The Homogeneity of Helium. 



321 



derived from leakage of air, ifc would correspond to only 0'3 c.c. of 

 argon. 



The contents of jar ~No. 2 were also purified and. weighed. During 

 the purification hardly a trace of nitrogen was removed. The 

 density was 2*209. We have thns : — 



Jar JSTo. 1 contains gas of density 2*245 



2 „ „ 2-209 



6 „ „ 1-979 



The light gas which had previously been stored in tubes was now 

 mixed with the light gas from the second set of diffusions, and the 

 mixture w^as re-diffused fifteen times, involving ninety operations. 

 The density of the lightest portion of this helium was determined by 

 weighing and found to be 1*988. The helium had, therefore, not. 

 been made sensibly lighter by re-diffusion. The mean of the two 

 determinations may be taken as the true density of pure helium; it 

 is 1*98. The refractivity of this sample measured against hydrogen 

 and multiplied by the ratio between hydrogen and air, viz., 0*4564, 

 gives 0*1238. This specimen of light helium of density 1*988 was 

 placed in one of the refractivity tubes, and the lightest helium of the 

 former preparation (density = 1*979) in the other. They had the 

 same refractivity (1000 to 1004). The contents of No. 1, obtnined 

 from the mixture of light gases had the density 2*030, showing that 

 only a little heavier material had been withdrawn. 



The lighter fractions of helium were then sealed up in glass reser- 

 voirs and stored. The heavier portions were placed in the diffusion 

 apparatus and submitted to methodical diffusion. 



After fifteen rounds (ninety operations) the heaviest fraction had 

 density 2*275, the lightest 2*08. The refractivity of the heaviest 

 gas was next determined and found to be 0*1327. This gas examined 

 in a Pliicker's tube showed brilliantly pure helium lines, but along 

 with these the reds and green groups of argon. Calculating from 

 the density of this gas it should contain 1*63 per cent, of argon 

 according to the equation 1*96 la? + 20y = 2*275. Calculating from 

 the refractivity the percentage of argon should be 1*05, from the 

 equation 1'245-r + 0*9596?/ = 13*33. A mixture of 99 per cent, of 

 the purest helium and 1 per cent, of argon was made, and it showed 

 the argon spectrum with about the same or with somewhat less 

 intensity than the heaviest gas. Finally, the heavy gas was dif- 

 fused to the last dregs, so that only about 0*5 c.c. remained undif- 

 fused ; and this small residue, transferred to a Pliicker tube, showed 

 the argon spectrum with only a trace of the spectrum of helium. 

 The yellow line and the bright green line were visible, but feeble. 

 This spectrum was compared with that of a mixture of argon with a 

 trace of helium, and nearly the same appearance was to be seen. With 



