NEGATIVE ELECTRICITY FROM HOT CALCIUM AND FROM LIME. 153 



In some cases the attainment of the constant state was hastened by repeatedly 

 washing the apparatus out with dry air and heating the platinum in that gas. 

 When the pressure did not increase on heating the platinum, the apparatus was 

 pumped down to as low a pressure as possible, and helium was let in to a pressure of 

 3 or 4 millims. The sodium-potassium alloy was then let into the special discharge 

 tube through a well fitting tap, and the helium purified in the manner already 

 described. 



The relation between the current and the electromotive force was first investigated. 

 The results obtained were similar to those found by other observers for the negative 

 leak in air, oxygen, or nitrogen. For instance, at a pressure of O'OOS mi Him. the 

 current was saturated with a potential difference of 30 volts, the distance between 

 the electrodes being about 2 centims. With higher pressures of gas in the apparatus 

 the potential difference required to saturate the current was much greater, on account 

 of the formation of new ions by collisions with the gas molecules. With pressures 

 above about O'Ol millim. the current never became saturated, but increased more 

 and more rapidly with the potential as the latter was raised. 



Mention must be made of a curious increase in the negative leak which was 

 obtained whenever the cathode was allowed to remain for some time in a good 

 vacuum. For instance, if the apparatus was left at a low pressure (0'005 millim.) 

 over night, the leak was always found to be much larger when tested on the following 

 day. This increase was sometimes as much as a thousand times the normal current. 

 It gradually died away when the cathode was left at a bright red heat for some time. 

 On investigating this effect it was found that the increased leak was connected with 

 the appearance of a dark substance on the surface of the platinum strip. The amount 

 of this was very small, and it was only visible when the platinum was heated. It 

 disappeared on long continued heating, and its disappearance was always accompanied 

 by a huge decrease in the negative leak. This phenomenon only occurred at low 

 pressures. When the apparatus was left at a pressure of 2 or 3 millims., the negative 

 leak was found to remain practically constant. An effect similar to this seems to 

 have been obtained by Professor H. A. WILSON in his experiments on the discharge 

 of electricity from hot platinum. He says* : " If the wire is simply left standing in 

 air at a low pressure for a long time, the leak is often greater than before on again 

 heating the wire." I think that the black stuff which could be seen on the platinum 

 strip in my apparatus must have been some compound formed by the action of the 

 mercury vapour upon the platinum. WILSON found that mercury vapour increases 

 the negative leak very considerably at high temperatures. This seems to indicate 

 that there is some action between the two metals. 



The leak would sometimes increase enormously when the apparatus was left at a 

 low pressure for only a few minutes ; sometimes even in the course of an observation 

 it would increase to ten or twenty times the normal value. In one case a platinum 



* H. A. WILSON, ' Phil. Trans.,' A, vol. 202, p. 243, 1903. 

 VOL. CCVII. A. X 



