Law of the Rectilinear Diameter. 423 



Traube has proposed a method to which Lewis does not 

 refer. From considerations of the refractive index he arrives 

 at the conclusion that the co-volume of all liquids at the same 

 temperature and pressure is 24*5 c.cm. per gramme molecule, 



22 400 

 and thence obtains tt from 9 * = 970 atmospheres. The 



co-volume, however, is not constant by any means, and con- 

 sequently the method fails. It must be noted though that 

 24*5 is very near to the mean value of my determinations, 

 which is 25*67 c.cm. It should be noted also that all the 

 above figures relate to liquids which are not at all, or very 

 slightly, associated. 



Lewis deals very fully with the Dupre" relation between tt 

 and the internal latent heat of vaporization. He shows that 

 a better result would probably be obtained from a relation of 

 the form 



tt = Z + tJ, (20) 



where I is the latent heat per unit volume, but he made no 

 calculations as no information was available with regard to 

 dir 



2T' 



If in (20) we supply % from either (15), (16), or (17) 

 we get dL 



ir = Z-T.^ >,.,,. (21) 



= Z--,T ,,.,.. (22) 

 v 



= Z~14-8&.T (23) 



Either of the above forms will enable the necessary calcur 

 lations to be made from data for I and the critical constants 



dir 

 of the substance. The actual values of -^ are given in 



column 4 of Table IY. The values of tt obtained from 

 (21) for a few liquids are shown below with those from 

 Table IV, 



2 F 2 



