Action of a Welinelt Cathode. 249 



to the heating o£ the gas by the glowing cathode itself was 

 not more than one-tenth of the increase observed above, and 

 was hardly measurable. 



After the lime-covered platinum tube had been heated for 

 some hours it was always found that the passage of the 

 luminous discharge was accompanied by a liberation of gas, 

 and that subsequent heating with no thermionic current 

 passing decreased the pressure; the actual numbers were 

 not very regular, but the same general result was always 

 obtained. 



The most obvious explanation of this result is that oxygen 

 is liberated by electrolysis of the lime, and that recom- 

 bination of oxygen and calcium goes on when the cathode is 

 afterwards warmed in tbe liberated gas. Assuming that 

 this is the correct explanation, it follows that recombination 

 is also going on at the same time as the electrolysis, and the 

 question arises: Is this recombination the cause of the electron 

 emission ? To test this point observations of the pressure 

 were taken at intervals of 15 minutes with the high potential 

 continually applied to the terminals of the discharge-tube. 

 During the first 15 minutes a temperature of about 1400° 0. 

 was maintained, and a thermionic current of about 4 milli- 

 amperes passed. The temperature was then lowered to about 

 600° C, and kept at that value for 15 minutes, after which 

 it was raised again to 1400° C, several observations at these 

 two temperatures being taken alternately. With the cathode 

 at the lower temperature, no thermionic current could be 

 detected with a galvanometer giving L division deflexion for 

 a current of 1*94 x 10~ 9 ampere; but there was an average 

 decrease of pressure of '00063 mm. during these periods of 

 15 minutes. At the higher temperature, with the thermionic 

 current passing, there was an average increase of pressure of 

 about the same magnitude. Since the effect of the decreased 

 temperature of the cathode upon the gas pressure is almost 

 inappreciable, it is evident that there was an absorption of 

 gas taking place while the cathode was heated at 600° C. 

 This absorption is presumably due to the union of electro- 

 lytically liberated oxygen and calcium, and it was not accom- 

 panied by any detectable electron emission. It follows, 

 therefore, that the electron emission observed at the higher 

 temperature cannot be due simply to the recombination 

 of electrolytically separated calcium and oxygen ; at all 

 events the existence of the high temperature is an essential 

 condition. 



A similar conclusion also follows from the results of some 

 experiments made by the author in 1906, when comparing 



