230 Prof. W. Ramsay and Mr. M. W. Travers. 



mercuric oxide, of which he does not give the method of preparation ; 

 it may have contained oxides of nitrogen ; and for some reason, not 

 explained, he passed the gas through a vacuous porcelain tube, pre- 

 sumably red-hot, which, as recent experiments of Messrs. Bone and 

 Jerdan have shown,* is not impervious to furnace gases. Dulong, 

 on the other hand, who, as already remarked, prepared his oxygen 

 from chlorate, obtained the number 0*924 for white light, coincident 

 with our determinations. 



The refractive index of air, calculated from our determinations, 

 viz., 



Oxygen 0*9243 



Nitrogen 1*0163 



Argon 0-9596 



and the densities of the constituent gases,f gives the following 

 numbers : — 



(1-0163 x 78-15) + (0-9243 x 20-91) + (0-9596 x 0'94) = 99-653. 



Observers sometimes find the percentage of oxygen in air to be 

 about 20'98, or even 21'0. This would hardly affect the result ; with 

 20'96 per cent, of oxygen the calculated refractivity is 99*647, instead 

 of 99-653. 



There can be no doubt as to the refractivity of oxygen from our 

 ratios, as well as from Dulong's determinations. The question is as 

 regards nitrogen. It would require the refractivity of nitrogen to be 

 1*208, a number greatly above any of our values, in order that the 

 sum of the refracfcivities of oxygen, nitrogen, and argon should equal 

 100. The presence of argon would also make an almost inappreciable 

 difference. Taking Mascart's determination of the refractivity of 

 atmospheric nitrogen to be correct, that of pure nitrogen would be 

 1*0181, instead of 1*0178. And an error in the refractivity of argon 

 would also not affect the result, inasmuch as the total amount of 

 argon is so small. 



We are thus driven to conclude that the refractivity of the 

 mixture, air, is somewhat less than that of the sum of the refrac- 

 tivities of its constituents, taken in the proportion in which they 

 occur. 



It appeared advisable to try other mixtures; and a mixture of 

 hydrogen and helium was first selected, because these are both very 

 " perfect " gases, inasmuch as their critical points lie very low. It 

 was to be expected that if a difference between calculated and found 

 values should exist, it should be of the inverse character to that of a 



* < Chem. Soc. Trans.,' 1897, p. 42. 



f Argon, ' Phil. Trans.,' A, 1895, p. 202, foot-note. 



