COLLIERY EXPLOSIONS. 
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to definite laws, that the various gases,* solids, and liquids are 
built up. In the case of fire-damp, a molecule of the gas con- 
sists of one atom of carbon combined with four of hydrogen. 
It is colourless, lighter than air, combustible, and becomes highly 
explosive when mixed with air in certain proportions. Sir 
Humphrey Davy found that when mixed with 3.5 volumes of air 
fire-damp does not explode but burns, quietly in contact with the 
atmosphere; when mixed with 6.5 volumes it explodes slightly, 
but it becomes highly explosive with from 8 to 9 volumes of air. 
Mr. J. W. Thomas, F.C.S., in his book on “ Coal Mine Gases 
and Ventilation ” (page 170) finds, that the force of explosion is 
greatest when the gas is mixed with air in the proportion for 
complete combustion, that is, with 9.6 parts of air to 1 of gas. 
Mr. W. Gallaway, from observations made in No. 3 Pit, 
Llwynypid Colliery, found at a temperature of 57° F., humidity 
79, bar. 30.15 in., that a mixture composed of 1 of fire-damp to 14 
of air was slightly explosive, and that in the proportion of 1 of 
fire-damp to 15 of air it was inflammable. 
The direct cause of an explosion is the disintegration, so to 
speak, of the molecules of the gas, owing to the chemical affinities 
of its carbon and hydrogen atoms for oxygen at a certain 
temperature. The tendency of the carbon atom is to combine 
with two atoms of oxygen and form carbonic acid gas, more 
generally termed “ black-damp ” or “ after-damp.” The hydrogen 
atoms also combine with oxygen, producing water. If, however, 
there . is not sufficient oxygen to satisfy the full combining 
affinities of the carbon atom, a gas is formed having the com- 
position of one atom of carbon combined with one atom of 
oxygen ; this is known as carbonic oxide gas. 
The ‘‘ after-damp ” is a very different gas to that of the 
explosive “ fire-damp.” Instead of being lighter than air, it is 
heavier. Its effect . on animal and human life is to suffocate, 
and upon flame tO; extinguish ; hence it is that the after-damp is 
