unwillingness than this. The experiments of

Young and Fresnel had convinced a large number of philosophers that

light is a vibration and not a substance; but so great an authority as

Biot clung to the old emission idea to the end of his life, in 1862, and

held a following.

Meantime, however, the company of brilliant young men who had just

served their apprenticeship when the doctrine of conservation came upon

the scene had grown into authoritative positions, and were battling

actively for the new ideas. Confirmatory evidence that energy is a

molecular motion and not an "imponderable" form of matter accumulated

day by day. The experiments of two Frenchmen, Hippolyte L. Fizeau and

Leon Foucault, served finally to convince the last lingering sceptics

that light is an undulation; and by implication brought heat into the

same category, since James David Forbes, the Scotch physicist, had shown

in 1837 that radiant heat conforms to the same laws of polarization

and double refraction that govern light. But, for that matter, the

experiments that had established the mechanical equivalent of

heat hardly left room for doubt as to the immateriality of this

"imponderable." Doubters had indeed, expressed scepticism as to

the validity of Joule's experiments, but the further researches,

experimental and mathematical, of such workers as Thomson (Lord Kelvin),

Rankine, and Tyndall in Great Britain, of Helmholtz and Clausius in

Germany, and of Regnault in France, dealing with various manifestations

of heat, placed the evidence beyond the reach of criticism.

Out of these studies, just at the middle of the century, to which

the experiments of Mayer and Joule had led, grew the new science

of thermo-dynamics. Out of them also grew in the mind of one of the

investigators a new generalization, only second in importance to the

doctrine of conservation itself. Professor William Thomson (Lord Kelvin)

in his studies in thermodynamics was early impressed with the fact that

whereas all the molar motion developed through labor or gravity could

be converted into heat, the process is not fully reversible. Heat can,

indeed, be converted into molar motion or work, but in the process a

certain amount of the heat is radiated into space and lost. The same

thing happens whenever any other form of energy is converted into molar

motion. Indeed, every transmutation of energy, of whatever character,

seems complicated by a tendency to develop heat, part of which is

lost. This observation led Professor Thomson to his doctrine of the

dissipation of energy, which he formulated before the Royal Society of

Edinburgh in 1852, and published also in the Philosophical Magazine the

same year, the title borne being, "On a Universal Tendency in Nature to

the Dissipation of Mechanical Energy."

From the principle here expressed Professor Thomson drew the startling

conclusion that, "since any restoration of this mechanical energy

without more than an equivalent dissipation is impossible," the

universe, as known to us, must be in the condition of a machine

gradually running down; and in particular that the world we live on has

been within a finite time unfit for human habitation, and must again

become so within a finite future. This thought seems such a commonplace

to-day that it is difficult to realize how startling it appeared half a

century ago. A generation trained, as ours has been, in the doctrines

of the conservation and dissipation of energy as the very alphabet

of physical science can but ill appreciate the mental attitude of a

generation which for the most part had not even thought it problematical

whether the sun could continue to give out heat and light forever. But

those advance thinkers who had grasped the import of the doctrine of

conservation could at once appreciate the force of Thomson's doctrine

of dissipation, and realize the complementary character of the two

conceptions.

Here and there a thinker like Rankine did, indeed, attempt to fancy

conditions under which the energy lost through dissipation might be

restored to availability, but no such effort has met with success, and

in time Professor Thomson's generalization and his conclusions as to the

consequences of the law involved came to be universally accepted.

The introduction of the new views regarding the nature of energy

followed, as I have said, the course of every other growth of new ideas.

Young and imaginative men could accept the new point of view; older

philosophers, their minds channelled by preconceptions, could not get

into the new groove. So strikingly true is this in the particular case

now before us that it is worth while to note the ages at the time of the

revolutionary experiments of the men whose work has been mentioned as

entering into the scheme of evolution of the idea that energy is merely

a manifestation of matter in motion. Such a list will tell the story

better than a volume of commentary.

Observe, then, that Davy made his epochal experiment of melting ice by

friction when he was a youth of twenty. Young was no older when he

made his first communication to the Royal Society, and was in his

twenty-seventh year when he first actively espoused the undulatory

theory. Fresnel was twenty-six when he made his first important

discoveries in the same field; and Arago, who at once became his

champion, was then but two years his senior, though for a decade he had

been so famous that one involuntarily thinks of him as belonging to an

elder generation.

Forbes was under thirty when he discovered the polarization of heat,

which pointed the way to Mohr, then thirty-one, to the mechanical

equivalent. Joule was twenty-two in 1840, when his great work was

begun; and Mayer, whose discoveries date from the same year, was then

twenty-six, which was also the age of Helmholtz when he published his

independent discovery of the same law. William Thomson was a youth just

past his majority when he came to the aid of Joule before the British

Society, and but seven years older when he formulated his own doctrine

of the dissipation of energy. And Clausius and Rankine, who are usually

mentioned with Thomson as the great developers of thermo-dynamics, were

both far advanced with their novel studies before they were thirty.

With such a list in mind, we may well agree with the father of inductive

science that "the man who is young in years may be old in hours."

Yet we must not forget that the shield has a reverse side. For was not

the greatest of observing astronomers, Herschel, past thirty-five before

he ever saw a telescope, and past fifty before he discovered the heat

rays of the spectrum? And had not Faraday reached middle life before he

turned his attention especially to electricity? Clearly, then, to make

this phrase complete, Bacon should have added that "the man who is

old in years may be young in imagination." Here, however, even more

appropriate than in the other case--more's the pity--would have been the

application of his qualifying clause: "but that happeneth rarely."

