364 Prof. J. J. Thomson on the Passage 



experiments which I shall describe later I am inclined to think 

 that in order for the electricity to get with ease from the 

 gas to the electrodes the latter must be glowing (and there- 

 fore disintegrating?). 



The results obtained by using the platinum tube are given 

 in the following table. The results for metallic vapours are 

 not given here, as with this arrangement the temperature 

 could not be raised high enough to volatilize many metals, so 

 that a different method had to be used for these. 



Behaviour when heated to a 

 Substance. yellow heat# 



Air Small deflexion, about 10 or 12 di- 

 visions for 156 Daniells. 



Nitrogen Much the same as air. 



Carbonic acid Rather less than air. 



Ammonia Rather less than air. 



Steam Considerably less than air. 



Hydrochloric acid Large deflexion ; spot driven right off 



scale by 12 Daniells. 



Hydriodic acid Very large deflexion indeed : larger 



than for HC1. 



Sulphuric acid Small deflexion ; very little more 



than air. 



Nitric acid About the same as air. 



Iodine , Very large deflexion ; comparable 



with HI. 



Bromine Large deflexion. 



Potassium iodide Conducted fairly well, but not so 



well as HI, HC1, or I. 



Sal-ammoniac ...,».. Conducted well. 



Sulphur (surrounded by nitrogen). Small deflexion. 



Sulphuretted hydrogen (ditto) Very small deflexion until the tem- 

 perature was raised to a white 

 heat. 



Mercury Very small deflexion indeed ; much 



smaller even than air. 



Sodium chloride Large deflexion. 



Potassium chloride Large deflexion. 



In this table the gases which conduct well are printed in 

 italics. Several of these are known to dissociate at high 

 temperatures ; thus iodine and bromine are known to do so 

 to a very large extent, and the dissociation of the vapour of 

 hydriodic acid, perhaps the best conductor in the list, is 

 rendered very striking by the change in the colour of the 

 vapour, which is brown at low temperatures and purple at 

 high. There are, however, many substances in the list whose 

 dissociation has not been observed, and in some cases cannot 

 be great: thus the experiments on the vapour-density of 



