146 HEATING. 



The presence of so large a quantity of nitrogen being the una* 

 voidable condition of obtaining the oxygen through the instru- 

 mentality of atmospheric air, 



It is to be observed that the process of combustion here 

 described is the most perfect that could be produced, either in a 

 furnace or lamp. Any deviation, therefore, by means of excess 

 or deficiency, or from any interruption or interference, such as 

 the interposition of another gas, must be more or less destructive 

 to the desired effect, viz., the generation of the greatest quantity 

 of available heat. 



5. When we speak of mixing a given quantity of oxygen 

 with a given volume of smoke, (or coal gas,) we do so because 

 we know that such quantity of the former is required to saturate 

 the latter, and by such saturation every atom of both gases 

 enters into union, without excess or deficiency of either, pro- 

 ducing entire and complete combustion. 



So, when we speak of mixing a given volume of atmospheric 

 air with a given volume of smoke, we do so for the same pur- 

 pose, knowing that the precise quantity of air will provide the 

 required quantity of oxygen. 



Thus, if we know that two cubic feet of oxygen are the exact 

 saturating equivalent, or combining volume, for effecting the en- 

 tire combustion of one cubic foot of coal gas, we know that ten 

 cubic feet of atmospheric air will effect the same purpose, 

 because ten cubic feet of air contain the required two cubic feet 

 of oxygen. 



We require ten cubic feet of air to supply two cubic feet of 

 oxygen, which, if the air be pure, effects the combustion of one 

 cubic foot of coal gas, emanating from coals in the process of 

 combustion in a furnace ; but if this quantity of air does not 

 contain this 20 per cent., or one-fifth, of oxygen, it is clear we 

 cannot obtain it. The air, in this case, may be said to be viti- 

 ated, or impure. It is therefore desirable that the air admitted 

 into a furnace should be direct from the atmosphere ; otherwise, 

 the oxygen contained may be deficient, although the volume 

 of air admitted be sufficiently large. 



