In lithography the design to be printed, which may be engraved on stone or copper, or written with pen on paper, is transferred by a greasy ink upon the smooth surface of a stone of peculiar fineness and firmness. This stone, which is found in its best state only in Bavaria, where the art was invented, is a variety of slate, which faithfully responds in printing to the slightest touch of a graver or a crayon, and permits the use of fine shades and tints which cannot be produced on wood or on copper. The transferred lines of the design cling to and dry upon the surface of the stone, which is then subjected to the action of a weak acid, which hardens the ink in the transferred lines, while it slightly etches and lowers the surface where it is unprotected. The process of printing begins by dampening the stone with a moist sponge, the water in which is absorbed by the [p022] unprotected face of the stone, while it is repelled by the hard greasy matter in the transferred lines. The inking roller is then applied to the stone with a contrary result; the moistened surface repels the greasy ink, but the transferred lines attract and retain it. When an impression on paper is taken, the only part of the paper which receives ink is that part which touches the transferred lines. The theory of lithography is based upon the repulsion between grease and water. Lithographic printing is chemical printing. [Illustration: Surface Exposed to Impression by the Copper-plate Method. The entire surface of the plate is covered with ink until the white lines are filled. The surface around the figures is wiped clean before the impression is taken.] [Illustration: Surface Inked and Exposed to Impression by the Typographic Method.] [Illustration: Surface Exposed to Impression by the Lithographic Method. This surface is rolled twice: once with water, which is absorbed only by the surface here shown in dull black tint; once with ink, which is retained only on the figures.]