THE FINAL UNIFICATION

There are only a few great generalizations as yet thought out in any

single field of science. Naturally, then, after a great generalization

has found definitive expression, there is a period of lull before

another forward move. In the case of the doctrines of energy, the

lull has lasted half a century. Throughout this period, it is true, a

multitude of workers have been delving in the field, and to the casual

observer it might seem as if their activity had been boundless, while

the practical applications of their ideas--as exemplified, for example,

in the telephone, phonograph, electric light, and so on--have been

little less than revolutionary. Yet the most competent of living

authorities, Lord Kelvin, could assert in 1895 that in fifty years he

had learned nothing new regarding the nature of energy.

This, however, must not be interpreted as meaning that the world has

stood still during these two generations. It means rather that the rank

and file have been moving forward along the road the leaders had

already travelled. Only a few men in the world had the range of thought

regarding the new doctrine of energy that Lord Kelvin had at the middle

of the century. The few leaders then saw clearly enough that if one

form of energy is in reality merely an undulation or vibration among the

particles of "ponderable" matter or of ether, all other manifestations

of energy must be of the same nature. But the rank and file were not

even within sight of this truth for a long time after they had partly

grasped the meaning of the doctrine of conservation. When, late in

the fifties, that marvellous young Scotchman, James Clerk-Maxwell,

formulating in other words an idea of Faraday's, expressed his belief

that electricity and magnetism are but manifestations of various

conditions of stress and motion in the ethereal medium (electricity a

displacement of strain, magnetism a whirl in the ether), the idea met

with no immediate popularity. And even less cordial was the reception

given the same thinker's theory, put forward in 1863, that the ethereal

undulations producing the phenomenon we call light differ in no respect

except in their wave-length from the pulsations of electro-magnetism.

At about the same time Helmholtz formulated a somewhat similar

electro-magnetic theory of light; but even the weight of this combined

authority could not give the doctrine vogue until very recently, when

the experiments of Heinrich Hertz, the pupil of Helmholtz, have shown

that a condition of electrical strain may be developed into a wave

system by recurrent interruptions of the electric state in the

generator, and that such waves travel through the ether with the

rapidity of light. Since then the electro-magnetic theory of light has

been enthusiastically referred to as the greatest generalization of

the century; but the sober thinker must see that it is really only

what Hertz himself called it--one pier beneath the great arch of

conservation. It is an interesting detail of the architecture, but the

part cannot equal the size of the whole.

More than that, this particular pier is as yet by no means a very firm

one. It has, indeed, been demonstrated that waves of electro-magnetism

pass through space with the speed of light, but as yet no one has

developed electric waves even remotely approximating the shortness of

the visual rays. The most that can positively be asserted, therefore,

is that all the known forms of radiant energy-heat, light,

electro-magnetism--travel through space at the same rate of speed, and

consist of traverse vibrations--"lateral quivers," as Fresnel said of

light--known to differ in length, and not positively known to differ

otherwise. It has, indeed, been suggested that the newest form of

radiant energy, the famous X-ray of Professor Roentgen's discovery, is

a longitudinal vibration, but this is a mere surmise. Be that as it

may, there is no one now to question that all forms of radiant energy,

whatever their exact affinities, consist essentially of undulatory

motions of one uniform medium.

A full century of experiment, calculation, and controversy has thus

sufficed to correlate the "imponderable fluids" of our forebears, and

reduce them all to manifestations of motion among particles of matter.

At first glimpse that seems an enormous change of view. And yet, when

closely considered, that change in thought is not so radical as the

change in phrase might seem to imply. For the nineteenth-century

physicist, in displacing the "imponderable fluids" of many kinds--one

each for light, heat, electricity, magnetism--has been obliged to

substitute for them one all-pervading fluid, whose various quivers,

waves, ripples, whirls or strains produce the manifestations which in

popular parlance are termed forms of force. This all-pervading fluid the

physicist terms the ether, and he thinks of it as having no weight. In

effect, then, the physicist has dispossessed the many imponderables in

favor of a single imponderable--though the word imponderable has been

banished from his vocabulary. In this view the ether--which, considered

as a recognized scientific verity, is essentially a nineteenth-century

discovery--is about the most interesting thing in the universe.

Something more as to its properties, real or assumed, we shall have

occasion to examine as we turn to the obverse side of physics, which

demands our attention in the next chapter.

IX. THE ETHER AND PONDERABLE MATTER

"Whatever difficulties we may have in forming a consistent idea of the

constitution of the ether, there can be no doubt that the interplanetary

and interstellar spaces are not empty, but are occupied by a material

substance or body which is certainly the largest and probably the most

uniform body of which we have any knowledge."

Such was the verdict pronounced some thirty years ago by James

Clerk-Maxwell, one of the very greatest of nineteenth-century

physicists, regarding the existence of an all-pervading plenum in the

universe, in which every particle of tangible matter is immersed.

And this verdict may be said to express the attitude of the entire

philosophical world of our day. Without exception, the authoritative

physicists of our time accept this plenum as a verity, and reason about

it with something of the same confidence they manifest in speaking of

"ponderable" matter or of, energy. It is true there are those among them

who are disposed to deny that this all-pervading plenum merits the name

of matter. But that it is a something, and a vastly important something

at that, all are agreed. Without it, they allege, we should know nothing

of light, of radiant heat, of electricity or magnetism; without it there

would probably be no such thing as gravitation; nay, they even hint that

without this strange something, ether, there would be no such thing as

matter in the universe. If these contentions of the modern physicist are

justified, then this intangible ether is incomparably the most important

as well as the "largest and most uniform substance or body" in the

universe. Its discovery may well be looked upon as one of the most

important feats of the nineteenth century.

For a discovery of that century it surely is, in the sense that all

the known evidences of its existence were gathered in that epoch.

