One of the chief dangers to which workmen in the mines are subject arises from the gases given off by minerals and metals. Though these deleterious gases are commonly found in more or less abundance in the coal mines, and are usually considered in connection with such mines, they are, nevertheless, not confined to the coal measures. They have been noticed also in mines of lead, sulphur, salt, and other substances. It is said that anthracite contains a much larger proportion of these gases than do bituminous or other coals, but that being hard it holds them more tenaciously, and is therefore worked with less risk. The soft coals, on the contrary, being porous as well as soft, allow the gases to escape from them much more readily, and so increase the danger at the working faces of the mines. The gas given out most abundantly by the coal is light carbureted hydrogen, known as marsh gas, from the fact that being a product of vegetable decomposition under water, bubbles of it rise to the surface on stirring the waters of a marsh. This is the gas that is known to miners as fire damp. The French call it grisou. Marsh gas, in its simple form, consists of four parts of hydrogen to one of carbon. It is about one half the weight of air, and therefore rises and gathers at the roof of a mine chamber, extending downward as it accumulates. When it is mixed with from four to twelve times its volume of atmospheric air it becomes violently explosive. If the mixture is above or below this proportion it is simply inflammable, burning without explosive force, with a pale blue flame. The value of a perfect ventilating current across the faces of chambers which are making gas rapidly can now be appreciated. It is not only necessary that the supply of air should be sufficient to make the gas non-explosive, but that it should be sufficient to dilute it beyond even the point of inflammability. For to its inflammable more than to its explosive quality is due most of the disasters with which it is accredited. A peculiar and dangerous feature of this gas is that it does not always escape from the coal at a uniform rate, but often comes out suddenly in large compact bodies. These are called “blowers.” They are found most commonly in faults, in cracks in the coal seams, or in open spots in the body of coal, where they have opportunity to accumulate. They contain, besides marsh gas, less than one per cent. of carbonic acid, and from one to four per cent. of nitrogen. It is impossible to anticipate their coming; the miner’s drill may strike into one and free it at any time without a moment’s warning. It may even burst through the face by its own power. In such cases danger is imminent, disaster is most common.