Asbestos, a, fibrous form of amphibole or hornblende, is composed principally of silica, magnesia, lime, and oxide of iron. Sometimes asbestos is a compact substance, the fibres being stiff and brittle, whilst in other samples the fibres are easily separable, being then elastic and flexible; the fibres may be reduced to a powder which is soft to the touch. In colour, asbestos varies, and is found in whitish shades of green and grey, passing into brown, red, or black. Asbestos is nearly incombustible, and being a very low conductor of heat, finds its application in almost every department of industry. It is mined in Siberia, Switzerland, Spain, Italy, the United Kingdom, and in many Dry parts of Canada and the United States. On being detached from the surrounding rock by blasting, the blocks of asbestos are examined, pounded in such a manner as not to break the fibres, and these are then sorted into different lengths. The fibres, which in good specimens may be 20in. long, are treated in much the same way as are ordinary textile threads; asbestos cannot, however, be felted, and the process of concentration through which, in consequence, the fibres must pass renders the manufacture of asbestos tissues very difficult.

Rock-cork asbestos resembles vegetable cork, is soft and easily cut, and is sufficiently light to float on water. Rock-leather or mountain-leather and rock-wood or mountain-wood resemble rock-cork, but are heavier; rock-wood has somewhat the structure of wood. Other varieties are fossil-paper and fossil-flax, which have respectively a papei'-like and a flax-like texture. Amianthus asbestos is a very superior kind, and is capable of being woven into the finest of tissues. Blue asbestos is more correctly termed crocidolite, which is a mineral composed of silica, iron, and sodium; it has a fibrous structure and a delicate blue colour.