True dreamers of all ages have, for metaphysical reasons, imagined the

existence of intangible fluids in space--they had, indeed, peopled

space several times over with different kinds of ethers, as Maxwell

remarks--but such vague dreamings no more constituted the discovery of

the modern ether than the dream of some pre-Columbian visionary that

land might lie beyond the unknown waters constituted the discovery

of America. In justice it must be admitted that Huyghens, the

seventeenth-century originator of the undulatory theory of light, caught

a glimpse of the true ether; but his contemporaries and some eight

generations of his successors were utterly deaf to his claims; so

he bears practically the same relation to the nineteenth-century

discoverers of ether that the Norseman bears to Columbus.

The true Columbus of the ether was Thomas Young. His discovery was

consummated in the early days of the nineteenth century, when he brought

forward the first, conclusive proofs of the undulatory theory of light.

To say that light consists of undulations is to postulate something that

undulates; and this something could not be air, for air exists only in

infinitesimal quantity, if at all, in the interstellar spaces, through

which light freely penetrates. But if not air, what then? Why, clearly,

something more intangible than air; something supersensible, evading all

direct efforts to detect it, yet existing everywhere in seemingly

vacant space, and also interpenetrating the substance of all transparent

liquids and solids, if not, indeed, of all tangible substances. This

intangible something Young rechristened the Luminiferous Ether.

In the early days of his discovery Young thought of the undulations

which produce light and radiant heat as being longitudinal--a forward

and backward pulsation, corresponding to the pulsations of sound--and as

such pulsations can be transmitted by a fluid medium with the properties

of ordinary fluids, he was justified in thinking of the ether as being

like a fluid in its properties, except for its extreme intangibility.

But about 1818 the experiments of Fresnel and Arago with polarization

of light made it seem very doubtful whether the theory of longitudinal

vibrations is sufficient, and it was suggested by Young, and

independently conceived and demonstrated by Fresnel, that the

luminiferous undulations are not longitudinal, but transverse; and all

the more recent experiments have tended to confirm this view. But it

happens that ordinary fluids--gases and liquids--cannot transmit lateral

vibrations; only rigid bodies are capable of such a vibration. So

it became necessary to assume that the luminiferous ether is a body

possessing elastic rigidity--a familiar property of tangible solids, but

one quite unknown among fluids.

The idea of transverse vibrations carried with it another puzzle. Why

does not the ether, when set aquiver with the vibration which gives us

the sensation we call light, have produced in its substance subordinate

quivers, setting out at right angles from the path of the original

quiver? Such perpendicular vibrations seem not to exist, else we might

see around a corner; how explain their absence? The physicist could

think of but one way: they must assume that the ether is incompressible.

It must fill all space--at any rate, all space with which human

knowledge deals--perfectly full.

These properties of the ether, incompressibility and elastic rigidity,

are quite conceivable by themselves; but difficulties of thought appear

when we reflect upon another quality which the ether clearly

must possess--namely, frictionlessness. By hypothesis this rigid,

incompressible body pervades all space, imbedding every particle of

tangible matter; yet it seems not to retard the movements of this matter

in the slightest degree. This is undoubtedly the most difficult to

comprehend of the alleged properties of the ether. The physicist

explains it as due to the perfect elasticity of the ether, in virtue

of which it closes in behind a moving particle with a push exactly

counterbalancing the stress required to penetrate it in front.

To a person unaccustomed to think of seemingly solid matter as really

composed of particles relatively wide apart, it is hard to understand

the claim that ether penetrates the substance of solids--of glass,

for example--and, to use Young's expression, which we have previously

quoted, moves among them as freely as the wind moves through a grove

of trees. This thought, however, presents few difficulties to the mind

accustomed to philosophical speculation. But the question early arose

in the mind of Fresnel whether the ether is not considerably affected by

contact with the particles of solids. Some of his experiments led him to

believe that a portion of the ether which penetrates among the molecules

of tangible matter is held captive, so to speak, and made to move along

with these particles. He spoke of such portions of the ether as "bound"

ether, in contradistinction to the great mass of "free" ether. Half a

century after Fresnel's death, when the ether hypothesis had become

an accepted tenet of science, experiments were undertaken by Fizeau

in France, and by Clerk-Maxwell in England, to ascertain whether any

portion of ether is really thus bound to particles of matter; but the

results of the experiments were negative, and the question is still

undetermined.

While the undulatory theory of light was still fighting its way, another

kind of evidence favoring the existence of an ether was put forward by

Michael Faraday, who, in the course of his experiments in electrical and

magnetic induction, was led more and more to perceive definite lines or

channels of force in the medium subject to electro-magnetic influence.

Faraday's mind, like that of Newton and many other philosophers,

rejected the idea of action at a distance, and he felt convinced that

the phenomena of magnetism and of electric induction told strongly for

the existence of an invisible plenum everywhere in space, which might

very probably be the same plenum that carries the undulations of light

and radiant heat.

Then, about the middle of the century, came that final revolution of

thought regarding the nature of energy which we have already outlined in

the preceding chapter, and with that the case for ether was considered

to be fully established. The idea that energy is merely a "mode

of motion" (to adopt Tyndall's familiar phrase), combined with the

universal rejection of the notion of action at a distance, made the

acceptance of a plenum throughout space a necessity of thought--so, at

any rate, it has seemed to most physicists of recent decades. The proof

that all known forms of radiant energy move through space at the same

rate of speed is regarded as practically a demonstration that but one

plenum--one ether--is concerned in their transmission. It has, indeed,

been tentatively suggested, by Professor J. Oliver Lodge, that there may

be two ethers, representing the two opposite kinds of electricity, but

even the author of this hypothesis would hardly claim for it a high

degree of probability.

