450 



Mr. W. Travers. 

 FIG. l. 



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 appa- 

 ratus for the introduction of gases into vacuum-tubes (' Trans. Chem. 

 Soc.,' 1895, p. 686). The tube B is connected with a tap on the Top- 

 ler'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 cur- 

 rent was turned on, making a the cathode. A deposit of platinum 

 quickly appeared on the walls of the tube round a, and the following 

 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 out- 



side 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 re- 

 moved. The tap was again closed, and the tube was warmed care- 

 fully 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 



