CONDITIONS OF CHEMICAL CHANGE 75 



clusively that the addition of a small volume either of hydrogen 

 or of chlorine to a mixture of chlorine and hydrogen in equiva- 

 lent amounts did not appreciably affect the sensitiveness of the 

 mixture. 1 It may here be mentioned that the hydrogen used by 

 Bunsen and Roscoe to dilute the mixture was prepared by the 

 electrolysis of dilute sulphuric acid and probably contained 

 oxygen derived from the electrolyte. 



As we have already seen, the power of retarding the photo- 

 chemical interaction of chlorine and hydrogen had been shown 

 to be a property of two substances, oxygen and nitrogen 

 chloride, the effect of the latter being incomparably greater than 

 that of the former. The question arose, Is the property com- 

 mon in some degree to all substances or is it limited to a 

 special class of gases and vapours related in some unknown way 

 to chlorine ? To answer this question, the effect of adding 

 small amounts of a large number of volatile substances to 

 electrolytic gas had to be investigated. Accordingly an ap- 

 paratus was devised by means of which a measured volume of 

 the gases to be tested could be introduced into the insolation 

 vessel of an actinometer which contained a sensitive mixture of 

 chlorine and hydrogen. A series of experiments disclosed the 

 fact that the inhibitors belong to a special class of substances 

 and that substances outside this class exert an inappreciable 

 influence on the rate of interaction. Moreover, all the sub- 

 stances which were proved to retard the action at all were also 

 shown to be capable of exerting an inhibitive influence of 

 surprising magnitude. The inhibitors discovered were nitric 

 oxide, chlorine peroxide and ozone. 2 In the case of nitric 

 oxide, it is not certain whether the true inhibitor is nitrosyl 

 chloride or peroxide of nitrogen. On entering the actinometer, 

 the nitric oxide would be converted almost immediately into 

 nitrosyl chloride but this compound would be acted upon, 

 perhaps very rapidly, by the water vapour present and con- 

 verted into nitrogen peroxide. Nitrosyl chloride undoubtedly 

 retards the interaction of dried chlorine and carbon monoxide ; 

 but it is doubtful if it can exist more than a short length of time 

 in the presence of moisture. It is not improbable, therefore, 

 that both nitrosyl chloride and nitrogen peroxide prevent the 

 interaction of chlorine and hydrogen. As might have been 



1 Chapman and MacMahon, Trans. Chan. Soc. 1909, 95, 135. 

 3 Ibid., Trans. Chem. Soc. 1909, 95, 1717, and 1910, 97, 845. 



