366 On the Passage of Electricity through Hot Gases. 



became black at once. Sal-ammoniac, another good con- 

 ductor, is known to dissociate into ammonia and hydrochloric 

 acid. In fact, whenever a gas became a good conductor when 

 heated, I was able to detect dissociation by purely chemical 

 means. 



The converse statement is not, however, true ; there are 

 many cases in which dissociation takes place without the gas 

 acquiring the power of conducting electricity. Thus, take the 

 case of ammonia; this at high temperatures is known to 

 dissociate into nitrogen and hydrogen ; but ammonia heated 

 to a temperature above that of dissociation, the dissociation 

 being proved by the absence of any effect on litmus-paper of 

 the hot gas rising from the tube, was an excessively bad con- 

 ductor, being worse than air. Again, steam at high tem- 

 peratures dissociates into hydrogen and oxygen ; but steam 

 even at these temperatures does not conduct electricity. The 

 dissociation of the steam which wo aid not conduct was proved 

 by sucking the hot gases quickly from the platinum tube ; when 

 a series of electric sparks were passed through the gases thus 

 sucked over, a large condensation took place. 



There are, however, two kinds of dissociation — the one 

 where atoms or unsaturated bodies are produced, as in the 

 dissociation of iodine, bromine, chlorine, hydrochloric acid, 

 hydrobromic acid, and hydriodic acid ; the other where a 

 complicated molecule is merely split up into simpler mole- 

 cules, as for example ammonia, which at temperatures below 

 that at which the molecules of nitrogen and hydrogen dis- 

 sociate, dissociates into nitrogen and hydrogen molecules ; or 

 steam, which at temperatures below that of the dissociation 

 of hydrogen and oxygen dissociates into molecules of these 

 gases. 



It is the first kind of dissociation which is always accom- 

 panied by conduction, while the second kind has little if any- 

 thing to do with it. This is just what we should expect if we 

 accept the view of the passage of electricity through gases 

 gh'en above, for in the first case we have the charged atoms 

 which can carry the electricity, while in the second we have 

 merely neutral molecules which have no such power. 



[To be continued.] 



