128 Prof. Thomson & Mr M'CleUand, On the Leakage of [Mar. 9, 



variable that they are useless for these measurements. We 

 generally found that a bulb got steadier after being used, indeed 

 we never found a bulb that was steady enough for our purpose 

 when first taken off the pump. 



The following table contains the rate of leak of different gases 

 at atmospheric pressure, and unless when otherwise stated at a 

 temperature of about 15° C, the rate of leak through air at this 

 temperature and at atmospheric pressure being taken as unity. 



Name of Gas. 



Hydrogen 



Coal gas 



Ammonia gas 



Sulphuretted Hydrogen 



Carbonic acid gas 



Sulphur dioxide 



Nitrogen peroxide (N 2 4 ) 



Chlorine 



Bromine (at the temperature 

 of boiling bromine) 



Iodine (at the temperature 

 of boiling iodine) 



Hydriodic acid gas 



Mercury vapour (at the tempera- 

 ture of boiling mercury) 200 10 



It will be seen from the preceding table that the electrical 

 conductivity of the heavier gases when exposed to the Rontgen 

 rays is greater than that of the lighter gases, the order for 

 electrical conductivity does not however exactly correspond with 

 the order of the molecular weight. A very marked feature of the 

 table is the high conductivity of the halogens, in fact the leakage 

 of electricity from a charged disc through a tube filled with 

 chlorine is a very sensitive and convenient measure of the inten- 

 sity of the Rontgen rays, and was used by us in many of the 

 experiments described in the later part of this paper. 



The high conductivity of mercury vapour is very remarkable, 

 for this gas only allows the ordinary electric discharge to pass 

 through with great difficulty, whereas under the influence of the 

 Rontgen rays it is one of the best conductors among the gases we 

 investigated. 



In the case of most of the preceding gases the rate of leak was 

 determined when the charged disc was electrified positively as 

 well as when it was electrified negatively, but no difference was 

 ever detected between the rate of leak in the two cases. 



