﻿Law 0] Molecular Attraction. Ill 



Further data for comparison can be obtained by use of the 

 thermodynamic equation 



^J^' "~ VLiquid PVapour' 



or 



r -c -^ + 2 



W'Liquid ^Vapour C tT M' 



where L is latent heat o£ the liquid, T is the temperature, 

 and M tlie molecular weight. In the following table the 



values of -77, have been obtained by smoothing the values 



ell 

 calculated from the Clapeyron-Clausius formula by Mills * 

 and then reading off tangent values from the curves. The 

 above equation gives approximate results. 



I 



Substance. 

 Ether 



Temp. 

 Kange. 





 0-50 



0-40 



0-50 

 0-50 

 0-60 

 0-60 



0-60 



dh 



c/T 



(mean). 



0244 



0-199 



0-160 

 0-123 

 0-114 

 0143 



0-071 



M. 







M 



Wt to/' 



P -C • ! 

 Liquid l Liquid 



72 



86 

 100 

 114 



78 



154 



0-027 

 0-028 



0023 

 0-020 

 017 

 0026 



0-013 



0-271 

 0-227 



0-183 

 0-143 

 0-131 

 0-169 



0-104 



0-132 at 20° 

 0-137 at 30° 



0-0918 at 0° 

 0878 at 20° 

 0-0872 at 40° 



132 at 20° 



1066 at 20° 



0-1027 at 20° 



0-139 at 20° 

 0-134 at 30° 

 0-133 at 50° 



0-068 at 20° 









Octane 



Benzene 



Carbon tetrachloride. . 



Although the results in the above tables can only be 

 regarded as approximate, they show clearly that the specific 

 heat of a substance at constant volume is considerably greater 

 in the liquid state than in the gaseous state. This means 

 that the energy change which takes place in an expanding 

 liquid is not due solely to potential energy of molecular 

 attraction, but must involve a liberation or intra-molecular 

 energy. Still further evidence of this is yielded by the 

 experiments of Joly f on the specific heat of C0 2 at constant 



* Loc. cit. 



t IVoc. Roy. Soc. lv. p. 300 (1894). 



