On the Kathode Fall of Potential in Gases. 



357 



and the current not so strong as to make the negative glow cover the 

 whole kathode or extend to the walls of the tube, the kathode fall 

 has a definite value for each gas a value that is independent of the 

 pressure of the gas, or of the current strength, and that appears, in 

 fact, to be a constant of the gas. 



This being the case, it is probable that the kathode fall will prove 

 to be connected with other physical and chemical constants of the 

 gas, and the aim of the experiments described below was to find 

 whether there is any intelligible relation amongst the kathode falls of 

 three gases, one of which is formed by the combination of the other 

 two. 



The choice of suitable gases is very limited, for there must be no 

 deposition of solid matter by the current, and no chemical action 

 between the gas and the electrode, so that organic gases and gases 

 containing a halogen cannot be used at least with metal electrodes. 

 The present investigation has been confined to water vapour, ammonia, 

 and nitric oxide and their constituents. 



The general plan of the apparatus was the same for all the gases, 

 and is shown in the figure below. 



The gas generating apparatus was sealed on at A, and a mercury 

 pump at L. In order to isolate the generating flask from the rest, 

 the mercury trap B was used. By raising the reservoir C, the mer- 

 cury could be made to rise above the bend at B, and thus everything 

 to the right could be exhausted. D is a small bulb to catch stray mer- 

 cury. E is the purifying apparatus, and F the vessel in which the 

 discharge took place. This vessel consisted of a globe 15 cm. in 

 diameter, into which were sealed three wires ; Gr, the anode of alumi- 

 nium, 2 mm. thick ; K, the kathode of platinum, 2 mm. thick, and 

 extending 10 cm. into the globe, so as to afford plenty of free space 

 for the negative glow ; and H, a thin platinum wire, covered with 

 glass to within a millimetre of the tip. 



