PHOTOGRAPHY ON GLASS. 457 of brom-iodide of silver is thus produced upon the plate, which, at a certain stage, possesses a high degree of sensitiveness to light. The plate is then transferred to a camera, and exposed at the focus of the lens to the light radiated from the object to be copied. After remaining a few seconds in the camera, it is withdrawn, and immediately exposed in a warm box to the vapor of metallic mercury. When the plate is taken from the camera, the film looks as uniform as ever, and no image is visible upon it; but the exposure to mercury vapor immediately brings out an image. The mercury fixes itself strongly upon those parts which have received the light, while it takes no hold upon those parts of the film which the light has not decomposed. A strong solution of hyposulphite of sodium is then poured over the plate, in order to dissolve off the undecomposed brom-iodide. The highly polished silver, beneath, forms the shades, and the amalgam of mercury with silver forms the lights. The plate is washed, and a very dilute solution of chloride of gold in hyposulphite of sodium is poured over its surface and gently warmed. A thin film of gold, which, as it were, varnishes the picture, is thus deposited upon the plate; another washing completes the operation. The daguerreotype is the most perfect of photographs; but the polish of the surface prevents the image from being seen in all lights, and the plate is liable to be tarnished and ruined by sulphuretted gases. In order to get a photograph upon glass, a transparent film capable of holding the necessary silver-salt must first be attached to the glass plate. Collodion (a solution of a variety of guncotton in a mixture of alcohol) and ether is the material of this film. To the collodion is added a solution of an iodide, either of potassium, cadmium, or ammonium, or a mixture of these; the bromides of ammonium and cadmium, or one of them, added in the proportion of one part of the bromides to three or four of the iodides, render the film more sensitive to yellow and red rays --a point of importance in cloudy weather or smoky towns. The collodion thus prepared is poured rapidly over a clean and dry surface of plate-glass; the volatile solvents evaporate rapidly, and as soon as the film is coherent the glass is plunged into a bath of nitrate of silver very slightly acidified with acetic or dilute nitric acid. This bath must be in a dark place; the plate remains in it for several minutes. A yellow layer of iodide or brom-iodide of silver is produced in the film, and nitrate of potassium, cadmium, or ammonium dissolves in the bath. The plate is then exposed in the camera for a few seconds. When removed no image is perceptible; but on pouring over the film a solution of gallic or pyrogallic acid in alcohol and acetic acid, or a solution of the green sulphate of iron, mixed with a few drops of a weak solution of nitrate of silver, the image will be developed, slowly or rapidly, according to the nature and strength of the developingliquid, the degree of exposure, and the intensity of the light. The illuminated portions of the picture will appear, under the action of the developer, more or less black, while the shaded portions will retain the yellow colour of the iodide. As soon as the details of the shaded portions appear, the liquid is washed off and the development arrested. A saturated solution of hyposulphite of sodium is then poured over the film to dissolve off the yellow iodide of silver where it has not been affected by the light; only the reduced portions of silver remain, and they appear more or less opaque. The plate must finally be very thoroughly washed to remove all traces of the hyposulphite, and then dried and varnished on the collodion side to protect the film from injury. Concerning the nature of the change which a film of iodide of silver undergoes when exposed to light, we cannot be said to have any exact knowledge. There is no perceptible alteration in the film; there is no loss of iodine; the iodide retains its solubility in hyposulphite of sodium; yet the impression is not of a temporary kind; for the invisible image produced on a plate may be developed many hours afterwards, if the plate is kept in the dark during the interval. The photograph on collodion may be employed directly as a positive picture, if not too strongly developed, by placing it on a black background. Those portions which are opaque to light, or in other words those in which silver is deposited, will reflect light, and furnish the lights of the picture; while those on which the light did not act, and which are therefore transparent, will appear black from the nature of the background, and these will form the shades of the picture. In the daguerreotype the finished picture PHOTOGRAPHY ON PAPER. 459 is inverted; in the collodion positive it is not inverted. If the development be pushed further, the image becomes so strongly defined that the deposited silver will more or less completely intercept the light. The collodion side of the plate is then placed in contact with the sensitive side of paper impregnated with chloride of silver by a process immediately to be described, and exposed to light in a pressure-frame. The light is arrested by the altered parts of the collodion, but is freely transmitted by the other portions; upon the paper, therefore, the lights of the real object are. light and the shades are dark. Such a negative collodion picture may of course be copied on a second sensitive collodion film. Two developing-solutions, used one after the other, produce a better effect than one. The green sulphate of iron may be used first, and pyrogallic acid with nitrate of silver subsequently; the iron solution must be completely washed off before the other is added. The picture may even be intensified by pyrogallic acid after the plate has been washed in hyposulphite of sodium. Photographs were made on paper long before the film on glass came into use; but the paper process is now, chiefly confined to the printing of positive impressions from collodion negatives on glass. The silversalt which is preferred for photographic paper is the chloride, with or without albumen, but always