But the safety lamp itself must be used with care and prudence, otherwise it may become no less an instrument of danger than the naked lamp. When it is carried into a chamber that contains fire damp the gas enters freely through the gauze into the cylindrical chamber, and is there ignited and consumed without communicating its flame to the outside body. The presence of gas is indicated by the conduct of the flame of the lamp. If the percentage of marsh gas is small the flame simply elongates and becomes smoky. If it is mixed with from eight to twelve or fourteen times its volume of atmospheric air the flame of the wick disappears entirely, and the interior of the cylinder becomes filled with the blue flame of burning gas. It will not do to hold the lamp long in this mixture, the wires will become red with heat, and the outer gas may then become ignited from them. Neither will it do to hold the lamp in a current of gaseous air moving at a greater rate of speed than six or eight feet per second, since in that case the flame is apt to be driven through the gauze and to set fire to the gas outside. There is also danger if the lamp be thrust suddenly into an explosive mixture that the force of the explosion inside the wire-gauze cylinder will force the flame through the mesh. It will be seen, therefore, that even the safety lamp is not an absolute protection against danger from explosive and inflammable gases. The position and duties of the fire boss at each colliery have already been referred to. He goes into the mine about four o’clock in the morning and makes his round before the men arrive. If gas has been found in an inflammable or explosive condition the workmen are not allowed to enter the place until it has been cleared out by the erection of brattices and other ventilating appliances. If only an insignificant quantity has been found in any chamber, the miner who works the place is warned of its existence and told to brush it out. In obedience to this order he goes to the working face, sets his lamp on the floor, and removing his coat swings that garment vigorously over his head, thus mixing and diluting the gas and driving it down into the current.