COAL-GAS. 189 



true coal, and contains a smaller percentage of carbon, but 

 as it continues to give off gases, there seems no reason why 

 it should not turn into true coal at last. 



Even then it will not cease to give off gases, two of 

 which are often fatal to the miner, one being the deadly 

 "choke-damp," or carbonic acid, which collects in old 

 and ill-ventilated mines; and the other, the "fire-damp," a 

 compound of carbon and hydrogen, which ignites with 

 violent explosion on the introduction of a light and the 

 admission of a certain proportion of air. 



Coal varies extremely, according to the amount of gas 

 it contains, and when at last little is left but carbon and 

 ash, it is called anthracite, which is so hard as not to soil 

 the fingers, and burns with a dull red, flameless, and smoke- 

 less glow, giving out great heat 



In the Wallsend Colliery so much inflammable gas 

 escapes from the coal, that on the insertion of a tin pipe in 

 a hole drilled for the purpose, and surrounded with clay, 

 the gas issuing from the pipe may be lighted just as at 

 an ordinary gas jet. 



It is by heating coal in large closed retorts that the gas 

 we burn is obtained, and the same thing may be done on a 

 small scale by filling the bowl of a tobacco pipe with coal- 

 dust, covering it close with clay, and holding it in the fire. 

 Enough gas will be driven up the stem of the pipe to burn 

 when a light is applied. 



Nature has in some cases used probably heat, and 

 certainly pressure to drive off the gas from large masses of 

 coal, for in the Appalachian coal-field, which covers at least 

 63,000 square miles, according to Prof. Bischof, the coal 



