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evolved in the dark. Hence we may conclude, that the light does 

 not effect a permanent modification, either of the chlorine or hydrogen, 

 but that the combination produced by the light must depend on 

 photo-chernical action affecting only the increasing attractions of the 

 chemically active molecules. 



All the curves representing the increase of the induction under 

 various conditions have a common form, and a point of flexure at 

 which the maximum increase occurs. In order to determine whether 

 this common property of the curves arises from the general mode of 

 action of affinity, or whether the light plays an essential part, we 

 have made experiments upon idio-chemical induction, i. e. action 

 in which pure chemical attractions alone effect the alteration. For 

 this purpose we employed a dilute aqueous solution of bromine with 

 tartaric acid, which mixture, when left to itself in the dark, under- 

 goes decomposition, hydrobromic acid being formed. By determi- 

 ning the amount of free bromine contained in the liquid at different 

 times, we became acquainted with the rate at which the decompo- 

 sition occurred. Analysis showed that the amount of hydrobromic 

 acid formed was not the same in equal spaces of time ; and curves 

 representing this increase were found to have the form obtained for 

 the photo-chemical induction. Hence the cause of this maximum 

 increase appears not to lie in any peculiar property of light, but 

 rather in the mode of action of affinity itself. 



One of the many interesting applications of the laws of photo- 

 chemical induction relates to the phenomena of photography. As 

 an instance of this application we cite the remarkable observations 

 of Becquerel, which induced him to assume the existence of certain 

 rays which can continue, but not commence, chemical action. In 

 order to explain the phenomenon observed by the French physicist, 

 we do not need to suppose the existence of a new property of light, as 

 the facts are easily explained by the laws of photo-chemical induction ; 

 and we are satisfied that these relations, which we have examined 

 only in the case of chlorine and hydrogen, occur in a slightly modi- 

 fied form in other photo-chemical processes. 



Having determined in this part of our investigation the most im- 

 portant phenomena of photo-chemical induction, we shall in the 

 next section consider the laws which regulate the chemical action of 

 light after the induction is completed. 



