LITHIUM 



3464 



LITHOGRAPHY 



LITHIUM, lith'ium, a soft, silver-white 

 metal, and the lightest known solid. It is a 

 little over one-half as heavy as water, and it 

 floats on petroleum. When cut, lithium has a 

 silvery luster, but it soon tarnishes when ex- 



posed to the air. It is softer than lead and 

 harder than sodium. When thrown on water it 

 decomposes the water and sets the hydrogen 

 free. Lithium does not occur in a free state, 

 but is found in solution in some mineral waters. 

 It is usually obtained by separating it from its 

 melted chloride by a strong electric current. 

 It forms numerous compounds, some of which 

 are valuable in medicine, while others are used 

 to impart a red color to fireworks. 



LITHOGRAPHY, lithog'rafi, the art of 

 drawing pictures, designs or writing on a stone 

 or metal surface, in such a manner that the 

 impression may be reproduced by printing. 

 The plate is so treated that only the design 

 traced by the pen or crayon will take up the 

 ink, while the rest of the plate remains clean. 



The Process. The stone used in lithography 

 is a porous variety of calcium carbonate, found 

 chiefly in Bavaria. The best stones are light 

 gray in color. They are delivered in slabs from 

 three to four inches in thickness and in sizes 

 ranging from six by eight inches to forty-four 

 by sixty-four inches. More recently zinc and 

 aluminum have been substituted for stone. Ex- 

 cellent plates are now made of aluminum; they 

 are convenient because they are flexible, uni- 

 form in quality and less easily broken. 



When stone is employed, the treatment of 

 the surface depends upon the instrument to be 

 used. The surface is ground and left slightly 

 roughened for the lithograph crayon; when a 

 pen is to be used, the stone is polished with 

 pumice stone. In one method, the design is 

 drawn on the grained surface with crayon; it is 

 an exact copy of the object, but turned upside 

 down, like type in printing. The plate is then 

 washed with a solution of gum arabic and acid. 

 The acid decomposes the soap of the crayon 

 and leaves the surface chemically prepared to 

 take up fatty ink. The gum water covers the 

 untouched portion of the stone with a film. 

 Turpentine is then applied and an inked roller 

 is run over the surface. The design readily ab- 

 sorbs the fatty ink, while the moist, gummed 

 surface resists it. When a pen is used, the de- 

 sign is drawn with small dots on the polished 

 surface. This is called pen-stippling. 



Another process is by engraving. A smooth 

 stone is first prepared with gum water. Its sur- 

 face is then covered with lampblack or some 

 other pigment, and the picture is scratched with 

 a steel needle. When the design is completed, 

 the stone is oiled, the oil being readily ab- 

 sorbed by those portions of the stone laid bare 

 by the needle. The gummed surface here again 



