Many efforts have been made to render chromatic acid an active agent in the production of photographs. M. Ponton used a paper saturated with bichromate of potash, and this was one of the earliest photogenic processes. M. Becquerel improved upon this process by sizing the paper with starch previous to the application of the bichromate of potash solution, which enabled him to convert the negative picture into a positive one, by the use of a solution of iodine, which combined with that portion of the starch on which the light had not acted. But by neither of these processes could clear and distinct pictures be formed. Mr. Hunt has, however, discovered a process which is so exceedingly simple, and the resulting pictures of so pleasing a character, that, although it is not sufficiently sensitive for use in the camera, it will be found of the greatest value for copying botanical specimens, engravings, or the like.

The paper to be prepared is washed over with a solution of sulphate of copper--about one drachm to an ounce of water--and partially dried; it is then washed with a moderately strong solution of bichromate of potash, and dried at a little distance from the fire. Paper thus prepared may be kept any length of time, in a portfolio, and are always ready for use.

When exposed to the sunshine for a time, varying with the intensity of the light, from five to fifteen or twenty minutes, the result is generally a negative picture. It is now to be washed over with a solution of nitrate of silver, which immediately produces a very beautiful deep orange picture upon a light dim colored, or sometimes perfectly white ground. This picture must be quickly fixed, by being washed in pure water, and dried. With regard to the strength of the solutions, it is a remarkable fact, that, if saturated solutions be employed, a negative picture is first produced, but if the solutions be three or four times their bulk of water, the first action of the sun's rays darkens the picture, and then a very bleaching effect follows, giving an exceedingly faint positive picture, which is brought out with great delicacy by the silver solution.

It is necessary that pure water should be used for the fixing, as the presence of any muriate damages the picture, and here arises another pleasing variation of the Chromatype. If the positive picture be placed in a very weak solution of common salt the image slowly fades out, leaving a faint negative outline. If it now be removed from the saline solution, dried, and again exposed to sunshine, a positive picture of a lilac color will be produced by a few minutes exposure. Several other of the chromates may be used in this process, but none is so successful as the chromate of copper.