143. But, in speculations like these, we have soared far beyond that which may be called the first refinement on ordinary gross matter; i.e. the luminiferous, probably also the electric and magnetic, medium, provisionally the Ether.
To the consideration of its principal properties we now turn our attention.
These are, at first sight at least, of an apparently incongruous character; for, from one point of view, the ether appears as a fluid, from another as an elastic solid. Nothing is more certainly established in physical astronomy than the excessive minuteness of the resistance offered by the ether to the planetary motions, if, indeed, there be such a resistance at all appretiable, even when the velocity is, as in the case of the earth, somewhere about 100,000 feet per second! On the other hand, we learn from physical optics that light, transmitted with a velocity of 188,000 miles per second, depends upon transverse disturbances of some kind or other; while several optical phenomena indicate that a disturbance of the nature of compression (if such be possible) would be transmitted with velocity almost infinitely great, in comparison even with this enormous velocity.
144. Stokes, however, has given a very ingenious illustration which enables us to see that such an extraordinary combination of apparently irreconcilable properties is by no means without analogy, even in common matter. He takes the case of a solution of glue, or isinglass, or jelly, in different relative amounts of water. When the quantity of water is small, we have the elastic solid; when large, a liquid little different from water. And Stokes shows that it is excessively improbable that there is any definite intermediate stage which we could assign as that at which the transition from the solid to the liquid takes place. Of course, any such analogy must necessarily be excessively imperfect; but a great deal is gained by our being able to trace even a very imperfect analogy in a case like this.
145. The ether, in fact, must be distorted as well as displaced by matter passing through it; but any distortion of the nature of a shear, such as would give rise in water to vortex-motion accompanied by friction (the whole energy being thus ultimately frittered down into heat), would in the ether be handed on at once, as vibratory motion, with the velocity of light. Thus vortex-motion of the ether may be conceived to be impossible, simply in consequence of the minuteness of its density in comparison with the great tangential force called into play by a shear; and a body moving in it with a velocity not so great as that of light would thus not have eddies in its wake, as in an ordinary fluid, but, on the contrary, would be a source of radiation, even although there may have been no heating either of the body or of the medium it is displacing, paradoxical as this result may appear. In this connection it is hardly possible to avoid quoting Milton—though there may be a suspicion of something analogous to a pun:—
‘ The grinding sword with discontinuous wound
Passed through him—but the ethereal substance closed
Not long divisible.’
146. Sir William Thomson has endeavoured to obtain at least an inferior limit to the density of the ether in planetary space. His method is based upon the measurements by Pouillet and Herschel of the whole amount of radiant energy received from the sun by a given amount of terrestrial surface in a given time, and upon an assumption that the extreme amplitude of distortion of the ether in any radiation is small compared with the length of a wave. In this way he finds that, as a cubic mile of the ether near the earth contains about 12,000 foot-pounds of radiant solar energy, the mass of the ether in that cubic mile must be at least 1 1,000,000,000 of a pound.[48] To show that this is not by any means a surprisingly small quantity he compares it with the mass of a cubic mile of air at a distance of only a few radii from the earth’s surface (supposing that the atmosphere extends so far; which, by the way, the recent calculations of the velocities of the particles of a gas render exceedingly improbable). This, he finds, will be probably represented by a fraction of a pound having unit for a numerator and 329 places of figures in the denominator!!!
147. In a very remarkable paper by Struve,[49] an attempt was made to settle the question, Is the ether perfectly transparent? or, as we may now put it, Is any radiant energy absorbed by the ether, whether to produce other forms of energy, or to be dissipated by radiation in all directions? Long ago it had been pointed out by Olbers and others, that if the stars be infinite in number, and be distributed with anything roughly approximating to an average density through infinite space, the sky ought, night and day, to be all over of a brightness of the same order as that of the sun. Is the number of stars, then, finite; or does the ether absorb their light? Now, it need not in the least surprise us to find that the number of stars is finite, even though matter be infinite in quantity, and distributed with something like uniformity through infinite space. For only a finite portion of it may yet have fallen together so as to produce incandescent bodies; or, the other extreme, only a finite portion of it may be left incandescent. Either of these altogether different hypotheses is perfectly reasonable and scientifically justifiable; so that, from this point of view, we are not at present likely to obtain any information. Struve’s reasoning, which, by the way, is not accepted by Sir J. Herschel, introduces another consideration, viz., the number of stars of each visible magnitude. To apply this: suppose for a moment we make the assumption (actually measured values of annual parallax show it is certainly at best a very rough one) that the brighter stars are the nearer, and that a set of stars, on the average one-fourth as bright as another set, are on the average twice as far off, etc. A great deal of what we know to be certainly false is here assumed as true, but it is possible that the general accuracy of the results of the reasoning from it may not be thereby much affected. On the supposition of a sort of rough uniformity of distribution through space, we can easily calculate approximately what ought to be the relative numbers of the stars, classed by astronomers as of the various different magnitudes, once we have obtained (as it is not difficult to do) an estimate of the relative brightness of typical stars of these (arbitrary) magnitudes. From their brightness we calculate at once their relative distances, and thence (according to our hypothesis of approximately uniform distribution) what ought to be the relative numbers of each magnitude. When this is done, it appears that there is a great excess of the calculated over the observed numbers, at least for telescopic stars, and the greater the smaller the magnitude. This is the gist of Struve’s method, and he arrives at the result that the light of stars of the sixth magnitude (the smallest visible to an ordinary unaided eye, and whose average distance from us is supposed to be somewhere about ninefold that of stars of the first magnitude) loses about eight per cent. in its passage to the earth. Thus the light of stars of the first magnitude does not lose so much as one per cent.; but, on the other hand, stars of the ninth magnitude are enfeebled to the extent of about 30 per cent. Struve shows that, if his result is to be accepted, W. Herschel’s idea that his 40-foot telescope would show him stars seven times farther off than those visible with the 10-foot, was erroneous. He would, in fact, have been able to see little more than twice as far.