Black damp is also given off by the coal in the same manner that fire damp is, and frequently the two mixtures are evolved together. Carbonic acid gas is also one of the products of burning coal, of burning oil, and of the respiration of man and beast. It is about one and a half times as heavy as air, and is therefore always found next to the floor of the mine. This gas is not inflammable. Its presence may be detected by the conduct of the flame of the lamp. In an atmosphere containing but a small percentage of it the lamp light will grow dim, and, as the proportion of gas increases, will become more and more feeble until it is finally extinguished. An atmosphere containing from eight to ten per cent. of this gas may be breathed without immediate danger; it will simply occasion dullness of intellect and numbness of body. This condition changes into one of insensibility as the inhalation continues, or as the percentage of gas is increased, and to enter an undiluted body of it means sudden death. It is stated that the workmen in the Creuzot mine, in France, descended the shaft one morning, on their way to work, not knowing that carbonic acid had formed in the mine during the night. Following one after another along the main passage, they had reached a point not far from the foot of the shaft when the leader suddenly entered into a body of black damp and fell, stricken with asphyxia, before he could utter a cry. The man following him fell also. The third, bending over to draw his comrade out of danger, was himself prostrated, and the fourth, by reason of a similar effort, shared the fate of the others. But the fifth, being an experienced master miner, turned quickly in his tracks and obliged those behind him to ascend the shaft. The black damp is thus quick and terrible in its effect. The greatest danger from it, however, exists, not at the working faces, where it is usually swept away in the ventilating current, but in abandoned workings, where it often accumulates unnoticed.