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the fpecies of ware according to the intendedlife. For enduring the higbeft heat withoutfinking, no veffels have-been found fc ufefui asthe Heilian crucibles, which are made accordingto Pott, by mixing a very refraCtory clay withfund, the fineft part of which is feparatedbyafieveand rejected. Thefe veflels are not turned inthe potter’s wheel as common pottery is, but■ the clay is made into a much ftiffer mafs withlets water, and is falhioned into the requiredihaps by . being ftrongly rammed into an ironmould. This gives them fuch a degree ofcompactncfs that they will retain faline fluxesfor a confiderable time.

With regard to this latter circumftar.ce, noearthy mixture can be made which will not beadded on more or lefs by faline fluxes, and alfoby the vitrified metallic oxyds, as has beenmentioned under the article Glafs, and this isparticularly the cafe with the vitrified oxyd oflead, which has long been ufed as a kind of tellof the retentive power of crucibles, as it paffesthrough the ordinary porous crucibles in afliort time almoft with as much eafe as liquidsthrough a. fieve.

Ordinary crucibles may he made more re-tentive by being lined on the in fide before theyare quite dry with a thin coating of pure claywithout any other mixture.

1 he moll refractory material known is amixture of unburnt with burnt clay, and onethat refills the operation of faline fluxe 'ongerthan any other, and hence it is employed m themaking of the large crucibles for the glafs houfeas deferibed under the article Glafs. Theclofe denfe porcelain alfo retains faline matterslong without being materially acted on.

It is of confiderable importance, ini fre-vefl'els that, they iliould be able to bear heatingand cooling with tolerable quicluiefs, but itunfortunately happens that fome of the moltperfeCl ware in every other refpeCt, is verydeficient in this, and will even hardly bear thedraught and flame of a wind furnace, howeverflowly heated, without danger of cracking.This is particularly the cafe with the valuableporcelain fire-ware invented byMr.Wedgewood.This property of cracking on fudde-n changes oftemperature, appears particularly to depend onthe two circumftances of liardnefs and clofenefsof texture, and the latter is the greateft (ceterisparibus) in proportion to the finenefs of divifion(■ f the materials before binning. Thus in theware juft mentioned, both the clay and flint arebrought, to a mo ft impalpable powder beforemixture, and the texture is uncommonly hard.

and clofe; and on the other hand, it has beenfound neceflary in making tlie belt Heffian andother crucibles, to feparate and reject the fitterpart of the filiceous ingredient, for the exprefspurpofe of enabling it to bear the ftrong draughtof the wind furnace. The Wedge wood wareHands fudden heating and cooling better whenit is covered with a thin coating of Windforloam, or of a fire-lute compofed of clay andcoarfe fand, and tow or horfe-dung.

Crucibles intended merely for the fufion ofmetals, are much improved by a mixture ofblack lead. This material is protected fromaccels of air by being involved in the clay, andis then incombuftible, and its abfolute infufibi-lity and want of affinity with earths at anytemperature, enable the clay to undergo a very-high heat without melting. Befides the foftnelsand fmoothnefs of the plumbago enables theveffel to afford a very uniform furface withoutknots or projections, which in the commonrough ware are apt to detain a portion of meltedmeta 1 whilft pouring out. This ware will alfobear fudden heating and cooling better than anyother, fo that fcarcely any precaution is requi-fite on this account, but its ffftnefs and porouf-nefs render it quite unfit fa, tinkling any falineor other flux.

The moft porous fubftance ufed for fire-veffelsis bone-afh, which is therefore only employedwhen this quality is an advantage, as in theprocefs of cupellation, already fully deferibedunder the article AJfay. Here it aCts like a,lpotige in abforbing the vitrified oxyd of lead,,and only the reguline metal remains.

Of the earthen veflels intended to hold acidor corrofive liquors in diftillation, no ware isfo perfect as the white Wedgewood porcelainware already mentioned, its texture being foclofe as not to require any lead-glazing. Theonly defeCt of this is the danger, of cracking onany fudden change of temperature.

But even this as all other pottery ware when ;unglazed, becomes in fome degree perviouswhen heated fully red-hot, fo that it cannotbe depended on at this heat to confine gaffeswhen under any preflure. This accounts forthe total difappearance of diamonds when en-clofed in lumps of fine porcelain-clay and bakedin ,an intenfe fire, as was found in fome of theearlier experiments on this fubftance, and couldnot be explained till the difeovery of the totalconverfion of this gem into carbonic acid gasby combuftion. This permeability of unglazedporcelain ware alfo explains fome objectionsbrougl* to the antiphlogiftic theory by the