294 



Drs. W. De La Rue and H. W. M filler. [June 14, 



smaller tube 23 inches (58*4 centims.) long, and 075 inch (1'9 centims.) 

 diameter (fig. 3), it was 1 millim., 1316 M. It is evident, therefore, 

 that not only the dimensions of the tube, but possibly also the shape 

 of the terminals, have an influence on the pressure of least resistance, 

 and it is very probable that in the atmosphere, where lateral expan- 

 sion is practically unlimited, the conditions of minimum resistance are 

 different from those which exist even in very large tubes, and that 

 this may influence the height of the aurora. 



The paper next deals with the discharge in miniature tubes § inch 

 (2*2 centims.) long, and J inch (0*63 centim.) diameter, with ter- 

 minals nearly touching (fig. 4) ; at first it required 2,400 cells to pass, 



then a single cell would do so, but after standing a short time it 

 required 4,800 cells to reproduce a discharge. In another tube 

 If inches (4'4 centims.) long, and f inch (0'95 centim.) diameter, with 

 the terminals distant 00104 inch t (0'0264 millim.), it required 2,240 

 cells to produce a discharge, then the potential had to be increased to 

 11,240 cells to do so. Ultimately even this number failed, but after 

 the tube had lain by for some days, 600 cells could pass. It is very 

 possible that the strong discharge in the first instance volatilized a 

 portion of the terminals which were of platinum, and that this 

 volatilized metal condensed afterwards, or else that the terminals 

 absorbed the gas so completely as to produce a vacuum too perfect to 

 admit of a discharge taking place; and that, ultimately, sufficient of 

 the occluded gas was again given off to render it again possible. 



In connexion with the occlusion of gas by terminals, a case is 

 described in which the terminals are of palladium, and the charge 

 hydrogen (fig. 5) . After a few discharges the terminals occluded some 

 of the gas, and when a fresh one was produced, a volatile compound of 

 hydrogen and palladium was given off, especially from the negative, 

 and produced a dense mirror-like coating on the inside of the tube 



