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



More diffusible. I Less diffusible. 



The densities were determined by weighing. 



These numbers show that no important separation has been 

 effected. The difference in density of the two portions may possibly 

 be attributed to experimental error. When the density of the heavier 

 portion was taken the weather was damp, and we have found it difficult 

 to obtain concordant results under such circumstances, owing doubt- 

 less to the uneven deposition of moisture on the surfaces of the bulb 

 and its counterpoise. But as it stands, the difference is an extremely 

 minute one, and it may, we think, be taken that any separation of 

 argon, if effected at all, is very imperfect. 



The Fractional Diffusion of Helium. 



Two hundred c.c. of helium from fergusonite of density 2'13 were 

 separated into two nearly equal portions by diffusion. The rate of 

 diffusion was 7*14" per millimetre as a mean of two experiments, 

 giving 7'13" and 7'15" respectively. The most diffusible portion of 

 this gas gave the rate 7'12" per millimetre. The more diffusible 

 half of this gas had the rate 7'48", and the least diffusible of the 

 remainder 7 '38", the temperature being lower. A second specimen 

 of helium from mixed sources, samarskite, fergusonite, broggerite, &c., 

 which showed the nitrogen spectrum strongly, gave a rate for the 

 first portion of 8'29". This half on rediffusion had the rate 7'64", 

 and the residue of 8'39", showing that a separation was being effected. 

 The heavier residue of the remainder from that portion which 

 showed the rate 8 "39" was too small to make it possible to diffuse it 

 by the usual method. A second method was therefore resorted to, 

 and it was directly compared with hydrogen under the same circum- 

 stances. While hydrogen took 12'14" per millimetre, the residue 

 took 21*00", and calculating its density from these rates, we have 



21-00") 2 x 1-0082 QAO 



(12-14'T 



This would correspond, if it be granted that the impurity is nitro- 

 gen, to a percentage of 8'5 of that gas. This residue showed a 



