468 THE POPULAR SCIENCE MONTHLY. 



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, notwith- 

 standing the fact that their difference in weight is more than a third 

 greater than that of lead and water. A further study of this phe- 

 nomenon reveals the significant fact that just as much of each gas dif- 

 fuses 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 glohe 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 tempera- 

 ture 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 de- 

 gree," for even liquids do spring into the gaseous form and mingle 

 their vapors with other gases at common temperatures. This is evapo- 

 ration. 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 receiv- 

 ing it is increased. Whenever a body of gas, freed from opposing 

 pressures, expands, in obedience to this molecular agency, its own tem- 

 perature is reduced. Moreover, the most exact quantitative relation 

 can be traced. This molecular energy and heat are, therefore, cor- 

 relative. All this is suggested by the facts of expansion, vaporization, 

 and diffusion. But I have no time to give even an outline of the clas- 

 sic 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 con- 

 ceptions 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 with- 

 out changing the nature of the substance, separated by distances in 



