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silver in contact with charcoal or an earthy carbonate, was soon reduced to the metallic state, under the action of strong sunshine.

(77.) There are some peculiar effects produced when we place a stick of charcoal in a solution of nitrate of silver, to which I have devoted some attention, and on which I have still a series of experiments in progress. A few facts will, however, prove the peculiar nature of the phenomenon of the revival of silver by carbon.

If a stick of charcoal is placed in a bottle in which is some solution of nitrate of silver, so that one half of the charcoal is in the solution, and the other half above it, there will in a short time appear little spangles of silver upon the upper portion of the charcoal, if it is exposed to diffused Light. In full sunshine the effect is greatly retarded. If the bottle is placed in a dimly illuminated place, there will, in the course of a few weeks, form in the solution around the charcoal, a series of the most delicate thread-like crystallisations of the silver. After these have formed, if the bottle is exposed to sunshine, they are gradually redissolved in the fluid. An examination of the influence of the different rays upon the charcoal in these conditions, appears to indicate a more intense action in the least refrangible rays, than in those of higher refrangibility. An extensive series of experiments is, however, necessary to determine many important points connected with the action of carbon on the salts of silver and gold; and by these we should learn more perfectly the action of organic compounds in general.

(78.) A remarkable analogy between the effects of heat and Light deserves notice, and it is also of some practical importance in the preparation of the papers. If a piece of nitrated paper is placed upon hot iron, or held near a good fire, it will be found that at a heat just below that, at which the paper chars, the salt is decomposed. Where the heat is greatest, the silver is revived; and immediately around it, the paper becomes a deep blue; beyond this a

pretty decided green colour results; and beyond the green, a yellow or a yellow brown stain is made.

(79.) Prismatic Analysis. The first published examination of the effects produced by the spectrum on any considerable variety of chemical preparations, was that of Sir John Herschel, to whom we are greatly indebted for a very large amount of the most valuable information on this branch of the inquiry. I feel it a duty which I owe to this distinguished philosopher, to use his own words in describing the phenomena of spectral actions observed by him, as far as is practical; and to distinguish such from my own experiments and remarks, I shall in every case affix his name.

(80.) In the experiments made by myself on the spectrum, I have allowed the solar rays to pass, either a fine slit formed by two knife edges, or a small round hole. This beam fell upon a prism placed at the proper angle; and the spectrum formed was received upon an achromatic lens, and the preparations under examination placed in the focus of the lens.

The prisms employed were very fine pieces of flint glass, which enabled me to see all the principal fixed lines of the luminous spectrum; a very perfect crown-glass prism; one hollow prism, which was commonly filled with castor-oil; and a quartz prism of superior character manufactured by Mr. Darker.

(81.) Paper washed with the solution of nitrate of silver already mentioned, was placed still damp in the proper position, and in few a minutes a good chemical image resulted. The colour of the impressed spectrum on this paper is, at first, a pale brown, which passes slowly into a deeper shade; that portion corresponding with the blue rays, becoming a blue brown; and under the violet of a peculiar pinky shade, I have sometimes observed a very decided green tint, on the point which corresponds with the least refrangible blue rays. (Herschel.) Its limit of action, under the most favourable circumstances, commences very

near the centre of the yellow ray; its maximum appears about the centre of the blue, although the action up to the edge of the violet ray is continued with very little diminution of effect; it extends beyond the violet, but the action of the invisible rays is feeble.

(82.) When the spectrum is made to act on nitrated paper, which has been previously darkened by exposure to to sunshine under a solution of the cupro-sulphate of ammonia, the phenomena are materially different. The photographic spectrum is lengthened out on the red or negative side by a faint but very visible red portion, which extends fully up to the end of the red rays, as seen by the naked eye. The tint of the general spectrum, too, instead of brown is dark gray, passing, however, at its most refracted or positive end into a ruddy brown. (Herschel.)

