Art. LXVI.—On a new Theory of the Mode by which Photographic Effects are produced with Silver Salts.
[Read before the Wellington Philosopliical 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 consideration 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—even 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 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 Cl) + H2 O =
1st process—Ag2 Cl + Ag O + 2 (H Cl).
2nd process—Ag2 Cl + Ag Cl + H2 O + Cl.
[Footnote] * “Trans, N.Z. Inst.,” vol. xii., art. lxi.
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 simultaneously produced with it. Thus well-known chemical effects are produced, decomposition and recomposition ensue, and there is obtained argenticchloride, 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 compounds 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 photographic effect in case of iodide and bromide of silver should be explainable by a theory similar in kind to it.