GASEOUS REFRACTIVE INDICES 293 



important fact into account. The case of the sulphur com- 

 pounds, H 2 S, S0 2 , and S0 3 , is very interesting. The first of 

 these loses nearly 1 1 per cent, of its refractive power on com- 

 bination, the second 20 per cent., and the third 23 per cent.; 

 and it is significant that the refractivity of gaseous sulphuric 

 acid, calculated from the liquid, is also extremely low. Here we 

 have cases in which, both when sulphur is negative, as in H 2 S, 

 and when it is positive, as in S0 2 , and S0 3 and SF 6 , a large 

 reduction of refractivity takes place as a result of combination. 

 The phosphorus compounds given in the table are also inte- 

 resting. Though there is a reduction of refractivity on com- 

 bination, it is so small compared with the analogous figures for 

 sulphur that it suggests a tendency on the part of phosphorus to 

 form compounds of unduly high refractivity like nitrogen. 



It would take too long to discuss the interesting features 

 presented by even the slender list of carbon compounds given 

 in Table IV. Our ignorance of the refractivity of free carbon 

 detracts from their value, and all that can be done is to draw 

 some rough conclusions by calculating what the value for 

 carbon would be if the additive law held. From column 5 we 

 see that the average value for one atom of carbon is about 170, 

 which is considerably higher than that of one atom of nitrogen, 

 147. Marked differences arise, however, in different compounds. 

 When a carbon atom is united with four hydrogens, whose re- 

 fractivity is low, the additive value comes out low (163), as if the 

 carbon had somehow lost some of its refractive power. When 

 the four hydrogen atoms are replaced by four chlorine atoms, 

 the refractivity of which is higher than that of carbon, the 

 additive value comes out higher (232), and still higher in CHC1 

 (273). Unless this anomaly turns out to be due to the adoption 

 of different standards of density or to experimental errors, it is 

 an important clue. 



One of the most striking facts which can be deduced from 

 the carbon compounds is that when a compound is unsaturated, 

 as in CO, C 2 H 2 , the refractivity of the compound is greater than 

 it would have been otherwise. This has been very fully proved 

 in the case of liquids, and Bruhl has assigned definite increases 

 to the refractivity of the carbon and oxygen atoms under these 

 circumstances. The number of gaseous compounds of this 

 nature whose index has been determined with accuracy is not 

 sufficient to enable us to confirm Bruhl's law; but existing 



