COMBUSTION OF FUEL. 



attention, to comprehend clearly the origin of the power of steam, 

 and the physical conditions which determine its maintenance and 

 its limits. 



6. The general principles upon which heat is developed in the 

 combustion of fuel have been already explained in our Tract on 

 FIRE. It appears from what is there stated, that the varieties of 

 coal are chiefly combinations of carbon and hydrogenous gases ? 

 the proportion varying in different sorts, but the carbon entering 

 into its composition in very large proportions in all cases. In 

 different sorts of mineral combustibles, the proportion of carbon 

 varies from 75 to 90 per cent. 



When carbon is heated to a temperature of about 700 in an 

 atmosphere of pure oxygen, it will combine chemically with that 

 gas, and the product will be the gas called carbonic acid. In this 

 combination heat is evolved in very large quantities. This effect 

 arises from the heat previously latent in the carbon and oxygen 

 being rendered sensible in the process of combustion. The carbonic 

 acid proceeding from the combustion is by such means raised 

 to a very high temperature, and the 'carbon during the process 

 acquires a heat so intense as to become luminous ; no flame, 

 however, is produced. 



Hydrogen, heated to a temperature of about 1000, in contact 

 with oxygen, will combine with the latter, and a great evolution 

 of heat will attend the process ; the gases will be rendered 

 luminous, and flame will be produced.* 



If coals, therefore, or other fuel exposed to atmospheric air be 

 raised to a sufficiently high temperature, their combustible con- 

 stituents will combine 'with the oxygen of the atmospheric air, 

 and all the phenomena of combustion will ensue. In order, how- 

 ever, that the combustion should be continued, and should be 

 carried on with quickness and activity, it is necessary that the 

 carbonic acid and other products should be removed from the 

 combustible as they are produced, and fresh portions of atmospheric 

 air brought into contact with it; otherwise the combustible would 

 soon be surrounded by an atmosphere composed chiefly of carbonic 

 acid to the exclusion of atmospheric air, and therefore of uncom- 

 bined oxygen, and consequently the combustion would cease, and 

 the fuel be extinguished. To maintain the combustion, there- 

 fore, a current of atmospheric air must be constantly carried 

 through the fuel : the quantity and force of this current must 

 depend on the quantity and quality of the fuel to be consumed. 

 It must be such that it shall supply sufficient oxygen to the fuel 

 to maintain the combustion, and not more than sufficient, since 



* For the full explanation of this process, see Tract on Fire. 



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