Perhaps the best way to convey some idea of this order of magnitudes to the ordinary reader is to quote Sir Wm. Thomson's illustration, that if we could suppose a cubic inch of water magnified to the size of the earth - i.e., to a sphere 24,000 miles in circumference - the dimensions of its ultimate particles, etherons, magnified on the same scale, or, as he expresses it, its degree of coarse-grainedness, would be something between the size of rifle-bullets and cricket-balls.
Extraordinary as these dimensions are, they are not more so than those at the opposite extremity of the scale, where the distance of stars and nebulae has to be measured by the number of thousand years their light, traveling at the rate of 192,000 miles per second, takes to reach us. Infinitely small, however, as those dimensions appear to our original conceptions derived from our natural senses, they are certain and ascertained facts, if not as to the precise figures, yet beyond all doubt as to the orders of magnitude. In dealing with them also we are to a great extent on familiar ground. Molecules are nothing more or less than small pieces of ordinary matter; and atoms are also matter, for they obey the law of gravity, have definite weights, and build up molecules as surely as molecules build up ordinary matter, and as squared stones build up pyramids.
But to understand the constitution of the material universe we must go a step further, part from the familiar world of sense, and deal with an all-prevading medium, which is at the same time matter and not matter, which lies outside the laws of gravity, and yet obeys other laws intelligible and calculable by us; of which it may be said we know it and we know it not. We call it Ether.
Ether is a medium assumed as a necessary consequence from the phenomena of light, heat, and elctricity - primarily from those of light. Respecting light two facts are known to us with absolute certainty.
1st. It traverses space at a rate of 192,000 miles per second.
2nd. It is propagated not by particles actually traveling at this rate, but, like sound through air, by the transmission of waves.
The first fact is known from the difference of time at which eclipses of Jupiter's satellites are seen according as the earth is at the point of its orbit nearest to or farthest from Jupiter - i.e., from the time light takes to traverse the diameter of the earth's orbit, which is about 180 millions of miles; and this velocity of light is confirmed by direct experiments, as by noting the difference of time between seeing the flash and hearing the sound of a gun, which gives the velocity of light compared with the known velocity of sound.
The second fact is equally certain from the phenomena of what are called interferences, when the crest of one wave just overtakes the hollow of a preceding one, so that, if the two waves are of equal magnitude, the oscillations exactly neutralize one another, and two lights produce darkness. This is shown in a thousand different ways, and for all the different colors depending on different waves into which white light is analyzed when passed through a prism. It is a certain result of wave-motion, and of wave-motion only, and therefore we know without a doubt that light is propagated by waves.
But waves inply a medium through which waveforms are transmitted, for waves are nothing but the rhythmic motion of something which rises and falls, or oscillates symmetrically about a mean position of rest, slowly or quickly according to the less or greater elasticity of the medium. The waves which run along a large and slack wire are large and slow, those along a small and tightly stretched wire are small and quick; and from the data we possess as to light, its velocity of transmission, its refraction when its waves pass from one medium into another of different density, and from the distance between the waves as shown by interference, it is easy to calculate the lengths and vibratory periods of the waves, and the elasticity of the medium through which such waves are transmitted.
The figures at which we arrive are truly extra ordinary. The dimensions and rates of oscillations of the waves which produce the different colors of visible light have been measured and calculated with the greatest accuracy, and they are as follows:
| Color | # of Waves in | No. of oscillations |
| one inch | in one second | |
| Red | 39,000 | 477,000,000,000,000 |
| Orange | 42,000 | 506,000,000,000,000 |
| Yellow | 44,000 | 535,000,000,000,000 |
| Green | 47,000 | 577,000,000,000,000 |
| Blue | 51,000 | 622,000,000,000,000 |
| Indigo | 54,000 | 658,000,000,000,000 |
| Violet | 57,000 | 699,000,000,000,000 |
The elasticity of this wonderful medium is even more extraordinary.
