I.

Man is prone to idealization. He cannot accept as final the phenomena of

the sensible world, but looks behind that world into another which rules

the sensible one. From this tendency of the human mind, systems of

mythology and scientific theories have equally sprung. By the former the

experiences of volition, passion, power, and design, manifested among

ourselves, were transplanted, with the necessary modifications, into an

unseen universe from which the sway and potency of those magnified human

qualities were exerted. "In the roar of thunder and in the violence of

the storm was felt the presence of a shouter and furious strikers, and

out of the rain was created an Indra or giver of rain." It is

substantially the same with science, the principal force of which is

expended in endeavoring to rend the veil which separates the sensible

world from an ultra-sensible one. In both cases our materials, drawn

from the world of the senses, are modified by the imagination to suit

intellectual needs. The "first beginnings" of Lucretius were not objects

of sense, but they were suggested and illustrated by objects of sense.

The idea of atoms proved an early want on the part of minds in pursuit

of the knowledge of nature. It has never been relinquished, and in our

own day it is growing steadily in power and precision.

The union of bodies in fixed and multiple proportions constitutes the

basis of modern atomic theory. The same compound retains, for ever, the

same elements, in an unalterable ratio. We cannot produce pure water

containing one part, by weight, of hydrogen and nine of oxygen, nor can

we produce it when the ratio is one to ten; but we can produce it from

the ratio of one to eight, and from no other. So also when water is

decomposed by the electric current, the proportion, as regards volumes,

is as fixed as in the case of weights. Two volumes of hydrogen and one

of oxygen invariably go the formation of water. Number and harmony, as

in the Pythagorean system, are everywhere dominant in this under-world.

Following the discovery of fixed proportions we have that of _multiple_

proportions. For the same compound, as above stated, the elementary

factors are constant; but one elementary body often unites with another

so as to form different compounds. Water, for example, is an oxide of

hydrogen; but a peroxide of that substance also exists, containing

exactly double the quantity of oxygen. Nitrogen also unites with oxygen

in various ratios, but not in all. The union takes place, not gradually

and uniformly, but by steps, a definite weight of matter being added at

each step. The larger combining quantities of oxygen are thus multiples

of the smaller ones. It is the same with other combinations.

We remain thus far in the region of fact: why not rest there? It might

as well be asked why we do not, like our poor relations of the woods and

forests, rest content with the facts of the sensible world. In virtue of

our mental idiosyncrasy, we demand _why_ bodies should combine in

multiple proportions, and the outcome and answer of this question is the

atomic theory. The definite weights of matter, above referred to,

represent the weights of atoms, indivisible by any force which chemistry

has hitherto brought to bear upon them. If matter were a _continuum_--if

it were not rounded off, so to say, into these discrete atomic

masses--the impassable breaches of continuity which the law of multiple

proportions reveals, could not be accounted for. These atoms are what

Maxwell finely calls "the foundation stones of the material universe,"

which, amid the wreck of composite matter, "remain unbroken and unworn."

A group of atoms drawn and held together by what chemists term affinity

is called a molecule. The ultimate parts of all compound bodies are

molecules. A molecule of water, for example, consists of two atoms of

hydrogen, which grasp and are grasped by one atom of oxygen. When water

is converted into steam, the distances between the molecules are greatly

augmented, but the molecules themselves continue intact. We must not,

however, picture the constituent atoms of any molecule as held so

rigidly together as to render intestine motion impossible. The

interlocked atoms have still liberty of vibration, which may, under

certain circumstances, become so intense as to shake the molecule

asunder. Most molecules--probably all--are wrecked by intense heat, or

in other words by intense vibratory motion; and many are wrecked by a

very moderate heat of the proper quality. Indeed, a weak force, which

bears a suitable relation to the constitution of the molecule, can, by

timely savings and accumulations, accomplish what a strong force out of

relation fails to achieve.

We have here a glimpse of the world in which the physical philosopher

for the most part resides. Science has been defined as "organized common

sense;" by whom I have forgotten; but, unless we stretch unduly the

definition of common sense, I think it is hardly applicable to this

world of molecules. I should be inclined to ascribe the creation of that

world to inspiration rather than to what is currently known as common

sense. For the natural history sciences the definition may stand--hardly

for the physical and mathematical sciences.

The sensation of light is produced by a succession of waves which strike

the retina in periodic intervals; and such waves, impinging on the

molecules of bodies, agitate their constituent atoms. These atoms are so

small, and, when grouped to molecules, are so tightly clasped together,

that they are capable of tremors equal in rapidity to those of light and

radiant heat. To a mind coming freshly to these subjects, the numbers

with which scientific men here habitually deal must appear utterly

fantastical; and yet, to minds trained in the logic of science, they

express most sober and certain truth. The constituent atoms of molecules

can vibrate to and fro millions of millions of times in a second. The

waves of light and of radiant heat follow each other at similar rates

through the luminiferous ether. Further, the atoms of different

molecules are held together with varying degrees of tightness--they are

tuned, as it were, to notes of different pitch. Suppose, then,

light-waves, or heat-waves, to impinge upon an assemblage of such

molecules, what may be expected to occur? The same as what occurs when a

piano is opened and sung into. The waves of sound select the strings

which respectively respond to them--the strings, that is to say, whose

rates of vibration are the same as their own--and of the general series

of strings these only sound. The vibratory motion of the voice, imparted

first to the air, is here taken up by the strings. It may be regarded as

_absorbed_, each string constituting itself thereby a new center of

motion. Thus also, as regards the tightly locked atoms of molecules on

which waves of light or radiant heat impinge. Like the waves of sound

just adverted to, the waves of ether select those atoms whose periods of

vibration synchronize with their own periods of recurrence, and to such

atoms deliver up their motion. It is thus that light and radiant heat

are absorbed.

