CHARCOAL-MAKING. 549 



are sifted from the refuse dust and sold separately, while 

 pieces that are not sufficiently charred are set aside for a fresh 

 kiln, or are charred again at once in small kilns. 



(). Properties of Charcoal 



Good charcoal has a heating power of 7,000 to 8,000 calories, 

 approaching that of pure carbon. Its composition is as 

 follows : 



Carbon .... 7580% 



- Hydrogen . . . T5 2'5 ,, 



Oxygen . 812 



Hydroscopic water . . 6 12 ,, 



Ashes .... 12-5 ,, 



Violette has shown that the quantity of carbon in charcoal 

 is lower, the greater the heat of combustion. Charcoal has a 

 higher heating-power, the heavier it is and the heavier the 

 wood from which it is made ; while the higher the temperature 

 at which it is burned the lower is its sp. weight. Violette 

 therefore assigned to charcoal of elder-wood : 



At 350 a sp. weight of 150 

 ,, 1,025 184 



,, 1,500 187 



On the average the sp. weight of charcoal varies from 140 

 to 200, that of water being 100. 



The sp. weight of charcoal in pieces, with numerous air 

 interstices, according to Hassenfratz is : 



Birch 20'3 



Ash 20-0 



Beech 18'7 



Hornbeam 18'3 



Elm 18-0 



Spruce 17'6 



Oak 15-5 



According to Mayr's researches, the sp. weight of the best 



