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basis of explosive mixture, is the carburetted hydrogen so 
generally given out by coal, and the strata with which it is 
in immediate contact. This gas has the remarkable property 
of being one of the lightest substances in nature, and 
possesses a specific gravity very little more than one-half 
that of atmospheric air ; and hence (to use again the words 
of Mr. Thorp) its " constant tendency (particularly if the 
coal bed rises to an acute angle) to creep along the roof to 
the upper and higher portions of the excavated mine, and 
thereby accumulating to form a most dangerous magazine." 
This constant and ever active tendency to creep along, and 
insinuate itself into every elevated cavity or space, is a fact 
far more familiar to those accustomed to mining operations, 
than are the expedients by which its dangerous presence 
may be counteracted or removed. Moreover, the activity 
with which it possesses itself of such elevations, pointing, as 
it does, in so conspicuous a manner, to the provision which 
should be made for its natural escape, is not unfrequently 
taken advantage of, and the gas is permitted to accumulate 
in such situations, from its inherent hostility to the downward 
course by which the ordinary ventilation is forced through 
the mine. Surely it is requisite to show that every other 
principle has been exhausted, before a system is propagated, 
which so essentially opposes itself to the natural habit of the 
agent under consideration. It is not intended to overlook 
the affinity which the gas possesses for the atmospheric air, 
and the fact that, in union, the two may be passed off 
together along the steepest descents, by the ordinary expe- 
dients of ventilation now in use. But there is nothing in 
this affinity which at all compensates for the separate 
attribute of gravity, imparting so active a tendency to the 
hydrogen gas to isolate itself from all such admixture, and 
so to elude the chances which otherwise might lead to its 
expulsion. 
