in mineralogy, a species of fossil arranged under the magnesian earths. In Magellan's edition of Cronstedt's Mineralogy, it is considered as a species of Mica, and has accordingly been mentioned by us under that article. On the other hand, Dr Kirwan has classified the mica under the siliceous earths, while he places talc under the magnesian. According to the analysis of Dr Kirwan, "talc consists of pure magnesia, mixed with nearly twice its weight of silica, and less than its own weight of argill." It is composed of broad, flat, and smooth laminae, or plates. There are two varieties of it, the Venetian talc and Muscovy talc; for the difference of which, see the article MICA.

The Venetian talc has not derived its name from being a production of the territories of Venice (for it is not often to be met with in that country), but probably from being an article of Venetian commerce. It abounds in England, Norway, Hungary, Bohemia, Spain, and in many countries of Asia. Venice talc, with half its weight of alkaline salt, may, in a strong fire, be brought into perfect fusion, though not to perfect transparency: with equal its weight, or less, of borax, it runs into a beautiful, pellucid, greenish yellow glass. Talc does not melt with any other earth, nor even bake or cohere with any but the argillaceous: Mixtures of it with them all are nevertheless brought into fusion by a remarkably less quantity of saline matter than the ingredients separately would require. Thus equal parts of talc and chalk, with only one-fourth their weight of borax, melt in no very vehement heat into a fine transparent greenish glass, of considerable hardness and great lustre. On substituting gypseous earths to chalk, the fusion was as easy, and the glass as beautiful; in colour not green, but yellow like the topaz. Talc, with half its weight of sand, and a quantity of nitre equal to both, yielded also a transparent topaz yellow glass. Several further experiments on talc may be seen in a memoir by Mr Pott in the Mem. de l'Acad. de Berlin, 1746.

Muscovy talc, called also lapis specularis, is found in many parts. The island of Cyprus abounds with it. It is very common also in Russia, and has of late been discovered to abound in the Alps, the Apennines, and many of the mountains of Germany. It is imported in large quantities into England, and is used by the lanthorn-makers instead of horn in their nicer works; by the painters to cover miniature pictures; and by the microscope-makers to preserve small objects for viewing by glasses. The ancients used it instead of glass in their windows. Some take the lapis specularis to have been a species of gypsum, and composed of the acid of vitriol and calcareous earth. It came into use at Rome in the age of Seneca*; and soon after its introduction was applied not only to lighten apartments, but to protect fruit-trees from the severity of the weather; and it is recorded, that the emperor Tiberius was enabled, principally by its means, to have cucumbers at his table during almost every month in the year. Dr Watson apprehends it is still used in some countries in the place of glass: however, it is well known, that it was so used in the time of Agricola; for he mentions† two churches in Saxony which were lighted by it.

Agricola esteemed it to have been a species of plaster-stone; and in speaking of it he remarks, that though it could bear, without being injured, the heat of summer and the cold of winter, yet the largest masses of it were wasted by the rain. It differs from plaster-stone in this property, that it does not, after being calcined and wetted with water, swell and concrete into a hard stony substance*.

Although we have treated of Muscovy talc and lapis specularis as the same, we are not ignorant that a distinction has been made between them by some chemists; but as we have found a greater degree of confusion on this subject in several valuable systems of mineralogy than we had reason to expect, we continue the old names as formerly, till a more satisfactory analysis make it proper to apply them differently.

Talc is employed, in those places where it is found in any considerable quantity, in compositions for earthen vessels; and by some for tests and cupels. From its smoothness, uncleanness, and brightness, it has been greatly celebrated as a cosmetic; and the chemists have submitted it to a variety of operations, for procuring from it oils, salts, tinctures, magisteries, &c. for that intention. But all their labours have been in vain; and all the preparations sold under the name of talc have either contained nothing of that mineral, or only a fine powder of it.