EUCALYPTUS TREES. 51 
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 
quarter 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 volumen 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 
*For extensive details consylt yon Berg’s Anleitung zum Pertcohten : also, 
Muspratt’s Chemistry, 
