Laws of Molecular Force. 



247 



or the molecular latent heat is proportional to the absolute 

 boiling-point. 



(It is to be noted that Wl—l\^0v i r Y b gives a rough means 

 of obtaining I from the boiling-points and the volumes at the 

 boiling-points of liquids, which might be convenient when 

 better data are wanting.) 



Eobert SchifF {Ann- der Chem. ccxxxiv.) has made the 

 most accurate determinations to test this relation between 

 molecular latent heat and boiling-point. For 29 compounds 

 of the form C n H 2w 2 , from ethyl formiate up to isoamyl vale- 

 rate, he finds M\=208T & inthe mean, the greatest departures 

 being 20*4 for propyl isobutyrate, and 21*1 for propyl for- 

 miate ; for 8 hydrocarbons of the benzene series he finds a 

 mean coefficient 20 with 19*8 for cymene and 20*6 for benzene 

 as the greatest departures. To these 37 examples we will 

 add the following from T ronton' s paper, doubling his numbers, 

 as he used density instead of M. 





1 



'abli 



- XXIII.— Values of MtyT g . 





2 H 5 C1. 



OH01 3 . 



OCl 4 . 



AsCl 3 . 



SnCl 4 . 



S0 2 . 



CS 2 . 



21 



22 21 



1 



21 



20 



23 



21 



(C 2 H 5 ) 2 0. 



(C 5 H n ) 2 0. 



(CH 3 ) 2 CO. 



O 10 H 16 . , (C 2 H 5 ) 2 CA- 



22 



24 



23 



22 23 



The mean value of the coefficient is higher than that deduced 

 theoretically above (19*4), because in round fraction we wrote 

 T 6 = 2T c /3, but the general truth of the relation is well enough 

 brought out. 



Fourth method of finding the virial constant I : from the 

 critical temperature and pressure. Now we have (Section 4), 



T c = 120Z/409R&, p =3Ql/mk% 



... TJ Pc =10k/3R and Z=409R 2 T;!/400p c , 



this is for compounds; for elements Z = 27R 2 T^/64p c . Where 

 both the critical pressure and temperature are known, this 

 gives I theoretically with accuracy, but practically the diffi- 

 culties in measuring the critical pressure introduce inaccuracy. 

 In the relation T c //? o =10A;/3R,, as E varies inversely as the 

 molecular weight, we see that the molecular domains (Mole- 

 cular volumes) of bodies at the critical temperature are 



