404 Transactions. — Chemistry. 



The first effect is to produce argentic-dioxide, argentous-chloride, and 

 hydrochloric acid, by re-grouping the elements present in a portion of the 

 argentic-chloride and water used ; were free alkali present, no secondary 

 effect would take place, as the newly-formed acid would combine with it ; 

 thus we should have, in course of time, most or all of the argentic-chloride 

 represented by argentic-dioxide. But in the present case we suppose no 

 free alkali present to absorb the hydrochloric acid just formed. This acid, 

 therefore, is free to act upon the argentic-dioxide which has been simul- 

 taneously produced with it. Thus well-known chemical effects are produced, 

 decomposition and recomposition ensue, and there is obtained argentic- 

 chloride, water, and chlorine. If the impingement of light is continued, 

 action does not end there, but the newly-made argentic- chloride is in its 

 turn decomposed, a portion of it is passed by the first process described 

 into a further quantity of subchloride, while of the remainder, in conjunction 

 with water, compounds are again formed which, by their mutual action, set 

 chlorine free. Thus the original argentic-chloride is gradually, and by two 

 alternative processes, depleted of one equivalent of chlorine and the pure 

 subsalt ultimately left on our hands. The final result is, it may be stated, 

 in harmony with the fact first observed by Professor Vogel, that chlorine is 

 one of the products of the action of light upon argentic-chloride. 



The part which light plays in the photographic effect may be viewed as 

 purely a mechanical one — thus, the molecules of this compound, together 

 with those of water (which we have seen to be necessary for effect), are 

 oscillated by certain of the rays which make up light to such an extent that 

 they get to positions in which they are attracted less by their companion 

 molecules than by the stranger ones with which they are thus associated. 

 Therefore those combinations are effected which we observe. 



The direct effect of light, in fact, decomposes both the argentic chloride 

 and water, producing, as I have said, argentous chloride, argentous oxide, 

 and hydrochloric acid ; but secondary action between certain of these com- 

 pounds prevents any accumulation of argentous oxide by decomposing it as 

 fast as it is made. 



I have only to add that, allowing this theory to be correct, the photo- 

 graphic effect in case of iodide and bromide of silver should be explainable 

 by a theory similar in kind to it. 



