400 Cause of Period of Chemical Induction. 



" Note on the Cause of the Period of Chemical Induction in the 

 Union of Hydrogen and Chlorine." By D. L. CHAPMAN and 

 C. H. BURGESS, Demonstrators in Chemistry in the University 

 of Manchester. Communicated by Professor H. B. DIXON, 

 F.K.S. Received January 14, Eead January 26, 1905. 



The induction period, which, in certain circumstances, is observed 

 when a mixture of hydrogen and chlorine is exposed to light, has been 

 commonly attributed to the preliminary formation of an unstable 

 intermediate compound. The authors have for some time held that 

 this view does not accord with the observed facts. This contention is 

 confirmed by recent experiments, which have established that the 

 phenomenon in question is due to the presence in the gas (or in the 

 aqueous solution in contact with it) of substances capable of reacting 

 with chlorine. 



At an early stage in the investigation it was proved that the 

 retardation of chemical action did not depend on any condition of the 

 hydrogen, thus making it only necessary to consider the condition of 

 the chlorine itself and of the other substances immediately in contact 

 with it. 



Water, and particularly solutions of salts in water, possess the power 

 of rendering active chlorine inactive towards hydrogen. On long 

 contact with chlorine in the presence of light, or on boiling with 

 chlorine, these solutions lose this property even after subsequent 

 removal of the chlorine by exhaustion. Numerous experiments have 

 recently been made in order to find out if these solutions recover their 

 power of rendering chlorine inactive, and it has been found that the 

 only method of effecting this is by the introduction of substances 

 which react with chlorine. Of the compounds investigated the most 

 efficient to use for this purpose is ammonia, minute traces of which are 

 capable of preventing the action between hydrogen and chlorine for a 

 period of time many times longer than had been previously observed. 

 Sulphur dioxide acts in the same sense as ammonia, but it is more 

 easily removed on exposure to light. 



It has been hitherto supposed that an active mixture of hydrogen 

 and chlorine, after standing in the dark for several hours, returned to 

 its original inactive condition. If this were really the case it would be 

 an objection to the view that the induction period is due to the presence 

 of impurities. By the employment of a quartz actinometer we have, 

 however, succeeded in showing that no decay of activity takes place ; 

 and, after keeping the mixture for days in the dark, the gases immediately 

 combine on exposure to light without any period of gradual 

 acceleration. 



