REFRACTIVE INDICES OF THE ELEMENTS. 339 



The compounds of P whose refractive indices have been measured are more 

 numerous than those of mercury. From the following table it will be seen that, in 

 the case of PH, and PCI,, whose indices have been measured in the gaseous form, 

 the value now obtained satisfies with great exactness the rule so firmly established 

 that the refractivity of a compound is slightly less than the sum of the refractivities 

 of its components in the free state. In the case of the liquid compounds, where 

 additional uncertainty is introduced by the transformation of the indices by the 

 formula of LORENZ and LORENTZ, the agreement is not so close, but the discrepancies 

 are not abnormal in amount. The substitution of LE Koux's value would give worse 

 results in six, and that of GLADSTONE in five, cases out of seven. 



A comparison of the indices of the triethyl compounds of P and N strongly 

 supports the same view. 



2 x 'J 



