EUCALYPTUS TREES. 87 



wood a lighter coal. Slow combustion also renders 

 the coal heavier than a more rapid burning process, 

 because in the latter case more carbon is consumed 

 for various volatile products formed from the wood. 

 As a rule, the quantity of coal obtained is about a 

 qua[*pr of the weight of wood employed. Good coal 

 has a slight metallic lustre, is firm, not friable, caus- 

 ing a clear sound when thrown on the ground. It 

 must burn without flame and smoke. For trade pur- 

 poses coal must be kept dry, as its absorption of hu- 

 midity is considerable.* The heating power of coal 

 as compared to wood is ascertained to be as one hun- 

 dred to fifty-five or sixty. An equal ^olumen of wood 

 produces less heating effect than the same space of 

 coal. For technic operations the equable and more 

 lasting heat, and the great power of radiation, give 

 to charcoal its special value. Igniting wood for char- 

 coal in caverns is wasteful, through the great access 

 of air. 



By the method of carbonizing wood in ovens, tar 

 and other volatile products can be secured. The wood 

 chosen for coal intended for gunpowder is chiefly 

 that of Willows, Poplars, Alder, and Lime. It must 

 be healthy, and is preferred from young trees. Woods 

 which contain a good deal of hygroscopic salts — such 

 as that of Elms, Firs, Oaks — are not adapted for the 

 purpose. Extreme degrees of heat in producing coal 

 for gunpowder or blasting powder should be avoided, 

 otherwise the best wood will not serve the purpose, 

 because the powder would be less ready to ignite. 

 The yield of this coal is sixteen to seventeen one 



. ~-g~ ■ ' 



*For extensive details consult von Berg's Anleitung mm VerJcohlen ; also, 

 Muspratt's ChemUtry, 



