SS RESULTS OF COMBUSTION. 



have combined with oxygen, and they cannot be sep- 

 arated again from it except by the action of something 

 which, having a more powerful attraction for the oxy- 

 gen than it has for the elements of the candle, causes 

 it to relinquish them. 



32. There are substances which have sufl5cient 

 attraction for the oxygen to effect this : we cannot 

 get back the tallow, it is true, but we may obtain its 

 elements, or the simple substances of which it was 

 composed. What has been said with regard to the 

 burning of a candle is equally applicable to the burn- 

 ing of wood, coal, or in fact any combustible matter. 

 In all ordinary cases they burn in consequence of their 

 affinity for the oxygen of the air, and they are never 

 destroyed when burnt, for their elements may always 

 afterwards be found combined with oxygen, in the air 

 in which they have been burnt. 



33. It must also be remembered that when the 

 candle goes out for want of air, it does not do so 

 because all the oxygen is burnt, but because the 

 elements of the candle having combined with all the 

 oxygen of the air, or, as it were, saturated it, there 

 is no more free oxygen left to keep up the combustion 

 of the candle. 



34. The changes occasioned by chemical action fre- 

 quently proceed slowly and quietly, but in many cases, 

 and especially when substances combine together 

 which have a strong affinity for each other, a great 

 deal of heat is given out. Sometimes, as soon as two 

 substances are brought together, they combine di- 



