224: INDUSTRIAL MINERALS DIAGRAM 20. 



might happen that the galleries would be inundated, and work 

 become impossible. Another danger is fire-damp. The coal in 

 some mines emits a gas which explodes as soon as it touches a 

 light, shatters the works, and kills the miners. This danger is 

 avoided by using lamps, the flame of which is protected by wire 

 gauze, and the fire-damp burns within the gauze, but does not 

 communicate the flame to that without. Another danger is 

 choice-damp, a gas likewise emitted by the coal, which does not 

 explode like fire-damp, but which suffocates the miners. 



Another danger is that of a coal mine taking fire, which is less 

 dangerous to the life of the miners, but which leads to enormous 

 losses. Whenever it occurs, the mine must be closed immediately, 

 and several years must sometimes elapse before work can be resumed. 



Coal is a source of immense wealth to the countries where it 

 occurs. It is not only used in fire-places, for its principal use is 

 to heat the water in the boilers of steam-engines. But coal is 

 also used for many other purposes. The gas which is used to 

 light our streets and houses is made of it. For this purpose it is 

 heated in iron retorts out of contact with air, which is called the 

 destructive distillation of coal. The gas which it produces is 

 collected in large receptacles called gasometers, and coke is found 

 in the retorts when they are opened. Other products besides coal 

 gas are obtained by the distillation of coal, such as salammonic 

 and coal tar. "When coal tar is distilled anew, a number of 

 useful substances are extracted from it, such as the beautiful dyes 

 called mauve, magenta, aniline, &c. 



Bituminous Schists. These are laminated rocks like slate, but 

 mixed with a large quantity of bitumen. When this stone is 

 distilled, rock oil for lamps is obtained from it, as well as sub- 

 stances analogous to coal tar. 



Bitumen or Asphalte. In some countries this substance flows 

 from strata formed of bituminous schists, either alone or mixed 

 with water. At other times it is found solid, but it then melts at 

 a low temperature. On distilling it, oils similar to rock oil, or to 

 petroleum are obtained. 



