250 Dr. F. Horton on the 



the electron emission from calcium with that from lime *. 

 In these experiments a platinum strip was covered with 

 calcium by sublimation in a vacuum ; an excess of oxygen 

 was let into the discharge-tube and the calcium was oxidized 

 to lime. No detectable ionization occurred during this 

 process of oxidation in the cold, nor even at a temperature 

 of 500° or 600° C, under which conditions the oxidation 

 must have been very rapid. It was only when the lime 

 formed had been raised to 700° 0. that a measurable therm- 

 ionic current was obtained. 



In this part of the present paper the term "luminous 

 discharge " has so far been used to indicate that faint 

 luminosity of the residual gas which appears gradually as 

 the temperature of the cathode is raised, but which is not 

 accompanied by luminous pencils of cathode rays. At 

 higher temperatures, or with greater applied potential dif- 

 ferences, the nature of the discharge alters; brightly luminous 

 pencils of cathode rays appear, starting from points of lime 

 which are visibly hotter than the rest of the cathode, 

 and at the same time there is a sudden increase in the 

 thermionic current. The alterations of pressure which occur 

 during the passage of such a discharge are generally quite 

 different from those recorded above. In the early stages of 

 heating a cathode newly coated with lime there is usually 

 a considerable evolution of gas ; but if a brightly luminous 

 discharge is sent from a cathode which has been used for 

 some time, through air at a low pressure, there is nearly 

 always a diminution of pressure produced. This effect was 

 observed by the author some time ago f, and a spectroscopic 

 examination was made to see if the passage of the discharge 

 alters the chemical constitution of the residual gas. Some 

 new lines in the red part of the spectrum were observed, but 

 these were shown to be due to mercury {. 



During experiments with this type of discharge the current 

 passing is several times as large as that with no luminous 

 pencils of cathode rays ; and it is certainly remarkable that 

 a thermionic current of 4 milliamperes is accompanied by an 

 increase of gas pressure in the apparatus, whereas a diminution 

 of pressure is produced by a current of 20 milliamperes. 

 The effect of these large currents is, however, uncertain; on 

 some occasions, even after long-continued heating, an increase 

 of pressure was produced. The diminution of pressure when 



* Phil. Trans. A. ccvii. p. 149 (1907). 



t Phil. Mag. [6] xi. p. 505 (1906). 



X Proc. Camb. Phil. Soc. xiv. p. 501 (1908). 



