Page:The American Cyclopædia (1879) Volume X.djvu/532

526 LITHOGRAPHY medicine. Of these the first two are more pow- erfully diuretic than the corresponding salts of sodium or potassium, and from their low com- bining numbers a smaller dose suffices to render the urine alkaline. The compound formed by lithia with uric acid is very soluble, and these salts have accordingly been recommended and used in the treatment of gout (in which disease an excess of uric acid is found in the blood), being administered either in the form of an ordinary pharmaceutical solution or mixture, or as natural or artificial mineral waters. The action of the bromide of lithium is similar to that of the bromide of potassium, but is said to be effective in cases in which the latter fails. LITHOGRAPHY (Gr. M6of, a stone, and ypd- $eiv, to write), a method of producing printed copies of a writing or drawing on stone without the usual process of engraving. It was invent- ed about 1796-'8, in Munich, by Aloys Sene- felder. As originally proposed by him, it was merely an etching in relief upon stone, a pro- cess which had long before been practised both upon stone and metal, although he was probably ignorant of the fact. As early as 1728 Dufay, a member of the French academy, described and practised a method of etching upon stone. He made a drawing with varnish, and used an acid to eat down the unprotected parts of the stone, leaving the lines in relief, and is said to have produced some exquisite work. About 1788 William Blake, the English painter, in- vented (or as he believed was spiritually taught) a similar process, only he used plates of copper, and in this manner produced his most famous works. Senefelder's use of stone was wholly accidental. Being, like Blake, too poor to pay for printing his works, he endeav- ored to devise some means of doing this him- self from plates etched in relief, and to avoid expense he used smooth slabs of stone instead of plates of copper. Being ignorant of the composition of the varnish used by engravers for their etching ground, he invented a kind of crayon composed of wax and tallow. One day his mother wished him to write out a list of clothes to be sent to the laundress. Paper and ink not being at hand, he wrote the list upon a stone with his crayon. When he was about to clean off the stone it occurred to him, as it had to Dufay, that the body of the stone could be eaten down by aquafortis, leaving the lines in relief, so that impressions could be taken in the usual manner. His experiments in this direction were partially successful, al- though less so than those of Blake. In 1798 he thought of the availability of the chemical principle which is the foundation of the art of lithography properly so called, namely, the mutual repulsion between oily substances and water. The material upon which the drawing is usually made is an argillaceous limestone. Stones more or less adapted for the purpose occur in various parts of Europe and America ; but the best are found in the quarries of So- lenhofen in Bavaria, and these are almost ex- clusively used, being exported to all parts of the world where lithography is practised. The rock belongs to the upper oolite, is very closely grained, and is evidently formed from the finest sediments, the color varying from a light buff to a pearl gray. The stones, being quarried in mass, are split or sawn into slabs two or three inches in thickness and of any required size. To prepare them for use, they are ground to a perfectly uniform face and polished. If the drawing is to be in crayon, they are " grained " by rubbing two together, with the intervention of fine sand, the grain- ing being finer or coarser according to the nature of the work to be produced. If the drawing is to be in ink, the surface is left polished. The crayons are composed mainly of tallow, wax, hard soap, and shell lac, color- ed with lamp black; other ingredients being sometimes added. The mixture is thoroughly incorporated in a closed vessel over a fire, and then moulded into the usual crayon form. Crayons, technically called "chalk," are re- quired of different degrees of hardness ; an in- crease of tallow makes them softer, of shell lac harder. With these crayons the drawing is made upon the stone precisely as upon pa- per. For pen-and-ink drawings a piece of the chalk is rubbed down with water upon a mar- ble or porcelain slab, so as to form a liquid ink, which is applied with a fine pen or a camel's hair pencil. Both methods are fre- quently used in the same drawing, the fine strokes and sharp outlines being made with a pen, the coarser ones with crayons, while broad masses and tints are washed in with the pencil. But owing to the presence of the alkali of the soap, the chalk is soluble in water, and the drawing can be washed off with a moistened sponge. Diluted nitric or hydro- chloric acid is therefore poured over the stone ; the acid unites with and neutralizes the alkali, leaving the remainder of the chalk insoluble in water. The acid attacks the uncovered por- tions of the stone, rendering them more porous and more capable of absorbing water, and also eats it down, leaving the lines in slight relief, and thus facilitates the process of printing. The stone is next washed with pure water and afterward with gum water; the object of the latter is to prevent the coloring matter from spreading under the pressure to which the stone is to be subjected in printing, and to re- tain the acid that adheres to the greasy sub- stance, the quality of which it is designed to change. Being then rolled over with printers' ink, it is ready for printing, although it is sometimes necessary to lay it aside for a day or two in order that the chalk may become thoroughly hardened. Sometimes the drawing is made upon transfer paper, which is merely paper coated on one side with a solution of gum, starch, and alum; thus the drawing is not directly upon the surface of the paper, but upon the preparation with which it is covered. The paper is then laid upon the stone, and