THE CONQUEST OF NATURE 



less so. Obviously a cavity of the earth where hundreds 

 of men are constantly consuming the atmosphere and 

 vitiating it, and where thousands of lights are burning, 

 would become like the black hole of Calcutta in a few 

 minutes if some means were not adopted to relieve 

 this condition. But besides this vitiation of the at- 

 mosphere caused by the respiration of the men and the 

 burning of lamps there are likely to be accumulations 

 of poisonous gases in mines, that are even more dan- 

 gerous. Of the two classes of dangerous gases 

 those that asphyxiate and those that explode or burn- 

 it may be said in a general way that the suffocating 

 or poisonous gases, such as carbonic acid, which is 

 known as black damp, or choke damp, are more likely 

 to occur in ore mines, while the explosive gases are 

 found more frequently in coal mines. 



Choke damp, which is a gas considerably heavier 

 than the atmosphere, is usually found near the bottom 

 of mines, running along declines and falling into holes 

 in much the same manner as a liquid. It kills by suffo- 

 cation, and, as it will not support combustion, it may 

 be detected by lowering a lighted candle into a sus- 

 pected cavity, the light being extinguished at once if 

 the gas is present. To rid the cavity of it, forced 

 ventilation is used where possible, the gas being scat- 

 tered by draughts of fresh air. If this is impracticable, 

 and the cavity small, the choke damp may be dipped 

 out with buckets. 



But the problem of the mining engineer is not so 

 much to rid cavities of gas as to prevent its accumula- 

 tion. In modern mining, with proper ventilation and 



