32 PHYSICAL PROPERTIES AND COMPOSITION 



Referred to the molecular quantity in grams with P in 

 atmospheres, according to p. 23, 



E = 82, 

 so that 



- 



This relation may be tested in certain cases as follows : 



One is inclined to go a step further, and use the above 

 relation to determine molecular weights by means of the 

 density of liquids ; thus from 



M may be determined by means of D T k P k and T. This 

 would serve in the case of the above-mentioned substances, 

 and from a collection of forty-two l it appears that the 

 maximum error is 9 / o , which is usually not too great for 

 a molecular weight measurement. 



The important question as to double molecules in liquids 

 is, however, not satisfactorily answered in this way. If 

 the doubling has already set in at the critical point, which 

 appears to be the case with acetic acid, the values of Pk and 

 T k are influenced by it, and the calculated molecular volume 

 or weight becomes abnormal : 



T D T k P, V^ M D V(calc.} 



C 2 H 4 2 273 1.08 594-6 57-1 57 75 



But if the doubling occurs first below the critical point, 

 as is apparently the case for ethyl alcohol (p. 26), it all 

 depends on whether this fact affects the liquid density, 



1 D. Berthelot, Compt. Eend. 128. 606. 



