A process of an analogous character to that which has just been described, and in which the chloride of gold is an agent, must be next described : this was discovered at the same time as the cyanotype, and has been termed the chrysotype.
" In order to ascertain whether any portion of the iron in the double ammoniacal salt employed had really undergone deoxida-tion, and become reduced to the state of protoxide, as supposed, I had recourse to a solution of gold exactly neutralised by carbonate of soda. The proto-salts of iron, as is well known to chemists, precipitate gold in the metallic state. The effect proved exceedingly striking, issuing in a process nowise inferior in the almost magical beauty of its effect to the calotype process of Mr. Talbot, which in some respects it nearly resembles ; with this advantage, as a matter of experimental exhibition, that the disclosure of the dormant image does not require to be performed in the dark, being not interfered with by moderate daylight. As the experiment will probably be repeated by others, I shall here describe it ab initio. Paper is to be washed 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 impressed 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, etc. 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 farther 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 structure 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 all 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 photographic 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 this 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. Light, 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, which quantity must be greater the greater the amount of free acid present in the chloride. Under these conditions, the gold is precipitated by light as a black powder if the liquid be in any bulk, and if merely washed over paper a stain is produced, which, however feeble at first, under a certain dosage of the chloride, oxalate, and free acid, goes on increasing from day to day and from week to week, when laid by in the dark, and especially in a damp atmosphere, till it acquires almost the blackness of ink; the unsunned portion of the paper remaining unaffected, or so slightly as to render it almost certain that what little action of the kind exists is due to the effect of casual dispersed light incident in the preparation of the paper. I have before me a specimen of paper so treated in which the effect of thirty seconds' exposure to sunshine was quite invisible at first, and which is now of so intense a purple as may well be called black, while the unsunned portion has acquired comparatively but a very slight brown. And (which is not a little remarkable, and indicates that in the time of exposure mentioned the maximum of effect was attained) other portions of the same paper exposed in graduated progression for longer times, viz. 1 min., 2 min., and 3 min., are not in the least perceptible degree darker than the portion on which the light had acted during thirty seconds only.
" If paper prepared as above recommended for the crysotype, either with the ammonio-citrate or ammonio-tartrate of iron, and impressed, as in that process, with a latent picture, be washed with nitrate of silver instead of a solution of gold, a very sharp and beautiful picture is developed, of great intensity. Its disclosure is not instantaneous ; a few moments elapse without apparent effect ; the dark shades are then first touched in, and by degrees the details appear, but much more slowly than in the case of gold. In two or three minutes, however, the maximum of distinctness will not fail to be attained. The picture may be fixed by the hyposulphite of soda, which alone, I believe, can be fully depended on for fixing argentine photographs.
The best process for fixing any of the photographs prepared with gold is as follows :—As soon as the picture is satisfactorily brought out by the auriferous liquid, it is to be rinsed in spring water, which must be three times renewed, letting it remain in the third water five or ten minutes. It is then to be blotted off and dried, after which it is to be washed on both sides with a somewhat weak solution of hydriodate of potash (iodide of potassium). If there be any free chloride of gold present in the pores of paper, it will be discoloured, the lights passing to a ruddy brown ; but they speedily whiten again spontaneously, or at all events on throwing it (after lying a minute or two,) into fresh water, in which, being again rinsed and dried, it is now perfectly fixed".