COMBUSTION. 61 



ciimstances, it shows very little tendency to combine 

 with them; under particular circumstances, however, 

 it does form compounds (147, 163), and some of them 

 are very curious and important. Common air consists 

 of one part of oxygen to four parts of nitrogen : it is 

 a mixture, not a compound. 



30. When a candle burns, it gradually disappears ; 

 it grows shorter and shorter, and at last, when all 

 the tallow is burnt, the candle goes out ; but we must 

 not therefore suppose that it is utterly destroyed. A 

 change has taken place ; the tallow, or rather its ele- 

 ments, have combined with the oxygen of a portion 

 of air, and two new compounds, one of which is a gas 

 or kind of air, are produced. If we put a piece of 

 salt into water, it will get less and less, and at last 

 will disappear altogether, having wholly dissolved ; but 

 the salt is not destroyed, it is only dissolved in the 

 water. 



31. Now we may compare the burning of a candle 

 to dissolving a piece of salt ; for all the solid matter 

 of the candle remains diffused throughout the air, 

 after it is burnt, just as the salt remains dissolved in 

 the water; but with this difference, the salt is dissolved 

 in the water, but not combined with it. The elements 

 of the tallow are dissolved in the air, but they have 

 combined with a quantity of oxygen, because they have 

 a strong affinity or attraction for it. If the solution 

 of salt is left for some time in a warm place, the 

 water evaporates, and we get the salt again un- 

 changed ; but in the case af the candle its elements 



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