igo^] Mills — Molecular Attraction. 157 



without the excess of pressure doubles the amount of energy 

 required. Thus he has, 



[9] h = 2 J V ^8* = 2RT log- e ^-. 



Mr. Crompton makes no supposition as to the true cause of 

 the change of density. But he proceeds on the principle that 

 in effecting- a given change of condition the process pursued 

 is immaterial if the total energy change is alone to be consid- 

 ered, the idea being that the change in density could, theo- 

 retically, have been produced by pressure. The law govern- 

 ing the change of pressure with the density is known, there- 

 fore the amount of energy involved in the change of state 

 can be calculated. 



Mr. Crompton uses as the law governing the change of' 

 pressure with the volume, the gas equation, PV = RT. But 

 since Mr. Crompton deals with an ideal condition, from which 

 the action of forces other than the pressure are by assumption 

 removed, his equation is not limited to those temperatures for 

 which that equation holds true. It is necessary, however, 

 that PV = RT should represent the true law of pressure for 

 the substance, which is ideally considered only in a limited 

 sense. (The material size of the molecules or some effect of 

 the temperature, etc., might, therefore, affect the exactness 

 of the law.) It will be further rtcognized that Mr. C romp- 

 ton's equation, no less than equation 4 above, involves the 

 assumption that the only energy change is that involved in a 

 change in density — that is a change of potential energy, — 

 and the total kinetic energy of the molecules of the liquid 

 and of the gas must be the same. As a consequence it is to 

 be expected, though not with certainty owing- to possible 

 compensation, that the theory would not apply to substances 

 more associated in the liquid than in the gaseous condition. 

 In the tables below, 1 to 21, in the column headed "'Cromp- 

 ton" we give the results obtained from this equation for the 

 substances under examination. For additional evidence bear- 



