114 OF THE GENERATION AND [SECT. in. 



PROCESS OF COMBUSTION. 



192. The elementary bodies -require very different degrees of heat to cause 

 them to form new combinations. Sir H. Davy has rendered it probable that 

 charcoal and oxygen combine at about 700, when common air is not present ; and 

 hydrogen and oxygen at about 800. But when the oxygen is afforded by the 

 common air, about 800 for carbon, and 950 1 for hydrogen, seems to be nearer the 

 temperatures at which they inflame readily ; and when the fuel affords in- 

 combustible gases, the intensity will still require to be increased. Hence we need 

 not be surprised to find, in the common mode of applying heat, that except in as 

 far as it increases the draught through the fire, it is of little or no advantage for a 

 fuel to contain a large proportion of hydrogen. On the other hand, if the intensity 

 of the heat be too great, the earthy parts of the fuel combine with some portion of 

 the carbon and fuse, forming the glassy scoriae called clinkers, by which some 

 combustible matter is lost. We may expect this effect to take place in a con- 

 siderable degree whenever the heat approaches to about 1500, and therefore infer 

 that an average heat not exceeding about 1200 is the best for the production of 

 effect. 



The circumstances which must be attended to, that the fuel and its products 

 may remain in this temperature till they be consumed, are next to be considered. 



193. First. A quantity of air sufficient to supply the oxygen required for 

 combustion must have as free access as possible to all the parts of the burning 

 mass, and with as little exposure of the surface of the mass to the cooling effect of 

 other air as the draught of the chimney will allow. 



194. Secondly. The quantity or mass of fuel in combustion must be of such a 

 proportion to the quantity and temperature of the surface to which it communicates 

 heat, that it can only lose as much heat as it generates when it arrives at the best 

 temperature for combustion ; allowing for the cooling effect of the surface acted on 

 by the air required in the process. 



195. Thirdly. The flame and smoke must be kept in contact with the vessel 

 as long as it is capable of affording heat. 



196. Fourthly. The fluid to be evaporated should enter so as first to receive 

 the heat where the smoke last acts on the fluid, so that there may be the greatest 

 possible difference of temperature between the smoke and the fluid ; and, con- 

 sequently, that the fluid may deprive the smoke of heat, as it becomes gradually 

 heated to the temperature of the vapour before it arrives over the fire. 



1 Dr. Thomson says about 1000 from his own experiments. System of Chemistry, vol. i. p. 224. 



