278 
DR. FRANK HORTON ON THE ORIGIN OF THE 
This chemical action theory has received considerable support from the experiments 
of Haber and Just,* and of FredenhagenI on the alkali metals, and of Pring and 
Parker! on carbon. The experiments with the alkali metals have shown that by 
repeated purification (re-distillation) and continued improvement of the vacuum the 
electron emission from these substances can be continually reduced. If, however, a 
small amount of air or of any gas having chemical action upon the metal under test 
is allowed to enter the apparatus a large increase in the thermionic current at once 
occurs. 
Similarly with carbon, the negative thermionic current decreases continuously 
as the gas pressure is reduced and with progressive purification of the carbon, and, 
moreover, by admitting small amounts of different gases to the carbon an increased 
current is obtained, the increase being proportional to the known chemical activity of 
these gases. 
Richardson § has recently described some experiments with a tungsten filament 
heated to a high temperature in a good vacuum, and has considered the following 
different hypotheses as to the possible mode of origin of the electronic emission :— 
(1) The emission is due to the evolution of gas by the filament; 
(2) The emission is caused by chemical action or by some other cause depending on 
impacts between gas molecules and the filament; 
(3) The emission is the result of some process involving consumption of tungsten. 
Experiments were devised for the purpose of testing each of these theories, and in 
each case it was found that the observed emission was very much greater than could 
be accounted for by the hypothesis. It was, therefore, concluded “that the emission 
of electrons does not arise from any interaction between the filament and the 
surrounding gases or vapours, nor from any process involving consumption of the 
material of the filament. It thus follows that the emission of electrons from hot 
tungsten, which there is no reason for not regarding as exhibiting this phenomenon 
in a typical form, is not a chemical but a physical process.” These experiments 
cannot be regarded as conclusive, for there is a possible cause of the electron emission 
which has not been considered, namely, that it arises from an action between the 
metal filament and gases contained in it. In testing the second hypothesis mentioned 
above, Richardson considers only the possibility of chemical action between the 
metal filament and the gas surrounding it. The fact that the thermionic emission 
from a hot wire in a good vacuum decreases 011 driving out the gas by long continued 
* Haber and Just, ‘Ann. der Phys.,’ vol. XXX., p. 411, 1909, and vol. XXXVL, p. 308. 1911. 
f Fredenkagen, ‘ Verh. d. Deutsch. Phys. Ges.,’ vol. 14, p. 384, 1912. 
f Pring and Parker, ‘Phil. Mag.,’ vol. 23, p. 192, 1912, and Pring, ‘Roy. Soc. Proc.,’ A, vol. 89, 
p. 344, 1913. 
§ Richardson, ‘Phil. Mag.,’ vol. 26, p. 345, 1913. 
