454 On Leakage of Electricity from Charged Bodies. 



substance acts as an iron body at low red-heat does, retaining 

 negative but not retaining positive electricity. 



In some instances — such as potassium permanganate, 

 potassium chlorate with manganese dioxide, potassium iodide 

 with bromine on zinc — the phenomenon is complicated by 

 the fact that the heating causes solid particles to fly about to 

 such an extent as to give no steady reading on the quadrant- 

 electrometer when these substances are insulated without 

 charge and heat applied. These substances also, when heated, 

 discharge negative as well as positive electricity even when 

 the electricity is at a potential of 200 volts. 



For substances such as salt with iodine on zinc, which are 

 found to insulate at higher voltages for negative but not for 

 positive charges, an explanation may be offered founded on 

 Enrioht's* and Townsend's f experiments. Townsend shows 

 that hydrogen given off when metal is dissolved in an acid 

 carries off with it a positive charge. If, then, we assume that 

 a gas is given off by these substances, and that it carries a 

 positive charge with it, we can understand why a positive 

 charge leaks away. 



In the case of salt with iodine on zinc strips we have the 

 possibility of the contemporaneous existence of common salt, 

 sodium iodide, zinc iodide, zinc chloride. It has been shown 

 that the first two, when heated, do not give the peculiar effect 

 observed. The second two do show this effect even in the 

 absence of zinc ; and the fact that to restore the capability of 

 discharging positive electricity more iodine has to be added, 

 might lead one to explain the result by saying that zinc iodide 

 was formed and afterwards at a higher temperature this gave 

 off iodine which carried a positive charge with it. 



An attempt was made to solve the problem by chemical 

 analysis, but the results were not promising. 



It is proposed to test by Kelvin's electric-filter method 

 whether or not the gas is charged ; and, if so, how. It may 

 also be possible to collect sufficient of the gas to examine it 

 directly. 



I have to thank one of my assistants, Mr. Michell, for 

 much help in the laborious work of observing. 



Physical Laboratory, 



South African College, Cape Town. 



June 1900. 



* Enright, " Electromotive Force of Contact between a Liquid and a 

 Gas," Phil. Mag. xxix. p. 56 (1890). 



f Townsend, Proceedings of the Cambridge Philosophical Society, 

 vol. x. part .5, 1897. 



