COMBUSTION. 201 



which goes up in vapour with the smoke ; and 

 various other substances which form solid pro- 

 ducts 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 oxy- 

 gen, 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 temperature 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 the sun so expand the air that the 

 current to the fire is greatly diminished, and stops 

 altogether. We hence see how very unskilfully 

 many persons blow the fire with bellows. They 

 put the nose of the bellows close to the fire, and 

 thus drive the expanded air upon all parts of the 

 fire, except the little space on which the blast 

 acts ; and the consequence is, that that little por- 

 tion is very rapidly and unprofitably consumed, 

 and the rest of the fire is not at all improved ; 

 whereas, if the bellows were kept further off, they 

 would blow a much more effective current of air 

 against the whole fire. The position of the bel- 

 lows should be sloping upward to the fire, because 



