82 PHYSICAL PROPERTIES AND COMPOSITION 



colour, it would necessarily partly fail for other wave- 

 lengths. 



Accordingly the approximate equality of refraction shown 

 by organic isomers is very striking, more so than in the 

 case of volumes, e. g. 



Substance M ^ r 



f Valerianic acid 26-72 



(Methyl buty rate 26-79 



f Ethylene chloride 20-96 



| Ethylidene chloride 21-08 



(Athyl alcohol 16-85 



( Propyl aldehyde *5'93 



/o-Cresol 3252 



j m-Cresol 32-56 



(p-Cresol 32-57 



[Fumaric acid 70-89 



(Maleicacid 7029 



Since then equal differences in composition are associated 

 with equal differences in molecular refraction, the atomic 

 refractions may be determined indirectly and were found 

 by Landolt for n a to be 



H = 1-04, O = 2, C = 2-48. 



But then, just as in the relations of volume, the mode of 

 combination appears to have an essential influence, so that 

 e.g. for aldehyde 



H ' CC <H 



M a = ii 5, and deducting 2 x 2-48 -f 4 x 1-04, there remains 

 for oxygen 2-38, on the other hand for acetic acid 



MI = 12-93, an( l deducting 11-5, there remains for oxygen 



1-43- 



Double-linked oxygen regularly shows a higher atomic 

 refraction, and the same is true for double-linked carbon as 

 compared with single-linked. 



