60 PHYSICAL PROPERTIES AND COMPOSITION 



plete indications for judging of the extent of molecular 

 association, since the latter is merely proportional in 

 amount to the deviation. 



Finally it is to be noted that since ~ at corresponding 



temperatures is constant, and T then the same fraction of 

 the critical temperature, q at the temperatures in question 

 must be proportional to the critical temperature. Here 

 also a special interest attaches to the absolute zero as being 

 both a corresponding and a constant temperature, at which 

 molecular movement ceases. The molecular latent heat of 

 evaporation, which is then nothing else than the work 

 done against internal attractions in expansion to infinity 

 should be proportional to the critical temperature, a con- 

 clusion to which the arguments of pp. 21 and 63 also lead. 



B. Specific Heat. 



When heat is communicated to a body which does not 

 change its state of aggregation, in general both its volume 

 and temperature change. The former involves a consump- 

 tion of heat in the performance of both internal and 

 external work, so that the whole amount is divided into 

 three parts, each of which may be considered separately. 

 It is convenient to begin with gases at extreme dilution, or 

 so-called perfect gases, since there the internal work of 

 expansion is zero. 



i. Specific Heat of Perfect Gases. 



The heat, which on warming a perfect gas, serves only 

 for rise of temperature and for performing external work, 

 may be separated into these parts both experimentally and 

 by calculation. 



Experimentally by preventing any change of volume in 

 a calorimetric determination, so as to measure the specific 

 heat at constant volume. This we shall denote by C. c 

 referring to the kilogram-molecule. 



