other chemicals possessing analogous photographic properties, denominate them ‘the sensitizer;’ and in referring to the colored gelatinous solution, I will denominate this mixture ‘the tissue-compound.’ When the tissue to be produced is required for immediate use, I add the sensitizer to the tissue compound; but, where the tissue is required to be preserved for some time before using, I prefer to omit the sensitizer from the tissue-compound, with a view to the tissue being made sensitive to light subsequently, by the application of a solution of the sensitizer.
“With respect to the composition of the tissue-compound, it will be understood by chemists, that it may be varied without materially affecting the result, by the addition or substitution of other organic matters, similarly acted upon by light, when combined with a salt of chromium, such as I have referred to. Such other organic matters are gum arabic, albumen, dextrine; and one or more of these may be employed occasionally to modify the character of the tissue-compound, but I generally prefer to make it as follows: I dissolve, by the aid of heat, two parts of gelatine, in eight parts of water, and to this solution I add one part of sugar, and as much coloring matter in a finely divided state, or in a state of solution, or both, as may be required for the production of a photographic print with a proper gradation of light and shade. The quantity required for this purpose must be regulated by the nature of the coloring matter employed, and also by the character of the negative to be used in the printing operation. Where it is desired that the coloring matter of the print should consist entirely or chiefly of carbon, I prefer to use lampblack finely ground and prepared as for water-color painting, or I use Indian-ink; and where it is desired to modify the black, I add other coloring matter to produce the color desired. For instance, I obtain a purple black by adding to the carbon, indigo and crimson-lake, or I add to the carbon an aniline dye of a suitable color; where the coloring matter used is not a solution or dye, but solid matter in a fine state of division, such as Indian-ink or lampblack, I diffuse such coloring matter through water, or other inert liquid capable of holding it in suspension; and after allowing the coarser particles to subside, I add, of that portion which is held in suspension, as much as is required, to the gelatine solution. In preparing tissue to be used in printing from negatives technically known as ‘weak,’ I increase the proportion of coloring matter relatively to that of the tissue-compound; and I
