B L A 



Blast is alfo applied, in amove general ferfe, to any for- 

 cible ilream of wind, ov air, excited by the mouth, bellows, 

 or the like. 



Blast, in /Igrlculture, a difeafe in grain, trees, &c. See 

 Blight. 



Tlie fugur-cane in the Weft Indies is fubjeft to a difeafe 

 eallcd the blall, and faid to be occafioned by thi; aphis of 

 Linnxus, which is dilliiiguifhcd into two kinds, the black 

 and the yelkuv ; and of thefe the latter is the moft dellniflive. 

 It confifts of myriads of little infedls, invifiblc to the nalvcd 

 eye, whofe proper food is the juice of the cane, in learch of 

 which they wound the tender blades, and confequently de- 

 ftroy the vcffels. Hence the circulation being impeded, the 

 growth of the plant is checked, until it withers and dies in 

 proportion to the degree of the ravage. It is frequently 

 affirmed, fays Mr. Bryant Edwards (Hift. Weil Indies, 

 vol. ii. p. 215.) that the blaft never attacks thofe plantations 

 where colonies have been introduced of that wonderful little 

 animal, the carnivorous ant, or " formica omnivora" of 

 Linnifius, called in Jamaica the " raffles ant," from its being 

 fuppofed to have been introduced there by one Thomas 

 Raffles from the Havannah, about the year 1762. Thefe 

 minute and bufy creatures foon clear a fugar plantation of 

 rats ; and their natural food leems to conlill of all kinds of 

 infefts and animalcules. 



Fire Blast. See Blight. 



Blasts, among y)f;««-j-. See Damps. See Mines, and 

 Mining. 



Blast, the term ufed at iron founderies to denote the 



B L A 



and limeftone, heated by finiple atmofpheric preffure to a 

 bright red or white heat, and the iron ilone to a melting 

 heat. This temperature is foon increafcd throughout the 

 furnace, after the blail is applied. The blowing orifices or 

 tuyeres of the furnace exhibit the fuel increaling in white- 

 ncl's, and the iron-llone rapidly diifolving before the blaft, 

 of a blackilh colour. At this period, the lava whicii flows 

 from the furnace, in confequence of the reduttion of the 

 ore and hme-ftone, is conliderably charged with iron, and is 

 of a black, blackifli brown, or greenifh brown colour. 

 Thefe appearances will continue for twelve, twenty-four, or 

 thirty-fix hours, according to the mode of treatment in 

 bringing forward the furnace after blowing. The tuyere (if 

 a bright tuyere furnace) will appear like a blaze of uncom- 

 monly pure light, at times very offenhve to the eye ; it foon, 

 however, becomes accuftomed to it, and can w itli facility 

 difcern the individual mafles of coke, as they are forced 

 away, with the rapidity of lightning, before the irrel'iflible 

 force of the air. The concrete ore and lime-ftone are no 

 longer viiible ; but a fine metallic fpray is conltantly delcend- 

 ing, and, forced from the fuel, precipitates itlelf to the 

 bottom of the furnace. The fcoria formed by the fuliou 

 and union of the lime-ftone, with the immetallic parts of the 

 ore, is carried before the blaft in a fimilar manner and form, 

 but eafily diltinguilhable from the fluid metal by its buoy- 

 ancy, want of velocity when impelled, and by its dull colour. 

 In this ftate, the furnace is deemed in excellent fmelting or- 

 der. The iron is generally revived with little lofs ; and the 

 colour and purity of the cinder or lava fufficiently indicate 



column of air introduced into the furnace for the purpofe of the perfeftion of the feparation. When at any time the 



combullion. Its velocity is occafioned by the impelling 

 power of the blowing machine forcing the whole contents 

 of the air-pump through one or two fmall apertures called 

 nofe-pipes ; and, according to the abfolute power of the 

 engine, air of various denfities will be produced, fo that 

 denfity and velocity are always intimately connefted, and 

 mutually implied. 



