450 Dr. F. W. Aston on the 



Ramsay and Tr avers, and was accepted as an elementary 

 monatomic gas of the helium group. Its density was mea- 

 sured with extreme care by Watson (J. C. S. Trans, vol. i. 

 p. 810 (1910)), and found to correspond with an atomic 

 weight 20*200 (0 = 16), making it the lightest element to 

 diverge from the whole number rule in an unmistakable 

 manner. 



Neon has many very remarkable properties, its com- 

 pressibility, viscosity, and dielectric cohesion are all ab- 

 normal ; but the first suggestion that it might be a mixture 

 was the observation in 1912 by Sir J. J. Thomson of a faint 

 bat unmistakable parabola at a position corresponding 

 roughly to an atomic weight 22^ in addition to the expected 

 one at 20, in positive ray photographs, whenever neon was 

 present in the discharge-bulb (v. 'Rays of Positive Elec- 

 tricity,' p. 112). The first plate which showed this was 

 obtained from a sample of the lighter constituents of air 

 supplied by Sir James Dewar ; other specimens of impure 

 neon gave a. similar result. So also did a portion of the gas 

 used by Watson in the atomic weight determinations, which 

 fact, together with the complete invisibility of any parabola 

 at 22 on hundreds of plates where neon was known to be 

 absent, was very strong evidence that the line was ascribable 

 to neon and to neon alone. 



These facts led the author to undertake a searching 

 investigation on the constitution of the gas by two distinct 

 lines of attack, firstly attempts at separation, secondly 

 accumulation of the evidence obtainable by positive rays. 



Evidence of Separation. 



The experiments on fractional distillation and fractional 

 diffusion through pipeclay have already been described 

 (F. A. Lindemann and F. W. Aston, Phil. Mag. vol. xxxvii. 

 May 1919). The former were completely negative and only 

 succeeded in confirming Watson's value of the density 

 already referred to. It has recently been shown (F. A. 

 Lindemann, Phil. Mag. July 1919) that this negative result 

 was theoretically inevitable. 



The diffusion results were more hopeful, an apparent 

 change of density of about '7 per cent, being obtained in 

 the first set of experiments. On the other hand, the more 

 elaborate automatic apparatus started in 1914 has given very 

 disappointing results, a difference of only '3 per cent, being 

 obtained. This is doubtless due to the initial mistake in 

 designing the apparatus to work at atmospheric pressure, 