And here the statement, though elementary, must not be omitted, that the

colors of the prismatic spectrum, which are presented in an impure form

in the rainbow, are due to different rates of atomic vibration in their

source, the sun. From the extreme red to the extreme violet, between

which are embraced all colors visible to the human eye, the rapidity of

vibration steadily increases, the length of the waves of ether produced

by these vibrations diminishing in the same proportion. I say "visible

to the human eye," because there may be eyes capable of receiving visual

impression from waves which do not affect ours. There is a vast store of

rays, or more correctly waves, beyond the red, and also beyond the

violet, which are incompetent to excite our vision; so that could the

whole length of the spectrum, visible and invisible, be seen by the same

eye, its length would be vastly augmented.

I have spoken of molecules being wrecked by a moderate amount of heat of

the proper quality: let us examine this point for a moment. There is a

liquid called nitrite of amyl--frequently administered to patients

suffering from heart disease. The liquid is volatile, and its vapor is

usually inhaled by the patient. Let a quantity of this vapor be

introduced into a wide glass tube, and let a concentrated beam of solar

light be sent through the tube along its axis. Prior to the entry of the

beam, the vapor is as invisible as the purest air. When the light

enters, a bright cloud is immediately precipitated on the beam. This is

entirely due to the waves of light, which wreck the nitrite of amyl

molecules, the products of decomposition forming innumerable liquid

particles which constitute the cloud. Many other gases and vapors are

acted upon in a similar manner. Now the waves that produce this

decomposition are by no means the most powerful of those emitted by the

sun. It is, for example, possible to gather up the ultra-red waves into

a concentrated beam, and to send it through the vapor, like the beam of

light. But, though possessing vastly greater energy than the light

waves, they fail to produce decomposition. Hence the justification of

the statement already made, that a suitable relation must subsist

between the molecules and the waves of ether to render the latter

effectual.

A very impressive illustration of the decomposing power of the waves of

light is here purposely chosen; but the processes of photography

illustrate the same principle. The photographer, without fear,

illuminates his developing room with light transmitted through red or

yellow glass; but he dares not use blue glass, for blue light would

decompose his chemicals. And yet the waves of red light, measured by the

amount of energy which they carry, are immensely more powerful than the

waves of blue. The blue rays are usually called chemical rays--a

misleading term; for, as Draper and others have taught us, the rays that

produce the grandest chemical effects in nature, by decomposing the

carbonic acid and water which form the nutriment of plants, are not the

blue ones. In regard, however, to the salts of silver, and many other

compounds, the blue rays are the most effectual. How is it then that

weak waves can produce effects which strong waves are incompetent to

produce? This is a feature characteristic of periodic motion. In the

experiment of singing into an open piano already referred to, it is the

accord subsisting between the vibrations of the voice and those of the

string that causes the latter to sound. Were this accord absent, the

intensity of the voice might be quintupled, without producing any

response. But when voice and string are identical in pitch, the

successive impulses add themselves together, and this addition renders

them, in the aggregate, powerful, though individually they may be weak.

It some such fashion the periodic strokes of the smaller ether waves

accumulate, till the atoms on which their timed impulses impinge are

jerked asunder, and what we call chemical decomposition ensues.

Savart was the first to show the influence of musical sounds upon liquid

jets, and I have now to describe an experiment belonging to this class,

which bears upon the present question. From a screw-tap in my little

Alpine kitchen I permitted, an hour ago, a vein of water to descend into

a trough, so arranging the flow that the jet was steady and continuous

from top to bottom. A slight diminution of the orifice caused the

continuous portion of the vein to shorten, the part further down

resolving itself into drops. In my experiment, however, the vein, before

it broke, was intersected by the bottom of the trough. Shouting near the

descending jet produced no sensible effect upon it. The higher notes of

the voice, however powerful, were also ineffectual. But when the voice

was lowered to about 130 vibrations a second, the feeblest utterance of

this note sufficed to shorten, by one half, the continuous portion of

the jet. The responsive drops ran along the vein, pattered against the

trough, and scattered a copious spray round their place of impact. When

the note ceased, the continuity and steadiness of the vein were

immediately restored. The formation of the drops was here periodic; and

when the vibrations of the note accurately synchronized with the periods

of the drops, the waves of sound aided what Plateau has proved to be the

natural tendency of the liquid cylinder to resolve itself into

spherules, and virtually decomposed the vein.

I have stated, without proof, that where absorption occurs, the motion

of the ether-waves is taken up by the constituent atoms of molecules. It

is conceivable that the ether-waves, in passing through an assemblage of

molecules, might deliver up their motion to each molecule as a whole,

leaving the relative positions of the constituent atoms unchanged. But

the long series of reactions, represented by the deportment of nitrite

of amyl vapor, does not favor this conception; for, were the atoms

animated solely by a common motion, the molecules would not be

decomposed. The fact of decomposition, then, goes to prove the atoms to

be the seat of the absorption. They, in great part, take up the energy

of the ether-waves, whereby their union is severed, and the building

materials of the molecules are scattered abroad.

Molecules differ in stability; some of them, though hit by waves of

considerable force, and taking up the motions of these waves,

nevertheless hold their own with a tenacity which defies decomposition.

And here, in passing, I may say that it would give me extreme pleasure

to be able to point to my researches in confirmation of the solar theory

recently enunciated by my friend the President of the British

Association. But though the experiments which I have made on the

decomposition of vapors by light might be numbered by the thousand, I

have, to my regret, encountered no fact which prove that free aqueous

vapor is decomposed by the solar rays, or that the sun is reheated by

the combination of gases, in the severance of which it had previously

sacrificed its heat.