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a composition of ^2 4 ^*2 6 The rapid evolution of car- 
buretted hydrogen in situations where its escape was pre- 
viously impossible, has frequently caused the fearful explosions 
so often following the opening by the miners of cavities in 
the coal strata filled with carburetted hydrogen, liquified by 
pressure. 
The older the formation of coal, the more is its composi- 
tion analogous to pure carbon, till we arrive at last at the 
anthracitic beds, which, as I have before noticed, contain 
extremely little hydrogen or oxygen. 
The coal formations may, therefore, be divided into three 
classes : — 
1st. The coal formation, subdivided into the older (an- 
thracite), and the younger coal formation (coal). 
2nd. The secondary formations found in the neighbour- 
hood of the chalk. 
3rd. The tertiary formations, consisting of alternations 
of brown coal with freshwater limestone. 
In the latter or tertiary formations the coal still retains 
all the forms of its vegetable origin, consisting generally 
of the flattened stems and leaves of ancient plants. It 
possesses no peculiar excellences as a fuel, yielding on an 
average from 3 to 7 per cent, of ashes, 60 to 70 per cent, 
of carbon, 4 to 6 of hydrogen, and 18 to 36 of oxygen. 
Its combustion is not attended with a good flame, and 
though much valued in Germany, where fuel is compara- 
tively rare, would be considered as of very inferior quality 
in England. 
Mineral coal, which we have next to consider, is so well 
known that I shall only remark, that it may be divided 
into fibre coal, appearing in thin layers, parallel with the 
plane of stratification, richer in carbon than the rest of 
the mass ; pitch coal, associated with bituminous hydro-car- 
bons ; and slate coal, so named from its fissile nature. 
