THE GASES OF THE ATMOSPHEEE 125 



leigh of the relative densities of the principal gases, an 

 investigation which extended over some twelve years, 

 commencing in 1882. In that year Rayleigh had called 

 .attention to the statement which had so long passed 

 under the name of Front's Law. According to this 

 statement the atomic weights and hence the densities ! 

 of the simple gases stand in a simple numerical relation- 

 ship to the atomic weight and density of hydrogen. The ' 

 first result of this investigation was the demonstration 

 that the atomic weight of oxygen is approximately 15*8 

 and therefore less than the whole number 16 required 

 by Prout's "law," which was thus shown to be illusory. 



Unexpected difficulties were encountered in dealing 

 with nitrogen gas, but the upshot of the numerous 

 experiments undertaken was the discovery that the 

 gas left when oxygen, water vapour and carbon dioxide 

 were completely removed from atmospheric air was 

 appreciably heavier than nitrogen prepared from am- 

 monia by passing a mixture of this gas with air or 

 oxygen over a surface of heated copper. 



With reference to this anomaly Lord Rayleigh 

 addressed a letter to Nature on 29th Sept., 1892, in 

 which the following passage occurs : 



" I am much puzzled by some recent results as to the density 

 of nitrogen and shall be obliged if any of your chemical readers 

 -can offer suggestions as to the cause. According to two methods 

 of preparation I obtain quite distinct values. The relative 

 xlifference, amounting to about TT5 Vfr part, is small in itself; but 

 it lies entirely outside the errors of experiment, and can only be 

 ^attributed to a variation in the character of the gas." 



