Discharge from an Electrified Point. 601 



Changes of Pressure accompanying Point Discharge 

 in Hydrogen. 



This view, that the negative ions in pure hydrogen are 

 corpuscular in nature, also throws light on the net changes 

 of pressure which result in closed vessels from point discharge 

 in hydrogen. 



If there is any oxygen present water is formed, and there 

 is in consequence a small decrease in pressure. It was 

 found, however, that even if oxygen is very carefully ex- 

 cluded, a very small contraction still occurs (loc. cit. Phil. 

 Mag. 1908). This effect was attributed to an absorption of 

 ions at the surfaces of the metal electrodes. With a copper 

 plate this contraction amounted to 2 atoms per ion — the ex- 

 pression " per ion "ibeing denned as " per hydrogen atom set 

 free in a water voltameter placed in series with the discharge 

 vessel/' At the time it was generally supposed that an ion 

 was a charged molecular cluster. From the results of the 

 work of Wellisch and others, on the mobilities of the ions, 

 there is now however considerable evidence for the theory 

 that an ion is either molecular or atomic in size at atmospheric 

 pressure. Now, if in hydrogen the negative ion is a corpuscle, 

 there can only be absorption of gas at one electrode, the 

 plate in positive discharge and the point in negative dis- 

 charge. The two atoms absorbed, " per ion," must thus be 

 carried in the gas by a single positive hydrogen ion. This 

 is, therefore, evidence for the view that the positive ion in 

 pure hydrogen at atmospheric pressure is a charged molecule. 



But if there was an evolution of gas from the cathode and 

 an absorption of gas at the anode, as has been found by 

 Skinner * to occur in glow-discharge in hydrogen at low 

 pressures, the observed fall in pressure might have been 

 merely a small difference effect. The author rejects this 

 view for the following reason. The point used was a piece 

 of the finest platinum, wire and therefore of very small 

 surface. By making it the cathode it could be easily de- 

 nuded of surface gases by discharge — at any rate temporarily. 

 The same electrodes were used for months without change, 

 and for the greater part of that time the point was the 

 cathode. If the Skinner effect were responsible for the result, 

 one would expect that after a long negative discharge from 

 the point, the contractions per coulomb in following dis- 

 charges would differ widely according to whether the 

 discharge was positive or negative — that is to say, according 

 to whether the plate or the denuded point was the cathode. 



* Skinner, Phil. Mag. xii. p. 481 ; Tliys. Rev. xxi. pp. 1 & 169. 

 Phil. Mag. S. 6. Vol. 21. No. 125. May 1911. 2 \\ 



