332 SECTION MECHANICS. 



smithy fire, a cupola, or an ordinary coke furnace for melting- steel 

 or brass in crucibles, an intense local heat is required in the mass of 

 the fuel itself, and any heat developed above its surface is useless. 

 In flame furnaces, on the other hand, such as those for glass melting, 

 or for puddling or heating iron, in which the materials to be heated 

 are not embedded in the fuel, but placed in a chamber above or at 

 the side of it, the heat made use of is that of the flame ; the heat 

 that is carried into the working chamber by the current of gases 

 rising from the fire, together with that due to the further combustion 

 of these gases, on admixture with an additional amount of air. 



Thus, for furnaces of the first class, the most suitable fuel is one, 

 such as charcoal, coke, or anthracite, consisting of nearly pure carbon, 

 free from volatile matter, as this is useless in them as a source of 

 heat, and the driving of it off renders latent a certain amount of that 

 generated by the combustion of the carbon, and so lowers the tem- 

 perature >f the fire. 



In flame furnaces, a lowering of the temperature at the fire- 

 grate, where the air and the solid fuel meet, is immaterial, or may 

 be even advantageous, as tending to diminish loss by radiation and to 

 preserve the furnace from injury by excessive heat. The only use of 

 the heat at the grate is to generate a full supply of combustible or 

 partly burned gases, at a high temperature, which in completing their 

 combustion, as they pass over the working bed, shall heat as strongly 

 as possible the matters placed there. The fuel preferred for use in such 

 furnaces, is thus either a combustible gas, or a solid fuel containing 

 hydrogen as well as carbon, such as coal or dried wood, that will pro- 

 duce on burning a long and powerful flame. A flame, it is true, may 

 also be obtained from fuels that contain little else than carbon and 

 mineral matter, by burning them in a thick bed, so that the greater 

 part or nearly the whole of the CO 2 formed in the first instance by 

 the combustion of the carbon, is transformed into CO as it passes up 

 through the mass ; and by introducing with the air as large a pro- 

 portion of steam as can be used without lowering too much the tem- 

 perature of the fire. The steam is decomposed by the hot carbon, 

 producing, according to the temperature and thickness of the fire, a 

 mixture of either H and CO 2 or H and CO. The gases thus gene- 