The most recent speculations regarding the properties of the ether have

departed but little from the early ideas of Young and Fresnel. It is

assumed on all sides that the ether is a continuous, incompressible

body, possessing rigidity and elasticity. Lord Kelvin has even

calculated the probable density of this ether, and its coefficient of

rigidity. As might be supposed, it is all but infinitely tenuous as

compared with any tangible solid, and its rigidity is but infinitesimal

as compared with that of steel. In a word, it combines properties of

tangible matter in a way not known in any tangible substance. Therefore

we cannot possibly conceive its true condition correctly. The nearest

approximation, according to Lord Kelvin, is furnished by a mould of

transparent jelly. It is a crude, inaccurate analogy, of course, the

density and resistance of jelly in particular being utterly different

from those of the ether; but the quivers that run through the jelly when

it is shaken, and the elastic tension under which it is placed when its

mass is twisted about, furnish some analogy to the quivers and strains

in the ether, which are held to constitute radiant energy, magnetism,

and electricity.

The great physicists of the day being at one regarding the existence of

this all-pervading ether, it would be a manifest presumption for any one

standing without the pale to challenge so firmly rooted a belief. And,

indeed, in any event, there seems little ground on which to base such

a challenge. Yet it may not be altogether amiss to reflect that the

physicist of to-day is no more certain of his ether than was his

predecessor of the eighteenth century of the existence of certain

alleged substances which he called phlogiston, caloric, corpuscles of

light, and magnetic and electric fluids. It would be but the repetition

of history should it chance that before the close of another century the

ether should have taken its place along with these discarded creations

of the scientific imagination of earlier generations. The philosopher of

to-day feels very sure that an ether exists; but when he says there is

"no doubt" of its existence he speaks incautiously, and steps beyond the

bounds of demonstration. He does not KNOW that action cannot take place

at a distance; he does not KNOW that empty space itself may not perform

the functions which he ascribes to his space-filling ether.

Meantime, however, the ether, be it substance or be it only dream-stuff,

is serving an admirable purpose in furnishing a fulcrum for modern

physics. Not alone to the student of energy has it proved invaluable,

but to the student of matter itself as well. Out of its hypothetical

mistiness has been reared the most tenable theory of the constitution of

ponderable matter which has yet been suggested--or, at any rate, the

one that will stand as the definitive nineteenth-century guess at

this "riddle of the ages." I mean, of course, the vortex theory of

atoms--that profound and fascinating doctrine which suggests that

matter, in all its multiform phases, is neither more nor less than ether

in motion.

The author of this wonderful conception is Lord Kelvin. The idea was

born in his mind of a happy union of mathematical calculations with

concrete experiments. The mathematical calculations were largely the

work of Hermann von Helmholtz, who, about the year 1858, had undertaken

to solve some unique problems in vortex motions. Helmholtz found that

a vortex whirl, once established in a frictionless medium, must go on,

theoretically, unchanged forever. In a limited medium such a whirl may

be V-shaped, with its ends at the surface of the medium. We may imitate

such a vortex by drawing the bowl of a spoon quickly through a cup

of water. But in a limitless medium the vortex whirl must always be

a closed ring, which may take the simple form of a hoop or circle, or

which may be indefinitely contorted, looped, or, so to speak, knotted.

Whether simple or contorted, this endless chain of whirling matter (the

particles revolving about the axis of the loop as the particles of a

string revolve when the string is rolled between the fingers) must, in

a frictionless medium, retain its form and whirl on with undiminished

speed forever.

While these theoretical calculations of Helmholtz were fresh in his

mind, Lord Kelvin (then Sir William Thomson) was shown by Professor

P. G. Tait, of Edinburgh, an apparatus constructed for the purpose

of creating vortex rings in air. The apparatus, which any one may

duplicate, consisted simply of a box with a hole bored in one side, and

a piece of canvas stretched across the opposite side in lieu of boards.

Fumes of chloride of ammonia are generated within the box, merely to

render the air visible. By tapping with the band on the canvas side

of the box, vortex rings of the clouded air are driven out, precisely

similar in appearance to those smoke-rings which some expert

tobacco-smokers can produce by tapping on their cheeks, or to those

larger ones which we sometimes see blown out from the funnel of a

locomotive.

The advantage of Professor Tait's apparatus is its manageableness and

the certainty with which the desired result can be produced. Before Lord

Kelvin's interested observation it threw out rings of various sizes,

which moved straight across the room at varying rates of speed,

according to the initial impulse, and which behaved very strangely when

coming in contact with one another. If, for example, a rapidly moving

ring overtook another moving in the same path, the one in advance seemed

to pause, and to spread out its periphery like an elastic band, while

the pursuer seemed to contract, till it actually slid through the

orifice of the other, after which each ring resumed its original size,

and continued its course as if nothing had happened. When, on the other

hand, two rings moving in slightly different directions came near each

other, they seemed to have an attraction for each other; yet if they

impinged, they bounded away, quivering like elastic solids. If an effort

were made to grasp or to cut one of these rings, the subtle thing shrank

from the contact, and slipped away as if it were alive.

And all the while the body which thus conducted itself consisted simply

of a whirl in the air, made visible, but not otherwise influenced, by

smoky fumes. Presently the friction of the surrounding air wore the

ring away, and it faded into the general atmosphere--often, however, not

until it had persisted for many seconds, and passed clear across a large

room. Clearly, if there were no friction, the ring's inertia must make

it a permanent structure. Only the frictionless medium was lacking to

fulfil all the conditions of Helmholtz's indestructible vortices. And

at once Lord Kelvin bethought him of the frictionless medium which

physicists had now begun to accept--the all-pervading ether. What

if vortex rings were started in this ether, must they not have the

properties which the vortex rings in air had exhibited--inertia,

attraction, elasticity? And are not these the properties of ordinary

tangible matter? Is it not probable, then, that what we call matter

consists merely of aggregations of infinitesimal vortex rings in the

ether?

