A Manual Of Photography

BBBHBPBBBBBBBB®®BMBWB®®®®®®®®®®®®®®® 130 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. Muriate of Iron.—A solution of this salt appears in the first- instance to answer remarkably well ; but, unfortunately, the pictures formed perish slowly, however carefully guarded from the influence of light. Chlorate of Potash.—Mr. Cooper recommends a solution of this salt, and a silver wash of sixty grains to the ounce of water, as capable of forming a good paper. Some of the specimens prepared with it are of exceeding beauty, the ground being of a very pretty blue, or rather lilac ; but these papers cannot be used where any considerable degree of sensitiveness is desired. Muriatic Acid.—A slightly acidulated solution of this acid produces a very tolerable paper, but it is extremely difficult to hit the best proportions for use. If too weak, the paper fails in sensibility, and a slight increase occasions a very injurious action on the paper, raising the pile like a down over the sheet. This kind of paper loses its sensitiveness with great rapidity: in about six or seven days, however carefully kept, it is scarcely susceptible to luminous influence. By washing the paper, after it is prepared, in pure water, it keeps much better ; but, after being washed, light changes it to a rather disagreeable brick- red, prior to which the colour in general is a fine brown. Dr. Schafhaeutl has proposed the use of the muriatic acid in a different way, to be noticed in a future chapter, and certainly his process has some advantages: when it is carefully attended to, the liability to spots or patches appears to be less than in any of the ordinary methods, and a very sensitive paper results, ,*but it will not keep. Aqueous Solution of Chlorine gives rise to a paper possessing in an eminent degree the merits of that prepared with muriatic acid, I and it has the advantage of retaining its sensibility much longer. Solutions of Chlorides of Zinc and Soda.—Either of these solutions may be used indiscriminately, provided the strength of the silver is regulated so as to give an excess of the nitrate ,: 1 over the chlorine in solution: the effects are not generally pleasing. Section III.—Iodide of Silver. If iodide of silver is precipitated by mixing together solutions of iodide of potassium and nitrate of silver in a concentrated state, a heavy yellow powder falls, which will scarcely change in colour by an exposure of many days to sunshine. But if the solutions are infinitely diluted, so that on mixing they only become milky, and the light powder which occasions the opacity