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May 1, 1863.] THE PHOTOGRAPHIC NEWS. 211 as the sulpho-cyanide of silver, they present a surprising stability: this substance may be boiled with nitric acid ; even calcined with nitrate of potassa, and, in those circum stances, decomposes only after a very prolonged contact with those reagents. Thus, from our present point of view, the sulpho-cyanides present themselves at once under very favourable conditions: the dilute mineral acids, the concen trated organic acids, very concentrated nitrate of silver, solid even, can never cause with the sulpho-cyanides a dis engagement of sulphur, or sulphuretted hydrogen com parable, with that produced by hyposulphite. Action of the Fixing Salt uponthePicture.—This established, the first question that presents itself is the following:—Can the sulpho-cyanide of ammonium remove all the argenti ferous compounds? We have no hesitation in replying in the affirmative. For a long time it has. been known that the alkaline sulpho-cyanides dissolve the salts of silver ; this fact is recorded in all treatises on chemistry ; but expe riment has shown us that its solvent faculty is more absolute than that of the hyposulphite. The action of this salt upon nitrate and chloride of silver is more decided, as every one may ascertain for himself. Take a solution of nitrate of silver, and pour into it some drops of the alkaline sulpho-cyanide, a white precipitate of sulpho-cyanide of silver will appear : upon your adding an excess of the reagent, the precipitate will quickly disappear. Chloride of silver, shaken into a solution of sulpho-cyanide, is dissolved in the same manner. The clearness of these two reactions admits of our passing them over rapidly; we therefore devoted all our attention to the solution of albuminate of silver. We recognized that the solution was complete, and that the action of the sulpho cyanide was, from this point of view, more energetic even than that of hyposulphite of soda. For, on the one hand, we precipitated some albumen by nitrate of silver ; then, after -well washing the precipitate with distilled water, we treated several times, one portion with sulpho-cyanide of ammonium, the other with hyposulphite. Again, well washed, the precipitates were then calcined; that which had been treated with sulpho-cyanide furnished traces of silver only; while that treated with hyposulphite of soda furnished very notable quantities. On the other hand, we fixed, com paratively, by means of these two agents, some albumenized papers, which we afterwards burned ; and, in this case, we recognize a difference of the same kind quite favourable to the employment of the sulpho-cyanide, but which, we must say, was less marked than in the first case. The sulpho-cyanide then attacks albuminate of silver more energetically than hyposulphite of soda, and, conse quently, it presents a decided superiority over the latter with respect to the absolute fixing of the whites of the proofs. Next to this first question we place a second no less im portant. The sulpho-cyanide of ammonium, when employed under suitable conditions, leaves on the proof no portion of its substance capable of change, either immediately or ulti mately. In that respect it behaves like fresh hyposulphite of soda, but like the latter, it exposes the proof to some risk in the case where the manipulations have not been conducted with the necessary care These dangers consist, 1st, In the precipitation of a small quantity of sulpho-cyanogen, cither in the case where, by accident, some concentrated acid is present at the moment of fixing, or in the case where the bath full of proofs is exposed to contact with the atmosphere for an extreme length of time. 2nd, In a feeble quantity of sulpho-cyanideof silver remaining in the proof in consequence of incomplete fixing. In the first case, the bath, becoming thick, deposits a yellow powder, and the proof is immediately stained: in the second case, the sulpho-cyanide of silver, sensitive^ light, quickly blackens ; in both cases the photo- grapheris immediately aware of what takes place, an advan tage he is far from possessing with hyposulphite of soda, the injurious .effects, of which are not immediately developed, and in this particular the purchaser has an excellent com mercial guarantee. Limit of Saturation of the Sulpho-cyanide of Ammonium.— The study of this part of the subject, so complex with regard to hyposulphite of soda, is very simple with the sulpho cyanide. Upon contact with the salts of silver, whichever they be, the alkaline sulpho-cyanides give rise to a white precipitate absolutely insoluble in water, the sulpho-cyanide of silver corresponding to the formula C>NS, Ag S. This salt combines with the sulphocyanide of ammonium to yield a double salt. O’ NS, Ag S+CNS, NH, S, uncrystallizable in water, which it decomposes ; but crystalli zing with facility in an excess of sulpho-cyanideof ammonium. Isomorphous with this latter, the double salt always acquires a certain quantity in crystallizing, and we are thus able to obtain salts varying according to their composition from 2 (O' NS, Ag S) +3 (C‘ NS, N H- S) to that corresponding with the formula, C: NS, AgSq-6 (C‘ NS, NH‘ S). Between all these there exists no other difference of proper ties than the instantaneous decomposition of the first by water, opposed to the solubility of the latter in a very minimum quantity of water, a greater quantity of this liquid rapidly producing an identical decomposition, and separating the double salt into soluble sulpho-cyanide of ammonium, and white in soluble sulpho-cyanide of silver. This decomposition of the double sulpho-cyanide by water is, as we shall soon see, upon studying the practical condi tions of fixing, a very serious inconvenience inasmuch as it compels the operator to submit each proof to two successive fixings, separated by a washing in water. This property also brings notable variations in the limit of saturation of the solutions of sulpho-cyanide of ammonium of different degrees of strength, and it is very evident a priori, that an equal quantity of sulpho-cyanide of ammonium will dissolve much less of the salts of silver, of chloride for example, as the quantity of water in which it is itself dis solved is increased. This we have proved by direct experi ment. F or, 100 parts of sulpho-cyanide of ammonium dissolved in 100 parts of water, dissolve 26 parts of chloride of silver. ICO parts of sulpho-cyanide of ammonium, dissolved in 200 parts of water, dissolved only 19 parts. 100 parts of sulpho-cyanide of ammonium dissolved in 400 parts of water, dissolved only 14 8 parts. So that there is an advantage and economy in dissolving a given quantity of sulpho-cyanide of ammonium in as little water as possible. Preservation of the Bath.—An important consideration which demands all our attention is this :—Do the baths of sulpho-cyanide deteriorate like those of hyposulphite of soda when they are charged with salts of silver ? Experience has shown that a solution of sulpho-cyanide undergoes a similar change, whether it contains silver or not. Besides the compounds formed in it are not, in any respect, of a nature to injure the permanence of a proof; a small quantity of a yellow powder (sulpho-cyanogen) is precipitated, and the bath exhales a faint odour of cyanuret compounds. A simple filtering will remove the sulpho-cyanogen which renders the bath turbid, but the cyanide of ammonium will re main dissolved. Perhaps this compound, the action of which upon the proof is necessarily energetic, may exercise some influence, good or bad, upon the proof. Of this, experience will instruct us, but what theory at present teaches us is, that a bath of sulpho-cyanide does not acquire, by becoming old, any element destructive to the proof. The stability of the compounds under consideration is such, that the fixing baths may even, without danger, be acidulated with acetic, tartaric, citric, and other acids. Practical conditions of Fixing*—The proof, before being submitted to the bath of sulpho-cyanide of ammonium, must be washed under the same conditions as a proof in- * If the proof be fixed after it is toned, it will be well to carry this a little further than ordinary, for the sulpho-cyanide of ammonium appears to attack the proof a little more than the hyposulphite of soda.