Under the lead of this law, theory, or assumption, discoveries have been made that deeply and practically interest the most abject mortal who anywhere swings a hoe or shoulders a hod, as well as the lords of the land. For example, it has been ascertained that heat is converted into motion, or motion into heat, according to a fixed or constant ratio or equivalent. To be more particular, the heat which will raise the temperature of a pound of water one degree of Fahrenheit's scale, when converted into mechanical motion, is equivalent to the force which a weight of seven hundred and seventy-two pounds would exert by falling one foot. This is a wonderfully small quantity of heat to balance so heavy a blow, but the careful experiments of Mr. Joule of Manchester, the discoverer, confirmed by Regnault, Thomson, Rankine, Clausius, Mayer, Rennie, and others, have, we believe, satisfied scientific men that it is not far from the correct measure. Were the same, or a far less amount of heat, concentrated on a minute chip of steel struck off by collision with a flint, it would be visible to the eye as a spark, and show us how motion is converted into light as well as heat. It is not our vocation to dive into the infinities, either upward or downward, in search, on the one hand, of the ultimate atoms of the rarest ether, by whose vibrations the luminous waves run through space at the rate of more than ten millions of miles a minute, or, on the other, of the nebulous systems, worlds in the gristle, so far off that the light just now arriving from them tells only how they looked two hundred thousand years ago. All we have to say is, that, if we do not now absolutely know, we do reasonably suspect, that heat and light are mere mechanical motions, alike in nature and interconvertible in fact. The luminiference seems to behave itself, not like infinitely small bullets projected from Sharpe's rifles of proportionately small bore, as was once supposed, but rather after the manner of the sound-waves, which we know travel through the air from the sonorous body to the ear. They have also a resemblance, not so close, to the waves which run in all directions along the surface of a pond of water from the point where a stone falls into it. These three classes of waves, differing so immensely in magnitude and velocity, all agree in this,--that it is the wave that travels, and not the fluid or medium. The rapidity of the luminous wave is about nine hundred million times that of the sound-wave; hence we may suppose that the ether in which it moves is about as many times rarer or lighter than air, and the retina of the eye which it impresses as many times more delicate and sensitive than the drum of the ear. It can hardly be unreasonable to suppose that a fluid so rare as this luminiferous ether will readily interflow the particles of all other matter, gaseous, liquid, or solid, and that in such abundance that its vibrations or agitations may be propagated through them.