OXIDE OF SILVER exposed for a few hours to good sunshine, passes into a more decided olive color, than characterises it when first prepared by precipitation from nitrate of silver. Longer exposure renders this color very much lighter, and the covered parts, are found much darker, than those on which the light has acted directly. In some instances where the oxide of silver has been spread on the paper a decided whitening process in some parts, after a few days exposure, is noticed. Oxide of silver dissolved in ammonia is a valuable photographic fluid; one application of a strong solution forming an exceedingly sensitive surface. The pictures on this paper are easily fixed by salt or weak ammonia.

NITRATE OF SILVER.--This salt in a state of purity, does not appear to be sensibly affected by light, but the presence of the smallest portion of organic matter renders it exceedingly liable to change under luminous influence.

If a piece of nitrated paper is placed upon hot iron, or held near the 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 color results, and beyond the green, a yellow or yellow brown stain is made. This exhibits a remarkable analogy between heat and light,--before spoken of in chap. II--and is of some practical importance in the preparation of the paper.

PRISMATIC ANALYSIS.--The method of accomplishing the prismatic decomposition of rays of light by the spectrum has already been described on pages 22 and 23. The color of the impressed spectrum, on paper washed with nitrate of silver, 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 pinkey shade, a very decided green tint, on the point which corresponds with the least refrangible blue rays, may be observed, its limits of action being near the centre of the yellow ray, and its maximum 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; beyond this point the action is very feeble.

When the spectrum is made to act on paper which has been previously darkened, by exposure to sunshine under 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 grey, passing, however, at its most refracted or positive end into a ruddy brown.

In its Photographic application, the nitrate of silver is the most valuable of the salts of that metal, as from it most of the other argentine compounds can be prepared, although it is not of itself sufficiently sensible to light to render it of much use.

CHLORIDE OF SILVER.--This salt of silver, whether in its precipitated state, or when fused, changes its color to a fine bluish grey by a very short exposure to the sun's rays. If combined with a small quantity of nitrate, the change is more rapid, it attains a deep brown, then slowly passes into a fine olive, and eventually, after a few weeks, the metalic silver is seen to be revived on the surface of the salt. Great differences of color are produced on chlorides of silver precipitated by different muriates. Nearly every variety in combination with the nitrate, becomes at last of the same olive color, the following examples, therefore, have reference to a few minutes exposure, only, to good sunshine; it must also be recollected that the chloride of silver in these cases is contaminated with the precipitant.

Muriate of ammonia precipitates chloride to darken to a fine chocolate brown, whilst muriate of lime produces a brick-red color. Muriates of potash and soda afford a precipitate, which darkens speedily to a pure dark brown, and muriatic acid, or aqueous chlorine, do not appear to increase the darkening power beyond the lilac to which the pure chloride of silver changes by exposure. This difference of color appears to be owing to the admixture of the earth or alkali used with the silver salt.

The prismatic impression on paper spread with the chloride of silver is often very beautifully tinted, the intensity of color varying with the kind of muriate used. Spread paper with muriate of ammonia or baryta and you obtain a range of colors nearly corresponding with the natural hues of the prismatic spectrum. Under favorable circumstances the mean red ray, leaves a red impression, which passes into a green over the space occupied by the yellow rays. Above this a leaden hue is observed, 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 throughout the invisible rays. At the least refrangible end of the spectrum, the very remarkable phenomenon has been observed, 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. Not only the extreme red ray exerts this very peculiar property, but the ordinary red ray through nearly its whole length.

In photographic drawing this salt is of the utmost importance. Mr. Talbot's application of it will be given hereafter in another portion of this work.

IODIDE OF SILVER--Perfectly pure, undergoes very little change under the influence of light or heat; but if a very slight excess of the nitrate of silver be added it becomes infinitely more sensitive than the chloride.

The spectrum impressed upon paper prepared with a weak solution of the hydriodate of potash 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, varying slightly according to the kind of paper used, and the quantity of free nitrate of silver 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 color, 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 beginning of the violet rays of the luminous spectrum. Beyond the extreme violet rays, the action rapidly diminishes, but the darkening produced by these invisible rays, extends a very small space beyond the point at which they cease to act on the chloride of silver.

