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which the rays

a yellow picture not be expos

paper is to be immersed in a solution of bichromate of potash, and dried by the fire; by this it assumes a fine yellow colour, and it may be kept for any length of time without injury, and is always ready for use. When an engraving is laid on this paper, and it is exposed to the sunshine, it passes rapidly, over all the parts through which the rays can act, into a light brown; consequently the first result is a yellow picture upon a brown ground. In this state the photograph cannot be exposed to the Light, as all the yellow parts would become brown. If, however, the paper is soaked in water, all the unchanged salt is dissolved out, but that which is browned is not disturbed. We have thus a delicate negative picture, from which positive copies may be taken. If the paper is exposed too long to sunshine, it loses colour. A pleasing variety may be made by mixing sulphate of indigo, with the bichromate of potash, the colour of the object and of the ground being different shades of green.

(317.) The change which the solar rays produce upon this salt is one of deoxidation. Chromic acid is liberated, and it combines with the organic matter of the size of the paper. Mr. Ponton states that the neutral chromate exhibits no such change; it has, however, been since discovered that even the chromate of potash on paper will darken; but it is only by long exposure that much effect can be produced; and the ultimate degree of darkness falls very far short of that given by the bichromate. This change is effected by the blue rays of the spectrum, and their action appears to be confined within rather narrow limits.

(318.) M. E. Becquerel has investigated, with considerable care, the action of chromic acid on organic bodies under the influence of Light, and he has shown that the darkening is dependent upon the nature of the size used on the paper. It occurred to him, therefore, that by the application of starch as a size to the paper, pleasing

effects might be produced by the agency of iodine, and the result was satisfactory. According to Becquerel's method, a sizing of starch is applied very evenly over the paper; it is then steeped in a concentrated solution of the bichromate of potash, and dried. Pictures are taken in the usual way, and the paper is washed and dried. When dry, it is immersed in a weak alcoholic solution of iodine, and afterwards, when it has remained in it for some time, it is rinsed in water, and carefully dried between folds of blotting paper. If the drawing is not considered to be sufficiently distinct, the immersion may be repeated until it becomes so. The effect is not improved by using a more concentrated solution of iodine. When the paper is wet, the shades of the picture are of a very fine blue; but when it is dry, they become of a deep violet. If, while the photograph is still wet, it be covered with a layer of gum-arabic, the colour of the drawing is greatly preserved, and it is more beautiful when dry.

(319.) The metallic chromates have been thought to be compounds of too permanent a character to change under solar influence. Many of them, however, deepen in colour by exposure; and the chromate of mercury has been found to undergo a very remarkable change. Paper was prepared with the bichloride of mercury (corrosive sublimate) and the chromate of potash, and exposed with an engraving upon it for some hours. There was evidently some change of colour, but it was very slight, over the exposed parts. This was placed aside, and remained in a dark drawer for two or three months without being noticed. It was then found to have become through its substance semi-metallic, and, both on the front and back of the paper, a tolerably good impression of the engraving was visible.

(320.) FERROCYANIDES. The photographic uses of these salts have already been the subject of consideration (140.); and it only remains to state, in this place, that



a solution of the ferrocyanide of potassium mixed with a solution of the iodide of potassium, applied to paper, speedily changes in the sunshine, and may be used for the production of a very pretty variety of negative photographs. M. Fischer pointed out that a solution of ferrocyanide of potassium precipitated by alcohol, and rapidly dissolved in water, when exposed to Light passes into a green and then a blue colour, depositing Prussian blue, and giving a strong smell of hydrocyanic acid.





(321.) RESINS.Heliography.—By this name M. Niepce distinguished, in 1827, the first process by which the images of the camera obscura were rendered permanent, after having impressed themselves upon prepared tablets. Although the process of the philosopher of Chalons is not likely to attract much attention from the photographer, who is now in possession of processes which infinitely surpass it in sensitiveness, yet, as it develops some important operations of the solar rays, it could not be allowed to pass without notice. This process is now becoming more important, since it is employed for the purpose of obtaining impressions upon metal plates and lithographic stones, which can be etched, or prepared for printing. M. Niepce has given directions, which are essentially as follows:

(322.) Into a glass is put a small portion of asphaltum, upon which is dropped essential oil of lavender till the asphaltum is impregnated with it, and as much additional oil is added as will cover it to a slight depth. The mix. ture is then submitted to a gentle heat, until the whole of the essential oil is saturated with the colouring matter of the bitumen. A highly polished plate of silver is procured, and with a soft roll of skin some of this varnish is applied, in a very thin and equal coating; the plate is then placed upon heated iron, and when the varnish has ceased to simmer, it is withdrawn, and left to cool and dry in a gentle temperature, secured against any moisture. The plate thus prepared is placed in the camera,

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and in bright summer sunshine a period of four or six hours is necessary to produce anything like the proper effcct. The images are exceedingly faint at first, but they are brought out by the action of a solvent, which removes from the plate, or renders perfectly transparent, those parts upon which the solar rays have not acted. This solvent consisted of one part, by volume, of essential oil of lavender, poured upon ten parts, by measure also, of oil of white petroleum. The varnished tablet is placed in a proper vessel, which has been filled with the solvent, and the operator, by reflected Light, watches the development of the images, and removes the plate when the proper effect is produced.

The process is then completed, by placing the plate upon an inclined plane, and washing it with very clean water, to remove all the softened parts of the varnish which may still adhere to it. This varnish may be spread upon metal, glass, or stone. Engravings are more easily copied by this method than pictures from nature can be procured.

(323.) Niepce appears to have advanced this process considerably; but his partner in this inquiry, M. Daguerre, suggested the use of materials by which the operation was greatly improved, as it regards sensitiveness and general effect. These improvements consisted in applying the residuum obtained by the evaporation of the essential oil of lavender to the plates, instead of the asphaltum; and instead of dipping the plate, after exposure, into a solvent, it is so placed that the vapour of petroleum acts upon it, by which the portions of the varnish that have been acted on by the Light are rendered transparent.

(324.) Daguerre remarks that all bitumens, all resins, and all residua of essential oils, are decomposable by sunshine in a very sensible degree. To exhibit this action, very thin coatings of them should be spread over fitting surfaces; and it is a curious fact, and well worthy the inquiry of chemists, that different solvents act differently

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