Thus the vortex theory of atoms took form in Lord Kelvin's mind, and its

expression gave the world what many philosophers of our time regard as

the most plausible conception of the constitution of matter hitherto

formulated. It is only a theory, to be sure; its author would be the

last person to claim finality for it. "It is only a dream," Lord Kelvin

said to me, in referring to it not long ago. But it has a basis in

mathematical calculation and in analogical experiment such as no other

theory of matter can lay claim to, and it has a unifying or monistic

tendency that makes it, for the philosophical mind, little less than

fascinating. True or false, it is the definitive theory of matter of the

twentieth century.

Quite aside from the question of the exact constitution of the ultimate

particles of matter, questions as to the distribution of such particles,

their mutual relations, properties, and actions, came in for a full

share of attention during the nineteenth century, though the foundations

for the modern speculations were furnished in a previous epoch. The most

popular eighteenth-century speculation as to the ultimate constitution

of matter was that of the learned Italian priest, Roger Joseph

Boscovich, published in 1758, in his Theoria Philosophiae Naturalis.

"In this theory," according to an early commentator, "the whole mass of

which the bodies of the universe are composed is supposed to consist

of an exceedingly great yet finite number of simple, indivisible,

inextended atoms. These atoms are endued by the Creator with REPULSIVE

and ATTRACTIVE forces, which vary according to the distance. At very

small distances the particles of matter repel each other; and this

repulsive force increases beyond all limits as the distances are

diminished, and will consequently forever prevent actual contact. When

the particles of matter are removed to sensible distances, the repulsive

is exchanged for an attractive force, which decreases in inverse ratio

with the squares of the distances, and extends beyond the spheres of the

most remote comets."

This conception of the atom as a mere centre of force was hardly such

as could satisfy any mind other than the metaphysical. No one made a

conspicuous attempt to improve upon the idea, however, till just at the

close of the century, when Humphry Davy was led, in the course of

his studies of heat, to speculate as to the changes that occur in the

intimate substance of matter under altered conditions of temperature.

Davy, as we have seen, regarded heat as a manifestation of motion among

the particles of matter. As all bodies with which we come in contact

have some temperature, Davy inferred that the intimate particles of

every substance must be perpetually in a state of vibration. Such

vibrations, he believed, produced the "repulsive force" which (in common

with Boscovich) he admitted as holding the particles of matter at a

distance from one another. To heat a substance means merely to increase

the rate of vibration of its particles; thus also, plainly, increasing

the repulsive forces and expanding the bulk of the mass as a whole. If

the degree of heat applied be sufficient, the repulsive force may become

strong enough quite to overcome the attractive force, and the particles

will separate and tend to fly away from one another, the solid then

becoming a gas.

Not much attention was paid to these very suggestive ideas of Davy,

because they were founded on the idea that heat is merely a motion,

which the scientific world then repudiated; but half a century later,

when the new theories of energy had made their way, there came a revival

of practically the same ideas of the particles of matter (molecules they

were now called) which Davy had advocated. Then it was that Clausius in

Germany and Clerk-Maxwell in England took up the investigation of

what came to be known as the kinetic theory of gases--the now familiar

conception that all the phenomena of gases are due to the helter-skelter

flight of the showers of widely separated molecules of which they are

composed. The specific idea that the pressure or "spring" of gases is

due to such molecular impacts was due to Daniel Bournelli, who advanced

it early in the eighteenth century. The idea, then little noticed, had

been revived about a century later by William Herapath, and again with

some success by J. J. Waterston, of Bombay, about 1846; but it gained

no distinct footing until taken in hand by Clausius in 1857 and by

Clerk-Maxwell in 1859.

The considerations that led Clerk-Maxwell to take up the computations

may be stated in his own words, as formulated in a paper "On the Motions

and Collisions of Perfectly Elastic Spheres."

"So many of the properties of matter, especially when in the gaseous

form," he says, "can be deduced from the hypothesis that their minute

parts are in rapid motion, the velocity increasing with the temperature,

that the precise nature of this motion becomes a subject of rational

curiosity. Daniel Bournelli, Herapath, Joule, Kronig, Clausius, etc.,

have shown that the relations between pressure, temperature, and density

in a perfect gas can be explained by supposing the particles to move

with uniform velocities in straight lines, striking against the sides of

the containing vessel and thus producing pressure. It is not necessary

to suppose each particle to travel to any great distance in the same

straight line; for the effect in producing pressure will be the same

if the particles strike against each other; so that the straight line

described may be very short. M. Clausius has determined the mean length

of path in terms of the average of the particles, and the distance

between the centres of two particles when the collision takes place. We

have at present no means of ascertaining either of these distances;

but certain phenomena, such as the internal friction of gases, the

conduction of heat through a gas, and the diffusion of one gas through

another, seem to indicate the possibility of determining accurately the

mean length of path which a particle describes between two successive

collisions. In order to lay the foundation of such investigations on

strict mechanical principles, I shall demonstrate the laws of motion

of an indefinite number of small, hard, and perfectly elastic spheres

acting on one another only during impact. If the properties of such a

system of bodies are found to correspond to those of gases, an important

physical analogy will be established, which may lead to more accurate

knowledge of the properties of matter. If experiments on gases are

inconsistent with the hypothesis of these propositions, then our theory,

though consistent with itself, is proved to be incapable of explaining

the phenomena of gases. In either case it is necessary to follow out

these consequences of the hypothesis.

