The Homogeneity of Helium and Argon. 213 



The rate of diffusion of the gas of density 2*057 was determined 

 .finally, so as to afford a check on its density. It took 657*9" for a 

 quantity to diffuse ; while the same volume of hydrogen under pre- 

 cisely similar circumstances took 492'3". Reducing these numbers 

 to density, if hydrogen be taken as 1*0082, the helium possesses the 

 density 1*801, which compares very favourably with the number 

 already found, 1*826. 



As a final check on these results, a sample of helium from an 

 entirely different source, samarskite, was so diffused, that first nine- 

 tenths were removed by diffusion ; from the residue nine- tenths was 

 again removed, and the process was repeated a third time. The 

 more diffusible portion was tested as regards rate; while hydrogen 

 took 492*3" to diffuse, this sample took 652*6". Stated as density, 

 ths number is 1*771. 



The actual density was next determined, with the following 

 result : 



Volume of globe 162*843 c.c. 



Pressure at filling 691*6 mm. 



Temperature 19*85 



Weight 0-02567 gram 



Density 2*080 



This number closely coincides with the density of the previous 

 specimen, freed from argon by diffusion ; and in this case it must be 

 remembered, no systematic process for separating two possible con- 

 stituents was carried out, but the heavier portion only was removed. 

 The heavier gas separated by diffusion was examined for argon, and 

 it was possible to see the green group of five lines, but not the red 

 lines. And with a jar and spark-gap, argon could just be detected. 



The rate of diffusion of this gas, which, stated as density, gives the 

 number 1*8, differs from the density determined by weighing, viz., 

 2*08, or thereabouts. This might be caused (1) by a lighter portion 

 passing over first during diffusion, leaving a heavier portion behind . 

 or (2) by the hypothesis that the rate of diffusion of helium is ab- 

 normal ; and helium has already shown such very remarkable pro- 

 perties in relation to refractivity for light, and conductivity for 

 electricity, that the hypothesis is not unwarrantable. The first 

 supposition, however, is the more probable, and was put to the test 

 in the following manner. 



A smaller apparatus was made for measuring the rate of diffusion 

 of 10 to 20 c.c. of gas ; and the rates of the sample of density 2*08, 

 and of the less diffusible residues from this sample were determined. 

 Both the hydrogen and the helium were carefully measured and 

 diffused under precisely similar conditions. While the hydrogen took 

 181" to diffuse, the helium of density 2*08 took 246*6", implying a 



