538 
HISTORY  OF  PETROLEUM  OR  ROCK  OIL. 
oxygen,  and  more  carbon  and  hydrogen  than  cellulose,  so  that 
the  mean  composition  of  recent  woods,  as  deduced  from  numer- 
ous analyses  of  various  kinds,  may  be  represented  by  C24  H18>4 
0164.  We  may  conceive  of  four  different  modes  of  transforma- 
tion of  woody  fibre,  all  of  which  probably  intervene  to  a  greater 
or  less  degree  in  the  production  of  mineral  combustibles  ;  and 
in  considering  these  changes  we  shall  for  greater  simplicity 
adopt  for  the  composition  of  woody  fibre,  the  first  named  form- 
ula, C24H20O20. 
I.  When  wood  is  exposed  to  the  action  of  moist  air,  oxygen 
is  absorbed,  and  carbonic  acid  and  water  are  evolved  in  the 
proportion  of  one  equivalent  of  the  first  for  two  of  the  last. 
We  may  suppose  that  for  H2  which  is  oxidised  by  02  from  the 
air,  the  wood  loses  C02,  so  while  the  carbon  increases  in  amount 
the  proportions  of  oxygen  and  hydrogen  are  unchanged.  In 
this  way  an  equivalent  of  cellulose,  by  absorbing  sixteen  equiv- 
alents of  oxygen,  and  losing  eight  of  carbonic  acid  (8C02,)  and 
sixteen  of  water  (16 HO,)  would  leave  C16H404.  Such  is  the 
nature  of  the  decay  of  wood  when  exposed  to  the  air,  and  the 
process,  could  it  be  carried  out,  would  leave  a  residue  of  carbon 
only.  If,  however,  the  wood  is  deeply  buried  and  excluded 
from  the  oxygen  of  the  air,  two  reactions  are  conceivable. 
II.  The  whole  of  the  oxygen  of  the  wood  may  be  given  off 
in  the  form  of  carbonic  acid,  while  the  hydrogen  remains  with 
the  residual  carbon.  The  abstraction  of  ten  equivalents  of  car- 
bonic acid  from  one  of  woody  fibre  would  leave  a  hydrocarbon 
C14  H20' 
III.  Instead  of  combining  exclusively  with  the  carbon,  a  part 
of  the  oxygen  of  the  wood  may  be  set  free  as  water,  in  combi- 
nation of  the  hydrogen.  The  abstraction  from  an  equivalent  of 
woody  fibre  of  four  equivalents  of  carbonic  acid  and  twelve  of 
water  would  leave  a  hydrocarbon  C20H8. 
IV.  These  decompositions  are,  however,  never  so  simple  as 
we  have  supposed  in  II.  and  III.,  for  a  portion  of  hydrogen  is 
at  the  same  time  evolved  in  combination  with  carbon,  chiefly  as 
marsh-gas,  C2H4.  The  amount  of  this  gas  evolved  from  decay- 
ing plants  submerged  in  water,  and  the  immense  quantities  of  it 
condensed  in  coal-beds  and  other  rocky  strata  (forming  fire- 
damp,) show  the  great  extent  to  which  this  mode  of  decomposi- 
tion prevails. 
