the iron salts, under the influence of light, being reduced by the tartaric acid, restoring the organic matter to its natural solubility.
The sheet is then washed in hot water, which removes the ferruginous compound and develops the picture.
Swann, of Newcastle-upon-Tyne, about 1861, was the first to introduce into a practical process the transfer of the film, after exposure, to another surface, with the face of the film downwards, so as to admit of the dissolving off of the unaltered gelastance.
With such tints, — when the solution takes place from the face, — when the free gelatine comes to be dissolved, the thin coating of insoluble gelatine and pigment representing the more delicate shades becomes undermined and floats away.
Swann, to avoid this, transferred the film with its affected side downward on a sheet of paper, washed from the back of the film, and transferred back again to the paper on which it remained.
Argentotype is a modified form of carbon picture introduc