494 



THE STEAM-ENGINE. 



pure oxygen, it will combine chemically with that gas, and the product will 

 be the gas called carbonic acid. The volume of carbonic acid produced by 

 this combination, will be exactly equal to that of the oxygen combined with 

 the carbon, and therefore the weight of a given volume of the gas will be in- 

 creased by the weight of carbon which enters the combination. It is found 

 that two parts by weight of oxygen combined with three of carbon, form car- 

 bonic acid. The weight of the carbonic acid, therefore, produced in the com- 

 bustion, will be greater than the weight of the oxygen, bulk for bulk, in the 

 proportion of five to two, the volume being the same and the gases being com- 

 pared at the same temperatures and under equal pressures. In this combina- 

 tion 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 1,000°, in contact with oxygen 

 will combine with the latter, and a great evolution of heat will attend the pro- 

 cess ; the gases will be rendered luminous, and flame will be produced. The 

 product of this process will be water, which being exposed to the intense heat 

 of combustion, will be immediately converted into steam. Hydrogen combines 

 with eight times its own weight of oxygen, producing nine times its own 

 weight of water. 



Hydrogen gas is, however, not usually disengaged from coal in a simple 

 form, but combined chemically with a certain portion of carbon, the combina- 

 tion being called carburetted hydrogen. Pure hydrogen burns with a very 

 faintly luminous blue flame, but carburetted hydrogen gives that bright flame 

 occasionally having an orange or reddish tinge, which is seen to issue from 

 burning coals : this is the gas used for illumination, being expelled from the 

 coal by the process of coking, and conducted to the various burners through 

 proper pipes. 



The sulphur, which in a very small proportion is contained in coals, is also 

 combustible, and combines in the process of combustion with oxygen, forming 

 sulphurous acid : it is also sometimes evolved in combination with hydrogen, 

 forming sulphuretted hydrogen. 



Atmospheric air consists of two gases, azote and oxygen, mixed together in 

 the proportion of four to one ; five cubic feet of atmospheric air consisting of 

 four cubic feet of azote and one of oxygen. Any combustible will combine 

 with the oxygen contained in atmospheric air, if raised to a temperature some- 

 what higher than that which is necessary to cause its combustion in an at- 

 mosphere of pure oxygen. 



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

 sufficiently high temperature, their combustible constituents will combine with 

 the oxygen of the atmospheric air, and all the phenomena of combustion will 

 ensue. In order, however, 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 uncombined oxygen, and consequently the combustion would cease, 

 and the fuel be extinguished. To maintain the combustion, therefore, a cur- 

 rent of atmospheric air must be constantly carried through the fuel : the quanti- 

 ty and force of this current must depend on the quantity and quality of the fuel 



