1867.] The Ventilation of Coal Mines. 185 



Bischof,* u consists chiefly of the stems of Stigmaria, Sigillaria, 

 Lepidodendra, and Calainites, in the more or less perfectly preserved 

 bark of which may be recognized the characteristic leaf cicatrices." 

 Wherever these plants have been discovered in our coal-beds, the 

 only portion which has been converted into coal, has been the bark ; 

 the woody portion has, by the operation of the law of substitution, 

 become stone, but never coal. The writer of this article has sought 

 over every coalfield of the United Kingdom for an example of wood 

 converted into coal, in vain ; nor has he been successful in obtain- 

 ing such a specimen from any of the coalfields of Europe or 

 America. 



It is not denied that woody matter may have played its part in 

 the formation of coal ; but if it has done so, every trace of wood 

 has been lost by the changes which it has undergone, previously 

 to its being deposited as part of a bed of coal. It may not be 

 amiss, as showing the chemical evidence adduced, to place together 

 the results of a few analyses, omitting the earthy matter, which 

 exhibit the difference in chemical constitution between Wood, 

 Turf, Lignite, and Coal. 









Oxygen 





Carbon. 



Hydrogen. 



and 

 Nitrogen. 



Wood — mean composition 



. 49-1 



6-3 



44-6 



Turf from Dartmoor 



. 60-0 



6-0 



33-8 



Brown coal from Bovey . 



. 679 



5-8 



24-0 



Lignites from the Khone 



. 74-2 



5-9 



22-5 



Coal, Newcastle 



. 87-9 



6-5 



12-6 



"It may be some time before we fully understand the pro- 

 cesses in operation during the conversion of woody fibre into the 

 curious mineral substance, coal. The only way of attaining such 

 knowledge is to examine carefully the gases eliminated during its 

 progressive alterations, and endeavour to follow out the proximate 

 changes which have produced these ultimate products of transform- 

 ation."! However, without admitting that ligneous tissues have 

 been principally concerned in the formation of coal, which is 

 Bischof 's assumption, it is not denied that they may have formed no 

 inconsiderable portion of the vegetable mass, which was to be, by a 

 series of changes, eventually converted into Bituminous Coal. 



Vegetable matter in its various stages of decay, and under 

 different conditions, is found to exhibit — 



1. The separation of carbonic acid and carburetted hydrogen. 



2. „ ,, of carbonic acid and water. 



3. „ ,, of carburetted hydrogen and water, or 



4. „ „ of all three. 



Consequently, except where the oxidation of the carbon alone takes 



* * Elements of Chemical and Physical Geology,' vol. i., p. 260. 

 t ' On the Gases evolved during the Formation of Coal/ by Dr. Lyon Play- 

 fair. * Memoirs of the Geological Survey of Great Britain,' vol. i.,p. 460. 



