But I knew the plan, and by its means I discovered the presence of firedamp and consequently that of a new seam of coal in the Dochart pit." All that the old overman had related of the so-called "monk" or "fireman" was perfectly true.

The air in the galleries of mines was formerly always purified in the way described.
Fire-damp, marsh-gas, or carburetted hydrogen, is colorless, almost scentless; it burns with a blue flame, and makes respiration impossible.
The miner could not live in a place filled with this injurious gas, any more than one could live in a gasometer full of common gas.

Moreover, fire-damp, as well as the latter, a mixture of inflammable gases, forms a detonating mixture as soon as the air unites with it in a proportion of eight, and perhaps even five to the hundred.

When this mixture is lighted by any cause, there is an explosion, almost always followed by a frightful catastrophe.
As they walked on, Simon Ford told the engineer all that he had done to attain his object; how he was sure that the escape of fire-damp took place at the very end of the farthest gallery in its western part, because he had provoked small and partial explosions, or rather little flames, enough to show the nature of the gas, which escaped in a small jet, but with a continuous flow.
An hour after leaving the cottage, James Starr and his two companions had gone a distance of four miles.

The engineer, urged by anxiety and hope, walked on without noticing the length of the way.