contain a mixture of the gases. What must have happened? Evidently a portion of the heavier gas has risen into the upper jar, and a portion of the lighter gas has fallen into the lower jar, and this too, notwithstanding the fact that their difference in weight is more than a third greater than that of lead and water. A further study of this phenomenon reveals the significant fact that just as much of each gas diffuses into the space of the other as would expand into a vacuum of the same size. In fact each gas is, at all temperatures, a vacuum to every other. This fact remains an unsolved mystery, except we admit the existence of molecules and of molecular spaces far outmeasuring the molecules themselves.
Vapors and permanent gases are, therefore, not unlike in this respect. But when we compare this diffusion of the latter with the production and commingling of the former, as shown in the globe experiment, we discover this difference: whereas the molecules of the vapor are driven into mixture by the application of heat, those of the permanent gases spring spontaneously each into the spaces of the other without it. Plainly there exists among the molecules of the gases at low temperature an energy to drive them asunder, such as must be introduced by artificial means among those of vapors to enable them to manifest the same action in the same degree. We need to say, "in the same degree," for even liquids do spring into the gaseous form and mingle their vapors with other gases at common temperatures. This is evaporation. And when we remember, further, that many solids, notably ice, camphor, iodine, yield vapors to the atmosphere on similar exposure thereto, we can feel justified in saying that there exists in gaseous and liquid and solid bodies alike an energy by which their molecules are urged asunder.
This molecular energy bears the closest relationship to heat. Of this the facts already stated are sufficient evidence. Every variation in one is accompanied by a corresponding variation in the other. Whenever heat is expended this molecular energy in the body receiving it is increased. Whenever a body of gas, freed from opposing pressures, expands, in obedience to this molecular agency, its own temperature is reduced. Moreover, the most exact quantitative relation can be traced. This molecular energy and heat are, therefore, correlative. All this is suggested by the facts of expansion, vaporization, and diffusion. But I have no time to give even an outline of the classic researches of Rumford, Mayer, Joule, and others, which prove that heat and molecular energy is the energy of molecules in motion.
Molecules, molecular space, and molecular motions—these three conceptions stand as the modern translation of the symbols on which the facts in regard to the constitution of matter are written. According to the theory, in every material body these three fundamental elements are embodied. It consists of particles which can not be divided without changing the nature of the substance, separated by distances in