The rapidity with which wave-motion is transmitted depends, other things being equal, on the elasticity of the medium, which is proportional to the square of the velocity with which a wave travels through it. As the velocity of the sound-wave in air is about 1,100 feet a second, and that of the light-wave about 192,000 miles in the same time, it follows that the velocity of the latter is about a million times greater than that of the former, and if the density of ether were the same as that of air, its elasticity must be about a million million times greater. But the elasticity is the same thing as the power of resisting compression, which in the case of air we know to be about 15 pounds to the square inch; so that the ether, if equally dense, would balance a pressure of 15 million million pounds to the square inch - that is, it would require a pressure of about 750 millions of tons to the square inch to condense ether to the density of the air. On the other hand, its density, if any, must be so infinitesimally small that the earth moving through it in its orbit with a velocity of 1,100 miles a minute suffers no perceptible retardation.
Consider what this means. Air blowing at the rate of 100 miles an hour is a hurricane uprooting trees and levelling houses. If ether were as dense as air the resistance to the earth in passing through it would be 600 times that of going dead to wind in a tropical hurricane. But in point of fact there is no sensible resistance, for the earth and heavenly bodies move in their calculated paths according to the law of gravity exactly asthey would do if they were moving in a vacuum. Even the comets, which consist of such excessively rare matter that when one of them got entangled among the satelittes of Jupiter it did not affect their movements, are not retarded by the ether, or so slightly, that any retardation in the case of one or two ofthem is suspected rather than proved. But, if the ether has no weight, how can we call it material, weight being, as we have seen, the invariable test and measure of all matter down to the minutest atom? And yet how can we deny its existence when it is demonstrably necessary to account for undoubted facts revealed to us every day by the prism, the spectroscope, electricity, and chemical action, and deductions from these facts based on the strict laws of mathematical calculation? For the existence of the ether is not based only on phenomena of light: it is an equally necessary postulate to explain those of heat, electricity, and chemical action. We must conceive of our atoms and molecules as forming systems and performing their movements, not in vacuo, but in an all-prevading medium of this ether, to which they impart, andfrom which they receive, impulses.
These impulses are excessively minute, and when they occur in irregular order they produce no appreciable effect; but when the vibrations of the ether keep time with those of the atoms, the multitude of small effects becomes summed up into one considerable enough to produce great changes. Just so a rhythmic succession of tiny ripples may set a heavy buoy oscillating, and the footfalls of a regiment of soldiers marching over a suspension-bridge may make it swing until it breaks down, while a confused mob could traverse it in safety. The latter affords a good illustration of the way in which molecular structures may be broken down, and their atoms set free to enter into other combinations, by the action of heat, light, or chemical rays beyond the visible end of the spectrum.
Conversely the phenomena of the spectroscope all depend on the fact that the vibrations of atoms and molecules can propagate waves through the ether, as well as absorb ether-waves into their own motions, and thus spectra distinquished by bright or dark lines peculiar to each substance, by which it can be identified. Whatever ether may be, this much is certain about it: it prevades all space. That it extends to the boundaries of the infinitely great we know from the fact that light reaches us from the remotest stars and nebulae, andthat in this light the spectroscope enables us to detect waves propagated and absorbed by the very same vibrations ofthe same familiar atoms at the enormous distances as at the earth's surface. Glowing hydrogen, for instance, is a principal ingredient of the sun's atmosphere and ofthose distant suns we call stars, and it affects the ether and is affected by it exactly in the same manner asthe hydrogen burning in an ordinary gas-lamp.
In the direction also of the infinitely small, ether permeates the apparently solid structure of crystals, whose molecules perform their limited and rigidly definite movements in an atmosphere of it, as is shown by the fact that in so many cases light and heat penetrate through them. A whole series of remarkable phenomena arise from the manner in which the vibrations of ether which cause light are affected by the structure of the molecules of crystals through which they pass. In certain cases they are what is called polarised, or so affected that while they pass freely if the crystal is held in one direction, they are stopped if it is turned round through an angle of 90° to its former position, so that one and the same crystal may be alternately transparent and non-transparent. It would seem as if its structure were like that of wood, grained, and more easy to penetrate if cut with the grain than against it, so that when a ray of light attempted to penetrate, its vibrations were resolved into two, one with the grain which got through, the other against it which was suppressed; so that the emeging ray, which entered with a circular vibration, got out with only one rectilinear vibration parallel to the diameter which coincided with the grain.
