Skey. — On Production of Photographic Effects ivith Silver Salts. 403 



Abt. LXVI. — On a new Theory of the Mode by ivhich Photographic Effects are 

 produced with Silver Salts, By William Skey, Analyst to the Greological 

 Survey Department. 



[Read before the Wellington PMlosopMcal Society, 11th February, 1882.] 

 The photographic effect of the actinic rays upon argentic chloride has not 

 within my knowledge been thoroughly explained ; it is known what the 

 ultimate effect is, — namely, to split this salt into sub-chloride of silver 

 which remains as a pale-reddish substance, and chlorine which escapes ; but 

 the exact process or processes by which this comes about is not, I believe, 

 understood, though many theories in regard to it have been produced. 

 This being the case, I beg to lay before you a theory which I have formed 

 on the subject, a theory which I have been led to entertain by a consider- 

 ation of certain phenomena that have been hitherto unnoticed. 



You may remember that in a paper upon certain argentiferous salts in 

 relation to light,* I showed that the iodide of silver like the chloride requires 

 the presence of water m conjunction with light, in order that photographic 

 change be produced, and besides I showed " that the product of sunlight 

 upon argentic chloride varies both in colour and composition according as 

 to whether this substance is alkalized or not." " Thus in weak solution of 

 potash the product is dark brown or black," and as to composition it is 

 largely made up of argentic oxide (according to more recent investigations of 

 mine), as it evolves oxygen when brought into contact with any acid — even 

 carbonic acid; while on the other hand if free alkaU is absent the argentic 

 chloride passes into argentous chloride. 



These facts, few and simple though they are, nevertheless appeared to 

 me explanatory of the whole process by which the fullest photographic effect 

 with this salt is reached. 



Now, all the efforts made to attain this hitherto, all the equations made 

 to represent the chemical action which is effected therein, have, as far as I 

 know, been with the elements of water left out of cognizance or calculation, 

 though, as we know, these are necessary to such action. 



The formula which I append to express the theory I here propose, shows 

 the processes by which argentic chloride is split into argentous chloride and 

 chlorine by the influence of light. In them I assume that water is present 

 and decomposed. For perspicuity, I take three equivalents of argentic 

 chloride and one of water, thus : — 

 3 (Ag 01) +mo = 



1st process— Ag2 CI -}- Ag -t- 2 (H CI). 

 2nd process— Ag2 CI + Ag CI -t- H^ + CI. 



* " Trans. N.Z. Inst.," vol. xii., art. Ixi. 



