of nitro-glycerine and disinfectants. Sheets of mica are used as a surface for painting, especially in India ; for lantern slides; for carrying photographic films ; as a protective covering for pictures and historical documents ; for mounting soft and collap sible natural history specimens preserved in spirit ; for vanes of anemometers ; for mirrors of delicate physical instruments ; for various optical and many other purposes. Being a bad conductor of heat it is used for the packing and jackets of boilers and steam-pipes. Other applications depend on the strength of its resistance to acids.
The most extensive application of mica at the present day is for electrical purposes. Being a bad conductor of electricity it is of value as an insulator, and the smooth flexible sheets are much used in the construction of armatures of dynamos and in other electrical machinery. "Micanite" or "micanite cloth"—small sheets of mica cemented with shellac or other insulating cement on cloth or paper—is used for various purposes.
Muscovite and phlogopite are practically the only species used commercially. Phlogopite is rarely found as colourless transpar ent sheets and is therefore almost exclusively used for electrical purposes. Many other uses of mica might be mentioned : the potassium it contains renders it of value as a manure, and the species lepidolite is largely employed in the manufacture of lithium and rubidium salts.
Mining, Preparation and Value.—Mica mining is an indus try of considerable importance, especially in India where, how ever, methods are very primitive and wasteful. In working down wards in open quarries and in tortuous shafts and passages much of the mica is damaged and a large amount of labour is expended in hauling waste material to the surface. Since the mineral occurs in definite veins a more satisfactory and economical method of working would be that adopted in metalliferous mines, with a ver tical shaft, cross-cuts, and levels running along the strike of the vein ; the mica could then be extracted by overhead stopping, and the waste material used for filling up the worked-out excava tions.
In dressing mica the "books" are split along the cleavage into sheets of the required thickness, and the sheets trimmed into rectangles with a sharp knife, shears or guillotine, stained and damaged portions being rejected. The dressed sheets are sorted according to size, transparency, colour and freedom from spots or stains. Scrap mica is ground to powder or used in the manufacture of micanite.