Other crystals of more complicated structure affect transmitted light in a more complex way, developing a double polarity very similar to that induced in the iron filings when brought under the influence of the two poles of the magnet. With this polarised light the most beautiful colored rings can be produced from the waves of the different colors into which the white (undifferentiated) light has been analyzed in passing through the crystal, which alternately flash out and disappear as the cyrstal is turned round its axis, and which present a remarkable analogy to the curves into which the iron filings form themselves under the single or double poles of the magnet.
The importance of this will appear afterwards, but for the present it is sufficient to show that the waves of ether which cause light really penetrate through the molecules of crystals, but in doing so may be affected by them.
An attempt has recently been made, based on abstruse mathematical calculations, to carry our nowledge of the constitution of matter one step further back, and identify atoms with ether. This is attempted by the vortex theory of Helmholz, Sir W. Thomson, and Professor Tait. It is singular how some of the ultimate facts discovered by the refinements of science correspond with some of the most trivial amusements. Thus the blowing of soap-bubbles gives the best clue to the movement of waves of light, and through them to the dimensions of molecules and atoms; and the collision of billiard-balls, knocked about at random, to the movements ofthose minute bodies, and the kinetic theory of gases. In the case of the vortex theory the idea is given by the rings of smoke which certain adroit smokers amuse themselves by puffing into the air. These rings float for a considerable time, retaining their circular form, and showing their elasticity by oscillating about it and returning to it if their form is altered, and by rebounding and vibrating energetically, just as two solid bodies would do, if two rings come into collision. If we try to cut them in two, they recede before the knife, or bend around it, returning, when the external force is removed, to their original form without the loss of a single particle, and preserving their own individuality through every change of form and of velocity. This persistance of form they owe to the fact that their particles are revolving in small circles at right angles to the axis or circumference of the larger circle which forms the ring; motion thus giving them stability, very much as in the familiar instance of the bicycle. They burst at last because they are formed and rotate in the air, which is a resisting medium; but mathematical callculation shows that in a perfect fluid free from all friction these vortex rings would be indivisible and indestructible: in other words, they would be atoms.
The vortex theory assumes, therefore, that the universe consists of one uniform primary substance, a fluid which fills all space, and that what we call matter consists of portions ofthis fluid which have become animated with vortex motion. The innumerable atoms which form molecules, and through molecules all the diversified forms of matter of the material universe, are therefore simply so many vortex rings, each perfectly limited, distinct, and indestructible, both as to its form, mass, and mode of motion. They cannot change or disappear, nor can they be formed spontaneously. Those of the same kind are constituted after the same fashion, and therefore are endowed with the same properties.
The theory is a plausible one, and the reputation of its authors must command for it respectful consideration; but it is as yet a long way from being an established theory which can be accepted as a true representation of facts. In the first place it is based solely on mathematical theory, and not, as in the case of atoms and light-waves, upon actual facts of weight and measurement tested by experiment, andto which mathematical reasoning affords only an aid and supplement. No one has proved the existence of such a medium or of such vortex rings, much less weighed or measured them.
Moreover the theory is open to some very obvious objections. How can aggregations of imponderable matter acquire weight, and become subject to the law of gravity, which, as we have seen, is one of the essential and permanent qualities of atoms? If a cubic millionth of a millimeter of ether formed into a big vortex ring of, say, an atom of mercury, has a weight equal to 200 times that of an atom of hydrogen, which itself has a definite weight, why has it no weight in its original form? And if it had weight, however small, how could the enormous mass of ether filling all space produce no perceptible effect on bodies, even of attenuated cometic vapor, revolving through it with immense velocities? Again, how could these innumerable vortex rings be formed out of the ether without disturbing the uniformity and continuity of the medium, which are essential for the propagation of the light-waves through it? And how could the motions requisite to form the vortex rings be impressed on the them de novo consistently with the principle of the conservation of energy? Energy can no more be created out of nothing than matter, by any process known in nature or conceivable by the human intellect; and to assume it is simply a more refined manner of falling back on the supernatural, which is itself only a more refined manner of saying that we know nothing.
For the present, therefore, we must be content with atoms and ether as the ultimate terms or our knowledge of the material or quasi-material components of the universe. (Laing, Stephen; A Modern Zoroastrian; Chapman and Hall, London, 1891)
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