356 



PALAEOZOIC SYSTEM OF ROCKS. 



the carbon unites with some of the oxygen, forming carbonic acid 

 (C0 2 ) ; some of the carbon unites with some of the hydrogen, forming 

 carbureted hydrogen or marsh-gas (CH 4 ) ; and some of the hydrogen 

 unites with some of the oxygen, forming water (H 3 0). These products 

 are probably formed in all cases of vegetable decomposition under these 

 conditions : If, for example, we stir up the mud at the bottom of stag- 

 nant pools where weeds are growing, the bubbles which rise always 

 consist of a mixture of C0 2 and CH 4 . In every coal-mine these same 

 gases are constantly given off ; the one being the deadly choke-damp 

 and the other the terrible fire-damp of the miners. Now, by varying 

 the relative amounts of these products, it is easy to see how all the 

 principal varieties of bituminous coal may be formed. I have given 

 below the approximate composition of typical varieties of bituminous 

 coal, and of graphite, and constructed formulae expressing the chemical 

 change by which they are formed : 



Vegetable matter — cellulose C 36 H6o0 30 * 



( 9C0 2 J 

 Subtract \ 3CH 4 [■ C 12 H 34 29 



( 11H 2 ) 



And there remain C 24 H 26 = cannel. 



Again, vegetable matter C 36 H6o03o 



( 7CO- 2 ) 

 Subtract \ 3CII 4 > C 10 H 40 O 28 



( 14H 2 > 



a~,i +v,™~ „:~ n tt r\ S bituminous coal from 



And there remain C 26 II 20 O 2 = -j Staffordshire 



Again, vegetable matter . . , C 36 H 6 o0 30 



i 10CO 2 ) 

 Subtract -j 10CH 4 [ C 20 H G(> 3 o 



( 10'HaO ) 



And there remains C 16 = graphite. 



The composition of vegetable matter varies considerably. The com- 

 position of the varieties of coal is differently given by different authori- 

 ties. Different reactions from those above given might be contrived 

 which would give as good results. These reactions, therefore, are not 

 given as certainly the actual reactions which take place. They are only 

 intended to show the general character of the changes which take place 

 in the formation of coal. 



Metamorphic Goal. — It is probable that bituminous coal is the nor- 



* The composition of wood — timber — is usually given as about Ci 2 H, 8 8 . I have 

 taken the formula of cellulose instead, viz., C 6 H 10 5 ; or, taking six equivalents for con- 

 venience of calculation, C 36 H 6 o0 30 . I believe this to be much nearer the composition of 

 vegetable matter of the Coal period than is the formula of hard wood like oak or beech. 

 All the results may be worked out, however, with equal ease by the use of either formula 

 for vegetable matter. 