"Instead of saying that the particles are hard, spherical, and elastic,

we may, if we please, say the particles are centres of force, of which

the action is insensible except at a certain very small distance, when

it suddenly appears as a repulsive force of very great intensity. It is

evident that either assumption will lead to the same results. For the

sake of avoiding the repetition of a long phrase about these repulsive

bodies, I shall proceed upon the assumption of perfectly elastic

spherical bodies. If we suppose those aggregate molecules which move

together to have a bounding surface which is not spherical, then the

rotatory motion of the system will close up a certain proportion of the

whole vis viva, as has been shown by Clausius, and in this way we may

account for the value of the specific heat being greater than on the

more simple hypothesis."(1)

The elaborate investigations of Clerk-Maxwell served not merely to

substantiate the doctrine, but threw a flood of light upon the entire

subject of molecular dynamics. Soon the physicists came to feel as

certain of the existence of these showers of flying molecules making up

a gas as if they could actually see and watch their individual actions.

Through study of the viscosity of gases--that is to say, of the degree

of frictional opposition they show to an object moving through them

or to another current of gas--an idea was gained, with the aid of

mathematics, of the rate of speed at which the particles of the gas are

moving, and the number of collisions which each particle must experience

in a given time, and of the length of the average free path traversed

by the molecule between collisions, These measurements were confirmed

by study of the rate of diffusion at which different gases mix together,

and also by the rate of diffusion of heat through a gas, both these

phenomena being chiefly due to the helter-skelter flight of the

molecules.

It is sufficiently astonishing to be told that such measurements as

these have been made at all, but the astonishment grows when one hears

the results. It appears from Clerk-Maxwell's calculations that the mean

free path, or distance traversed by the molecules between collisions in

ordinary air, is about one-half-millionth of an inch; while the speed of

the molecules is such that each one experiences about eight billions

of collisions per second! It would be hard, perhaps, to cite an

illustration showing the refinements of modern physics better than

this; unless, indeed, one other result that followed directly from these

calculations be considered such--the feat, namely, of measuring the size

of the molecules themselves. Clausius was the first to point out how

this might be done from a knowledge of the length of free path; and the

calculations were made by Loschmidt in Germany and by Lord Kelvin in

England, independently.

The work is purely mathematical, of course, but the results are regarded

as unassailable; indeed, Lord Kelvin speaks of them as being absolutely

demonstrative within certain limits of accuracy. This does not mean,

however, that they show the exact dimensions of the molecule; it means

an estimate of the limits of size within which the actual size of the

molecule may lie. These limits, Lord Kelvin estimates, are about

the one-ten-millionth of a centimetre for the maximum, and the

one-one-hundred-millionth of a centimetre for the minimum. Such figures

convey no particular meaning to our blunt senses, but Lord Kelvin has

given a tangible illustration that aids the imagination to at least a

vague comprehension of the unthinkable smallness of the molecule. He

estimates that if a ball, say of water or glass, about "as large as

a football, were to be magnified up to the size of the earth, each

constituent molecule being magnified in the same proportion, the

magnified structure would be more coarse-grained than a heap of shot,

but probably less coarse-grained than a heap of footballs."

Several other methods have been employed to estimate the size of

molecules. One of these is based upon the phenomena of contact

electricity; another upon the wave-theory of light; and another upon

capillary attraction, as shown in the tense film of a soap-bubble! No

one of these methods gives results more definite than that due to the

kinetic theory of gases, just outlined; but the important thing is that

the results obtained by these different methods (all of them due to Lord

Kelvin) agree with one another in fixing the dimensions of the molecule

at somewhere about the limits already mentioned. We may feel very sure

indeed, therefore, that the molecules of matter are not the unextended,

formless points which Boscovich and his followers of the eighteenth

century thought them. But all this, it must be borne in mind, refers

to the molecule, not to the ultimate particle of matter, about which we

shall have more to say in another connection. Curiously enough, we shall

find that the latest theories as to the final term of the series are

not so very far afield from the dreamings of the eighteenth-century

philosophers; the electron of J. J. Thompson shows many points of

resemblance to the formless centre of Boscovich.

Whatever the exact form of the molecule, its outline is subject to

incessant variation; for nothing in molecular science is regarded as

more firmly established than that the molecule, under all ordinary

circumstances, is in a state of intense but variable vibration. The

entire energy of a molecule of gas, for example, is not measured by its

momentum, but by this plus its energy of vibration and rotation, due

to the collisions already referred to. Clausius has even estimated

the relative importance of these two quantities, showing that the

translational motion of a molecule of gas accounts for only three-fifths

of its kinetic energy. The total energy of the molecule (which we call

"heat") includes also another factor--namely, potential energy, or

energy of position, due to the work that has been done on expanding,

in overcoming external pressure, and internal attraction between the

molecules themselves. This potential energy (which will be recovered

when the gas contracts) is the "latent heat" of Black, which so long

puzzled the philosophers. It is latent in the same sense that the energy

of a ball thrown into the air is latent at the moment when the ball

poises at its greatest height before beginning to fall.

It thus appears that a variety of motions, real and potential, enter

into the production of the condition we term heat. It is, however,

chiefly the translational motion which is measurable as temperature;

and this, too, which most obviously determines the physical state of the

substance that the molecules collectively compose--whether, that is to

say, it shall appear to our blunt perceptions as a gas, a liquid, or a

solid. In the gaseous state, as we have seen, the translational motion

of the molecules is relatively enormous, the molecules being widely

separated. It does not follow, as we formerly supposed, that this

is evidence of a repulsive power acting between the molecules. The

physicists of to-day, headed by Lord Kelvin, decline to recognize any

such power. They hold that the molecules of a gas fly in straight lines

by virtue of their inertia, quite independently of one another, except

at times of collision, from which they rebound by virtue of their

elasticity; or on an approach to collision, in which latter case, coming

within the range of mutual attraction, two molecules may circle about

each other, as a comet circles about the sun, then rush apart again, as

the comet rushes from the sun.

