8o PHYSICAL PROPERTIES AND COMPOSITION 



sumption of spherical molecules in the interstices of which 

 light moves with the same velocity as in vacuo. The rela- 

 tion between the density D and the refractive index n 

 referred to vacuum, then becomes 



This formula, though not in all respects satisfactory, 

 expresses satisfactorily the observations on the influence of 

 temperature and states of aggregation, with which it is in 

 the first instance concerned, as is shown by the following 

 observations of Eykmann l taken at widely different temper- 

 atures : 



r ^ - 



Substance Temperature 



Isosafrol . . C 10 H 10 2 j I7 ' 6 29 * 5 



(141.1 0-2962 



Diphenylethylene , C 14 H 12 I 



(143.4 0.3382 



Quinoline .... C 9 H 7 N \*f %% 



For change of state, Lorenz, e.g. found the following 

 numbers : 



Substance 



Liquid at 10 Vapour at 100 



Water 0-2068 0-2061 



Carbon disulphide .... 0-2898 0-2805 



Methyl iodide 0-1571 0-1557 



Ethyl ether 0-3068 0-3026 



D. Additive Relations. 



The close relation of the refractive index to the occu- 

 pation of space by matter harmonizes with the additive 

 relations found to hold for both quantities, only that the 

 volume to be considered is essentially that at the absolute 

 zero, or the values in the critical or other corresponding 

 states, which are proportional to the former; whereas 



1 Eec. des trav. Chim. des Pays-Bas, 12. 163 ; 14. 185. n a refers to the 

 red hydrogen line. 



