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of purpofes and capable of being wrought withthe greateft facility.
It is not eafy to obtain a perfect union ofzinc and copper by mere fufion in open veflels,for at a heat lefs than is required to melt thecopper, the zinc readily takes fire and much ofit burns off before it has time to mix with theother metal, fo that the proportion of zinc isconftantly lefl'ening by volatilization. Even afterboth metals are fufcd, the zinc continues toburn off in uncovered vefTels, and at laft fcarcelyany thing but copper would be left. In ordertherefore to combine copper moft intimatelywith zinc, and yet to preferve its malleability,the ingenious procefs of cementation has beenreforted to in the manufacture of brafs, whichis performed by heating in a covered pot alter-nate layers of copper in fmall pieces, with zincore and charcoal, and continuing the fire till thecopper is thoroughly impregnated, with thezinc.
Zinc being a volatile metal can only be pro-cured from its ores by fublimation ; the procefsfor obtaining it (which will be defcribed moreat length under that article) being to heatftrongly a mixture of its ore with charcoal in aVefl'el clofed on all fides, except where it admitsa tube, the lower end of which dips in water:as foon as the charcoal reduces the oxyd, themetal rifes in vapour through the tube and con-denfes in the water below. A fimilar reductiontakes place in brafs-making, only the vapour ofthe zinc inftead of being conveyed out of thecrucible in which it is formed, unites with thecopper encloled in the fame vefl'el, and the wholemelts down into brafs. A lefs heat is requiredin brafs-making than that which fufes copper,the zinc being able to penetrate the copperwhen thoroughly red hot, and melting it downas foon as it becomes brafs.
Brafs is manufactured in many countries, butno where more extenfively and better than inEngland, in which both the materials are ingreat abundance. The ores of zinc are feveralfpecies of calamine and' of blende , called by theminers Black-Jack, which are found abundantlyin Devonfhire, Derbyfhire, and North-Wales ,generally accompanying lead ores. Thefe arechiefly oxyds or carbonated oxyds of zinc, andrequire a previous calcination before they arefit for brafs-making.
At Holywell, in Flintfliire, the calaminewhich is received raw from the mines in theneighbourhood, is firft pounded in a flampingnfill, and then walhed and fifted in order to
feparate the lead, with which it is largely ad-mixed. It is then calcined on a broad {hallowbrick hearth, over an oven heated to rednefs,and frequently ftirred for fome hours. In fomeplaces a conical pile is compofed of horizontallayers of calamine alternating with layers ofcharcoal, the whole refting on a layer of woodin large pieces, with fufficient intervals for thadraught of air. It' is then kindled, and theftack continues to burn till the calamine isthoroughly calcined. The calamine 1 thus pre-pared is then ground in a mill, and at the fametime mixed with about a third or a fourth partof charcoal, and is then ready for the brafs fur-nace. In fome places pit-coal is ground withthe calamine inftead of charcoal, but this isfound to injure the malleability of the brafsobtained.
The brafs-furnace a has the form of the fruf»turn of a hollow cone, or a cone with the apexcut off horizontally. At the bottom of thefurnace is a circular grate or perforated ironplate, coated with clay and horfe-dung, to de-fend it from the aftion of the fire. The cm*cibles ftand upon the circular plate, forming acircular row with one in the middle. The fuel,which in England is coal, is thrown round thecrucibles, being let down through the upperopening or fmaller end of the cone: over thisopening is a perforated cover made of fire-bricksand clay, and kept together with bars of ironfo as to fit clofely. This cover ferves to regulatethe heat in the following manner : the draughtof air is formed through an under-ground vaultto the afh-hole, thence through the grate andround the crucibles, and through the fmallerupper opening into an area where the workmenftand, which is covered by a large dome and achimney to convey the fmoke into the outer air.When the draught is the ftrongeft, and theheat is required of y the greateft intenfity, thecover is entirely removed and the flame thendraws through'the upper opening of the furnaceto a confiderable height into the outer brickdome; when the heat is to be lefiened thecover is put on, which intercepts more or lefsof the draught from the furnace, as more orfewer of the 1 holes of the-cover are left uu-ftopped.
The crucibles are charged with the mixedcalamine and charcoal, together with copperclippings and refufe bits of various kinds, andfometimes brafs clippings alfo, moft of whichare previoufly melted and run into a final! funkciftern of water through a kind of cullender*