CONDITIONS OF CHEMICAL CHANGE 77 



inert substances such as carbon dioxide and nitrogen are in- 

 capable of reducing the rate of the photochemical process nor 

 has any reducing substance been discovered which possesses 

 inhibitory properties. 



Chlorine monoxide and nitrous oxide, though oxidising 

 agents, exert no influence on the change. That chlorine 

 monoxide should be incapable of modifying the rate of inter- 

 action of moist chlorine and hydrogen is not astonishing, as 

 moist chlorine gas almost certainly contains a small proportion 

 of the lower oxide of chlorine : a solution of chlorine in water 

 consists largely of hypochlorous acid and it would be surprising 

 if the vapour of the latter substance were not to some extent 

 dissociated into chlorine monoxide and water vapour. 1 



Nitrous oxide, although usually classified as an oxidising 

 agent, since it supports combustion, is probably incapable of 

 parting with its oxygen at the ordinary temperature. There is, 

 in fact, reason to suppose that the molecules of this gas are so 

 stable that it may be regarded as an inert substance except at 

 elevated temperatures. 



Oxygen, ozone, nitrogen chloride and nitrosyl chloride also 

 retard the interaction of chlorine and carbon monoxide, the 

 effect of the nitrosyl chloride being in this case permanent, 

 since it is not destroyed by light and no water is present to 

 effect its removal. Luther and Goldberg have shown that 

 oxygen retards the interaction of chlorine and benzene ; and in 

 a research which has not yet been published, Mr. R. Atkin has 

 found that some of the other substances which retard the inter- 

 action of chlorine and hydrogen act inhibitively towards the 

 union of chlorine and benzene. Each known inhibitor appears 

 to be capable of exerting a retarding influence on all photo- 

 chemical actions in which chlorine takes part. 



We may here draw the attention of the reader to a remark- 



1 Either chlorine monoxide or hypochlorous acid may be an intermediate 

 product in the formation of hydrogen chloride from moist chlorine and hydrogen. 

 The course of the interaction would then be represented by the equations : 



CI, + H 2 = HCl + HCIO (instantaneous) 

 2HCIO + H 2 - 2H,0 + HCl (photochemical) 



The circumstance that an increase in the partial pressure ot the hypochlorous 

 acid makes no difference to the rate at which hydrogen chloride is produced can- 

 not at present be regarded as a valid objection to this view, as it is not improbable 

 that the rate of a photochemical change is regulated almost entirely by the rate 

 at which the light is absorbed and degraded. 



