PRINCIPLES OF COMBUSTION. 147 



Let us now inquire how far the ordinary mode of constructing 

 and managing our furnaces enables us to satisfy this condition. 



In ordinary furnaces, the supply of air is obtained by means 

 of the ash-pit ; and the larger the ash-pit, the greater the quan- 

 tity of air admitted. The ash-pit is made larger, under the 

 mistaken notion that the more air we give, the better will be the 

 draught, the more complete the combustion, and the greater the 

 quantity of heat produced. 



There can scarcely be a more absurd practice than is involved 

 in this one-sided view of the principles of combustion, even sup- 

 posing that the introduction of air is tantamount to the introduc- 

 tion of oxygen. It is manifest, however, that there are two 

 different processes going on in the furnace, and two different 

 combustibles, requiring their respective volumes of oxygen to 

 consume them, namely, the gas or smoke generated in the body 

 or cavity of the furnace, and passing off by the flues, and also, 

 the solid carbon resting on the bars, both of which require sepa- 

 rate volumes of oxygen to effect their combustion. 



All that seems to be concluded in practice is, that air is 

 essential to combustion ; and that if air be admitted to the fuel, 

 through between the bars, it will work out the process of com- 

 bustion satisfactorily in its own way. And hence the many 

 errors and absurdities of the present system of practice. 



There can be no greater mistake than letting a large quantity 

 of air act directly on the burning fuel, which acts like a blast 

 upon the red-hot mass, driving off the gases more rapidly, but 

 also driving off the contained heat, and consuming the fuel with 

 unnecessary rapidity. 



It seems to be taken for granted, that if air, by any means, be 

 introduced to the fuel in the furnace, it will, as a matter of 

 course, mix with the gas, or other combustible, in a proper man- 

 ner, and assume the state suitable for combustion, whatever be 

 the nature or state of such fuel, and without regard to time or 

 other circumstances. Now, it might as well be supposed, that 

 by bringing large masses of nitre, sulphur, and charcoal to- 

 gether, we could form gunpowder. We know that it is by the 

 proper mixture and incorporation of the different elementary 

 atoms that simultaneous action is imparted to the whole ; and 

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