COAL AND COLLIERY EXPLOSIONS. 
357 
the trunks in both cases, leaves have fallen around the trunks, 
bog and morass have formed, cutting off all supply of oxygen 
from the roots, the gas which forms so essential a constituent of 
the sap, and without which the tree cannot live; the decomposi- 
tion going on, the fallen vegetable material must have robbed 
the soil of its oxygen until all the iron in it was reduced to 
protoxide, and an effectual barrier to the entrance of all fresh 
oxygen produced : no less than 4*73 per cent, of protoxide of iron 
being found in fireclays by Dr. Frankland. After the death of 
the trees, the trunks appear to have been snapped off by preva- 
lent winds along the line of junction of water and air, and both 
in the coal measures and in the mosses there is generally a 
determinate direction in the lie of the trunks. The growth 
of peaty material gradually covered up the trunks, and subsidence 
placing the surface beneath the water level, the whole became 
covered up with sand and mud, borne by currents which occa- 
sionally have completely cut out the coal seams, since filled in 
with sandy material forming the “ rock faults ” of the miner. 
Through subsequent pressure the coal has obtained an elabo- 
rate system of jointing, which maintains an average general 
direction over extensive districts, but differs in different coal- 
fields, doubtless running parallel to the main lines of move- 
ment which traverse the district, some of which have again 
brought the coal-field to the surface, or to comparatively short 
distances beneath the cover of underlying rocks. 
Bischoff, Graham, and Playfair have shown that the gases 
evolved by coal contain from 66 to 94 per cent, of light carbu- 
retted hydrogen, and that nitrogen is always present, sometimes 
amounting to from 14 to 21 per cent, of the entire volume, which 
can only be explained by “ supposing that air has permeated 
the fissures of the coal and, acting upon it, has been robbed of 
its oxygen, partly by union with hydrogen, partly with carbon.” 
The process of decomposition of the coal is still going on 
under conditions of considerable pressure and tolerably high 
temperature; the elimination of carburetted hydrogen, carbonic 
acid, and water, forming the much-dreaded “ fire damp ” of the 
collier, is the result. It will be remembered that the composi- 
tion of our atmosphere in a state of purity is, in 1,000 parts, 
788 nitrogen, 197 oxygen, 14 moisture, and 1 carbonic acid, 
combined mechanically, and that an adult vitiates a cubic foot 
of air a minute. Then the capacity of the air for holding 
moisture increases with its temperature, holding 2 grains per 
cubic foot at 30° F. and 7 grains at 70°. 
The ventilation of a colliery has not only to provide a fresh 
supply of air to replace that vitiated by the colliers, each of 
whom exhales 6*3 gallons of carbonic acid hourly, but has to be 
of sufficient strength to sweep away all inflammable gases issuing 
