on the Physical Properties of Compounds. 181 



V. Influence of Atomic Arrangement on Molecular 

 Refraction. 



The molecular refraction of an element = — ? — x At; where 



a 



A = the index of refraction, d — specific gravity, and M = 



molecular weight. 



Now Bruhl has recently shown, in a most important series 

 of papers, that the molecular refraction of a compound con- 

 taining polyatomic atoms is dependent on the manner in which 

 these atoms are combined in the compound, whilst that of 

 monatomic elements is quite independent of this. 



The atom of a given element has a definite refractive equi- 

 valent, which it retains in all those compounds in which there 

 are no double linkings ; so that the refraction-equivalent of 

 such a compound is equal to the sum of the refraction-equiva- 

 lents of its constituent atoms ; consequently, if we know the 

 refraction-equivalents of each of the constituent atoms, we can 

 readily calculate that of the compound. Thus, the refraction- 

 equivalents of the following elements are: — 



H = l-29: = 2-71; C=4-86; 



and therefore the refraction-equivalent of alcohol, CH S .CH 2 . OH, 

 which contains only single linkings, is 



(2 x 4-86) + (6 x 1-29) + 2-71 =20*17. 



But we find that, in the case of those compounds containing 

 double linkings between carbon atoms, the observed refrac- 

 tion-equivalent is always in excess of the calculated quantity, 

 as shown in the table, p. 188. 



Here we see that, for every double linking of carbon atoms, 

 the refraction-equivalent is raised by two units. 



Bruhl has also shown that a similar thing holds good in the 

 case of other polyatomic elements besides carbon. Thus, 

 oxygen, when combined with carbon by a double linking, as 

 in acetic acid, CH 3 . C — OH, has a higher refraction-equiva- 







lent (3'29) than when combined with onlv a single link, as in 

 alcohol, CH ? . CH 2 . OH, viz. 2-71. 



ATonatoinic elements, on the other hand, possess refractive 



equivalents which are constant in all their compounds. The 



above will show at once what an important influence the 



double or single linking of atoms has on the refractive equiva- 



Q.2 



