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the advantage of the nitrate of silver over the chloride is, that it is more readily reduced to the metallic state than the chloride; and that the accelerating power of moisture is due to the decomposition of water, and the influence of nascent hydrogen in reducing the argentine salt.

(120.) The principal point now to ascertain was the condition of the chloride of silver after it had been thoroughly darkened by exposure. Many experiments were tried in various ways, but to the most satisfactory only I will confine attention. Pure chloride of silver was prepared with great care, well washed with boiling distilled water, until neither nitrate of silver nor muriatic acid produced any precipitate, and then dried. Five grains of the salt were put into a long test tube full of distilled water, and placed in the sunshine to darken, the powder being frequently moved that every part might be acted upon by the sun's rays. It was found, even after an exposure of a few minutes, that the water contained chloride; it became opake on the addition of nitrate of silver, and this very gradually increased as the chloride darkened. The darkening process was continued for several hours, after which the solution was filtered to free it from chloride of silver, and nitrate of silver added to the filtered solution; this precipitated chloride of silver, which, when collected, dried, and weighed, gave 14 grains on one occasion, 1 grain on another, and 1.5 grains on a third trial. From this it is evident that chlorine is liberated during the process of darkening.

(121.) The exposure in the water was in another case continued for several days, but no greater degree of darkening occurred; but a curious fact was noticed. It was found that during the night nearly all the chlorine which had been liberated during the day was recombined, and that the darkened powder became lighter. This is an analogous case to placing pure chloride of silver in a tube filled with chlorine, which was first pointed out by Mr. Shaw. The chloride will darken by exposure in sunshine;

the chlorine again attacks the silver in the dark; and restores it to its white colour.

(122.) In these experiments the presence of organic matter had been carefully avoided. It now became necessary to inquire into the condition of the chloride of silver darkening by the solar rays on paper. Bath postpaper, highly glazed, was covered with chloride of silver in the usual way, all free nitrate of silver being washed off. The paper was then exposed to sunshine for fortyeight hours, in which time it had passed to a fine olivebrown metallic colour. The paper was now cut into pieces; some parts were immersed in very dilute nitric acid, which produced no change; others in ammonia, which had not the slightest effect upon them; therefore it was evident that no oxide of silver was present. On putting fragments of the paper into nitric acid diluted with equal parts of water, all the darkened portion was rapidly dissolved off, and the paper was left of a lilac colour. Hence we have very satisfactory proof that metallic silver is eventually formed on the surface of the chloridated photographic papers, and that the under sensitive surface is preserved in the condition of a subchloride of silver by the opacity of the superficial coat.

(123.) From other experiments, I am inclined to believe that the first action of the solar ray is to liberate one half the combined chlorine, which is very readily, moisture being present, replaced by oxygen. By the continued action of the exciting cause the oxide is decomposed, and metallic silver in a state of fine division is formed over the surface.

(124.) It must not be forgotten that whenever organic matter is present, even the stable oxide of silver, chloride and iodide readily give way under the chemical action of the solar rays, and metallic silver is very readily formed.

(125.) The absorption of oxygen, or rather its combination with the decomposing chloride, is proved by another very easy experiment. Some pure chloride of

silver was arranged in a bent tube closed at one end, and the other end immersed in a bottle of distilled water. In this state the chloride was exposed for many days to the action of sunshine, during which time it was frequently shaken, for the purpose of exposing the whole of the powder to its influence. As the chloride darkened the water rose into the tube, and it gave a precipitate of chloride of silver on the addition of the nitrate, thus appearing to prove the substitution of oxygen for chlorine under the agency of solar radiation. It was quite evident that some absorption of atmospheric air had taken place. This explanation will also serve for the iodide, bromide, and some other salts of this metal.

(126.) IODIDE OF SILVER. The preceding experiments on the chloride of silver, as I have said, apply equally to the iodide, the action in all cases being similar, so that if we substitute iodine or bromine for chlorine, the rationale is correctly represented. Perfectly pure iodide of silver may be exposed for a long period to Light without undergoing any visible change; except, perhaps, in some cases a more decided yellow colour than is usual to it comes on; but this appears to depend on the influence of the calorific rays, as the same effect is produced by exposing it to a very moderate degree of heat. If, however, a slight excess of the nitrate of silver in solution is present, the iodide becomes infinitely more sensitive than the chloride of silver.

