A Manual Of Photography

ON THE PRODUCTION OE POSITIVE PHOTOGRAPHS. 291 Hydriodate of Ammonia, if used on unsized paper, has some advantage as to quickness over the salts either of potash or of soda. This preparation is, however, so readily decomposed, that the size of the paper occasions a liberation of iodine, and the consequent formation of brown-spots. Hydriodide of iron.—This metallic hydriodide acts with rapidity on the darkened paper ; but even in the shade its chemical energy is too great, destroying the sharpness of outline, and impairing the middle tints of the drawing. It also renders the paper very yellow. Hydriodate of Manganese answers remarkably well when it can be procured absolutely free of iron. When the manganesic solution contains iron, even in the smallest quantities, light and dark spots are formed over the picture, which give it a curious speckled appearance. Hydriodate of Baryta possesses advantages over every other simple hydriodic solution, both as regards quickness of action and the sharpness of outline. A solution may, however, be made still superior to it, by combining a portion of iron with it. Forty grains of the hydriodate of baryta being dissolved in one ounce of distilled water, live grains of very pure sulphate of iron should be added to it, and allowed to dissolve slowly. Sulphate of baryta is precipitated, which should be separated by filtration, when the solution is composed of hydriodate of baryta and iron. By now adding a drop or two of diluted sulphuric acid, more baryta is precipitated, and a portion of hydriodic acid set free. The solution must be allowed to stand until it is clear, and then carefully decanted off from the sediment, as filtering paper decomposes the acid, and free iodine is liberated. By this means we procure a photographic solution of every active character, ft should be prepared in small quantities, as it suffers decomposition under the influences of the atmosphere and light. Hydriodic Acid, if used on paper which will not decompose its aqueous solution, which is rallier difficult to find, acts wry readily on the darlcened silver. A portion of this acid, free in any of the solutions, most materially quickens the action. Trom the barytic solution it is always easy to set free the required portion, by precipitating the barytes by sulphuric acid. As the hydriodate of barytes is rarely kept by the retail chemist, it may be useful to give an easy method of preparing the solution of the required strength. Put into a Florence flask one ounce of iodine, and cover it with one fluid ounce and a half of distilled water ; to this add half a drachm of phosphorus cut into small pieces; apply a iery gentle heat until they unite, and the liquid becomes colourless;