Befractivities of Air, Oxygen, Nitrogen, Argon, Sfc. 229 



It will be seen on inspecting the above table that the numbers 

 obtained indirectly are in close agreement with those obtained by 

 direct comparison with air. 



Taking the value found directly by Mascart for D*, viz. {n— 1) D = 

 0*0002923, the value found by him for nitrogen (atmospheric) was 

 0"0002972, giving for nitrogen, on the basis air equal to unity, the 

 number 1'0178. Mascart did not determine the value for oxygen, 

 but calculated it from the above data and the known composition of 

 air. Nor did Lorenz determine the value for nitrogen; but taking 

 his own value for oxygen, viz. {n— 1)d = 000272, and for air (n— 1) D 

 = 000291, he deduced it, as Mascart had done for oxygen. So 

 that we have no determination of the three constants, or their com- 

 parison, by any one observer since Dulong in 1826. It has been 

 tacitly assumed that the refractive index for a mixture of gases is 

 that of those of their consituents, taken in the proportion in which 

 they occur. We have in our hands a means of verifying this 

 assumption, which is well known not to hold for compound gases, 

 nor for mixtures of liquids, even though change of density be taken 

 into consideration. 



Dulongf gives very careful accounts of the methods he used in 

 preparing the samples of gas that he employed. Oxygen, to which 

 he ascribed the refractivity 0'924, was obtained by heating potassium 

 chlorate. His result is identical with ours. Nitrogen was prepared 

 from air by absorbing the oxygen with phosphorus, first at a high 

 temperature and then in the cold. It was then washed with a solu- 

 tion of chlorine, and afterwards with potash. It is difficult to see 

 what object was to be gained by washing with chlorine water, unless 

 it was the removal of hydrogen. The number he obtained was 1'02, 

 somewhat higher than that which we have found. Dulong also 

 determined the refractivity of air, and allowing for that of the small 

 percentage of carbon dioxide, it is precisely the mean of that of its 

 constituents, taken in the proportion in which they are present. 



Returning to the results of Mascart and Lorenz, we have for the 

 D lines :— 



Air. Nitrogen. Oxygen. 



Mascart 1 10178 — 



Lorenz 1 — 0'9347 



From these data of Mascart and Lorenz it is possible to calculate the 

 refractivity of air : — 



(1-0178 x 79-1) + (0-9347 x 20-9) = 100-15. 

 There is reason to doubt the purity of Lorenz's oxygen. He heated 



* The dispersions for these gases are so small as not to affect the ratios of these 

 numbers (' Compt. Rend.,' 1874, vol. 78, p. 621). 

 f ' Ann. Chim. Phys.,' vol. 31, p. 176, 1826. 



