2±8 Dr. F. Horton on the 



filament are rather larger than those for the case of lime 

 heated upon platinum ; but considering the difficulties in 

 making the comparison the differences are small, and we 

 are justified in concluding that, in these two cases, the 

 emission from lime does not depend on the nature of the 

 material upon which it is heated. 



II. 1 he Liberation of Gas from a Wehnelt Cathode. 



Observations have been made of the rate at which the gas 

 pressure in the apparatus increases when the lime-covered 

 platinum tube is acting as a cathode, and when it is heated 

 but no thermionic current is allowed to pass. For this 

 purpose the gas pressure was taken down as low as possible 

 by means of a carbon tube cooled in liquid air ; this tube 

 was then shut off by a tap. The mercury-pump was 

 separated from the discharge-tube so that the volume into 

 which the evolved gas could diffuse was considerably 

 reduced, being now only that of the discharge-tube, the 

 phosphorus pentoxide drying- tube, the McLeod gauge, and 

 the connecting tubes — in all 550 c.c. Taking observations 

 at intervals of 15 minutes with the platinum tube at 1360° C, 

 alternately with no thermionic current passing and with a 

 current of an average value of 8 milliamperes, it was found 

 that the pressure continually increased during the first two 

 hours' heating, but at a rather greater rate when the discharge 

 was passing than when no discharge passed. The increase of 

 pressure per minute, however, in both cases became less as 

 the heating was continued, until after two hours there was 

 a small decrease of pressure in the absence of the discharge. 

 The largest observed increase of pressure in 15 minutes with 

 a thermionic current of 8 milliamperes was '0007 mm., the 

 mean increase during the periods with no discharge imme- 

 diately preceding and following this being '0002 mm. If 

 the difference between these two increases of pressure is due 

 to oxygen gas liberated by electrolysis of the lime under the 

 action of the thermionic current passing through it, the 

 amount of oxyger; so liberated is less than one-thousandth 

 of that which would be set free according to Faraday's law, 

 on the assumption that the conduction of the lime is entirely 

 electrolytic, and that no recombination of the separated 

 elements takes place. It was thought possible that part of 

 the difference between these two increases of pressure was 

 due to the extra heating of the residual gas by the passage of 

 the luminous discharge. Observations were therefore made 

 to test this; but it was found that the pressure alteration due 



