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Pruffiat of potafh, if pure, does not decom-pofe the falts of tellurium.

The alkaline hydro-fulphurets throw it downin the form of a black powder.

Tin&ure of galls produces a flocculent paleyellowifh brown precipitate.

Zinc and iron precipitate tellurium in, itsmetallic ftate from all its acid folutions. Tinand antimony likewife decompofe, in a fimilarmanner, the muriat of this metal. Phofphorusalfo feparates the metal from its muriatic fo-lution.

The white oxyd of tellurium, if heated bythe blowpipe on charcoal, is reduced with arapid effervefcence to the metallic ftate, and isimmediately after volatilized by combuftion,and converted again to an oxyd. If, on theother hand, it be treated without any inflam-mable matter in a retort, it fufes into a paleyellow ftriated mafs.

Tellurium is not made any ufe of, either inthe metallic or in any other ftate.

TERRA FOLIATA TARTARI. SeeAcetite of Potafj.

TERRA FOLIATA CRYSTALLIZATA.See Acetite of Soda.

TERRA JAPONICA. See Catechu.

TERRA MERITA. See Turmeric.THERMOMETER. See the Appendix .THONSCHIEFER. Argillaceous Schis-t.us. Slate. Argillite.

This mineral may be divided into the threefollowing fubfpecies.

r Sub/p. Common argillaceous fchiftus.—ThonJ,chiefer, Wern. Argillite , Kirw. Schifleargilleux, Broch.

Its colour is fmoke-grey, bluifh or afh-grey,greenifli, reddifh, or blackifh grey •, fometimesalfo flefh-red, reddifh-brown, yellowifh-brown,or olive-yellow. The colour is for the moftpart uniform, but it fometimes prefents fpots,bands, and dendritical delineations. It occursin mafs, difl’eminated, or in rounded fragments.Its luftre is filky or refinous, and varies fromglimmering to Rightly finning. Its fradture isflaty, generally ftrait, but fometimes curved orundulating; fome varieties are fo compact asto prefent a dull earthy fra£ture. Its fragmentsare tabular or rhomboidal, rarely large fplintery.It varies from very foft to moderately hard, buteven the hardeft may readily be fcratched by aknife. It is eafily frangible, but not very brit-tle. It gives a greyifh-white ftreak.

It is compofed of filex, alumine, and oxydof iron, with variable proportions of carbonatedlime and magnefia. It is largely ufed for

covering houfes, and the ftrait-foliated bluifh-grey varieties are employed as writing flates.The fofter and more compact varieties aremade into flate pencils.

2 Subfp . Hone flate. —- Novaculite, Kirw.Wetxfchiefer , Wern. Schifle a aiguifer , Broch.Its colour is greenifli-grey, or fmoke-grey, paf-fmg into olive and mountain-green. It occursin mafs, and has a glimmering luftre. Its frac-ture in the great is flaty, in the fmall fplintery.Its fragments are tabular. It is more or lefstranflucent on the edges. It is moderatelyhard, and not very frangible. Sp. gr. 2.72.

It does not effervefce with acids, neither is itfufible by the blow-pipe without addition. Ithas not been regularly analyfed.

It is cut into hones for fharpening the finerkinds of fteel inftruments.

It occurs at Lauenftein in Bareith, at Sei-fendorf in Saxony, in Bohemia , and the Le-vant (whence it is called Turkifh hone); alfoin the valley of Llanberris in North Wales .

3 Subfp. Black chalk.— Zeichenfchief er, Wern.Schife a dejflner , Broch.

Its colour is greyifh or bluifh-black. It oc-curs in mafs; the longitudinal fratture is flatyand glimmering, the crofs fracture earthy anddull. Its fragments are tabular or fplintery.It ftains the fingers, and gives a fomewhatgloffy grey ftreak. It is meagre, but fmooth tothe touch; is foft and very eafily frangible.

Before the blow-pipe, without addition, itacquires a thin varnifh, hut does not melt. Itscomponent parts, according to an analyfis byWiegleb, are 1

64. Silex11.25 Alumine11. Carbon2.75 Oxyd of Iron7.5 Water

Eli

It is employed for drawing and writing onpaper and other materials.

The beft kinds come from Italy ; it is alfomet with in Spain, France , Bareith, and theifland of Ifla in the Hebrides .

Of the above fubfpecies, the firft, namely,common argillaceous fchiftus, is the moft abun-dant. It belongs both to the primitive, tranfi-tion, and fecondary rocks, and occurs in moun-tain beds, often of vaft extent. It inclofes and•fometimes alternates wilh beds of chlorite flate,hone flate, alum flate, and black chalk, of whichthe two former peculiarly characterize the pri-