PRINCIPLES OF COMBUSTION. 129 



And here we are called on to inquire, wiih reference to the 

 gases under consideration, whether there are any peculiar 

 conditions which can influence the amount of heat to be ob- 

 tained from them ? and, if so, what they are ? This, again, 

 involves other questions in reference to air, and the part which 

 it has to act in the process ; and thus we find ourselves intro- 

 duced into the chemistry of combustion. 



One advantage of receiving the subject in this light, is, that 

 we shall see how idle would be any calculations or arrangements 

 as to the dimensions or details of a furnace, before we had well 

 examined and understood the rationale of that process on which 

 these details must necessarily be contingent. For what chemist 

 would begin by deciding on the dimensions of his retort, or other 

 apparatus, before he had considered the particular purposes to 

 which they were to be applied ? Yet such is the every-day 

 practice of those who profess to instruct us in these matters. 

 The absurdity of this practice, and the dangers into which it 

 leads practical men, will be more apparent when we come to 

 consider the nature of heating apparatuses, and the powers and 

 properties which belong to each. 



Combustibility, then, is not a quality of the combustible taken 

 by itself. It is merely a faculty which may be brought into 

 action through the instrumentality of a corresponding faculty in 

 some other body. It is, in the case now before us, the union of 

 the combustible with oxygen, and which, for this reason, is 

 called the supporter. Neither of which, however, when taken 

 alone, can be consumed. 



To effect combustion, then, we must have a combustible, and 

 a supporter of combustion. Strictly speaking, combustion 

 means union; but it means chemical union, one of the ac- 

 companying incidents of this union being the emission of heat 

 and light. What the nature of heat is, or how it is liberated 

 during chemical action, it is not our province to consider; 

 nor does it relate much to our present inquiry. Sufficient for our 

 present purpose, is the fact, that the chemical union of the com- 

 bustible, (the coal,) and the supporter of combustion, (the oxygen 

 of the air,) is the cause of heat being given off; and, further, 



