concerning the Gases of the Atmosphere. 1 05 



Concentration of Helium from the Atmosphere. 



In a footnote (p. 498) to a paper on the Separation of Gases 

 by Diffusion *, I suggested that the lighter constituent of 

 a mixture might be concentrated by causing it to diffuse 

 against a stream of an easily absorbable gas, such as carbonic 

 acid. In Jan. 1899 a good many trials upon these lines were 

 made with the object of putting in evidence the helium of 

 the atmosphere, and a certain degree of success was attained. 

 A stream o£ carbonic acid (prepared from marble and hydro- 

 chloric acid and reckoned at 3 litres per hour) is main- 

 tained for say 14 hours through a diffusion-tube open above 

 to the atmosphere. This tube, placed vertically, is about 

 40 cm. long and of about 5 cm. diameter. The gases of the 

 atmosphere diffuse downwards into the tube, but the heavier 

 constituents are held almost entirely at bay by the stream of 

 carbonic acid. If we draw off continuously a supply from a 

 point say halfway down the diffusion-tube, we shall obtain 

 carbonic acid with a small admixture of atmospheric gases 

 in which the lighter ingredients, e. g. water, hydrogen, and 

 helium, are relatively much concentrated. In my experiments 

 the lateral stream was about 250 c.c. per hour, and was mani- 

 pulated with the aid of a Sprengel pump. Between the 

 pump and the diffusion-tube was interposed a short length of 

 tobacco-pipe through the walls of which the gas had to pass 

 and which presented the right degree of obstruction. After 

 passage to the low-pressure side, the bulk of the C0 2 was 

 absorbed with alkali, and the residual gases collected over 

 alkali at the foot of the Sprengel in the usual way. 



The subsequent treatment for removal of nitrogen by the 

 electric discharge was conducted as usual, towards the close 

 in the tube described and figured above. The final residue on 

 the occasion when D 3 was best seen (under the jar discharge) 

 was about *25 c.c. Argon was also plainly visible and probably 

 constituted the greater part of the bulk. When the volume 

 was doubled by addition of oxygen, D 3 was seen less well. 



Success depended a good deal upon precautions to avoid 

 the presence of gases, and especially of argon, which had not 

 undergone diffusion. It was necessary to eliminate the dis- 

 solved gases of the dilute hydrochloric acid with which the 

 C0 2 was prepared, and to keep an atmosphere of C0 2 in the 

 supply-vessel. Until these precautions were taken, D 3 , though 

 frequently suspected, was not clearly and steadily seen. 

 Even at the best, good measurements could hardly have been 

 taken ; but the line appeared to be in the right place for 

 helium, as distinguished for example from neon. 

 * Phil. Mag. vol. xlii, p. 493 (1896). 



