262 CONSIDERATION OF CARBON COMPOUNDS. 



points and other properties. In these isomeric paraffines the 

 atoms are arranged differently from those in the normal hydro- 

 carbons, which fact may be proven by the difference in decom- 

 position which these substances suffer when acted upon by 

 chemical agents. 



Coal, Coal-oil, Petroleum. The name coal-oil is applied to a 

 mixture of the various liquid paraffines, containing often in solu- 

 tion the gaseous and solid members of the group, and also hydro- 

 carbons belonging to other series. Coal-oil is produced in 

 nature during the formation of coal. 



The various substances classed together under the name of 

 coal consist principally of carbon, associated with smaller quan- 

 tities of hydrogen, oxygen, nitrogen, sulphur, and certain inor- 

 ganic mineral matters which compose the ash. Coal is formed 

 from buried vegetable matter by a process of decomposition 

 which is partly a fermentation, partly a decay, and chiefly a slow 

 destructive distillation, the heat for this latter process being de- 

 rived from the interior of the earth or by the decomposition 

 itself. 



The principal constituent of the organic matter furnishing 

 coal is wood (or woody fibre, cellulose), and a comparison of the 

 composition of this substance with the various kinds of coal 

 gradually formed will help to illustrate the chemical change 

 taking place : 



Carbon. Hydrogen. Oxygen. 



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 the coal miners as fire-damp, frequently causing the formation 



