512 Dr. R. S. Willows on the 



much greater with hydrogen than with other gases ; generally 

 the opposite was the case. With B cathode, the resistance 

 was not very different whether the tube was filled with 

 hydrogen, nitrogen, or air. 



The appearance of the discharge was also peculiar. When 

 B was anode the discharge was bright green next to the 

 outer cylinder, and gradually shaded away until midway 

 between the electrodes the luminosity was very weak, finally 

 next to the cathode it was red. The cathode hydrogen 

 spectrum generally gives a large number of lines : in this 

 case it only gave the red, blue, and green. 



The difference between the rates of running down, accord- 

 ing to the direction of the discharge, although distinct, was 

 not so well marked when an induction-coil was used as the 

 source of current. This is probably what we should expect, 

 for with this means of producing the discharge the current is 

 not easily measured, and so it is very likely that the currents 

 in the two directions are very unequal, that from B to A 

 being the less, since it has been noticed that the resistance is 

 greater when this is its direction. 



The apparatus was refilled several times with hydrogen, 

 but the same peculiarities always showed themselves. 



On taking the tube to pieces, it was found that the outer 

 cylinder had become covered with a thin white film in some 

 parts, probably oxide of aluminium; while other places were 

 marked by the presence of sputtered aluminium, giving rise 

 to brilliant bands of different colours. 



8. So far no suggestion has been made as to what becomes 

 of the gas tbat disappears. As a raison d'etre for some of 

 the experiments which follow, I will briefly notice some 

 possible explanations. 



Either the gas combines chemically with the glass, the 

 mercury vapour, or the electrodes, or the carriers or ions are 

 shot at the sides of the tube where they are held either elec- 

 trically, if they do not lose their charge, or mechanically by 

 penetrating into the walls of the tube *. 



With reference to the first of these alternatives, the 

 mercury vapour may be left out of account since it was 

 found that gas disappeared even when care was taken to 

 exclude it from the tube. A cylindrical tube was pumped 

 down by means of a mechanical pump to about '75 mm., as 



* According to W. Rollins (Elec. Rev. April 11th, 1900, p. 358) it 

 appears that it has been thought that the gas escapes through the walls 

 of the tube, a not altogether impossible idea at first sight when we 

 remember that the carriers in Lenard rays can be shot through thin 

 layers of aluminium. 



