38 PHYSICAL PROPERTIES AND COMPOSITION 



4. Volume at Ordinary Temperatures. 



The unsatisfactory conclusion arrived at in comparing 

 molecular volumes at the boiling point has led to the aban- 

 donment of Kopp's fundamental idea, and Horstmann l in 

 particular has carried out a simple comparison at ordinary 

 temperatures or at o. 



As under these circumstances the densities of compounds 

 with fairly high critical temperatures are about three times 

 the critical densities, additive relations are suggested : 



but these fulfil the equation 



even less accurately than at the boiling point 2 . 



This is to be expected a priori, since the remark that the 

 volume at o is one-third of the critical volume is obviously 

 not even approximately true for bodies whose critical tem- 

 peratures are in the neighbourhood of o. If only for that 

 reason, comparison at the boiling point is to be preferred 

 in principle. 



Further, on directly testing the additivity, negative values 

 are often obtained for the atomic volume. As an example 

 of this we may take the molecular volumes of the paraffins 

 at o : 



1 Graham-Otto, 1893, vol. i ; Collection of data on the subject by 

 Horstmann. 



2 Mathias, Journ. de Phys. 



