A Manual Of Photography

FULMINATE OF SILVER. 135 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 117, and to this a few drops of am- monia were added. Pieces of Bank post paper dipped in this solution became, the instant they were presented to the declin- ing 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 pre- pared 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 Prankland, 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. We may sum up the amount of our knowledge of the chemical influences of the solar radiations as follows:— 1. The rays, having different illuminating or colorific powers, exhibit different degrees and kinds of chemical action. 2. The most luminous rays exhibit the least chemical action upon all inorganic matter. The least luminous and the non- luminous manifest very powerful chemical action on the same substances.. 3. The most luminous rays influence all substances having an organic origin, particularly exciting vital power. 4. Thus under modifications, chemical power is traced to every part of the prismatic spectrum ; but in some cases this action is positive, exciting ; in others negative, depressing. 5. The most luminous rays are proved to prevent all chemical