294 DR. F. HORTON: ORIGIN OF ELECTRON EMISSION FROM GLOWING SOLIDS. 
Summary and Conclusion. 
The experiments with Nernst filaments have shown that the oxides of which these 
are made suffer electrolysis when conducting an electric current, but that the electron 
emission which occurs at high temperatures is not due either directly or indirectly to 
this electrolysis, for the magnitude of the emission at a given temperature is the same 
whether the current through the material is great or small. It is therefore concluded 
that the emission from lime in a Wehnelt cathode is not due to the re-combination of 
electrolytically separated calcium and oxygen, as has been suggested. 
Owing to the increasing chenfical activity of substances at high temperatures, and 
to the impossibility of removing all traces of impurities from the discharge tube, it is 
very difficult to prove conclusively that any known case of electron emissions is purely 
a thermal effect; but, on the other hand, there is no satisfactory evidence that 
ionisation is ever produced by chemical action alone. It would seem, therefore, that 
the theory that thermal agitation is by itself sufficient to cause the emission of 
electrons from matter could only be disproved by reducing the thermionic current from 
a metal at the highest attainable temperature to zero. 
The increased ionisation from a glowing cathode which occurs when a chemically 
active gas is allowed to enter the discharge tube may arise in several ways :—- 
(1) It may be directly due to chemical action ; 
(2) It may be indirectly due to chemical action, arising as a consequence of an 
increase of temperature resulting therefrom, or from the product of the 
action having a greater activity than the original substance of the cathode ; 
(3) It may be due to an alteration of the surface conditions at the cathode in, such 
manner as to make it easier for the electrons to escape, for instance, by the 
formation of an electric double layer ; 
(4) It may be due to ionisation by collisions. 
It seems probable that the cause of the increased activity is different in different 
cases. 
The author wishes to acknowledge his indebtedness to the Government Grant 
Committee of the Royal Society for the means of purchasing some of the apparatus 
used in these experiments, and also to Prof. Sir J. J. Thomson for his, advice and 
interest in the research, which was carried out in the Cavendish Laboratory. 
