C'o = 0-2980 + logio '-^ . 1-200 lO-^e. 



296 Mr Rideal, On the latent heats of vaporisation 



We have also seen that the expression Nhv can be put equal to 

 the latent heat of evaporation {L). 



If p be measured in atmospheres (10^ bars.) and L in calories 

 the expression becomes 



The value of the chemical constant Cq is thus equal to 



2-3023 

 or taking the generally accepted value oiN = 6-062 10^3 we obtain 



vVM 



IF 



This relationship may be tested by calculation of the chemical 

 constant in those cases where both L and d are known, and com- 

 paring these values with those determined in the usual way from 

 the vapour pressure curve. • 

 In the case of COg 



ilf = 44, ^250 0. = 3100*, .-. X=2600, ?; = 2-61 . 10i3, 



and d = 4-56 10-^ cm.f 



vVM 

 ••- logio -^ . 1-200 10-26 = log.o 9-972 10^, 



or Co = 3-29. 



The actual value of Cq from vapour pressure data is 



Co = 3-20. 

 For hydrogen Eucken finds L - 229 cal. per gm. mol. " 

 d = 2-68 10-8 cm. 

 Hence v = 2-446 lO^^, 



^^^ Co = 2-10 (observed value 2-20). 



For water, inserting the values 



d = 4-54 10-8 cm., L = 10,200 cal, 

 we find V = 1-09 1014, 



we obtain Cq = 3-73 (observed value 3-70) 



The influence of molecular size is well exhibited in the case of 

 benzene. Surface tension data indicate that the molecular diameter 



* Landolt-Bornstein Tabellen. 



t Jeans, The dynamical theory of gases. 



,1 