It is obvious that the length of the mean free path of the molecules

of a gas may be increased indefinitely by decreasing the number of the

molecules themselves in a circumscribed space. It has been shown by

Professors Tait and Dewar that a vacuum may be produced artificially of

such a degree of rarefaction that the mean free path of the remaining

molecules is measurable in inches. The calculation is based on

experiments made with the radiometer of Professor Crookes, an instrument

which in itself is held to demonstrate the truth of the kinetic theory

of gases. Such an attenuated gas as this is considered by Professor

Crookes as constituting a fourth state of matter, which he terms

ultra-gaseous.

If, on the other hand, a gas is subjected to pressure, its molecules are

crowded closer together, and the length of their mean free path is thus

lessened. Ultimately, the pressure being sufficient, the molecules are

practically in continuous contact. Meantime the enormously increased

number of collisions has set the molecules more and more actively

vibrating, and the temperature of the gas has increased, as, indeed,

necessarily results in accordance with the law of the conservation

of energy. No amount of pressure, therefore, can suffice by itself to

reduce the gas to a liquid state. It is believed that even at the centre

of the sun, where the pressure is almost inconceivably great, all matter

is to be regarded as really gaseous, though the molecules must be so

packed together that the consistency is probably more like that of a

solid.

If, however, coincidently with the application of pressure, opportunity

be given for the excess of heat to be dissipated to a colder surrounding

medium, the molecules, giving off their excess of energy, become

relatively quiescent, and at a certain stage the gas becomes a liquid.

The exact point at which this transformation occurs, however, differs

enormously for different substances. In the case of water, for

example, it is a temperature more than four hundred degrees above zero,

centigrade; while for atmospheric air it is one hundred and ninety-four

degrees centigrade below zero, or more than a hundred and fifty degrees

below the point at which mercury freezes.

Be it high or low, the temperature above which any substance is always

a gas, regardless of pressure, is called the critical temperature, or

absolute boiling-point, of that substance. It does not follow, however,

that below this point the substance is necessarily a liquid. This is a

matter that will be determined by external conditions of pressure. Even

far below the critical temperature the molecules have an enormous degree

of activity, and tend to fly asunder, maintaining what appears to be

a gaseous, but what technically is called a vaporous, condition--the

distinction being that pressure alone suffices to reduce the vapor to

the liquid state. Thus water may change from the gaseous to the liquid

state at four hundred degrees above zero, but under conditions of

ordinary atmospheric pressure it does not do so until the temperature

is lowered three hundred degrees further. Below four hundred degrees,

however, it is technically a vapor, not a gas; but the sole difference,

it will be understood, is in the degree of molecular activity.

It thus appeared that the prevalence of water in a vaporous and liquid

rather than in a "permanently" gaseous condition here on the globe is a

mere incident of telluric evolution. Equally incidental is the fact that

the air we breathe is "permanently" gaseous and not liquid or solid,

as it might be were the earth's surface temperature to be lowered to a

degree which, in the larger view, may be regarded as trifling. Between

the atmospheric temperature in tropical and in arctic regions there is

often a variation of more than one hundred degrees; were the temperature

reduced another hundred, the point would be reached at which oxygen

gas becomes a vapor, and under increased pressure would be a liquid.

Thirty-seven degrees more would bring us to the critical temperature of

nitrogen.

Nor is this a mere theoretical assumption; it is a determination of

experimental science, quite independent of theory. The physicist in the

laboratory has produced artificial conditions of temperature enabling

him to change the state of the most persistent gases. Some fifty years

since, when the kinetic theory was in its infancy, Faraday liquefied

carbonic-acid gas, among others, and the experiments thus inaugurated

have been extended by numerous more recent investigators, notably

by Cailletet in Switzerland, by Pictet in France, and by Dr. Thomas.

Andrews and Professor James Dewar in England. In the course of these

experiments not only has air been liquefied, but hydrogen also, the most

subtle of gases; and it has been made more and more apparent that gas

and liquid are, as Andrews long ago asserted, "only distant stages of

a long series of continuous physical changes." Of course, if the

temperature be lowered still further, the liquid becomes a solid; and

this change also has been effected in the case of some of the most

"permanent" gases, including air.

The degree of cold--that is, of absence of heat--thus produced is

enormous, relatively to anything of which we have experience in nature

here at the earth now, yet the molecules of solidified air, for

example, are not absolutely quiescent. In other words, they still have

a temperature, though so very low. But it is clearly conceivable that

a stage might be reached at which the molecules became absolutely

quiescent, as regards either translational or vibratory motion. Such a

heatless condition has been approached, but as yet not quite

attained, in laboratory experiments. It is called the absolute zero

of temperature, and is estimated to be equivalent to two hundred and

seventy-three degrees Centigrade below the freezing-point of water, or

ordinary zero.

A temperature (or absence of temperature) closely approximating this

is believed to obtain in the ethereal ocean of interplanetary and

interstellar space, which transmits, but is thought not to absorb,

radiant energy. We here on the earth's surface are protected from

exposure to this cold, which would deprive every organic thing of life

almost instantaneously, solely by the thin blanket of atmosphere with

which the globe is coated. It would seem as if this atmosphere,

exposed to such a temperature at its surface, must there be incessantly

liquefied, and thus fall back like rain to be dissolved into gas again

while it still is many miles above the earth's surface. This may be the

reason why its scurrying molecules have not long ago wandered off into

space and left the world without protection.

But whether or not such liquefaction of the air now occurs in our outer

atmosphere, there can be no question as to what must occur in its entire

depth were we permanently shut off from the heating influence of the

sun, as the astronomers threaten that we may be in a future age.

