NOTES ON COAL AND FUEL. 161 
such cocoons, which I add to .the other skein of raw, which you will 
perceive has become nearly white, and will take any colour. I therefore 
am persuaded now, after seeing this silk boiled off, that it really is silk, 
and might well be used for tram in European manufacture, if reeled 
equal to this sample. 
NOTES ON COAL AND FUEL 
We think that a few general observations upon the descriptions of 
coal most commonly used in England for domestic and steam purposes, 
may not be out of place. For domestic use, those coals that do not 
corrode the fire-bars, and leave the least quantity of ashes, are the best ; 
but for steam purposes the value of them depends upon their power of 
converting water quickly into steam — that is, if a given weight of coal in 
a certain time converts a larger proportion of water into steam than the 
same weight of another kind of coal in the same time, the evaporative 
power of the one would be greater than that of the other. Coal for 
steam purposes should burn rapidly, should not be too bituminous and 
produce much smoke, should have a cohesive power so as not to fall to 
pieces by the rolling and pitching motions of a ship, and should have 
such a density and structure as to bear stowage in as small a space as 
possible ; it ought not to contain a large proportion of sulphur, nor be 
subject to quick decay, for in either case it might lead to spontaneous 
combustion. No coals as yet discovered, unite all these conditions. 
Several attempts have been made to convert anthracite coal into a 
patent fuel, but at present the tar used to cement the coal burnt so much 
more rapidly in the furnaces than the anthracite itself, that the latter ac- 
cumulated on the bars and obstructed the draught, or escaped through 
the grates unconsumed. 
The component parts of coal are principally carbon or charcoal, and 
bitumen. Some kinds of coal are laminar, others compact. They 
in general burn freely, with a bituminous odour, and leave a considera- 
able residuum. 
Thi3 invaluable mineral is found in beds, ot strata, frequently be- 
twixt clay, slate, and sandstone, and seldom betwixt those of limestone. 
It occurs in great quantities in Great Britain, Siberia, Germany, Sweden, 
France, North America, the northern portion of China ; and, in -smaller 
quantities, in Australia. Dr. F. Kraus3, of Stuttgard, met with it in great 
abundance during his travels in South Africa, specimens of which were 
exhibited on his return to England. No fewer than seventy different 
kinds of coal are now brought to the London market, the value and 
prices of which greatly differ. Of these the coals called Wallsend, from 
