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Light, or the opposite. By pouring over these pictures a solution of hyposulphite of soda, they disappear, but on washing them with pure water, and drying, they are restored, and assume much the air of a Daguerreotype, when laid on a black ground, and still more so when smoked at the back; so that its character is in fact changed from a negative to a positive picture. (Herschel.) It is necessary that the plate should be exposed wet, and when withdrawn plunged instantly into water.

(93.) The action of the solar radiations having produced the required impression, it becomes necessary to render these pictures permanent, that they should be subjected to some process, which should prevent the white parts from undergoing any change. The perfect fixing of the Photographic pictures depends upon the entire removal of the unchanged salt from the plate or paper employed: some solvent is therefore required. The best solvent for the chloride or iodide of silver is the hyposulphite of soda, for which we are indebted to Sir John Herschel. Some care is required in this fixing process, but with attention this salt will be found to be much more useful than any other agent. The drawing being produced, should be first soaked in clean water, to dissolve out as much as possible of the nitrate of silver. It is then to be immersed for a few minutes in water, to which a little common salt has been added, the object of which is to convert any portion of the nitrate that may remain in the paper into a chloride. When any nitrate of silver is present the hyposulphite changes it to a sulphuret, the brown colour of which is destructive to the beauty of the picture, and prevents its being used for multiplying originals. When nearly dry, it is to be brushed over, first on the face, and then on the back, with the solution of the hyposulphite of soda, and immediately immersed in clean water. Having been allowed to soak for a few minutes, it should be placed on a porcelain slab, and gently washed with a soft sponge

and clean water, until the fluid flows off without any sweetness of taste; the combination of the chloride of silver and the hyposulphite producing a salt,-hyposulphite of silver,-which is remarkable as producing a sensation of intense sweetness over the mouth. It is a

peculiarity of this method of fixing, that nearly all the delicate parts which may appear to have suffered in the process, develop themselves again with considerable sharpness on drying.

(94.) A very curious process of fixing was first noticed by Sir John Herschel, and nearly about the same time dropped upon by myself, while endeavouring to fix some pictures produced by a positive process, to be described under the Iodides. It has the peculiarity of completely obliterating the picture, "reducing it to a state of perfectly white paper, on which the nicest examination (if the process be perfectly executed) can detect no trace, and in which it may be used for any other purpose, as drawing, writing, &c., being completely insensible to Light."-(Herschel.) Where iodine is present, the paper becomes a deep yellow. This obliteration is effected by washing the picture with a solution of corrosive sublimate, soaking it in water, and drying it. Though invisible, the picture still exists, and may at any time be revived from its dormant state, by brushing it over with liquid hyposulphite of soda or caustic ammonia.

It should be noticed, that a very considerable difference will be found in the injury done to a photograph by the fixing process, according as it has been produced quickly under the action of a good sun, or by the prolonged influence of a more feeble light. Those produced in bright sunshine are not at all, or but slightly injured; whereas those which have been affected by a weak daylight, lose much of their sharpness, and indeed many parts are often destroyed.

(95.) We have now to consider the nature of the change produced on the salts of silver by solar influence,

about which it appears some misconception has existed. The following experiments will set the question in a clearer light. By examining the changes which take place on the oxide, nitrate and chloride of silver in connexion, we get a fairer representation of the chemical alterations than we should by any other arrangement.

(96.) OXIDE OF SILVER.-To a weak solution of nitrate of silver in distilled water a very dilute solution of pure baryta was added, and the resulting precipitated oxide of silver received upon plates of glass. When the whole of the oxide had fallen down, the fluid was removed very slowly, by means of a small glass siphon, so that the powder on the glass plates might not be disturbed. Without being removed from the vessel in which the precipitation was effected, the oxide was dried at a very moderate heat, and there resulted exceedingly thin films of the oxide of silver on the glass, quite free from all organic matter, every precaution being taken to prevent its pre

sence.

