Lithographic stones do not consist merely of carbonate of lime, but contain also silica, alumina, and some iron; the colour is a useful indication of quality, those presenting a bluish grey appearance being most highly prized, while those of a yellow colour are somewhat inferior. The stone may be split into various thicknesses, and prepared for use by grinding either one or both surfaces. They are placed face to face, and the first grinding is completed with coarse sand, for the second grinding a finer sand is employed between the two surfaces, and this is succeeded by a grinding with pumice stone and water, the stone being finally polished with snake stone. The so-called lithographic chalk, with which the drawing on stone is sometimes executed, is composed of finest lamp black (merely added for colour), soap, wax, tallow, and shellac; during the etching, these ingredients undergo a change, and form with the stone a chemical compound of great stability. Since the fatty acids of the soap and tallow-namely, oleic and margaric acid—set free by the nitric acid, combine in the nascent state with the lime to form oleo-margarate of lime. This adheres tenaciously to the stone, and the whole picture may, by the application of turpentine, to all appearances be completely removed from the stone. If the ink roller be again applied, it will be found that even the finest details remain intact, and will reappear of their original strength, the ink only adhering to the oleo-margarate of lime, and not to the surface of the stone, which owing to the etching is wet with nitrate of calcium. This is the theory of lithographic etching and printing, to which it has only to be added that soap and tallow must always be present in the inks or chalks used in drawing to produce the combinations referred to. Wax and shellac are only present as binding material. Transfer ink, as used in photo-lithography, must be slightly thinned, and this is best done by means of turpentine, and, if necessary, the occasional addition of a few drops of olive oil. Should the ink be too stiff, or be allowed to dry too much upon the transfer paper, the transfer will probably be faulty, as the ink will only partially leave the paper. If the ink remains in a soft condition it will be absorbed very readily by the clean dry litho stone (not etched). Owing to the avidity with which all grease is absorbed by the stone, it is necessary to keep it perfectly free from all such contaminations. To keep the stone in a clean condition while printing it must be etched, kept damp during working, and, on completion or suspension of work, the surface should be coated with a thick solution of gum arabic. Before resuming printing operations, it is of course necessary to remove the gum from the surface of the stone, which is done with a damp sponge.