Action of a Wehnelt Cathode. 251 



'the heavy discharge passes may perhaps be due to the high 

 temperature of the lime at the points from which the cathode 

 rays proceed ; and it is interesting to note in this connexion 

 -that some time ago Sir J. J. Thomson * suggested that the 

 origin of the large electron emission from barium oxide was 

 connected with a chemical transformation from BaO to Ba0 2 

 ■at the high temperature of the cathode. A peroxide of 

 calcium also exists, so that it is possible that a reversible 

 reaction of this nature goes on in the case of lime. It is, 

 however, very difficult to obtain accurate information about 

 chemical reactions which may be taking place at the high 

 temperatures and low pressures of these experiments. 



Conclusion. 



From experiments made by the author in 1906 it; appeared 

 that the activity of a Wehnelt cathode is not due to an 

 escape of electrons from the molecules of lime simply as a 

 result of an increase in their thermal energy; for on this 

 view we should expect that the presence of the electro- 

 negative oxygen atom in the lime molecule would hinder the 

 escape of electrons, and that, in consequence, the electron 

 -emission from lime would be less, at a given temperature, 

 than the emission from the same amount of calcium in the 

 -metallic state. The experiments referred to shewed that 

 the reverse was the case, and that the emission from calcium 

 is less than that from lime. The results of the experiments 

 -described in the present paper may be summarized as 

 follows : — 



(1) The electron emission from a Wehnelt cathode has its 

 origin in the lime itself, and the lime does not merely serve 

 to help the electrons to escape from the metal. 



(2) When an electric current passes through lime at a 

 high temperature, the amount of oxygen liberated is only 

 a very small fraction of what would be expected if the con- 

 ductivity of the lime were entirely electrolytic. It has been 

 suggested that the conductivity is entirely electrolytic, and 

 that the products of electrolysis diffuse through the lime 

 niul recombine; but it appears to the author to be improbable 

 that such recombination should go on, so rapidly and com- 

 pletely, through a laver of solid lime, especially ;is the 

 charged oxygen atoms would be liberated on the vacuum 



* 'Conduction of Electricity through Gases,' p. 427. Camb. Univ. 

 Press, 190(3. 



