This salt was employed very early by Talbot, Herschel, and others, and it enters as the principal agent into Mr. Talbot's calotype process. Paper is washed with a solution of the iodide of potassium, and then with nitrate of silver. By this Leans papers may be prepared which are very sensitive to luminous influence, provided the right proportions are hit; but, at the same time, nothing can be more insensible to the same agency than the pure iodide of silver. A singular difference in precipitates to all appearance the same, led to the belief that more than one definite compound of iodine and silver existed ; but it is now proved that pure iodide of silver will not change colour in the sunshine, and that the quantity of nitrate of silver in excess regulates the degree of sensibility. Experi-Lent has shown that the blackening of one variety of iodidated paper, and the preservation of another, depends on the simple admixture of a very minute excess of the nitrate of silver. The papers prepared with the iodide of silver have all the peculiarities of those prepared with the chloride, and although, in some instances, they seem to exhibit a much higher order of sensitiveness, they cannot be recommended for general purposes with that confidence which experience has given to the chloride. It may, however, be proper to state the best proportions in which the iodidated papers can be prepared, and the most approved method of applying the solutions.
The finest kind of paper being chosen, it should be pinned by its four corners to a board, and carefully washed over with a solution of six grains of the nitrate of silver to half an ounce of water : when this is dry, it is to be washed with a solution of iodide of potassium, five grains in the same quantity of water, and dried by, but at some little distance from, the fire; then, some short period before the paper is required for use, it must be again washed with the silver solution, and quickly dried, with the same precaution as before. If this paper is warmed too much in drying, it changes from its delicate primrose colour to a bright pink or a rosy brown, which, although still sensitive to solar influences, is not so readily changed as when in an unaltered state. The peculiar property of this salt to change thus readily by calorific influence, and some other very remarkable effects produced on already darkened paper when washed with a salt of iodine, and exposed to artificial heat, or the pure calorific rays of the spectrum, appears to promise a process of drawing of a new and peculiar character.
The few simple directions here given will be sufficient to guide the young experimentalist in his earliest essays; and it is particularly recommended that the first experiments should be confined to the salts named in this chapter. The minute details required for the more highly sensitive processes are described in immediate connection with the process to which they refer.
I would advise the amateur to start upon his studies with but three solutions—1st, Chloride of sodium; 2d, Nitrate of silver; 3d, Hyposulphite of soda. With the first he carefully washes several sheets of good letter paper on one side only, and dries ; with the second he, by another washing of the paper, forms the required chloride of silver, on which he may obtain pictures by simple exposure in the copying frame; with the third he gives permanency to the pictures which he produces.