But the problem of the mining engineer is not so much to rid cavities of gas as to prevent its accumulation. In modern mining, with proper ventilation and drainage, there is comparatively little danger of extensive accumulation of this gas. [Illustration: A FLINT-AND-STEEL OUTFIT, AND A MINER'S STEEL MILL. The upper picture shows a flint-and-steel outfit, the implements for lighting a fire before the days of matches. The lower picture shows a miner's steel mill, which was used for giving light in mines before the day of the safety-lamp. It consists of a steel disk which is rotated rapidly against a piece of flint, producing a stream of sparks. It was thought that such sparks would not ignite fire-damp--a belief which is now known to be erroneous.] The danger from this choke damp, therefore, is one that concerns the individual workman rather than large bodies of men or the structure of the mine itself. With fire damp, however, the case is different, as an explosion of this gas may destroy the mine itself and all the workmen in it. It is, therefore, the most dreaded factor in mining, and is the one to which more attention has been directed than to almost any other problem. This fire damp is a mixture of carbonic oxide and marsh gas which, being lighter than air, tends to rise to the upper part of the mines. For this reason explosions are more likely to occur near the openings of the mine, frequently entombing the workmen in a remote part of the mine even when not actually killing them by the explosion. As this gas is poisonous as well as explosive the miners who survive the explosion may succumb eventually to suffocation. Previous to the year 1816 no means had been devised for averting the explosions of fire damp except the uncertain one of watching the flame of the candle with which the miner was working. On coming in contact with air mildly contaminated with fire damp the candle flame takes on a blue tint and assumes a peculiarly elongated shape which may be instantly detected by a watchful workman. But miners were, and still are, a proverbially careless class of men even where a matter of life and death is concerned, and too frequently gave no heed to the warning flame. But in 1816 Sir Humphrey Davy invented his safety lamp, a device that has been the means of saving thousands of lives, and which has not as yet been entirely supplanted by any modern invention.