C.-POSITIVE PHOTOGRAPHS BV MEANS OF HYDRIODIC SALTS. 37

material chemical change. The results being curious, and illustrativeof some of the peculiarities of the hydriodic salts, it will be interestingto study a few of them.

Sulphate and Muriate of Iron. These salts, when used in small pro-portions, appeared to overcome many of the first difficulties, but all thedrawings on papers thus prepared faded out in the dark. If after thesephotographs have faded entirely out, they are soaked for a short timein a solution of the ferrocyanate of potash, and then are exposed to thelight, the picture is revived, but with reversed lights and shadows.

Acetate and Nitrate of Lead. These salts have been much used bySir John Ilerschel, both in the negative and positive processes, and, itappears, with considerable success. I found a tolerably good resultwhen I used a saturated solution; but papers thus prepared required astronger light than other kinds. When I used weaker solutions, thedrawings were covered with black patches. On these a little furtherexplanation is required. When the strong solution has been used, thehydriodic acid which has not been expended in forming the iodide ofsilver—which forms the lights of the picture—goes to form the iodideof lead. This iodide is soluble in boiling water, and is easily removedfrom the paper. When the weaker solution of lead has been used, in-stead of tho formation of an iodide, the hydriodate exerts one of itspeculiar functions in producing an oxide of the metal.

Muriate and Nitrate of Copper. These salts, in any quantities, ren-der the action of the hydriodates very quick, and, when used in moderatoproportions, they appeared to promise at first much assistance in quick-ening the process. I have obtained, with papers into the preparation ofwhich nitrate of copper has entered, perfect camera views in tenminutes, but experience has proved their inapplicability, the edges oftho parts in shadow being destroyed by chemical action.

Chlorides of Gold and Platina act similarly to each other. Theyremain inactive until the picturo is formed, then a rapid oxidation oftlieso metals takes place, and all tho bright parts of the picture aredarkened.

A very extensive variety of preparations, metallic and non-metallie,were used with like effects, and I am convinced that the only plan of ob-taining a perfectly equal surface, without impairing the sensitiveness ofthe paper, is careful manipulation with the muriated and silver solutions.

By attention to the following directions, simple in their character,but arrived at by a long series of inquiries, any one may prepare photo-graphic papers on which the hydriodic solutions shall act with perfectuniformity:—

Soak the paper for a few minutes in a muriated wash, removing with’ a soft brush any air-bubbles which may form on it. Tho superfluousmoisture must be wiped off with very clean cotton cloths, and thepapers dried at common temperatures. When dry, the paper mustbo pinned out on a board, and the silver solution spread overit boldly but lightly, with a very soft sponge brush. It is to be