Page:Proceedings of the Royal Society of London Vol 60.djvu/484

Rh 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 maximum intensity. Now, if any separation had taken place, the gas which had been absorbed by the platinum should contain a large proportion 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 appearance, 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 j 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 Sprengel’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 pumpingout 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, it was possible to maintain a constant pressure in the apparatus for a long time.