A Manual Of Photography

58 HISTORY OF PHOTOGRAPHY. with a moderately concentrated solution of ammonio-citrate of iron, and dried. The strength of the solution should be such as to dry into a good yellow colour, not at all brown. In this state it is ready to receive a photographic image, which may be im- pressed on it either from nature in the camera obscura, or from an engraving on a frame in sunshine. The image so impressed, however, is very faint, and sometimes hardly perceptible. The moment it is removed from the frame or camera, it must be washed over with a neutral solution of gold of such strength as to have about the colour of sherry wine. Instantly the picture appears, not, indeed, at once of its full intensity, but darkening with great rapidity up to a certain point, depending on the strength of the solutions used, &c. At this point nothing can surpass the sharpness and perfection of detail of the resulting photograph. To arrest this process and to fix the picture (so far at least as the further agency of light is concerned), it is to be thrown into water very slightly acidulated with sulphuric acid, and well soaked, dried, washed with hydrobromate of potash, rinsed, and dried again. "Such is the outline of a process to which I propose applying the name of Chrysotype, in order to recall, by similarity of struc- ture and termination, the Calotype process of Mr. Talbot, to which, in its general effect, it affords so close a parallel. Being very recent, I have not yet (June 10, 1842) obtained a complete command over al1 its details, but the termination of the session of the Society being close at hand, I have not thought it advisable to suppress its mention. In point of direct sensibility, the chrysotype paper is certainly inferior to the calotype ; but it is one of the most remarkable peculiarities of gold as a photo- graphic ingredient, that extremely feeble impressions once made by light go on afterwards darkening spontaneously and very slowly, apparently without limit, so long as the least vestige of unreduced chloride of gold remains in the paper. To illustrate tins curious and (so far as applications go) highly important property, I shall mention incidentally the results of some experiments made, during the late fine weather, on the habitudes of gold in presence of oxalic acid. It is well known to chemists that this acid, heated with solutions of gold, precipitates the metal in its metallic state ; it is upon this property that Berzelius has founded his determination of the atomic weight of gold. Bight, as well as heat, also operates this precipitation ; but to render it effectual, several conditions are necessary :—1st, the solution of gold must be neutral, or at most very slightly acid ; 2nd, the oxalic acid must be added in the form of a neutral oxalate; and 3rdly, it must be present in a certain considerable quantity,