ELECTRON EMISSION FROM GLOWING SOLIDS. 
293 
appear to be an inevitable result of an atom taking part in a chemical reaction, and if 
this does occur, it would be not improbable that such disturbance should lead, in 
some cases, to an electron emission. Experimental evidence shows that when atoms 
enter into chemical reactions at ordinary temperatures, it is rarely (if ever) that the 
accompanying disturbance increases the kinetic energy of any of the electrons 
sufficiently to enable them to escape ; but if the conditions are arranged so that the 
electrons already possess considerable kinetic energy in virtue of their thermal 
agitation, it would seem possible that the stimulus of chemical action should have 
the effect of increasing this energy sufficiently to cause some of the electrons to 
overcome the forces tending to retain them in the substance. It seems, therefore, 
that chemical action at a high temperature might be expected to result in an electron 
emission in cases where the temperature itself is not sufficient to produce this effect, 
or where it produces it only to a much smaller extent. This conclusion is supported 
by the fact that an increased emission occurs when certain chemically active gases 
are allowed to enter the discharge tube, but it must be remembered that there is 
another manner in which these gases may act in aiding the escape of electrons from 
the glowing cathode, namely, by causing an alteration in the surface conditions 
whereby the minimum amount of energy an electron must possess in order to escape 
from the cathode is reduced. Such a reduction might be caused by the formation of 
an electric double layer at the surface of the cathode which would create a force 
tending to drag out the electrons. There is already a considerable amount of 
evidence of the existence of these double layers in certain cases, and this view has 
been put forward to explain the action of hydrogen in increasing the electron 
emission from platinum.* 
The author is of opinion that at present there is not sufficient experimental 
evidence to enable us to explain the manner in which the presence of a gas in the 
discharge tube acts in increasing the negative thermionic emission, but it is not 
improbable that the method by which this increase is brought about is different in 
different cases; for instance, when oxygen comes in contact with a calcium cathode 
at a high temperature, the increased emission is doubtless due to the fact that the 
emission from the product of the chemical action—■lime—is greater than the emission 
from calcium at the same temperature. The possibilities of ionisation by collisions 
need also to be carefully investigated. Recent experiments have shown that the 
electric intensity required to produce this effect is very much smaller than has been 
supposed, and the process is probably always going on near a glowing cathode at low 
pressures. It must be remembered also that the gas pressure in the neighbourhood 
of a cathode which is continually evolving gas may be considerably greater than that 
measured in more remote parts of the apparatus, so that it is difficult to estimate the 
magnitude of the effect of ionisation by collisions. 
* ‘ Conduction of Electricity through Gases,’ Camb. Univ. Press, 1906, p. 203. 
