218 



Mr Monchnan, Discharge in Vacuo 



[Nov. 9, 



had been fused by exposing them to the heat of the electric arc. 

 With some of the gases other electrodes were used and it was 

 found that Pt, in all forms, Pd, and C, were effective, while Al 

 was only slightly so. The former bodies occlude gases, and when 

 the pressure is reduced, drive off small particles of their substance 

 into the surrounding space, while Aluminium does not do so. 



I have collected into one table the results obtained by the 

 previously described experiments. 



Table shelving the changes produced by variation of 

 electrodes, of gases, and of pressure. 



+ When the sparks were not passed until the pressure was reduced to 30 m., 

 they were greenish and reversals were obtained. When the sparks were passed as 

 the pressure varied, the greenish colour soon disappeared and reversals occurred at 

 from 18 to 25 m. pressure. 



* After using HC1 and N the electrodes became covered with a brown substance 

 and the reversals ceased to appear although trial was made with dry and damp air 

 and alcohol vapour, but they were again obtained after heating the platinum in the 

 blow-pipe flame. 



Further experiments showed this to arise from the Hydrochloric acid gas which 

 not only gave no reversal itself, but so acted upon the platinum as to prevent 

 reversal with other gases. The same action took place in a small degree with 

 Oxygen, which caused the reversal to fail in Hydrogen the first time after Oxygen 

 had been used. 



