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 in 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 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 windward 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 as they 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 satellites 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 of them 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 the 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-pervading medium of this ether, to which they impart, and from which they receive, impulses.