accompanied with free nitrate of silver. The paper is floated for five minutes on a solution of chloride of sodium or ammonium; when dried, it is floated in a dark room, for five minutes, on its salted surface, in a solution of nitrate of silver; again dried, it is fit for use. When such paper is used to obtain a positive impression from a collodion negative, or from a paper negative made transparent with wax or a mixture of wax and paraffine, it is exposed to light under the negative to be copied, until the lights of the picture are of a pale lilac hue, and the shades of a deep bronze color. being thoroughly washed, the paper is transferred to a "toning"-bath, which consists of a very dilute solution of bicarbonate of sodium, with a minute proportion of chloride of gold. The picture is kept in motion while in this bath; it remains there until its shades have acquired a deep purple-black color. It is only in those parts of the picture in which the silver has been well reduced that this toning effect is produced. The picture is again washed in water, and soaked for fifteen minutes in a solution of hyposulphite of sodium, in order to remove all the chloride of silver which is contained in the substance of the paper. Finally the picture must be soaked for twenty-four hours in After water which is constantly renewed, in order to wash away every trace of the compound hyposulphite of sodium and silver. No photograph will keep long, unless the chloride of silver has been completely dissolved by the hyposulphite, and the compound hyposulphite washed away with a thoroughness that leaves no trace behind. If the first condition is not fulfilled, diffused daylight will alter the picture; if the second condition is not complied with, yellow or brown stains will ultimately destroy the picture. As in every other art which embraces many details, and demands a trained eye and hand, eminent skill in photography can, as a rule, be acquired only by long practice. 538. Chloride of Silver (AgCl).—Native chloride of silver occurs, sometimes in cubical crystals, sometimes in compact semitransparent masses, which are sectile, and, from their general appearance, have given the mineral the name of horn-silver. Chloride of silver may be precipitated from any soluble silversalt by adding to the silver solution chlorhydric acid, or the solution of any soluble chloride; or it may be obtained by passing over a dry silver-salt a stream of dry chlorine gas. This last reaction is the basis of a method of preparing anhydrous nitric acid. When a stream of dry chlorine is made to pass over perfectly dry nitrate of silver heated to 50° or 60°, the following reaction takes place : Ag,N,O, + 2C1 = 2AgCl + N,0, + 0. 2 6 1 200000 The characteristics of precipitated chloride of silver have been already described (Exp. 272). The presence of an extraordinarily minute proportion of chloride of silver renders a clear liquid opalescent. It is easy to detect silver in a solution of which it forms only the part, by adding to the solution a drop of chlorhydric acid or of a soluble chloride. An admirable method of determining the amount of silver present in any solution depends upon the insolubility of chloride of silver, the density and peculiar curdy quality of the precipitate, and the visibility of the smallest trace of it in a clear fluid. This method, now generally employed in mints and assay-offices, is applicable to the quantitative analysis of silver alloys; it is volumetric, and depends upon the measurement of the amount of a standard solution of chloride of sodium which is required to effect the complete precipitation, as chloride, of the silver contained in a given ATOMIC WEIGHT OF SILVER. 461 weight of the alloy. In a solution which is acidulated with nitric acid, and which contains no excess of the soluble chlorides, the chloride of silver is easily coagulated into dense flocks by agitation; so that the exact point at which the precipitate ceases to be formed is readily perceived. Chloride of silver melts at about 260°. It is not decomposed when heated with carbon, but is easily reduced by hydrogen when heated in a current of the gas; zinc and iron reduce moist chloride of silver to metallic silver; when heated with carbonates or hydrates of sodium, potassium, or calcium, chloride of silver gives its chlorine to the other metal, and pure silver is set free. These methods of reducing chloride of silver, except that by hydrogen, are turned to account in the refining of silver on a large scale. The coin, or bullion to be refined is dissolved in nitric acid, and to the solution chloride of sodium is added; the precipitated chloride of silver is washed until the washings are tasteless, and is then slightly acidulated with sulphuric acid; bars of zinc are placed in the moist mass, and the whole left at rest for two or three days. Chloride of zinc and metallic silver are the products. As soon as the reduced silver is entirely soluble in nitric acid, the reduction is complete. The reduced metal is digested for two or three days in dilute sulphuric acid, to remove adhering zinc-salts, and is then thoroughly washed, dried, and finally melted and cast into ingots. If an absolutely pure metal is desired, the first reduction should be made with pure zinc, and this refined silver may be again dissolved in nitric acid, thrown down as chloride, and reduced again from the washed chloride by fusion with carbonate of calcium. 539. The reduction of chloride of silver by hydrogen is the basis of one of the several determinations of the atomic weight of silver; and since silver forms a large number of anhydrous salts with acids, and has little or no tendency to form more than one salt with each acid, the silver-salt is often the best one to prepare and analyze whenever the combining-weight of an acid is to be determined. But it is clear that the accuracy of these determinations depends upon the accuracy with which the atomic weight of silver is known; hence extraordinary pains have been taken to arrive at the true atomic weight of silver. It has been found, by the most careful experiment, by heating chloride of silver in a current of hydrogen, that in 132-856 parts of that compound, |