646 SCIENCE PROGRESS 



that the implication constantly set before students of chemistry 

 that combination or interaction takes place between two com- 

 pounds A and B is incorrect and that in all instances, to 

 promote change, a third substance must be coupled with these, 

 the third " substance " being of such nature that, if not itself an 

 electrolyte, it is one from which an electrolyte is produced by 

 association with either A or B, the presence of an electrolyte 

 being an essential feature in the occurrence of change. 



The student is invariably told that oxygen is a supporter of 

 combustion and is led to think that oxygen and the burning 

 substance are the two factors in the process. But as a matter 

 of fact, carefully dried carbon and phosphorus do not burn in 

 carefully dried oxygen ; they will take fire only in the moist gas. 

 And although ammonia and hydrogen chloride prepared in the 

 ordinary way combine with the greatest readiness, forming solid 

 ammonium chloride, if very special care be taken to dry both 

 gases nothing happens when they are mixed. 



Again, an ordinary mixture of hydrogen and oxygen is 

 exploded with the greatest readiness but if prepared with 

 special care by electrol3'Sing an alkaline solution of baryta, so 

 that it contains no trace of acid, the mixture, even if dried 

 simply by freezing out water from it by means of liquid air, is 

 no longer explosive. A coil of silver wire may be heated to 

 redness in the dried mixture without firing it — yet liquid water 

 gradually makes its appearance, the interaction taking place 

 slowly and without explosive violence. This result is one of 

 the very greatest importance, as it is a proof that the presence 

 of water is not in itself sufficient to bring about the change : 

 nor should it be, from the electrolytic point of view advocated 

 above, as water is not an electrolyte ; only " dirty " water 

 behaves as an electrolyte — that is to say, water containing some 

 substance dissolved in it — a trace of an acid, of an alkali or of 

 a salt, as the case may require. There can be little if any doubt 

 that in cases in which moisture alone has been observed to 

 condition change, it is because traces of "impurity" were 

 present together with water ; obviously it is impossible to 

 avoid such and it must therefore always be impossible to 

 prevent a change such as that involved in the formation of water 

 from the elementary gases hydrogen and oxygen from taking 

 place to some slight extent. 



It would be ungracious to pass from this subject without 



