doubts that matter can act where it is not. What light have recent researches shed upon this interesting question, heretofore little more than metaphysical?
The solar atmosphere has been found to extend to more than half a radius beyond its surface; at the top of its corona, high above the hydrogen, there are vast masses of a gas which emits a simple, green ray, not corresponding with that of any known substance. In auroral displays on earth, in the uppermost regions of our atmosphere, the same simple ray has been detected; whence it has been supposed that atmospheres are not restrictedly planetary nor solar, but continuous and cosmical; and that it may be a gas indefinitely rarefied that conveys to us through the depths of space not only light-motion, but the yet more inappreciable tremors of electricity and gravitation.
The ordinary definitions of the interstellar ether are open to the objections urged by Mill, because of a dread there seems to be abroad of ascribing materiality to it; while its infinitesimal materiality is not only within the bounds of possibility, but well agrees with the facts. All motion takes time; light has a measurable velocity; chemical action of the most violent kind and even explosions are not instantaneous. Were it otherwise, the hypothesis of no medium or of an immaterial one might be entertained. Now, the decidedness in amount of a body's weight as a mass, or in its particles, has no necessary connection with its efficiency as a medium of motion. Just the reverse: we find that as matter is smaller and lighter in its ultimate parts or gross masses, the more rapidly can it communicate motion, and the greater is its capacity for motion. It is a familiar fact that, in the use of machinery, a small wheel can, proportionately to its weight, contain and transmit more motion than a large one, the plain reason of which is that it can be driven at a higher peripheral speed, its smaller bulk causing less centrifugal strain at the axis than if it were larger.
Sound travels nearly four times faster in hydrogen than in air, and in quickness of elastic recoil it is, when compressed, preferable to air in the same degree. Its extraordinary chemical energy, far transcending that of denser gases, is a fact of parallel bearing.
If we can imagine a gas as much thinner than hydrogen as the square of light's velocity exceeds the speed of sound in hydrogen (about 4,000 feet a second), we have a reasonable presentation of what the luminous medium may be—its marvelous tenuity being vastly more than compensated by the mobility of its molecules. And, therefore, the most subtile aëriform fluid conceivable is of enormously more utility in propagating impulses from star to star than solid steel would be. The ether of space perhaps sustains some such relation to a gas as a gas does to a liquid; and the current disputes as to the materiality or immateriality of a cosmic medium recall very suggestively the days, not very distant, when wise men doubted the
