150 LECTURES 



the oxygen of the air. As in the formation of carbonic acid, oxygen 

 is consumed faster than the carbon ; if the decay goes on the residue 

 will be at least pure carbon. 



Wood =1 Cgg H33 O22 



Deduct C O3 -f- H (the H unites with of air) = Cgg B.^^ 0^^ rr partly decayed. 

 Deduct 5 (CO3) (=05 0^0) + 11 H = C30 H^q O^q r=: further decayed. 



But if decomposition take place out of contact, or with limited sup- 

 ply of air, the process is more complex. The carbon, hydrogen, and 

 oxygen combine with one another in various proportions, and the pro- 

 ducts of decomposition are : carbonic acid (C 0^,,) carburetted hydro- 

 gen (C H2 or C, H,,) and water (H 0,) and thus result the deadly 

 choke-damp (C 0^) and the dreaded fire-damp (C H.^) of the coal 

 mines. 



Let us now see how, according to this theory, the different varieties 

 of coal may be formed. 



Wood = C,6 H^, 0, 

 Deductll CO, =:Cii O22 I ^ r tt n 



Deduct 22 H oxydized by the air ] " ^n -^22 ^-'at 



and twenty-five atoms of carbon alone remain ; and this is the com- 

 postion of pure anthracite. Again : If decomposition takes place out 

 of contact of air, bitumen or bituminous coal is formed. Thus — 



Wood = C,6 H,, O22 



Deduct 9 C 0, = 0, 0,, ) 



Deduct 3 HO = " H, 3 f = C^^ H , O^i 



Deduct 3 C H^ = C3 H^^ ) ■ 



The remainder is cannel coal = 0,4 H^, ^ 



Again : 



Wood = C36 H^o O22 



and if from this we deduct 



10 atoms carbonic acid =Cio 0., 0? ^ -rr p. 



3 atoms defiant gas = C ^ H ^ " \ ~ ^'^ ^ <^ ^^'« 



the remainder is bitumen zi: C20 H^g 2 



In the same manner, by supposing the union of these three elements 

 to take place in various proportions, under circumstances of more or 

 less imperfect access of air, we may, without difficulty, account for all 

 the different varieties of coal. 



There can be no doubt, it seems to me, that bituminous coal is ac- 

 tually formed by this play of affinities. But with reference to the 

 extremes of this series, viz: anthracite and bitumen, naptha, &c., it 

 seems rnuch more probable that these have been the result of an after 

 change, the last of the three possible causes with which we started. 



In the third place,, then, we have many reasons for believing that 

 bituminous coal is really the normal coal, and that which is always 

 formed by the play of affinities, of which we have spoken above, and 

 that anthracite and bitumen are the result of the action of igneous 

 agency upon such bituminous coal. 



