98 ON THE EXPLOSIBrLITT OF COAL OILS. 



sidered as " coal oils ;" but there is an extraordinary difference in 

 their volatility and inflammability, from the explosive flash of volatile 

 spirits, resembling ether and alcohol, to the dull heavy blaze of a 

 smoking tar. The several peculiar qualities of the products of the dis- 

 tillation of bituminous coal will be most readily learned from a de- 

 scription of the processes practically adopted by the distillers for 

 obtaining them. 



The process of making common coal gas by the distillation of bitu- 

 minous coal in red-hot iron retorts or ovens, set over furnaces, is well 

 known ; but it may not be generally known that before the iron 

 retorts become heated red-hot, a tarry liquid or oil first comes ' over 

 into the main gas-pipes, and is collected in a separate cistern. This 

 dark liquid has an exceedingly strong and disagreeable odour, and the 

 only use of it at first was that of making coal tar. A nearly similar 

 tarry oil had previously been discovered in ancient times flowing 

 naturally out of crevices of rocks on the surface of the earth like 

 springs of water. This peculiar tarry oil received the name of " pe- 

 troleum," from the original Greek words signifying " rDck oil," as 

 being descriptive of the source from whence this extraordinary liquid 

 was first obtained. 



The natural petroleum so nearly resembles the artificial tarry oil 

 obtained by the distillation of bituminous coal, as above described, 

 that the same name will be applied to designate both of these products, 

 as being alike the results of the distillation of the. same material. 

 The question is often asked, Whence is this petroleum produced 1 



The region of our country where the oil-springs are found gushing 

 forth on the surface of the earth is near the frontier line dividing the 

 anthracite coal-fields of the seaboard of the United States and the bitu- 

 minous coal-field of the great valley of the Mississippi. In this 

 Intermediate region are vast beds of bituminous coal, originally com- 

 posed of the woody fibres of peat and other vegetable bodies, organ- 

 ised by the action of sunshine on the surface of the earth, and then 

 covered over securely from accidental combustion. The coal is, how- 

 ever, exposed to a heat in the depths of the earth, which is in- 

 creased regularly 1 deg. from fifty to seventy feet of descent, the 

 temperature at 200 feet depth having been found to be 85 deg. in an 

 English coal mine. The decomposition of the coal at great depths is 

 constantly going on, and the coal gas is often heard in the galleries of 

 coal mines rushing in hissing streams through crevices, and forming the 

 explosive " fire-damp," which renders the safety-lamp of Sir Humphrey 

 Davy a blessing for the preservation of the miners. 



One of the deserted coal mines near Newcastle, having a 4-inch 

 pipe connected with its chambers, has continued to distil sufficient 

 coal gas to form a jet of flame from its open aperture on the surface 

 of the ground " nearly sufficient to light a small town," as a recent 

 writer has stated. From the end of a similar pipe, inserted in a drill- 



