Ether proved by light—Light-waves—Elasticity of ether—Its universal diffusion—Influences molecules and atoms—Is influenced by them—Successive orders of the infinitely small—Illustrated by the differential and integral calculus—Explanation of this calculus—Theory of vortex rings. Perhaps the best way to convey some idea of this order of magnitudes to the ordinary reader is to quote Sir W. 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, 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 nebulæ has to be measured by the number of thousand years their light, travelling 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 nor 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-pervading 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 electricity—primarily from those of light. Respecting light two facts are known to us with absolute certainty. 1st. It traverses space at the rate of 192,000 miles per second. 2nd. It is propagated not by particles actually travelling 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.