The use of this salt appears to have been first suggested by Sir John Herschel ; it forms the basis of a process by the author, already described. It has lately been claimed as a new photographic agent by the French, but the date of publication determines this question in the author's favour.
Dr. Fyfe appears to have been the first to suggest the use of the phosphate of silver as a photographic material, but I am obliged to confess it has not, in my hands, proved anything like so successful as, from Dr Fyfe's description, it was in his own. Indeed, he himself observes, in speaking of its use in the camera obscura :—" Though representations may be got in this way, yet, so far as I have found, they have not the minute distinctness of those got by the method already mentioned (i. e. by application). Owing to the interference of the lens, the light does not act nearly so powerfully on the paper, as when it has to permeate merely a frame of glass."... For all practical purposes, the method which Dr. Fyfe has given of preparing these papers is, perhaps, the best : —" The paper is first soaked in the phosphate of soda, and then dried, after which the nitrate is spread over one side by a brush ; the paper again dried, and afterwards again put through the salt, by which any excess of silver is converted to phosphate. As thus prepared, it acquires a yellow tinge, which becomes black by exposure to light." It will be evident from these directions, that what was formerly said about the necessity of having the nitrate of silver in excess, is here, according to Dr. Fyfe, objectionable. It certainly does not appear to be so essential in this preparation, that anything but pure phosphate of silver should be used; yet I cannot help fancying that a slight advantage is gained, even here, by allowing a little excess of nitrate. Dr. Fyfe has given a process for applying the phosphate of silver, mixed as a paint, on metal, glass, or paper. It, however, requires the skill of an artist to produce an even surface, and unless a uniform ground is given, the picture is deformed by waving lines of different shades. A method of precipitating argentine salts on smooth surfaces will be given in the following pages, by which means the most uniform face is procured, and many beautiful effects produced.
Notwithstanding the extraordinaiy degree of sensibility which has been given to paper and to the metallic plates by the industrious experiments of chemists, I am convinced that we may hope to obtain agents of far higher natural sensibility than those we now possess ; and I look with much anxiety to some of the combinations of organic radicals with metallic bases. The fulminates and the ethyle compounds present a very promising line of inquiry. Mr. John Towson, of Devonport, who pursued, conjointly with myself, a most extensive series of researches on photographic agents, was endeavouring to form a solution of silver, in which the elements should be so delicately balanced as to be overturned by the action of the faintest light. To do this, he dissolved some very pure silver in nitric acid, to which spirits of wine was added somewhat suddenly in proportions equal to the acid used, and the precipitation of the fulminate prevented by a quick effusion of cold water, sufficient to bring the specific gravity of the solution to 1.17, and to this a few drops of ammonia were added. Pieces of Bank post paper dipped in this solution became, the instant they were presented to the declining light of an autumnal evening, a beautiful black having a purple tinge. This effect did not seem to come on gradually, but, as by a sudden impulse, at once. Both this gentleman and myself have often endeavoured to repeat this, but in no one instance have either of us succeeded in producing anything nearly so sensitive. It should be stated, that the solution prepared in the evening had become, by the following morning, only ordinarily sensitive, and that papers prepared with it were deliquescent and bad. In repeating any modification of this experiment, the greatest care should be taken, as explosions of considerable violence are otherwise likely to occur.
Another series of experiments on the fulminates of silver have produced very pleasing photographic results, but I am not enabled to specify any particular method of preparing them, which may be certain of reproducing the results to which I allude. Nothing can be more capricious than they are; the same salt darkening rapidly to-day, which will to-morrow appear to be absolutely insensible to radiation, and which will again, in a few days, recover its sensitiveness, to lose it as speedily as before.
The beautiful researches of Professor Frankland, of Owen's College, Manchester, however, most satisfactorily prove that a great many of the metals will combine with organic radicals in the sunshine which will not so combine in darkness.
With the exception of the carbonate, tartrate, acetate, citrate, oxalate, and one or two others, the salts of silver, besides those already described, do not appear to be sensibly influenced by light. Many have been mentioned by authors as absolutely insensible to its influence ; but recent experiments have produced modifications of these salts, which are delicately sensitive to the solar ray. Amongst others, the chromate has been named, and certainly it has not yet been rendered sensitive to an exposure of some hours to daylight; but one experiment of mine has proved that the solar beam will, in a few days, produce a fine revival of metallic silver from its chromate; and another experiment with it, has the most pleasing result of bringing within the range of probabilities, the production of photographic pictures in their natural colours.
Researches having this object in view led to the discovery of the chromotype; but this beautiful salt (chromate of silver) has not yet been applied directly as the photographic agent. In the present state of our knowledge, we cannot venture to affirm that any salt of silver, or, indeed, of any of the other metals, exists, having an absolute insensibility to light, or in which the required unstable equilibrium may not be induced, so that the sun's beam might change the character of its combinations. I am, indeed, convinced that no body in nature is entirely uninfluenced by the action of the sun's rays. Papers washed with either of the alkaline carbonates, and then with a solution of nitrate of silver, resemble in their character those prepared with the muriates, but are not darkened so readily.
The tartrate of silver possesses some very extraordinary peculiarities. Papers may be prepared, either by spreading the tartrate at once over the surface, or better, by soaking the paper in a solution of Rochelle salt (the tartrate of potash and soda), and then applying two washes of the solution of nitrate of silver. The first action of light is very feeble, but there gradually comes on a stronger discolouration, which eventually proceeds with rapidity, and at length blackens to an extent beyond almost every other paper. This discolouration may be wonderfully accelerated by washing over the tartrated paper with a very dilute solution of the hydriodate of potash, during the process of darkening. It is not easy to use this when copying anything, but there are cases in which the extreme degree of darkness which this preparation acquires, renders it valuable. The acetate of silver comports itself in the same manner as the tartrate. The citrate, oxalate, etc., are only interesting as forming part of the series of argentine preparations which exhibit decisive changes when exposed to light. The methods of rendering them available will be sufficiently understood from the foregoing details, and it would only be an unnecessary waste of words to give any more particular directions as it regards them.