Each molecule, not alone of the atmosphere, but of the entire earth's

substance, is kept aquiver by the energy which it receives, or has

received, directly or indirectly, from the sun. Left to itself, each

molecule would wear out its energy and fritter it off into the

space about it, ultimately running completely down, as surely as any

human-made machine whose power is not from time to time restored. If,

then, it shall come to pass in some future age that the sun's rays

fail us, the temperature of the globe must gradually sink towards the

absolute zero. That is to say, the molecules of gas which now fly about

at such inconceivable speed must drop helpless to the earth; liquids

must in turn become solids; and solids themselves, their molecular

quivers utterly stilled, may perhaps take on properties the nature of

which we cannot surmise.

Yet even then, according to the current hypothesis, the heatless

molecule will still be a thing instinct with life. Its vortex whirl will

still go on, uninfluenced by the dying-out of those subordinate quivers

that produced the transitory effect which we call temperature. For those

transitory thrills, though determining the physical state of matter as

measured by our crude organs of sense, were no more than non-essential

incidents; but the vortex whirl is the essence of matter itself. Some

estimates as to the exact character of this intramolecular motion,

together with recent theories as to the actual structure of the

molecule, will claim our attention in a later volume. We shall also

have occasion in another connection to make fuller inquiry as to the

phenomena of low temperature.

APPENDIX

REFERENCE-LIST

CHAPTER I

THE SUCCESSORS OF NEWTON IN ASTRONOMY (1) (p. 10). An Account of Several

Extraordinary Meteors or Lights in the Sky, by Dr. Edmund Halley. Phil.

Trans. of Royal Society of London, vol. XXIX, pp. 159-162. Read before

the Royal Society in the autumn of 1714. (2) (p. 13). Phil. Trans. of

Royal Society of London for 1748, vol. XLV., pp. 8, 9. From A Letter to

the Right Honorable George, Earl of Macclesfield, concerning an Apparent

Motion observed in some of the Fixed Stars, by James Bradley, D.D.,

Astronomer Royal and F.R.S.

CHAPTER II

THE PROGRESS OF MODERN ASTRONOMY

(1) (p. 25). William Herschel, Phil. Trans. for 1783, vol. LXXIII. (2)

(p. 30). Kant's Cosmogony, ed. and trans. by W. Hartie, D.D., Glasgow,

900, pp. 74-81. (3) (p. 39). Exposition du systeme du monde (included in

oeuvres Completes), by M. le Marquis de Laplace, vol. VI., p. 498. (4)

(p. 48). From The Scientific Papers of J. Clerk-Maxwell, edited by W.

D. Nevin, M.A. (2 vols.), vol. I., pp. 372-374. This is a reprint of

Clerk-Maxwell's prize paper of 1859.

CHAPTER III

THE NEW SCIENCE OF PALEONTOLOGY

(1) (p. 81). Baron de Cuvier, Theory of the Earth, New York, 1818, p.

98. (2) (p. 88). Charles Lyell, Principles of Geology (4 vols.),

London, 1834. (p. 92). Ibid., vol. III., pp. 596-598. (4) (p. 100). Hugh

Falconer, in Paleontological Memoirs, vol. II., p. 596. (5) (p. 101).

Ibid., p. 598. (6) (p. 102). Ibid., p. 599. (7) (p. 111). Fossil Horses

in America (reprinted from American Naturalist, vol. VIII., May, 1874),

by O. C. Marsh, pp. 288, 289.

CHAPTER IV

THE ORIGIN AND DEVELOPMENT OF MODERN GEOLOGY

(1) (p. 123). James Hutton, from Transactions of the Royal Society of

Edinburgh, 1788, vol. I., p. 214. A paper on the "Theory of the Earth,"

read before the Society in 1781. (2) (p. 128). Ibid., p. 216. (3)

(p. 139). Consideration on Volcanoes, by G. Poulett Scrope, Esq., pp.

228-234. (4) (p. 153). L. Agassiz, Etudes sur les glaciers, Neufchatel,

1840, p. 240.

CHAPTER V

THE NEW SCIENCE OF METEOROLOGY

(1) (p. 182). Theory of Rain, by James Hutton, in Transactions of the

Royal Society of Edinburgh, 1788, vol. 1, pp. 53-56. (2) (p. 191). Essay

on Dew, by W. C. Wells, M.D., F.R.S., London, 1818, pp. 124 f.

CHAPTER VI

MODERN THEORIES OF HEAT AND LIGHT

(1) (p. 215). Essays Political, Economical, and Philosophical, by

Benjamin Thompson, Count of Rumford (2 vols.), Vol. II., pp. 470-493,

London; T. Cadell, Jr., and W. Davies, 1797. (2) (p. 220). Thomas Young,

Phil. Trans., 1802, p. 35. (3) (p. 223). Ibid., p. 36.

CHAPTER VII

THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM

(1) (p. 235). Davy's paper before Royal Institution, 1810. (2) (p. 238).

Hans Christian Oersted, Experiments with the Effects of the Electric

Current on the Magnetic Needle, 1815. (3) (p. 243). On the Induction

of Electric Currents, by Michael Faraday, F.R.S., Phil. Trans. of Royal

Society of London for 1832, pp. 126-128. (4) (p. 245). Explication of

Arago's Magnetic Phenomena, by Michael Faraday, F.R.S., Phil. Trans.

Royal Society of London for 1832, pp. 146-149.

CHAPTER VIII

THE CONSERVATION OF ENERGY

(1) (p. 267). The Forces of Inorganic Nature, a paper by Dr. Julius

Robert Mayer, Liebig's Annalen, 1842. (2) (p. 272). On the Calorific

Effects of Magneto-Electricity and the Mechanical Value of Heat, by J.

P. Joule, in Report of the British Association for the Advancement of

Science, vol. XII., p. 33.

CHAPTER IX

THE ETHER AND PONDERABLE MATTER

(1) (p. 297). James Clerk-Maxwell, Philosophical Magazine for January

and July, 1860.

END OF VOL. III