1886.] Prof. Thomson, On some experiments, &c. 391 



The general belief of the people is that the Moors, when pressed 

 hard during their expulsion from the country, took refuge in these 

 caves. — TYis whole cave is about 30 ft. long. Near the further 

 end I found the almost complete half-fossilized skeleton of a 

 goat. 



May 10, 1886. 



Prof. Foster, President, in the Chair. 



The following communications were made to the Society : 



(1) Some experiments on the electric discharge in a uniform 

 electric field, with some theoretical considerations about the passage 

 of electricity through gases. By Professor J. J. Thomson. 



As the experiments which have hitherto been made on the 

 discharge of electricity through gases have in general been ar- 

 ranged in such a way that it is difficult to calculate what was the 

 state of the electric field before discharge took place, I have 

 thought it might be interesting to make some experiments when 

 the state of the field was accurately known. For this reason I 

 made the discharge take place between two parallel plates sepa- 

 rated by a distance which was but small in comparison with their 

 diameters. 



Fig. 1. 



The arrangement used is represented in fig. 1. A BCD, JEFGS 

 are two cast-iron plates, the flat portions of which are about 

 6 centimetres in diameter, and 1^ centimetres apart. They are 

 shaped as in the figure, special care being taken to make the 

 curved parts of the plate smooth and free from places of large 

 curvature; the object of this as well as the peculiar shape of the 

 electrodes is to make the electric field much less intense in those 

 places where it is not uniform than in those places where it is, 

 so that the discharge will take place in the uniform field and not 