In its photographic application, it is, alone, of very little use; but in combination with other reagents it becomes exquisitely sensitive. With gallic acid and the ferrocyanate of potash it forms two of the most sensitive photographic solutions with which we are acquainted. These are used in the calotype process.

IODURET OF SILVER.--If upon a plate of polished silver we place a small piece of iodine, and apply the heat of a lamp beneath the plate for a moment, a system of rings is speedily formed. The first ring, which spreading constantly forms the exterior of the circle, is of a bright yellow color; within this, there arises, successively, rings of green, red and blue colors, and then again a fine yellow circle, centred by a greyish spot on the place occupied by the iodine. On exposing these to the light, the outer yellow circle almost instantly changes color, the others slowly, in the order of their position, the interior yellow circle resisting for a long time the solar influence. These rings must be regarded as films of the ioduret of silver, varying, not only in thickness, but in the more or less perfect states of combination in which the iodine and metal are. The exterior circle is an ioduret in a very loose state of chemical agregation; the attractive forces increase as we proceed towards the centre, where a well formed ioduret, or probably a true iodide of silver, is formed, which is acted upon by sunlight with difficulty. The exterior and most sensitive film constitutes the surface of Daguerreotype plates. The changes which these colored rings undergo are remarkable; by a few minutes exposure to sunlight, an inversion of nearly all the colors takes place, the two first rings becoming a deep olive green; and a deep blue inclining to black.

The nature of the change which the ioduret of silver undergoes on Daguerreotype plates, through the action of light, Mr. Hunt considers to be a decided case of decomposition, and cites several circumstances in proof of his position. These with other facts given by Mr. Hunt in his great work on the Photographic art, but to voluminous to include in a volume of the size to which I am obliged to confine myself, should be thoroughly studied by all Daguerreotypists.

PRISMATIC ANALYSIS.--The most refrangible portion of the spectrum, (on a Daguerreotype plate) appears, after the plate has been exposed to the vapor of mercury, to have impressed its colors; the light and delicate film of mercury, which covers that portion, assuming a fine blue tint about the central parts, which are gradually shaded off into a pale grey; and this is again surrounded by a very delicate rose hue, which is lost in a band of pure white. Beyond this a protecting influence is powerfully exerted; and notwithstanding the action of the dispersed light, which is very evident over the plate, a line is left, perfectly free from mercurial vapor, and which, consequently, when viewed by a side light, appears quite dark. The green rays are represented by a line of a corresponding tint, considerably less in size than the luminous green rays. The yellow rays appear to be without action, or to act negatively, the space upon which they fall being protected from the mercurial vapor; and it consequently is seen as a dark band. A white line of vapor marks the place of the orange rays. The red rays effect the sensitive surface in a peculiar manner; and we have the mercurial vapor, assuming a molecular arrangement which gives to it a fine rose hue; this tint is surrounded by a line of white vapor, shaded at the lowest extremity with a very soft green. Over the space occupied by the extreme red rays, a protecting influence is again exerted; the space is retained free from mercurial vapor and the band is found to surround the whole of the least refrangible rays, and to unite itself with the band which surrounds the rays of greatest refrangibility. This band is not equally well defined throughout its whole extent. It is most evident from the extreme red to the green; it fades in passing through the blue, and increases again, as it leaves the indigo, until beyond the invisible chemical rays it is nearly as strong as it is at the calorific end of the spectrum.

Images on Daguerreotype plates which have been completely obliterated by rubbing may be restored, by placing it in a tolerably strong solution of iodine in water.

BROMIDE OF SILVER.--This salt, like the iodide, does not appear to be readily changed by the action of light; but when combined with the nitrate of silver it forms a very sensitive photographic preparation.

Paper prepared with this salt, blackens over its whole extent with nearly equal intensity, when submitted to the prismatic spectrum. The most characteristic peculiarity of the spectrum is its extravagant length. Instead of terminating at the mean yellow ray, the darkened portion extends down to the very extremity of the visible red rays. In tint it is pretty uniformly of a grey-black over its whole extent, except that a slight fringe of redness is perceptible at the least refracted end. Beyond the red ray, an extended space is protected from the agency of the dispersed light, and its whiteness maintained; thus confirming the evidence of some chemical power in action, over a space beyond the luminous spectrum, which corresponds with the rays of the least refrangibility.

