ASBESTOS must be considered in minera logy, rather as a term implying a peculiar form sometimes assumed by several minerals, than as a name denoting a particular species ; it is in fact applied to varieties of the amphi.. belie minerals, such as actinolite, tremolite, &c., which occur in long capillary crystals, placed side by side in parallel position, and thus giving rise to a fibrous mass.. Those the fibres of which are very delicate and regularly arranged, are called ainianthas, a Greek term signifying unpolluted, unstained. Of the finest kinds, the individual crystals are readily separated from each other, are very flexible and elastic, and have a white or greenish colour with a fine silky lustre. Though a single fibre is readily fused into a white enamel, in mass it is capable of resisting the ordinary flame, so that when woven it produces a fire-proof cloth. Those varieties in which the crystals are coarser, with scarcely any flexibility, are called common asbestos. It is generally of a dull green, and sometimes a pearly lustre, and readily fuses before tho blow-pipe flame. It occurs more frequently than the amianthus, or finer kind, and is usu ally found in veins traversing serpentine.
There are three other varieties known by the names of mountain feather, mountain wood, and mountain cork, which differ from the common asbestus by the fibres interlacing each other.
In 1815 Mr. Penny of Glasgow described a peculiar substance which had nearly all the characters of asbestus. It was found in a blast furnace, imbedded in the mass of matter which had collected at the bottom of the fur nace in the course of two years and a half, and which is technically called the hearth. It
was in a cavity, about eight inches below the level on which the liquid metal rested. This substance is colourless, iuodorous, and taste less ; and occurs in small masses, composed of extremely minute filaments, cohering lon gitudinally together ; these fibres are very easily detached, and are flexible, though not so much so as those of common asbestos ; they have a silky lustre, and are unattached by the common acids ; they remain unchanged in the flame of a spirit lamp, and are difficult of fusion even with the blow pipe. This sub stance consists chiefly of silica, protoxide of manganese, and alumina ; it differs from common asbestos chiefly in having about ten Per cent. more of silica, and in containing manganese instead of magnesia. Mr. Penny thinks that the occurrence of this substance in an iron furnace furnishes a proof of the igneous origin of asbestos.
Many remarkable proofs have been given, of the power of asbestos to resist heat. Che valier Aldini about twenty years ago had sonic asbestos woven into cloth, and the cloth made into garments ; lie clothed himself in these garments, and exhibited sundry astonishing feats before the Royal Society, suck as holding red-hot iron in a gloved hand, &c. This ma terial has sometimes been proposed for fire men's dresses, and for garments calculated to resist heat generally.