Sxey.—On Production of Photographie Effects with Silver Salts, 408 
Art. LXVI.—On a new Theory of the Mode by which Photographic Effects are 
produced with Silver Salts. By Wit1am Sxey, Analyst to the Geological 
Survey Department. 
[Read before the Wellington Philosophical Society, 11th February, 1882.} 
Tue 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 in 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—-oveH 
carbonic acid; while on the other hand if free alkali 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 ¢hloride 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 :-— : 
pacry se cl+AgO +2(HC)). 
* “Trans, N.Z, Inst.,” vol. xii., art. Ix. om | - 
