Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/860

824 PHOTOGRAPHY tive proof, if any were needed, that the daguerrean image is a relief. ?ox- Fo.r- Talbot Process. In January 1839 Fox Talbot Palbot described the first of his processes, photogenic drawing, in >rocess. a p a p er O tj ie R y a l Society. He states that he began experimenting in 1834, and that in the solar microscope he obtained an outline of the object to be depicted in full sunshine in half a second. We must turn, however, to the Philosophical Magazine for the account of the full details of his method, which consisted essentially in soak ing paper in common salt, brushing one side only of it with about a 12 per cent, solution of silver nitrate in water, and drying at the fire. Fox Talbot stated that by repeating the alternate washes of the silver and salt- always ending, however, with the former greater sensitive ness was attained. This is the same in every respect as the method practised by Wedgwood in 1802 ; but, when ?alotype. we come to the next process, which he called "calotype" or " beautiful picture," we have a distinct advance. This process Talbot protected by a patent in 1841. It may be briefly described as the application of iodide of silver to a paper support. Carefully-selected paper was brushed over with a solution of silver nitrate (100 grains to the ounce of distilled water), and dried by the fire. It was then dipped into a solution of potassium iodide (500 grains being dissolved in a pint of water), Avhere it was allowed to stay two or three minutes until silver iodide was formed. In this state the iodide is scarcely sensitive to light, but is sensitized by brushing " gallo-nitrate of silver" over the surface to which the silver nitrate had been first applied. This "gallo-nitrate" is not a chemical compound, but merely a mixture, consisting of 100 grains of silver nitrate dissolved in 2 oz. of water, to which is added one-sixth of its volume of acetic acid, and immediately before applying to the paper an equal bulk of a saturated solution of gallic acid in water. The prepared surface is then ready for exposure in the camera, and, after a short insolation in the dark, develops itself, or the development may be hastened by a fresh application of the "gallo-nitrate of silver." The picture is then fixed by washing it in clean water and drying slightly in blotting paper, after which it is treated with a solution of potassium bromide, and again washed and dried. Here there is no mention made of hyposulphite of soda as a fixing agent, that having been first used by Sir J. Herschel in February 1840. In a strictly historical notice it ought to be mentioned that development by means of gallic acid and nitrate of silver was first known to Rev. J. B. Reade. When impressing images in the solar microscope he employed gallic acid and silver in order to render more sensitive the chloride of silver paper that he was using, and he accidentally found that the image could be developed without the aid of light. The priority of the discovery was claimed by Fox Talbot ; and his claim was sustained after a lawsuit, apparently on the ground that Reade s method had never been legally published. It would be beyond the scope of the present article to give the slight improvements which Talbot afterwards made in the process. In one of his patents he recognizes the value of the proper fixing of his photogenic drawings by the use of hyposulphite of soda, and also the production of positive prints from the calotype negatives. We pass over his application of albumen to porcelain and its subsequent treatment with iodine vapour, as also his application of albumen in which iodide of silver was held in suspension to a glass plate, since in this he was undoubtedly preceded by Niepce de St Victor in 1848. Llbumen Albumen Process on Glass. It was a most decided step rocess j n a( j v ance when Niepce de St Victor, a nephew of Nice- phore de Niepce, employed a glass plate and coated it with iodized albumen. The originator of this method did not meet with much success. In the hands of M. Blanquart Evrard it became more practicable ; but it was carried out in its greatest perfection by M. Le Gray. The outline of the operations is as follows. The whites of five fresh eggs are mixed with about one hundred grains of potassium iodide, about twenty grains of potassium bromide, and ten grains of common salt. The mixture is beaten up into a froth with an egg- whisk or fork, and allowed to settle for twenty-four hours, when the clear liquid is decanted off. A circular pool of albumen is poured on a glass plate, and a straight ruler (its ends being wrapped with waxed paper to prevent its edge from touching the plate anywhere except at the margins) is drawn over the plate, sweeping off the excess of albumen, and so leaving an even film. The plate is first allowed to dry spontaneously, a final heating being given to it in an oven or before the tire. The heat hardens the albumen, and it becomes insoluble and ready for the nitrate of silver bath. One of the difficulties is to prevent crystallization of the salts held in solution, and this can only be effected by keeping them in defect rather than in excess. The plate is sensitized for five minutes in a bath of nitrate of silver, acidified with acetic acid, and exposed whilst still wet, or it may be slightly washed and again dried and exposed whilst in its desiccated state. The image is developed by gallic acid in the usual way. After the application of albumen many modifications were introduced in the shape of starch, serum of milk, gelatin, all of which were intended to hold iodide in situ on the plate ; and the development in every case seems to have been by gallic acid. At one time the waxed- paper process subsequently introduced by Le Gray was a great favourite. Paper that had been made translucent by white wax was immersed in a solution of potassium iodide until impregnated with it, after which it was sensi tized in the usual way, development being by gallic acid. This procedure is still followed in some meteorological observatories for obtaining transparent magnetograms, barograms, &c. Reflexion will show that in images ob tained by this process the high lights are represented by metallic silver, whilst the shadows are translucent. Such a print is technically called a " negative." When chloride of silver paper is darkened by the passage of light through a negative, we get the highest lights represented by white paper and the shadows by darkened chloride. A print of this kind is called a " positive." Collodion Process. A great impetus was given to photo- Collo- graphy in 1850, rendering it easy of execution and putting (Jlon - it into the hands of the comparatively untrained. This was the introduction of collodion, a vehicle which up to the present day holds its own against the more rapid pro cesses on account of the facility with which the plates are prepared, and also because it is a substance totally un affected by silver nitrate, which is not the case when any organic substance is employed, and, it may be said, in organic as well in many instances. Thus albumen forms a definite silver compound, as do gelatin, starch, and gum. The employment of collodion for use in photo graphy was first suggested by Le Gray, who has been already mentioned in connexion with the albumen process. He does not appear to have gone beyond suggestion, and it remained for Archer of London, closely followed by Fry, to make a really practical use of the discovery. Collodion is a solution of cotton or cellulose in which some atoms of its hydrogen have been replaced by NO,, by treatment with a more or less dilute mixture of sulphuric and nitric acids. The action of the sulphuric acid is to take up the molecules of water formed by elimination of the hydrogen from the cotton, which combines with oxygen from the nitric acid, the latter acid supplying the cotton with NO. 2 . According to the temperature of the acids and