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lence as to form a long pencil of blue flame,attended with a confiderable roaring noife. Thiscontinues as long as any alcohol is left in theboiler, which allows ample time for molt blow-pipe operations. The boiler is filled at theopening h. The centre hole i is nicely fittedwith a fmall brafs plug kept down by a thin-flip of iron /, the other end of which flips overthe top of the upright pillar d, and is confinedbetween two flat fcrew-plates m n. The ufe ofthis is as a fafety valve to take away all danger ofthe boiler burfting by the confined vapour notbeing able to efcape fall enough through thejet-pipe r, for when the internal preffure isgreat, the elafticity of the iron fpring l allowsthe valve i to rife fufficiently to let out part ofthe enclofed vapour. The ferew Hoppers h andk are made flill tighter by collars of leather, asis the part where the tube o joins the boiler.The jet-pipe r has a compleat rotatory motion,fo that the flame may be compelled in any di-rection. This is effected by turning in theform of a ball that part of the pipe which isenclofed in the hollow p q.

BURNING GLASS and MIRROR . SeeSolar Heat .

CALORIMETER. See the article Caloric ,p. 216 in the former part of this Dictionary.

CAPITAL, A capital or head, is the upperpart of an alembic, or other diftilling apparatus,in which the condenfation of the vapour begins.See Distilling Apparatus.

CARBOY is a very large glafs vefl’el of theform of a comprefled globe, with a fliort neck,ufed by the manufacturers for containing ful-phuric acid. It is generally protected by abafket of wicker work.

CRUCIBLE: PI. I. Fig. 22 and 2 6.

A crucible is a pot generally made of clay,but fometimes of other materials, and intendedto bear a ftrong heat. (For fome obfervationson the quality of thefe fire-veflels fee the articlePottery, in jhe former part of this work).The common fhape of the earthen ware cruci-bles, is either three-cornered or round, and theyare fitted with Hoppers of the fame materials,(Fig. 23 and 27) perforated with a fmall hole,opening obliquely to allow the efcape of anyvolatile matter, and to prevent any of the fuelfrom dropping in. As the lower part of thecrucible would efcape the greateft intenfity ofheat, and would be liable to crack by thedraught of air if it was let direCtly upon thegrate of the furnace, it is ufually raifed abouttwo inches by a fmall Hand, which may beeither folid Fig. 24, or hollowFig. 25, The latter

when inverted, alfo forms a convenient Handfor an earthen retort. The lid of the cruciblemay be luted on, when neceflarjr, with any kindof fire lute (as already deferibed in the articleCement , Vol. 1.) particularly with the mixtureof borax and clay. It is often of advantage toline the crucible with charcoal, for example, inthe redu£tion of metallic ores that require noflux, fuch as the black oxyd of manganefe.Chemifts have ufually done this by fitting intothe crucible a folid piece of clofe well-burntcharcoal, and fcooping out a hole in the latter;but it is a much more convenient way to mixfome powdered charcoal with a very little lin-feed meal, to moiften it with juft fo muchwater as will bring the mixture to a ftiff pafte,and to line the crucible with it when wet. Onapplying heat, the linfeed meal burns, and givesout a little flame and fmoke which efcapethrough the hole of the cover, but this does notderange the charcoal lining.

Crucibles are alfo made of call-iron, of fine fil-ver and of platina. The former are fometimesof advantage in analyfes where a very ftrongheat is required to be given to fubftances incontadl with alkaline fluxes, but they are feldomufed. Silver crucibles are employed with greatutility in the common anaiyfis or refolution ofminerals by cauftic alkalies; and they are alfoextremely ferviceable in a variety of other expe-riments, where a heat not exceeding moderaterednefs is required, and where the fubftancesdo not a£t upon this metal.

The ufe of platina for crucibles has fomuch increafed of late that it is confidered aseilential to a well-furnilhed laboratory, andindeed not without reafon, as there is no fub-ftance that unites fo fully the qualities of infu-fibility at almoft any heat with refiftance to theadtion of almoft every chemical reagent. Thecauftic alkalies however have fome diflblvingpower on this metal as is mentioned under thearticle Platina . Where a crucible of this metalis ufed for very high heats, particularly in firesfupplied by coal or coak, it fhould be loofeiyinclofed in a refradlory earthen crucible, other-wife the vitreous flag of the coal is apt to encruftthe outfide of the platina veffel, and adhere toit with fo much force that it can hardly be gotofl' without great violence.

CUCURBIT . PI. II. Fig. 39. «.

This is an oblong glafs vefl'el with a verywide mouth (nearly of the form of a long gourd,whence its name) ufed for various chemicalpurpoies, ami in particular it forms the body ofthe common glafs alembic.