196 COMBUSTION. 



of the current, except the artificial covering that 

 may be on them ; but there is self-protection in the 

 face. The breath which is expired is heated in the 

 lungs, and also charged with moisture. As in the 

 operation of breathing, carbon or charcoal, which 

 previously existed in probably a solid state, unites 

 with the oxygen of the inspired air, and with it forms 

 carbonic acid gas, it might be supposed that cold 

 would be the result, as is the case when most sub- 

 stances pass from any more condensed state into 

 the state of gas. Such, however, is not the fact ; 

 otherwise a common fire would cool the room in- 

 stead of heating it, and furnaces would harden metals 

 instead of melting them ; for the chief process Which 

 goes forward in the burning of fuel is the conversion 

 of the oxygen of the air and the carbon of the fuel 

 into carbonic acid gas. There are, indeed, generally 

 other matters in the fuel such as hydrogen, which 

 passes off, mixing with oxygen, in flame, and the 

 result is water, which goes up in vapour with the 

 smoke; and various other substances which form 

 solid products with oxygen. In these the whole of 

 the heat which held the oxygen of the air in a state 

 of gas becomes free and apparent to the senses ; and 

 as the carbon which combines with the oxygen does 

 not increase the volume, while it very much increases 

 the density, the oxygen which forms the carbonic 

 acid gas gives out a great quantity of its heat ; and 

 yet the gas which is formed may have a higher tem- 

 perature than the oxygen. Here we may see, by-the- 

 way, why fires burn brightest in cold weather ; and 

 why the sunbeams, or any other light, put out the fire, 

 or make it burn feebly. The colder the air is, it is 

 the more condensed, and of course has the more 

 oxygen in an equal bulk. Thus it moves faster to 

 the fire, and carries more of the element that feeds 

 the fire. The light expands the air, and that causes 

 it to come more slowly, and also to have less supply 

 of oxygen in the same bulk ; and the direct rays of 



