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 give spectra distinguished 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 pervades 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 nebulæ, and that in this light the spectroscope enables us to detect waves propagated and absorbed by the very same vibrations of the same familiar atoms at these enormous distances as at the earth’s surface. Glowing hydrogen, for instance, is a principal ingredient of the sun’s atmosphere and of those distant suns we call stars, and it affects the ether and is affected by it exactly in the same manner as the hydrogen burning in an ordinary gas-lamp.