(83.) Photographic Application.-Nitrate of silver, although the most valuable of the salts of that metal to the photographer, as from it most of the other argentine compounds can be prepared, is not of itself sufficiently sensible to Light to render it of much use. It may, however, in some few cases, be advantageously employed. If well sized paper is washed over with a solution of 100 grains of the salt in a fluid ounce of distilled water, and dried at a little distance from a warm fire, we form a paper which, for copying lace-work, feathers, and articles of which a perfect outline merely is required, answers well. By soaking the paper previously to applying the wash of nitrate of silver, in isinglass, parchment size, a solution of gum-arabic, or by rubbing it over with the white of egg, the silver darkens much more readily, and eventually acquires a much deeper colour. A pleasing variety of tints for the pictures, may be produced by varying these organic combinations; and a still more interesting series, by precipitating organic liquids with solutions of lead, applying them in the state of cream on paper, and drying, before the nitrate is applied, as recommended by Sir John Herschel. The influence of lead in exalting the oxidation of the silver

salts under the action of sunshine, will be further described in the section on lead. Pictures formed on the nitrated papers are rendered permanent by simply washing, first, in cold, and afterwards soaking in warm water, free from salt.

(84.) CHLORIDE OF SILVER is obtained by adding a solution of chloride of sodium (muriate of soda) to a solution of the nitrate of silver. This salt of silver, whether in its precipitated state, or when fused, by which operation we produce the horn silver of the alchymists, changes its colour to a fine bluish gray, by a very short exposure to to the sun's rays, or even by prolonged exposure in diffused Light. If the chloride is combined with a small quantity of the nitrate, the change is much more rapid, and the darkening process goes on to a deep brown, which slowly passes into a fine olive. Eventually, after a few weeks, metallic silver is seen to be revived on the surface of the paper. It is somewhat remarkable, that great differences are observed in the colour produced on chlorides of silver precipitate by different muriates. Nearly every variety in combination with the nitrate, becomes at last of the same olive colour; it must therefore be understood that the following notices apply to the colour produced by an exposure of a few minutes only to good sunshine, and it must also be recollected that the chloride of silver in these cases is contaminated with the precipitant.

(85.) Muriate of ammonia inclines the precipitated chloride of silver to darken to a fine chocolate brown, whilst chloride of calcium (muriate of lime) operates to the production of a brick-red colour. Chlorides of potassium and sodium afford a precipitate, which darkens speedily to a pure bark brown; while hydro-chloric (muriatic) acid, or aqueous chlorine, does not carry the darkening beyond the lilac to which the pure chloride of silver changes by expoAs far as my experiments have gone, it appears that this difference of colour is owing to the admixture of the earth or alkali used, with the silver salt, and not to the

sure.

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presence of organic matter, although it does, as in the case of the nitrate, produce similar varieties of colour. The chlorides of barium and strontium have some peculiar colorific properties, when in combination with the chloride and other salts of silver, which well deserve particular attention; however, as these peculiarities are more strikingly exhibited in some of the positive photographic processes which will be eventually described, it will be more in place to reserve the description of their effects.

(86.) Prismatic Analysis. The spectrum impressed upon a paper, spread with the chloride of silver, is often very beautifully tinted, the intensity of the colours varying considerably with the salt employed to precipitate the chloride. Papers prepared with the hydro-chlorate of ammonia or the chloride of barium, and then with two successive washes of the nitrate of silver, have given me, when the sunshine has been favourable, a range of colours very nearly corresponding with the natural hues of the prismatic spectrum. Under favourable circumstances the mean red ray leaves a red impression on the paper, which passes into green over the space occupied by the yellow rays. Above this a leaden blue is discovered, and about the mean blue ray, where the action is greatest, it rapidly passes through brown into black, and through the most refrangible rays it gradually declines into a bluish brown, which tint is continued through the invisible rays. At the least refrangible end of the spectrum, the very remarkable phenomenon has been observed,-in the first instance, by Sir John Herschel, of the extreme red rays exerting a protecting influence, and preserving the paper from that change which it would otherwise undergo, under the influence of the dispersed light which always surrounds the spectrum. Although this is very evident when the spectrum, concentrated by a good achromatic lens, is received on the muriated paper, it is still more strikingly shown, if we receive the spectrum directly from the prism, without