260 ECONOMIC GEOLOGY 
7000 to 16,000 B.T.U. Good coal is five times superior to 
wood when allowance is made for the difference in bulk. 
The large storage space required for wood renders it incon- 
venient in cities, while a steamship on a long voyage would 
require more firewood than it could carry. 
Dermvition or. Coar—Coal is not easily defined! The 
three meanings in Johnson's Dictionary (1755), * the common 
fossil fewel,” “the cinder of burnt wood, charcoal,” and 
“fire; anything inflamed or ignited,” illustrate the former 
wide meaning of the term. The coal of the Bible is charcoal, 
and though that term dates back to the fourteenth century, 
it was only restricted to carbonized wood in the seventeenth 
century. Mineral coal was called sea-coal, which as its use 
became general was abridged to coal. 
Coal is a mixed mineral of very complex constitution ; it 
is dark brown to black in colour; it consists of a mixture of 
carbon and hydrocarbons with earthy constituents or ash, 
of which the amount is not too high for use in fireplaces or 
furnaces; and it is insoluble in such solvents as turpentine, 
alcohol, chloroform, or benzine. Coal is usually defined as 
of vegetable origin, but the fuel value of some cannel coal is 
due to animal matter. Oil shales are an earthy variety of 
cannel coal, but are excluded from coal in ordinary usage 
just as sandstone containing coal fragments is classified as 
coaly sandstone. Cannel coal is different from other coals 
both in origin and use. It has been formed in lagoons and 
swamps by the accumulation of an organic mud which may 
be either animal or vegetable in origin. This mud has been 
called sapropel, and the cannel coals are conveniently separ- 
ated as the sapropelic coals. 
Humic CoaLs 
The humic coals are derived from plant tissues which 
consist of cellulose, C4H,,O;, with 50 per cent. of carbon. 
They form a series characterized by increase in the carbon 
percentage, the reduction of the oxygen and hydrogen—as 
shown in the following table, calculated free of ash and mois- 
! An excellent definition—* a solid fuel which occurs in seams, being 
the fossilized remains of organic matter” was given by F. D. Power. 
Coalfields of Australia, 1912, p. 402.