ATOM. 457 



fore equal to the energy of agitation multiplied by a certain factor. Thus the 

 energy communicated to a gas by heating it is divided in a certain proportion 

 between the energy of agitation and that of the internal motion of each 

 molecule. For a given rise of temperature the energy of agitation, say of a 

 million molecules, is increased by the same amount whatever be the gas. The 

 heat spent in raising the temperature is measured by the increase of the 

 whole kinetic energy. The thermal capacities, therefore, of equal numbers of 

 molecules of different gases are in the ratio of the factors by which the energy 

 of agitation must be multiplied to obtain the whole energy. As this factor 

 appears to be nearly the same for all gases of the same degree of atomicity, 

 Dulong and Petit's law is true for such gases. 



Another result of this investigation is of considerable importance in rela- 

 tion to certain theories *, which assume the existence of aethers or rare media 

 consisting of molecules very much smaller than those of ordinary gases. Ac- 

 cording to our result, such a medium would be neither more nor less than a gas. 

 Supposing its molecules so small that they can penetrate between the molecules 

 of solid substances such as glass, a so-called vacuum would be full of this 

 rare gas at the observed temperature, and at the pressure, whatever it may 

 be, of the setherial medium in space. The specific heat, therefore, of the 

 medium in the so-called vacuum will be equal to that of the same volume of 

 any other gas at the same temperature and pressure. Now, the purpose for 

 which this molecular sether is assumed in these theories is to act on bodies by 

 its pressure, and for this purpose the pressure is generally assumed to be very 

 great. Hence, according to these theories, we should find the specific heat of 

 a so-called vacuum very considerable compared with that of a quantity of air 

 filling the same space. 



We have now made a certain definite amount of progress towards a com- 

 plete molecular theory of gases. We know the mean velocity of the molecules 

 of each gas in metres per second, and we know the relative masses of the 

 molecules of different gases. We also know that the molecules of one and the 

 same gas are all equal in mass. For if they are not, the method of dialysis, 

 as employed by Graham, would enable us to separate the molecules of smaller 

 mass from those of greater, as they would stream through porous substances 

 with greater velocity. We should thus be able to separate a gas, say hydrogen, 

 into two portions, having different densities and other physical properties, 

 * See Gustav Hausemann, Die Atome und ihre Bewegungen. 1871. (H. G. Mayer.) 



VOL. II. 58 



