T At

1JA N

( 597 )

T

TALC. Talk , Wern. Talc, Broch.

Of this mineral there are the three followingfubfpecies.

Subfp. Earthy Talc. Talcite of Kirwan.

Its colour is greenilh-white palfing into red-dilh white, filver-white, pale apple-green, andbright grey. It occurs dilfeminated, fuperficial,or in i'mall kidney-lhaped mafi'es. It has agliftering pearly luftre, and is compofed offlighfiy agglutinated fcaly particles; it ftainsthe fingers, and is very undtuous to the touch.

It is found at Sylva in Piedmont; near Frey-berg in Saxony; and at Meronitz in Bohemia .

It has fometimes been confounded with agaricmineral.

2. Subfp. Common Talc. Venetian Talc.

Its colours are greenilh white, pale apple-green, filvery-white, leek-green, and reddifh oryellowifh white. It occurs in mafs, diffemin-ated, and very rarely cryftallized, in minutehexagonal tables confufedly grouped together.It has a confiderably firming pearly luftre, pafs-ing into femi-metallic. Its fradture is curvedand undulatingly lamellar; its fragments are inthe form of plates or wedges. It occurs fome-times in granular diftinft concretions. It istranflucent and in thin plates tranfparent; isfoft, fefitile, flexible but not elaflic, undtuousto the touch. It is infufible before the blow-pipe without addition. According to an analyfisby Hoepfer it confifts of

50. Silex t44. Magnefia6. Alumine

ICO.

Tt is rarely found except in primitive moun-tains particularly forming beds and veins in Ter-pentine, accompanied by adlynolite, fteatite,&c. It abounds in the mountains of Tyrol andSalzburg , whence it was brought to market atVenice, and from this circumftance receivedthe name of Venetian talc. It is alfo found atZoblitz in Saxony , in Silefia, and other places.

It is employed as a bafis for coloured crayons,and for the fineft rouge.

3. Subfp. Indurated Talc.

Its colour is greenifh-white of different de-grees of intenfity, alfo fnow white, and yel-lowifh white.

It occurs in mafs. It has a brightly fliiningluftre, between pearly and greafy, palfingfometimes into femi-metallic. Its fradture iscurved, lamellar, pafling into flaty, or coarfelyradiated. Its fragments are ufually in the formof blunt edged plates. It is tranflucent, and inthin pieces, femi-tranfparent; is very foft; fec-tile; fmooth and undtuous to the touch.

Its conftituent parts, according to Hoepfer,are,

38.54 Magnefia38.12 Silex6 .66 Alumine0.41 Lime15.02 Iron

98-7J

It forms beds in mountains of argillaceousand micaceous fchiftus, of gneifs and the neweftferpentine. It is accompanied by adlynolite,calcareous fpar, bitter fpar, pyrites, and mica.

It appears to form an intermediate link be-tween fteatite and potftone. French chalk(craie de Briangon) is by fome mineralogiftareferred to this fpecies.

It is found in the Alps, in Stiria, Auftriaand Hungary , and according to Jamefon inBreadalbane, and the Shetland Hlands in Scot-land .

TALCITE of Kirwan. See Talc earthy.

TAN or TANNIN. \

TANNING, (the Art of) f

The general properties of tannin particularlyas connected with the art of tanning, have beendefcribed pretty fully under the articles Lea-ther and Gelatin ; and the properties of thecompound of tannin, gallic acid, and extradt,with various earthy and other falts, which exiftsin the infufion of aftringent vegetables, havebeen given under the article Galls, to all ofwhich we mud refer the reader. It will beproper however in this place flightly to recapi-tulate the general properties of tannin, the me-thods by which chemifts have endeavoured toobtain it pure, and thofe by which its relativeproportion to the other contents of aftringentinfufions have been eftimated. We fiiall alfo addan account of the late experiments by whichMr. Hatchett has produced a fubftance clofclyrefembling tannin.