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DR, FRANK HORTON ON THE ORIGIN OF THE 
The arguments in support of the chemical action theory of the origin of the 
electron emission from glowing solids are based upon two experimental results :— 
(1) The reduction which takes place in the electron emission on purifying the 
element under test and on improving the vacuum in which the test is 
conducted. 
(2) The increased activity which is produced when substances known to have 
chemical action upon the cathode are introduced into the discharge 
chamber. 
These results point to a connection between electron emission and the possibility 
of chemical action, and it is claimed that the small value of the emission which is still 
obtained after continued purification must not be regarded as a true thermal effect, 
for it can be accounted for as being due to remaining traces of gaseous or other 
impurities. On the other hand, it can be urged with equal force that this remaining 
effect shoidd not be regarded as due to chemical action until it has been shown to 
occur under conditions which preclude the possibility of an emission of electrons as 
a result of an increase in their thermal energy by rise of temperature. Experiments 
which have been made with these precautions have almost invariably indicated that 
chemical action is not by itself the cause of ionisation. Evidence to this effect was 
obtained by the author some time ago when comparing 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 oxidised to lime. No detectable ionisation 
occurred during this process of oxidation in the cold, nor even at a temperature of 
500° C. 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° C. or 800° C. that a 
measurable electron emission was obtained. In the case of the strongly electro¬ 
positive alkali metals it is not impossible that an electron emission (as a result of 
thermal energy) occurs at the ordinary temperature of the laboratory, and if an 
increased emission does occur in the presence of a reacting gas, this may be due, not 
directly to the chemical action, but to the rise of temperature which accompanies it. 
This momentary increase of temperature may be very considerable in the case of the 
surface layer, although the rise in temperature of the bulk of the metal may be quite 
inappreciable. The fact that chemical action and rise of temperature are usually 
closely associated in the type of reaction with which we are concerned makes 
. it very difficult to prove that an observed thermionic emission is a result of either 
chemical action or temperature alone. 
A process which causes the liberation of electrons from matter must act either by 
increasing the kinetic energy of the electrons or by diminishing the work which 
an electron must do in order to escape. An increase of temperature acts in the 
former manner, but a disturbance of the motions of some of the electrons would also 