(97.) These plates were exposed to full sunshine for periods varying from one to twenty-four hours, one half of them being covered with an opake body. The brown oxide of silver (protoxide) was gradually darkened; so that after half an hour's exposure, the exposed part was a much deeper brown than that which had been covered. This darkening process was continued until a perfect black was produced, after which, by the prolonged action of sunshine, a very remarkable whitening of the oxide was observed to take place, and proceed slowly until it acquired a very fine olive colour.

(98.) On immersing one of these plates in strong ammonia, all the oxide which had been kept from the solar action was dissolved off, whereas the exposed part did not appear to be acted on. Another plate immersed in a solution of twenty grains of nitric acid in 300 grains of water, was acted on in precisely the same manner; the oxide being entirely removed from the unexposed portion

of the glass plate, whilst the changed part appeared to remain untouched. From this we might consequently infer that the oxide of silver had been reduced to the matallic state. Moderately strong nitric acid, dissolving the whole from the glass, seemed to render this in every way probable.

(99.) Metallic silver, in however fine a state of division. it may be, conducts electricity; consequently it was expected that the changed oxide would conduct the current of a voltaic battery. One of the glass plates covered with oxide was made part of the circuit, between a single pair and a galvanometer of great delicacy; but not the slightest indication of any conducting power could be detected. This may have arisen from a want of perfect continuity.

(100.) Films of oxide of silver were darkened all over by long exposure to sunshine. One was placed in ammonia, and another in very dilute nitric acid. It was now discovered that the ammonia did dissolve a portion of the charged oxide; it became of a pale brown colour, and was found to hold silver in solution. The dilute nitric acid also dissolved off some oxide from the plate, as was shown by its becoming milky on the addition of muriatic acid. On adding some potash to the ammonia, and dissipating the ammonia by heat, a black precipitate was formed. The precipitate from nitric acid by potash was dark brown. The general characters of this oxide were the same as those of the suboxide of silver examined by M. Wöhler.

(101.) After having removed as much as possible from the glass plate, by solution of ammonia, there was still a film upon the surface of the glass, which upon drying became nearly black, appearing of a fine olive green colour when looked through. It was now ascertained that this film conducted the electric current, and on pouring over the plates nitric acid diluted with an equal quantity of water, the films were dissolved off with the formation of nitrous acid fumes.

(102.) These results prove that the influence of the

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chemical rays (actinism) on the protoxide of silver, converts it first into a suboxide and into metallic silver. have not been able, by the longest exposure to sunshine, to reduce all the suboxide to the state of metal, unless organic matter was present.

(103.) Pure protoxide of silver being spread whilst moist upon paper, by means of a camel-hair pencil, and carefully dried, passes, in the course of a few hours, if the sun shines strongly, into a perfect black, the whole of which dissolves off in ammonia. This salt precipitated by potash, the ammonia being dissipated by heat, gave upon reduction the exact formula of Wöhler's oxide (Ag. 2 O.).

(104.) By continuing the exposure of these papers for a few days, their colours were changed from black to olive. Ammonia dissolved nothing, and diluted nitric acid was equally inactive. In moderately strong nitric acid, however, the paper was almost immediately rendered white with the formation of nitrous acid fumes. Any analytical results from preparations on paper are so liable to error, from the inconstant character of the composition of the paper itself, that they are scarcely to be depended upon. But by very carefully drying two weighed pieces of paper until they lost no further weight, and even heating them up to a point but a little below that at which the paper would scorch, placing them both in warm tubes and hermetically sealing them, exposing one to the actinic influence, whilst the other was preserved in the dark, opening the tubes under the same circumstances, burning the papers at the same temperature, and weighing the ashes against each other, I succeeded in reducing the sources of error considerably; and the result was, in several experiments, that the actinized oxide lost in weight very nearly the weight of the oxygen of the oxide employed.

(105.) Several trials were made to ascertain if the films of oxide of silver on the glass plates lost weight upon darkening; but although some evidence was obtained that they did so, it must be admitted that it was not at all satisfactorily proved to be the case by this means. The