TIN

TIN

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fcorite are not entirely exhaufted of metal, andare therefore transferred to the ftamping-mill,and afterwards waftied, in order to feparate thericher particles, which are then mixed with thenext parcel of roafted ore.

The pigs of tin thus procured are next put■without any addition, into a fmall reverberatoryfurnace where they are expofed to a very gen-tle heat, the pureft part of the tin firlt meltsand is drawn off, forming tire common graintin; the more refractory part containing a fmallbut variable portion of copper, arfenic, andiron, is then brought to a itate of fufion, andcall into pigs forming the common or ordinarytin.

2. The ftream tiuftone is not, we believe,found in any other part of Europe than Corn-wall . It differs from the former in its extremepurity and abfolute freedom from arfenic, andin its occurring in alluvial heds. The largeftitream-tin work is at Carn, about two miles tothe S. E. of Perran, not far from Redruth . Itis fituated in a valley, through which flows aftream, the courfe of which has been turnedfor the fake of getting at the treafure concealedbeneath its bed. The workmen firlt digthrough a ftratum about fifty feet thick of clay,{hells, and black earth, in which have beenfound hazel nuts, the antlers of an animal ofthe fta g kind, a human fcull, and a copperbattle-axe; to this fucceeds a layer of roundedftones, beneath which is the bed of tin ore ingrains and lumps of various fizes. The thick-nefs of this bed varies from one to five feet,but the thickelt part is comparatively the poor-eft. The whole of the fuperincumbent Stratais cut away as the workmen proceed, fo thatthe general appearance of the cavity is that of avalt gravel Gr fand pit, near half a mile long,and about 2oo feet broad, which is kept clearof water by the powerful action of two water-mill pumps. The tin ore, as it lies quite loofe,is merely {hovelled into barrows, and wheeled tothe head of the works, where it is thrownunder a thin flieet of water which wafties awaythe earth, leaving the pure ore behind. Afterthis fimple purification the ore is fent to St.Auftle, a diftance of about twenty miles, to befmelted. Here all the preparation for the fur-nace that it receives is being bruifed and palledthrough wire fieves containing fixteen rnelhesin the fquare inch. The furnace employed iscalled in Cornwall a blowing furnace, and is infa£t only a blaft furnace of the fimpleft con-ftrudtion, about feven feet high, and fupplied■with air from two cylinders, worked by an

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overfhot water-wheel. ' The only fuel madeufe of is charcoal, and after the furnace is fullyheated it is fed at {hort intervals with the fol-lowing charge, viz. three or four {hovelfulsof ore, and two or three half-bufhels of char-coal, no flux of any kind being employed. Atthe bottom of the furnace is a fmall channelthrough which the reduced tin is conftantlyflowing into a pit below, accompanied by afmall quantity of flag, which is removed fromtime to time, and thrown again into the fur-nace. When the pit is full of tin it is ladledout into an iron boiler, about three feet in dia-meter, with a fmall fire under it to keep themetal fufficiently fluid: two or three largepieces of charcoal are then laid upon the tin,,and plunged to the bottom by means of an ironinftrument refembling a wheel, with a longhandle fixed in the axle. A violent ebullitionis immediately excited, and a little flag, whichwas before mixed with the metal, riles to itsfurface, and is fcummed off. In a minute ortwo after, the metal is tried, by taking up aladleful and pouring it again into the mafs,when if it appears quite bright like filver, andof an uniform confiftence, the purification iscompleat, and nothing more is requifite thanto cool it to the proper degree, and lade it intothe moulds, by which it is formed into pigs,weighing from 2 to 3 cwt. each. If the metalis poured too hot into the moulds it is apt tobe brittle. Good ftream tin affords from 65 to75 per cent, of the very beft and pureft graintin.

None of the Cornifti tin may be fold till ithas been coined; for this purpofe a fmall pieceis cut off from every pig and allayed; if it ap-pears of the requifite purity it receives theftamp of the Duchy , and pays to the Prince ofWales, as Duke, four {hillings per cwt.

§. 4. Phyftcal and Chemical Properties.

Tin is a metal of a beautiful filvery whife-nefs when recently melted, but by fhort expo-fure to the air it acquires a fuperficial tarnifti,which does not much increafe by time. It hasa flight and naufeous tafte, and gives a peculiarfmell when rubbed. It may be cryftallized bymelting a quantity and pouring out the centerof the mafs after the fides have folidified. Itsfpecific gravity is about 7.9. It is an extremelyloft metal, fcarcely if at all elaftic, and when apiece of it is bent backwards and forwards itgives a peculiar crackling noife. It has very littletenacity, being in this refperit inferior even togold, and only fuperior to lead. It is ex-