274 
DR. HAROLD A. WILSON ON THE 
We may suppose also that the energy necessary to enable a corpuscle to escape 
from a hydrogen atom at the surface falls oft as the concentration of the hydrogen 
atoms oil the surface increases. In this way the diminution of Q by the presence of 
hydrogen can be explained. According to this view the number of corpuscles in a 
cubic centimetre of the metal cannot be deduced from the constant A. 
It seems prohalile tliat the temporary positive leak from platinum and palladium, 
when they are first heated, is due to the presence ot a small amount of some 
decomposable compound or volatile impurity. This view is supported by the fact that 
platinum wires, well cleaned with nitric acid, do not give this effect to an extent 
appreciable on the galvanometei'. 
The positive leak in gases at high pressures is no doubt due to ionization of the 
gas molecules at the surface of the platinum. 
In conclusion, I wish to say tliat my best thanks are due to Professor J. J. Thomson 
for advice and kindly interest in tliese experiments while they were being carried out 
at the Cavendish Laboratory. 
Appendix. 
Added Octoher 3, 1903.—Some interesting conclusions can be drawn from the way 
in which the negative leak from hot platinum or palladium in hydrogen varies with 
the pressure of the hydrogen. We have seen that when the pressure of the hydrogen 
is less than about 0-02 millim. the leak is nearly proportional to the pressure. 
Consider two temperatures and Let the leak at the temperature 6-^ be 
represented liy the ecpiation x, =: where rPi is the current per scpiare centim., 
p the pressure, and a constant. Let the corresponding equation at do be aq = Bop. 
If these expressions for .rq and x., are substituted in the equation 
^ ^ d, do 
1 d,-.r.o 
“ ^ dokci ’ 
we see that Q is independent of the pressure of the hydrogen. Consequently, vhen 
the pressure of the hydrogen is small, the energy required for the liberation of a 
corpuscle is independent of the pressure of the hydrogen. 
Consider now the formula x = Since Q is independent of p and .x = Bp, 
it follows that A is proportional to p. It is clear, therefore, that A cannot be 
regarded as proportional to the number of corpuscles in a cubic centimetre of the 
platinum, because this latter number must be Independent, or very nearly so, ofp. 
A is evidently proportional to the number of corpuscles which are in a position to 
contribute towards the leak, so that we may conclude that this latter number is 
proportional to the amount of hydrogen present when this amount is small, riius ve 
