702 



THE WONDERS OF GEOLOGY. Lect. VII. 



and true mineral coal containing combustible oils is the result. In 

 deposits of wood-coal changes are still going on, as is proved by the 

 issue from clefts in the rocks of the coal formation, of inflammable 

 gases; as carburetted hydrogen, nitrogen, and olefiant gas. Thus 

 from the continual removal of oxygen in the form of carbonic acid, 

 from layers of wood-coal, that substance gradually approaches in its 

 composition to mineral coal. From the latter hydrogen is disengaged 

 in the form of a compound of hydro-carbon ; and the removal of all 

 the hydrogen forms anthracite or stone-coal.* 



The chemical changes of this nature which are conti- 

 nually taking place in carboniferous deposits, give rise to 

 those evolutions of carburetted hydrogen, or fire-damp, 

 which are frequently so fatal to the miners. And it is a 

 fact worthy of remark, as corroborative of the opinions 

 above advanced, that the bituminous quality of the coal 

 depends on the nature of the bed which immediately covers 

 it. If this be argillaceous shale the escape of the gaseous 

 matter of the coal is prevented ; but if the roof be arena- 

 ceous, the gas is evolved from the coal, and collects in the 

 innumerable fissures and pores of the sandstone, which 

 become filled with carburetted hydrogen, and form as it 

 were a gasometer, ready to explode upon any occasion. 

 Mr. Hutton is of opinion that this gas exists in a highly 

 condensed, and even liquid state in the pores of the coal ; 

 and that the small explosions (termed by the miners eruc- 

 tations) which often take place when the coal is struck 

 with a pick, are due to the sudden expansion of the con- 

 densed gas.f 



According to the analysis of Dr. Thompson, the best 

 Newcastle coal consists of — Carbon, 75.28; Hydrogen, 4.18; 

 Oxygen, 4.58; Nitrogen, 15.96. 



Iron and copper pyrites abound in many of the beds of 

 coal ; and indeed, these metallic substances are very gene- 

 rally met with in accumulations of carbonized vegetables. 

 The carburetted hydrogen, with the acid and extractive 



* Licbig's Chemistry, translated by Professor Play fair, 

 f Sir H. De la Beche, Geological Manual, 3d edit. 



