The art, which was discovered by Senefelder towards the end of the eighteenth century, depends on the fact that grease and water are immiscible: a drawing made with a greasy pigment upon a suitable surface adheres very strongly, whilst those parts free from it retain water, so that when damped and rolled up (_i.e._, inked), the ink used will stick only to the lines, etc., of the drawing, but not to the other parts. Clearly the surface is all important, and this is provided by lithographic stone, a limestone occurring in Germany, France, England and Canada. The best stones occur at Solenhofen near Munich, those from other localities being inferior in quality. Incidentally it may be mentioned that zinc and aluminium plates are not infrequently used in place of stone. Lithographic stones vary in hardness, colour and grain. For the best work the stone should be homogeneous and of a hardness suitable for the subject; the colour affords an indication of the hardness, the lighter-coloured stones, which are much the commoner, being softer than the darker. There are two modes of procedure; the drawing may be made direct on the stone with lithographic ink or crayon--both being mixtures of tallow, wax, soap and shellac, with a sufficiency of pigment to render the drawing visible to the artist--or else the drawing may be made upon transfer paper. The former method, although the more satisfactory and often used by artists, is seldom pursued in scientific drawings except when professional draughtsmen are employed. In such cases it may be necessary to reverse the drawing, which is conveniently done by viewing it in a mirror, and, of course, all lettering must be reversed. The majority of amateur draughtsmen make their drawings in pencil or ink and these the lithographer traces upon lithographic transfer paper and transfers them to the stone; he, the lithographer, may merely trace the salient features and work the drawing up on the stone. The transfer papers are coated with gelatine, starch or gum, or mixtures of these substances, the idea being to interpose between the real paper and the pigment--in the form of lithographic crayon or ink--some substance soluble in water which will hold the pigment and prevent it soaking into the paper, so that a transfer has only to be damped through the back, pressed on to the stone and peeled off. The work, together with more or less of the film, will thus be transferred on to the stone and, of course, will be reversed, since the part uppermost on the stone will be the back of the original drawing. The original drawings may be made upon the transfer paper direct, and in so doing mistakes in tracing will be obviated. Suitable papers are made for various purposes, e.g., smooth for ink work and variously granulated for crayon (see Plate 1, which was drawn by Mr. Harry Becker on transfer paper).