FUR

FUR

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matter, and hence it never runs into a hardvitreous flag as coak does, but burns away intoa clean light alh which falls through the bars ofthe grate, without choaking and hindering thedraught of air, or melting down the clay wallsof the furnace. The flues or chimnies alfonever collect any foot or foulnefs, and thereforenever require cleaning. A much greater rangeof heat may be kept up by charcoal than by anyother fuel, for its utmoft intenfity of heat fullyequals that of coal or coak, and befides it affordsthe great advantage of burning away very flowly,with a gentle and pretty fteady temperature,when the fupply of air is juft fufficient to keepthe fire alive. Charcoal (befides being expen-five) has however the inconvenience of beingtoo light to bear a very ftrong blaft of air in com-mon blaft-furnaces, except it is in larger piecesthan is often convenient, and it alfo burnsout very rapidly in a ftrong draught, fo as torequire a conftant fupply. There appears tobe very little reafon for preferring the charcoalof one wood over another, provided both areequally well burnt. It may be added that thewood intended for charcoal is always previouflybarked; when this is neglected the charredbark as foon as it is kindled burns with a fhortbut prodigious eruption of fparks, often incon-venient to the operator.

Coal is the fuel almoft invariably ufed in thiscountry for common culinary purpofes, for allmanufacturing fires where a moderate heat iswanted, and the fubftance is not injured bythe fmoke, or can be put out of the reach of it(as in brewers and diftillers coppers) and in thelaboratory for heating fand-baths, for fmith’sforges, and alfo for the reverberatory furnace. Itgives a large, ftrong, and very lading flame, butfor any intenfity of fire it requires a pretty largeand high chimney, and a wide afti-hole.

Coak gives a very ftrong heat without flame,and this is the general material for ftrong windand blaft furnaces, where an intenfe and durableheat is required. Coak, in the way it is com-monly prepared, always gives out at firft a bluefulphureous flame, which ceafes when fullyred-hot. On account of the great denfity ofthis fubftance it bears the blaft extremely well,but being a much lefs pure combuftible thancharcoal, and containing a mixture of earthsand oxyd of iron, it is very apt when nearlyburnt out to cake together in an intenfe heat,and to run into a tough cohefive flag, quiteglafly and fonorous when cold, and which meltson the furface of the crucibles, and can hardlybe detached from them. Coak alone is not

eafily kindled, and requires a much more power-ful draught of air than any other fuel, fo thatit cannot be burned in an open grate except it isin very large quantity. Its combuftion is mate-rially aflifted, and its vitrification when nearlyburnt out is much prevented, by mingling it withabout its own bulk of charcoal, and thefe twotogether form the very beft material for thefurnaces that require an intenfe heat.

Coal cinders, picked from a common fire, byremoving the coal juft when it ceafes to blaze,and before it begins to burn to afhes, is oftenpreferable to coak for laboratory experiments,as it is of a much lighter and loofer texture, andmuch lefs fulphureous, and burns more readily,giving out a very intenfe heat. The beft New-caftle coal, of the kind that foftens in the fireand is highly bituminous, gives the beft cinderfor this purpofe, which, when cold, is gloffy,light and fonorous.

A mixture of refufe coal and charcoal madeup into irregular cakes and charred togetherhas been introduced of late years for malt-houfes and fimilar purpofes where a ftrongclear heat is required, and this, when brokeninto fmall pieces, anfwers extremely.well forthe laboratory.

Culm or Wellh Coal (which has been de-feribed under the article Coal ) differs from allthe common kinds of coal in burning withoutany flame or fmoke, or leaving much refidue.In this refpeft it more refembles charcoal, andit has been found to anfwer very well as a fuelfor fmall blaft-fires; and being much heavierthan charcoal, it is not fo readily difperfed bythe force of the ftream of air.

Kilkenny Coal is nearly of the fame natureas culm, but not fo pure.

Turf fhould not be unnoticed, though it isfeldom ufed. It is a ufeful material for light-ing fires, and it burns for a long time with alow fmothering heat, and much pungent fmokewhen the draught of air is very fmall. Itmight be employed for long digeftions with avery gentle warmth, even more effeftually thancharcoal, and at very fmall expence, but thefe-operations are now chiefly performed by thelamp, which is on the whole to be preferred.

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We {hall now deferibe fome of die furnaces-a&ually employed in the laboratory.

PI. VIII. Fig. 77 and 78 are a view and.feftion of Dr. Black’s Portable Furnace, as it iscalled, being, with fome variations and improve-ment, that which was invented by that eminent,pliiiofopher. It confifts of. an oval iron, cafe