(127.) Prismatic Analysis. The spectrum impressed upon a paper prepared with a weak solution of the iodide of potassium presents some very remarkable peculiarities. The maximum of intensity is found at the edge of the most refrangible violet rays, or a little beyond it: Sir John Herschel places it at a short distance beyond. The chemical action rarely extends below the green ray; hence the iodide of silver imperfectly receives impressions from foliage, or any bodies radiating yellow light. I have found that it varies slightly according to the kind of paper used,

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and also with the quantity of free nitrate of siver present. The action commences at a point nearly coincident with the mean red of the luminous spectrum, where it gives a dull ash or lead colour, while the most refrangible rays impress a ruddy snuff-brown, the change of tint coming on rather suddenly about the end of the blue or the beginning of the violet rays of the luminous spectrum. (Herschel.) Beyond the extreme violet ray, or rather beyond the maximum point, the action rapidly diminishes; but the darkening produced by these invisible rays extends over a small space beyond the point at which they cease to act on the chloride of silver.

The use of

(128.) Photographic Application. papers prepared with the iodide of silver alone can only be recommended as affording a very pleasing variety of pictures, having a primrose instead of a white ground. The best proportions in which the respective salts of iodine and of silver can be used, according to my own experience, are the following:- Twenty grains of nitrate of silver should be dissolved in half an ounce of distilled water, and this, with a very soft brush, carefully applied over the paper, and allowed to dry. Ten grains of the iodide of potassium dissolved in the same quantity of water, is next to be applied, and the paper quickly dried near the fire, care being taken, not to warm it too much, as heat changes this salt from its delicate primrose colour to a pink or rosy brown, which, although still sensitive, is not so much so as the parts which are not so altered.

(129.) In combination with other reagents, the iodide of silver becomes exquisitely sensitive, and from two such combinations, gallic acid and the ferrocyanide of potassium, result two exceedingly sensitive photographic applications. To these we must now direct attention; taking, in the first place, Mr. Talbot's description of the process, which he has named the Calotype.

(130.) THE CALOTYPE PROCESS. - Preparation of the

THE CALOTYPE PROCESS.

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Paper. Take a sheet of the best writing-paper, having smooth surface, and a close and even texture. The watermark, if any, should be cut off lest it should injure the appearance of the picture. Dissolve 100 grains of crystallized nitrate of silver in six ounces of distilled water. Wash the paper with this solution with a soft brush on one side, and put a mark on that side whereby to know it again. Dry the paper cautiously at a distant fire, or else let it dry spontaneously in a dark room. When dry, or nearly so, dip it into a solution of iodide of potassium, containing 500 grains of that salt dissolved in one pint of water, and let it stay two or three minutes in the solution. Then dip it into a vessel of water, dry it lightly with blotting-paper, and finish drying it at a fire, which will not injure it even if held pretty near; or else it may be left to dry spontaneously. All this is best done in the evening by candlelight. The paper so far prepared Mr. Talbot calls iodized paper, because it has a uniform pale yellow coating of iodide of silver. It is scarcely sensitive to Light, but nevertheless it ought to be kept in a portfolio or a drawer until wanted for use. It may be kept for any length of time without spoiling or undergoing any change if protected from the Light. When the paper is required for use, take a sheet of it, and wash it with a liquid prepared in the following manner:

Dissolve 100 grains of crystallized nitrate of silver in two ounces of distilled water; add to this solution onesixth of its volume of strong acetic acid. Let this mixture be called A.

Make a saturated solution of crystallised gallic acid in cold distilled water. The quantity dissolved is very small. Call this solution B.

Mix together the liquids A and B in equal volumes, but only mix a small quantity of them at a time, because the mixture does not keep long without spoiling. Mr. Talbot calls this mixture the gallo-nitrate of silver. tion must be washed over the iodized paper

This soluon the side