The well known combuftibility of iron, and the indifpen- 

 fible neceffity of exciting combufUon by the introduftion of 

 large quantities of condenfed air into the furnace, in contaft 

 with ore in various ftates of maturity as to feparation, into 

 contaft with iron exifting in all the modifications of qnahty 

 as to carbonation, and into contaA with an immenle body 

 of ignited fuel, render this fubjeft the moft important in 

 the major fcale of our manufaftures. Unfortunately for 

 art, as well as for fcience, few praftical deduftions can be 

 brought forward to eftabhih any one theory of blafl ; one 

 common principle only is acknowledged, that all reduction 

 in the furnace is in confequence of the combuftion excited by 

 the column of air introduced. 



To take a proper view of this interefting fubjcft, it will 

 be neceffary to lubmit it to the following dlvifions. 



I ft. Combuftion, as excited in this particular branch of 

 manufafture. 



2d. The nature of the fuel fubmitted to combuftion. 

 3d. The denfity of the air. 

 4th. The quantity. 



5th. The properties which follow as a confequence of 

 denfity and quality. 



ift. Combuilion in the blaft furnace confifts chiefly in the 

 rapid reduction of a given quantity of fohd fuel, and its ac- 

 companying portion of ore, in the fliortcft pofilble time. 

 That furnace, and tliat blaft, which can, in a given time, 

 reduce the greateft quantity of fuel, all things elfe being 

 alike, will always manufadlure the greateft quantity of iron. 

 In common, bef ire the introCutlion of the blaft, the furnace 



brightnefs of the tuyere fails, and becomes dull white or red- 

 difti white, then a change is indicated ; the iron-ftone and 

 lime-ftone will again appear in the folid unfepaiated ftate, 

 and the change of colour in the cinder infallibly betokens an 

 irregularity in the movements of the furnace. 1 hefe appear- 

 ances are fo general, as fcarcely to admit of an individual ex- 

 ception, and are luflicient to warrant the following explanation. 



Ax. the introduftion of the blaft, the interior of the 

 furnace at the tuyere was fimply a mixture of ignited mafles 

 of cokes and iron-ftone, the latter partly femifufed, but the 

 greateft part merely heated to a bright red heat. In the 

 defcent through the furnace, in contart with ignited coke, 

 the particles of metal in the ore may, by parting with the 

 oxygen, have received a difpofition to become revived. The 

 incrcafed temperature creates an additional tendency, by 

 eftablifhing a greater force of affinity betwixt the fuel and 

 the iron. But the metal approaching to its proper tlate, 

 meeting the current of blaft, is immediately fubjcft to a 

 partial combuftion. The portion thus oxydated conveys to 

 the lava, in proportion to its quantity and oxygenation, the 

 colour already mentioned. 



As foon as the continuation of the blaft conveys a higher 

 temperature to the fuperior regions of the furnace, the ap- 

 pearance of the folid matter at the tuvere ceafcs. The fufion 

 and feparation of the metal from the ore are effefled in fitua- 

 tions more remote from the blall, or chief fonrce of decompo- 

 fition, in a temperature more fuited to the nature and exift- 

 cnceof the metal. The iron, once formed into a fluid, audits 

 fluidity prelcrved, its defcent to the blaft is attended" with 

 little or no injury to its carbonation. 



To undcrfland this diftiniitly, it will be neceffary to ftate 

 two curious fads relative to caft-iron in a fluid ftate ; and 

 but for the cxiftence of thefe properties, the manufafture of 

 the metal in open furnaces or veflels would be totally im- 

 pradticable. ifl. Caft iron, while kept fluid, never decom- 

 pofes atmofpheric air, and never itfelf becomes oxydated. 



is previoufly filled with alternate ftrata of coke, iron-ftone, 2d. The degree of carbonation paftt.d upon the, metal at tiw 



moment 



