Page:Proceedings of the Royal Society of London Vol 60.djvu/483

450 long, are protected by a sheath of thin glass tube, the spirals at their ends being of thin platinum wire. The side-tube A is connected, by means of a tube containing pentoxide of phosphorus, with an apparatus for the introduction of gases into vacuum-tubes (‘ Trans. Chem. Soc.,’ 1895, p. 686). The tube B is connected with a tap on the Topler’s pump. The apparatus was first thoroughly exhausted and heated by a Bunsen’s flame, and then, after closing the tap on B, helium was introduced at about 3 mm. pressure. The electrodes were connected with the secondary terminal of a coil, and the current was turned on, making a the cathode. A deposit of platinum quickly appeared on the walls of the tube round a, and the folioAving changes took place in the colour of the glow :—

1. Yellow, with slight tinge of red. 2. Bright yellow. 3. Yellowish-green. 4. Green; green line very strong. 5. Green, with phosphorescence. 6. Phosphorescent vacuum; spark passed between electrodes outside the tube.

The tube was then connected with the pump by opening the tap on B, but, as might have been expected, no trace of gas could be removed. The tap was again closed, and the tube Avas warmed carefully with a Bunsen’s burner. The gas was slowly given off from the platinum, and on passing the discharge, colour-changes were observed to take place in the glow, from green to yellow.

From this experiment, it was obvious that the whole of the helium would be absorbed by the platinum splashed off, but it yet remained