Some Experiments on Helium. 



451 



to be proved that the change in colour in the glow was not due to 

 the absorption of the yellow constituent more quickly than the green 

 one. 



The vacuum-tube used in the last experiment was again filled with 

 helium to about 3 mm. pressure, and the discharge was passed till 

 the glow had become green, and the green line had reached its maxi- 

 mum intensity. Now, if any separation had taken place, the gas 

 which had been absorbed by the platinum should contain a large pro- 

 portion of the yellow constituent of helium, and should give a yellow 

 glow in a vacuum-tube, even at low pressure. The remaining gas in 

 the tube was, therefore, removed by pumping, and after closing the 

 tap on B, the gas was driven off from the platinum, by warming with 

 a Bunsen's flame. The current was then turned on, and a glow 

 appeared of the green colour invariably shown by helium at low 

 pressure. The change of colour in the tube during absorption of the 

 helium is, therefore, to be entirely attributed to the lowering of the 

 pressure. In describing these experiments I have used the term 

 absorption in its general sense, as it is impossible to say at present 

 whether we are dealing with a case of simple occlusion or not. The 

 platinum, when it is deposited, is black and non-metallic in appear- 

 ance, but, on heating, it assumes the colour and general character of 

 ordinary platinum, and sometimes breaks away from the tube in thin 

 scales. The change is probably the same as that which takes place 

 when platinum-black is heated. 



In a few of my experiments, I used helium containing traces of 

 hydrogen, nitrogen, and carbon compounds. In these cases I found 

 that not only was the helium absorbed, but also the other gases, to a 

 greater or less extent. Hydrogen is readily absorbed, and next in 

 order come carbon compounds and nitrogen. Argon is taken up only 

 in very small quantity ; in fact, this process serves as a method of 

 separation of helium from argon, even when the helium is present to 

 the amount of only 2 per cent. 



To carry out this separation, the gas is made to circulate at about 

 3 mm. pressure, through a vacuum-tube of the type used in the last 

 experiment. To effect this, the Topler's pump is replaced by a Spren- 

 gel's pump, arranged as shown in fig 2, to deliver the gas removed 

 from the vacuum-tube back into the tube C. To regulate the supply 

 of gas entering the apparatus, the tap F was carefully turned, till the 

 gas bubbled slowly through the mercury contained in the small tube 

 D. The tap E served as a by-pass during the preliminary pnmping- 

 out of the apparatus, and was closed during the experiment. By 

 carefully regulating the quantity of gas which entered the apparatus, 

 and the rate of flow of mercury in the Sprengel's pump, ifc was 

 possible to maintain a constant pressure in the apparatus for a long 

 time. 



