Refractivities of Air. Oxygen, Nitrogen, Argon, fyc. 231 



mixture of oxygen and nitrogen, for they are two somewhat " im- 

 perfect " gases. The result has borne out this idea. 



A mixture was made of 20*60 c.c. of hydrogen and of 20*12 c.c. of 

 helium free from argon, and of the density 1*960; and with the 

 refractivity of the mixture those of hydrogen and helium were com- 

 pared. Taking the refractivity of the mixture as unity, the follow- 

 ing ratios were found : — 



0*4495. 



Hydrogen/mixture 1*5977 Mean< ^ 1>5 g? 



1*5957 



Helium/ mixture 0*4513 



0*4478 



The calculated values are — 



(•04495 x 20*12) 



4072 - 2J J1 * 



(1-5967x20-60) 80*87 



40*72 102-99 



Here the calculated value of the refractivity of the mixture is 

 3 per cent, higher than the found value, while with air the calcu- 

 lated value is 0'35 per cent, too low. 



A third experiment was made, in which the " artificial air " was a 

 mixture of 19*13 c.c. of carbon dioxide with 19*29 c.c. of oxygen, 

 both gases supposed to be at 0° and 760 mm. Again, taking the 

 refractivity of the mixture as unity we found the following ratios : — 



Carbon dioxide/mixture 1*2450 



Oxygen /mixture 0*7525 



The calculated values are : — 



(1*2450x19*13) 



= 61-99. 



38*42 



(0-7525x19*29) _ 37-78 

 38*42 ~~ 99*77* 



Here, as with air, the total refractivity found is less than that 

 calculated. It is true the difference is not great, but we are per- 

 suaded that it is real, for it considerably exceeds the error of our 

 several determinations. 



The case is not bettered if Lorentz and Lorenz's formula be substi- 

 tuted for Gladstone and Dale's. Using their formula, n 2 —ljn 2 +2, 

 the calculated result is 99*72 per cent, of that found for air. 



The coefficient of compressibility of hydrogen is too small, while 

 that of other gases, such as oxygen and nitrogen, is too great. The 



