FUEL. 



117 



or fossil coals, in their crude state, which 

 it is proper to distinguish from charcoals 

 of the same substances. The difference 

 consists in their giving a copious and 

 bright flame, when plenty of air is admit- 

 ted to them, in consequence of which 

 they must be considered as fuels very 

 tlifferent from charcoal, and adapted to 

 different purposes. Flaming fuel cannot 

 be managed like the charcoal. If little 

 air be admitted it gives no flame, but 

 sooty vapor, and a diminution of heat. 

 And if much air be admitted to make 

 those vapors break out into flame, the 

 heat is too violent. 



These flaming fuels, however, have 

 their particular uses, for which the others 

 are far less proper. For flame, when 

 produced in great quantity, and made to 

 burn violently, by mixing it with a proper 

 quantity of fresh air, by driving it on the 

 subject, and throwing it into whirls and 

 eddies, which mix the air with every 

 part of the hot vapor, gives a most intense 

 heat. This proceeds from the vaporous 

 nature of flame, and the perfect miscibi- 

 lity of it with the air. As the immediate 

 contact and action of the air are necessary 

 to the buring of every combustible body, 

 so the air, when properly applied, acts 

 with far greater advantage on flame than 

 on the solid and fixed inflammable bodies; 

 for when air is applied to these last, it 

 can only act on their surface, or the 

 particles of them that are outermost; 

 whereas, flame being a vapor or elastic 

 fluid, the air by proper contrivances, can 

 be intimately mixed with it, and made to 

 act on every part of it, external and in- 

 ternal, at the same time. The great 

 power of flame, which is the consequence 

 of this, does not appear when we try 

 small quantities of it, and allow it to burn 

 quietly, because the air is not intimately 

 mixed with it, but acts only on the out- 

 side, and the quantity of burning matter 

 in the surface of a small flame is too small 

 to produce much effect. But when flame 

 is produced in large quantity, and is pro- 

 perly mixed and agitated with air, its pow- 

 er to heat bodies is immensely increased. 

 It is therefore peculiarly proper for heat- 

 ing large quantities of matter to a violent 

 degree, especially if the contact of solid 

 fuel with such matter is inconvenient. 



Flaming fuel is used, for this reason, in 

 many operations performed on large 

 quantities of metal, or metallic minerals, 

 in the making of glass, and in the baking 

 or burning of all kinds of earthen ware. 



The potter's kiln is a cylindrical cavity 

 filled from the bottom to the top with 

 columns of ware: the only interstices are 

 those that are left between the columns; 

 and the flame, when produced in sufficient 

 quantity, is a torrent of liquid fire, con- 

 stantly flowing up through the whole of 

 the interstices, which heats the whole pile 

 in an equal manner. Flaming fuel is also 

 proper in many works or manufactories, 

 in which much fuel is consumed, as in 

 breweries, distilleries, and the like. In 

 such works, it is evidently worth while 

 contrive the furnaces, so that heat may be 

 obtained from the volatile parts of the 

 fuel, as well as from the fixed; for when 

 this is done, less fuel serves the purpose 

 than would otherwise be necessary. 



But this is little attended to, or not un- 

 derstood, in many of those manufactories. 

 It is not uncommon to see vast clouds of 

 black smoke and vapor coming out of 

 their vents. This happens in conse- 

 quence of their throwing too large a 

 quantity of crude fuel into the furnace at 

 once. The heat is not sufficient to in- 

 flame it quickly, and the consequence is a 

 great loss of heat. The quantity of 

 watery fluid contained in fuel greatly 

 affects the amount of heat it produces; 

 much more, indeed, than is commonly 

 admitted in practice. It is a well known 

 law in chemistry, that the evaporation of 

 liquids, or their conversion into steamj 

 consumes and renders latent a great 

 amount of caloric. When green wood, 

 or wet coals, are added to the fire, they 

 abstract from it, by degrees, a sufficient 

 part of its heat to convert their own sap 

 or moisture into steam, before they aie 

 capable of being burnt. And as long 

 as any considerable part of this fluid 

 remains unevaporated, the combustion 

 goes on slowly, the fire is dull, and 

 the heat feeble. Green wood commonl)'^ 

 contains a third, or more, of its weight 

 of watery fluid, the quantity varying 

 according to the greater or less porosity 

 of different trees. Nothing is further from 

 true economy than to burn green wood. 



