HYDROCARBONS AND THEIR HALOGEN DERIVATIVES. 467 



zinc compound of the radicals on the iodides of the radicals. The radicals 

 may be the same or different ones; for instance: 



Zn(CH 3 ) 2 + 2CH 3 T = ZnI 2 + 2C 2 He, 

 Zinc methyl. Methyl iodide. Zinc iodide. Ethane. 



Zn(CH s ) 2 + 2C 2 H 5 I = ZnI 2 -f- 2C 3 H 8 . 

 Ethyl iodide. Propane. 



Coal. As methane is one of the products generated during the 

 formation of coal, it may be well to consider this process here briefly. 



The various substances classed togther under the name of coal con- 

 sist principally of carbon, associated with smaller quantities of hydro- 

 gen, oxygen, nitrogen, sulphur, and certain inorganic 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 derived 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. 



Wood 100 12.18 83.07 



Peat 100 9.85 55.67 



Lignite 200 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 



