A Manual Of Photography

ON FIXING THE PHOTOGRAPHIC PICTURES. 213 The iodide of silver, which is readily formed by washing the photograph with a solution of the iodide of potassium, is scarcely sensitive to light ; and tins salt, used in the proportions of five or six grains to four or five ounces of water, answers tolerably well where transfers are not required. It tinges the white lights of the picture of a pale yellow,—a colour which is extremely active in absorbing the chemical rays of light, and is therefore quite inapplicable where any copies of the original photograph are required; and, in describing the bydriodated photographs, other objections will be noticed. Of all the fixing agents, the hyposulphite of soda is decidedly the best. This was first pointed out by Sir John Herschel, who also recommended that it should be used warm in some cases, which was the plan adopted by Mr. Fox Talbot in the improve- ments of his calotype process. To use the hyposulphite of soda with effect, there are several precautions necessary. In the first place, all the free nitrate of silver must be dissolved out of the paper by well washing; the photograph being spread on a plane surface, is to be washed over on both sides with a saturated solution of the hyposulphite of soda. The picture must then be washed, by allowing a small stream of water to flow over it, at the same time dabbing it with a piece of soft sponge, until the water passes off perfectly tasteless. This operation should be repeated twice, or, in particular cases, even three times. The hyposulphite of soda lias the property of dissolving a large quantity of several of the salts of silver, but particularly of the chloride, witli which it ' combines, forming a triple salt of an exceedingly sweet taste. This salt is liable to spontaneous de- composition, accompanied witli separation of silver in the state of sulphuret: lienee the necessity of freeing the paper, by wash- ing, of every trace of it, the sulphuret of silver being of a dirty brown. It might appear that the use of warm water would more effectually cleanse the paper ; so far from it, it occasions the immediate formation of the sulphuret of silver. Some operators prefer leaving the picture in a bath of the hyposulphite of soda for some time, and then removing the salt by simple immersion in water, frequently changing it. The advantages of this appear to be, that the surface of the paper is not disturbed by any rubbing action or by the mechanical action of water flowing over the surface. For fixing the calotype pictures, Mr. Cundell, to whom we are much indebted for improvements in this particular process, recommends the fol- lowing mode of manipulation : , The picture, or as many of them as there may be, is to be soaked