manifest from the circumstances under which the specimen was issued. The details were not published, and remain to this day the secret of the inventor.
The next step in the history of carbon printing is based upon the discovery of another effect produced on certain bodies by the action of light, and is again due to M. Poitevin. It will be noticed that the processes already described depend on the action of light in rendering a soluble body insoluble. This is the effect on asphaltum, upon a mixture of some organic bodies and a chromic salt, and upon some other substances. In the new process discovered by M. Poitevin, a body previously insoluble is rendered soluble and hygroscopic by the action of light. The first results of this process were shown to the French Photographic Society, in July, 1860, the process having been patented the month previously. The details were published in the following November.[1] In this process a mixture of perchloride of iron and tartaric acid, ten parts of the former to four of the latter, dissolved in one hundred parts of water, is the sensitive preparation. It is poured on a plate of glass which has been previously coated with collodion or other suitable material; it is then left to dry in the dark, and becomes spontaneously insoluble. Submitted to the action of light, however, it again becomes hygroscopic. After exposure under a negative, if breathed upon, the parts upon which light has acted become moist and tacky, in degree and depth proportioned to the action of light. Finely-powdered carbon, applied with a brush, adheres to the image in greater or less proportion, just in the degree in which moisture is absorbed, thus giving an image with a just gradation of half-tone. For producing enamels, a vitreous powder has to be applied; and for
- ↑ “Photographic News,” vol. iv, p. 331.
