214 Drs. W. Ramsay and J. Norman Collie. 



density of T871 ; and the residue diffused in 266' 6", which corre- 

 sponds to a density of 2 % 187. In each of these experiments about 

 half the helium passed through the porous plug. 



The denser portion of this gas was again diffused five times, lighter 

 portions being removed. This corresponds to a residue of 30 c.c. 

 from 400 c.c. of the original gas. The rate of diffusion of this sample 

 compared with that of hydrogen was almost identical with the last, 

 namely 208" to 143", and corresponds to a density of 2*133. The 

 gas is therefore not increased in density by this process. 



The lighter gas was submitted to a similar fractionation, and the 

 ratio of its diffusion-rate to that of hydrogen was 24675" to 181 -0", 

 as a mean of several closely concordant experiments. This corres- 

 ponds to a density of 1*874. We have accordingly : 



Density. 



" Heavy " portion 2*133 



" Light " portion T874 



Not content with this, we pushed fractionation still further ; the 

 helium was divided into seven portions (by fractionation) and then 

 submitted to methodical fractional diffusion, in which the heavier 

 portions were transferred to the " denser " side, and the lighter 

 portions to the " lighter " side. This process was repeated four times, 

 and the end portions were each divided into two ; the lighter portion 

 of the "lighter" was collected separately, and its rate determined. 

 It took 258*5" to diffuse, compared with 189*5" for an equal volume of 

 hydrogen ; its density calculated from these rates was 1*876. It is 

 clear, therefore, that the limit has been reached in purifying the 

 lighter portion by diffusion. 



It should have been mentioned that the portion of 2*133 density as 

 well as that of T874 density had been sparked with oxygen in 

 presence of potash, and in a vacuum tube showed mere traces of 

 hydrogen, every other gas being absent. The spectrum of hydrogen 

 is still visible, even when 0*01 per cent, of that gas is present. 



At various times during the attempt to separate helium, the spec- 

 trum has been carefully examined. The very first portions of the 

 lightest gas gave an identical spectrum, seen with a hand-spectro- 

 scope, with the very last portions of the heaviest gas. Professor Ames, 

 of the Johns Hopkins University, has however kindly undertaken to 

 photograph the spectra using a dispersion -grating ; so that if any 

 difference can be detected, it will ere long be made known. 



Lord Rayleigh was so kind as to measure the refractivity of these 

 extreme portions of the fractionated gas. His process has been 

 described in the ' Proceedings,' vol. 59, p. 202. For the sample of 

 helium sent him in July, 1895, he found the number 0*146. The 

 lighter portion of the fractionated gas of density 1*876 had a refrac- 



