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whole of the air paffes in beneath the grate andgoes out above the fuel, whence the draught isproportionally ftronger and the heat more in-tenfe, and it alfo may be approached withouttrouble. If a furnace of this kind is fuppliedwith charcoal (which gives a much ftrongerheat than coal with a moderate draught of air)and the height above the grate is about double thediameter, a heat fully equal to the fufion of afmall bulk of copper will be produced.

Still this is far fhort of the temperature re-quired for many operations, and therefore,(where the natural draught of air is reforted to)it is neceffary to increafe this by inclofing thetop of the cylinder with a dome of the famematerials, which may be further elongated by achimney .of any height. The draught of air isnow prodigioufly increafed, and if che fules ofthe furnace are pretty thick, and the chimneybroad enough and high enough, an intenfity ofheat is produced, able to melt iron, reducemanganefe, &c. in fhort, to perform nearly allthat artificial fire can do. It is by no meansneceffary that the chimney fhould be perpendi-cular in every part, and it is often much moreconvenient that it fhould quit the furnace in anhorizontal or oblique direction, as it allows amuch readier accefs to the furnace and its con-tents.

The proportion which the diameter of thechimney fhould bear to that of the furnace, togive the greateft intenfity of heat, has been va-rioufly ftated, and it ftill feems not to be well de-termined. From fome experiments of Darcet andLavoifier it would appear that the greateft pof-fible heat obtainable by air furnaces, is procuredby making both furnace and chimney one verylong upright channel or flue, perfectly open atbottom and at top, the furnace itfelf (or thepart of the channel juft above the grate) beingbulged out in a moderate degree to give greaterroom for the fuel and the fire veflels. In gene-ral however the flue of the chimney is made alittle narrower than the afh-pit. A clofe fur-nace of this kind intended Amply for heatingany veffel in the midft of the fuel, requireseffentially no more than three openings, namely,the afn-hole, the chimney, and a fidc-door,through which to throw down fuel, and to intro-duce and take out the fubftance to be heated.

' Where diftillation by naked fire is wanted, afmall addition is required, and a hole mu ft becut level with the middle of the retort, throughwhich its beak may emerge, and conned): withthe receiver or other v-ffel intended to colled!

the product. The retort in this cafe is generallyearthen.

Another object in furnaces is to apply amoderate degree of heat to larger veflels, whichtherefore can only be partially in contadt with thefire, Moft of the furnaces ufed in manufacturesand for many common ptirpofes are of this kind,fuch as the common houfehold coppers for heat-ing water, brewing coppers, falt-pans, &c. &c.Sand pots are alfo of this kind, and in all thefe,the fubftance to be heated is placed over the fire,fometimes dipping into it. This alfo effentiallyrequires only three openings, viz. the alh-pit,chimney, and door for fuel, the furfacc of thefubftance heated beinf external to die fire.

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Another purpofe in furnaces is to inclofe andheat on all fides an earthen oven or muffle, themouth of which at the fame time comes in con-tad! with one of the inner fides of the furnace,and is freely acceflible from without by a cor-refponding hole cut through the fubftance of thefurnace. This therefore requires another open-ing befides the three former already mentioned.

It is fometimes required in experiments ofrefearch to place an earthen or iron tube infuch a manner that the middle of it fhall beftrongly heated whilft each end is cool, andprojecting beyond the furnace, fo that no partof the contents of the tube can come in contactwith the fuel. This is done in a clofe furnace,by having two holes through the fides directlyoppofite each other, through which the tubemay be thru!!.

One more mode of diftributing the heat of afurnace may be mentioned, which is, where thefubftance to be heated is neither inclofed in anyveffel, nor in actual contact with the fuel, butis fpread on a kind of floor immediately beyondthe fire in a fpace between the fire-place and thechimney, and receives the heat from the flamewhich draws over it. This forms the reverbe-ratory furnace, and requires a very peculiarcontraction, as will be afterwards explained.

The above are the principal objects to befulfilled in the contraction of chemical furnacesfor general purpofes, but another great diftinc-tion is in the mode of fupplying air, which maybe either by the natural draught of air, or bybellows or other artificial means. In the formercafe the rapidity of the current of air and confe-quent intenfity of the fire, depends on having afulficient fpace in the alh-pit and chimney, andefpecially a very great length of flue, and allthe fide-openings which would break the cur-rent, carefully clofed. But with an artificial3 u 2