222 INDUSTRIAL MINERALS DIAGRAM 19 



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 

 choke-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 enor- 

 mous losses. Whenever it occurs, the mine must be closed im- 

 mediately, 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 sal- 

 ammonic and coal tar. When coal tar is distilled anew, a num- 

 ber of useful substances are extracted from it, such as the beau- 

 tiful 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. 



Petroleum is now in universal use. Its name is derived fspm 

 two words meaning mineral oil. The use of petroleum has be- 