This salt is extensively used in photographic drawing.

PREPARATIONS OF GOLD.--Chloride of Gold, freed from an excess of acid is slowly changed under the action of light; a regularly increasing darkness taking place until it becomes purple, the first action of the light being to whiten the paper, which, if removed from the light at this stage, will gradually darken and eventually develope the picture. This process may be quickened by placing the paper in cold water.

Chloride of gold with nitrate of silver gives a precipitate of a yellow brown color. Paper impregnated with the acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-yellow hue. The first impression of light seems rather to whiten than darken the paper, by discharging the original color, and substituting for it a pale greyish tint, which by slow degrees increases to a dark slate color; but if arrested, while yet, not more than a moderate ash grey, and held in a current of steam, the color of the parts acted upon by light--and of that only--darkens immediately to a deep purple.

Here I must leave the subject of the action of light upon metalic compounds--referring to Mr. Hunts work for any further information the student may desire on the other metals--as I find myself going beyond my limits. I cannot, however, entirely dismiss the subject without giving a few examples of the action of light on the juices of plants, some of which produce very good photographic effect.

CORCHORUS JAPONICA--The juice of the flowers of this plant impart a fine yellow color to paper, and, so far as ascertained, is the most sensitive of any vegetable preparation; but owing to its continuing to change color even in the dark, photographic images taken on paper prepared with it soon fade out.

WALL FLOWER.--This flower yields a juice, when expressed with alcohol, from which subsides, on standing, a bright yellow finely divided faecula, leaving a greenish-yellow transparent liquid, only slightly colored supernatant. The faecula spreads well on paper, and is very sensitive to light, but appears at the same time to undergo a sort of chromatic analysis, and to comport itself as if composed of two very distinct coloring principles, very differently affected. The one on which the intensity and sub-orange tint of the color depends, is speedily destroyed, but the paper is not thereby fully whitened. A paler yellow remains as a residual tint, and this on continued exposure to the light, slowly darkens to brown. Exposed to the spectrum, the paper is first reduced nearly to whiteness in the region of the blue and violet rays. More slowly, an insulated solar image is whitened in the less refrangible portion of the red. Continue the exposure, and a brown impression begins to be percieved in the midst of the white streak, which darkens slowly over the region between the lower blue and extreme violet rays.

THE RED POPPY yields a very beautiful red color, which is entirely destroyed by light. When perfectly dried on paper the color becomes blue. This blue color is speedily discharged by exposure to the sun's rays, and papers prepared with it afford very interesting photographs.-- Future experiments will undoubtedly more fully develope the photogenic properties of flowers, and practically apply them.

Certain precautions are necessary in extracting the coloring matter of flowers. The petals of fresh flowers, carefully selected, are crushed to a pulp in a mortar, either alone or with the addition of a little alcohol, and the juice expressed by squeezing the pulp in a clean linen or cotton cloth. It is then to be spread upon paper with a flat brush, and dried in the air. If alcohol be not added, it must be applied immediately, as the air changes or destroys the color instantly.

Most flowers give out their coloring matter to alcohol or water--but the former is found to weaken, and in some cases to discharge altogether these colors; but they are in most cases restored in drying. Paper tinged with vegetable colors must be kept perfectly dry and in darkness.

To secure an eveness of tint on paper it should be first moistened on the back by sponging, and blotting off with bibulous paper. It should then be pinned on a board, the moist side downwards, so that two of its edges--the right and lower ones--project a little over those of the board. Incline the board twenty or thirty degrees to the horizon, and apply the tincture with a brush in strokes from right to left, taking care not to go over the edges which rests on the board, but to pass clearly over those that project; and also observing to carry the tint from below upwards by quick sweeping strokes, leaving no dry spaces between them. Cross these with other strokes from above downwards, leaving no floating liquid on the paper. Dry as quickly as possible, avoiding, however, such heat as may injure the tint.