128 HEATING. 



is its absorption by the coal, and the consequent disengagement 

 of gas, from which all that subsequently bears the character of 

 flame is exclusively derivable. This gas, whether it be in a 

 close retort, or in a furnace, is associated with several other 

 substances, more or less tending to deteriorate its inflammable 

 properties and powers of giving out heat and light. In the 

 preparation of gas, or smoke, for illuminating purposes, these 

 impurities are separated, and the pure gas alone is used. As, 

 however, this separation cannot be effected in a common furnace, 

 arid, as the entire gaseous products of the coal, good and 

 bad, are indiscriminately consumed together as they are gener- 

 ated, it is the more incumbent on us to be cautious, lest, by any 

 injudicious arrangement, we force these impurities into more 

 active energy, and thus increase their deleterious power. 



We will not stop here to consider the nature of those impuri- 

 ties arising out of the unions of sulphur, and the other injurious 

 constituents of coal, although they exercise a mischievous in- 

 fluence on the calorific effect of the gas burning in the furnace, 

 but will consider those constituents alone, which unite in form- 

 ing the useful gases, and from which we are to derive heat. 



These constituents are the hydrogen and the carbon. And 

 the unions which alone concern us here, are, first, carburetted 

 hydrogen; and, second, bi-carburetted hydrogen, commonly 

 called olefiant gas. These two, and their unions with the air, 

 in the process of combustion, we will shortly examine. 



Gases, as well as other bodies, endowed with the power of 

 giving out heat and light, have been called combustible. This 

 term has been a source of much error in practice, from a mis- 

 conception of its meaning, under the received impression that 

 combustibles possess, in some undefined manner, and within 

 themselves, the faculty of burning. And, though every person 

 knows that they will not burn without air, still the part which 

 air acts in the process is but little inquired into. It is but lately 

 that the nature of this union of the gas with the air has come to 

 be fully understood ; and, although the abstract question as re- 

 gards the immediate cause of that chemical action, which we 

 ,?all combustion, may continue to be disputed, and new theories 

 continue to be broached, still, for all practical purposes, it is 

 sufficiently defined and understood. 



