HYDROCARBONS. 303 



Carbon. Hydrogen. Oxygen. 



Wood 100 12.18 83.07 



Peat 100 9.85 55.67 



Lignite 100 8.37 42.42 



Bituminous coal . . . .100 6.12 21.23 



Anthracite coal 100 2.84 1.74 



This table shows a progressive diminution in the proportions of 

 hydrogen and oxygen during the passage from wood to anthracite. 

 These two elements must, therefore/ be eliminated in some form of 

 combination which allows them to move, viz., as gases or liquids. 

 The gases formed are chiefly carbon dioxide (which finds its way 

 through the rocks and soils to the surface either in the gaseous state 

 or after having been absorbed by water in the form of carbonic acid 

 springs) and methane, known to coal-miners as fire-damp, frequently 

 causing the formation of explosive gas mixtures in the coal mines, or 

 escaping, like carbon dioxide, through fissures to the surface of the 

 earth, where it may be ignited. 



Natural gas. While methane and other combustible gases are 

 undoubtedly formed during the formation of coal, the gas mixture 

 now generally termed natural gas (a mixture of methane, ethane, 

 propane, hydrogen, and a few other gases), and used largely for 

 heating and illuminating purposes, is most likely a product of the 

 complete decomposition of vegetable and animal matter which has 

 been precipitated from water, simultaneously with inorganic matter, 

 during the formation of certain rocks, chiefly slate and limestone. 

 The decomposition of this organic matter has been so complete that 

 the gaseous decomposition-products only are left, but no, or very 

 little of, solid residue similar to coal. 



Coal-oil, Petroleum. Similar to natural gas, coal-oil is a product 

 of the decomposition of organic matter, most likely of the fats and 

 oils of fish and other aquatic animals. While the albuminous con- 

 stituents of dead animal matter decompose rapidly, fats are compara- 

 tively stable. These oils and fats, after being precipitated simul- 

 taneously with other organic and inorganic matter from water, have 

 formed by their decomposition (which was chiefly a destructive dis- 

 tillation) the liquid we call coal-oil or petroleum. 



Coal-oil is a mixture of the various liquid paraffins, containing 

 often in solution the gaseous and solid members of this group, as also 

 small quantities of coloring and other matter. The boiling-points of 

 the constituents of coal-oil lie between and 300 C. (32 and 



