GASEOUS REFRACTIVE INDICES 



289 



subject, despite the fact that so important a law as that which is 

 here involved is still unguessed. Fortunately, however, the 

 determination of the indices of phosphorus and sulphur has 

 added a few compounds to the slender list of available data, 

 and though we are not yet far advanced beyond the negative 

 conclusions of Mascart and Briihl, it is certainly time that the 

 information we possess should be reviewed and examined afresh 

 in the light of work which has been done in other directions. 

 The following is a list of all the compounds whose gaseous 

 index, and the gaseous index of both whose constituents, have 

 been determined. Where observers differ, and are of approxi- 

 mately equal authority, more than one value is given. The 

 third column gives the actual refractivity observed (in general 

 for yellow light). The fourth column shows the sum of the 

 refractivities of the constituents. The fifth column shows the 

 difference between the third and fourth column, and it is 

 reckoned positive when the sum of the retardations of the 

 constituents is less than that of the compound. The sixth 

 column shows the percentage of the change to the additive 

 value. * 



Table III 



Refractivities of compounds of which the refractivities of all the components 



are known 



19 



