This solution applied to glass and dried is, in its dry condition, extremely sensitive to light. If printed under a negative, and then exposed to a damp atmosphere, it absorbs moisture, and becomes “tacky” upon the portions fully protected by the densest portions of the negative, while the parts exposed under the clear glass of the negative will be free from any such tendency. The gradations of the negative between these two extremes will be represented by the surface of the gum being more or less adhesive, and it is this property which renders it valuable in various processes. After exposure under a negative finely-ground colour—for instance, graphite—is carefully dusted over the surface of the plate by means of a camel-hair brush, and the powder will adhere in exact proportion to the protection afforded by the negative. A picture is thus produced in graphite powder possessing all the gradations of the original. From a negative a second negative is produced, while from an exposure under a positive a positive will result, each, however, reversed as regards left and right when compared with the original. Advantage is taken of this process to produce reversed negatives, a subject more fully dealt with in another chapter. =Bichromate and Sugar= are never used alone, but as an addition to heighten the sensitiveness and increase the hygroscopic properties of the gelatine or gum-chromate solutions. By the addition of sugar and a little ammonia to chromated gelatine the solubility of the gelatine in hot water is increased. The addition of acetic acid instead of ammonia acts in a contrary direction. It is advisable to become acquainted with the action of certain substances, the application of which to the printed layer increases its solubility after exposure to light, and of others having a contrary or hardening influence on the same, such oft-times proving of service in the case of under or over exposure. To the former class belong all alkalies, as chloride of lime and cyanide of potassium; while among the latter, which have a tanning or hardening effect on gelatine, may be particularly mentioned tannin, alum, chrome alum, and chloride of zinc. =To remove spots=, which print dark, from collotype plates a solution of chloride of lime is used. To produce dark spots without exposure to light a solution of tannin is used, which is also available for adding descriptive titles or other matter to the plates. The ink will “take” as well on the spots or parts touched with the tannin solution as if exposed to light. A collotype plate which has been prepared for printing from may also have writing executed upon it with a bichromated gum solution, once more exposed to light, the writing will then readily take the ink, and appear in the impression as a black inscription. Gum chromated solutions reduced to proper proportions, and applied to different textures and the same subsequently exposed to the light, render them completely waterproof. CHAPTER III.