Chalybeate springs, some of which deposit large quantities of yellow ochre, are common in every part of the Coal-field; and a water which flowed through the wooden pipes at Walker colliery, used to let fall a copious precipitate of gypsum. The substance formed during the twelve working hours of the mine was black, but at other times was as white, and had the same degree of hardness as chalk. A layer formed in twelve hours was about of an inch in, thickness. Specimens of this sediment are to be found in many cabinets, but are now no longer to be procured, the high main coal being there exhausted, and the colliery laid in.
The choak damp, the fire damp, and the after damp or stythe, are the miner's terms for the gases with which the coal mines are affected; and of these the second both from its immediate violence and as occasioning the other kinds of damps is the most to be dreaded. The accidents arising from it have become more common of late years, but it should not for a moment be supposed that they arise from any want of skill or attention in the professional surveyors of the mines. The following seem to be the causes in which the gas originates.
1st. The coal appears to part with a portion of carburetted hydrogene, when newly exposed to the atmosphere; a fact rendered probable by the well known circumstance of the coal being more inflammable when fresh from the pit than after long exposure to the air. 2d. The pyritous shales that form the floors of the coal-seams decompose the water that lodges in them, and this process is constantly operating on a great scale in the extensive wastes of old mines. In whatever mode we suppose the gas to be generated, it is disengaged abundantly from the High Main, but more particularly from the Low Main coal-seam, and that in a quantity and with a rapidity that are surprising. It is well known that the gas frequently
