40 
COLORING  MATTERS   FROM  COAL  TAR. 
constitutes  one  of  the  most  interesting  operations  of  chemistry. 
The  reactions  to  which  these  products  owe  their  origin  are  very 
complex,  and  some  of  them  have  been  but  little  studied,  as  in- 
deed is  the  case  with  many  of  the  substances  formed.  If  the 
body  submitted  to  dry  distillation  could  be  maintained  during 
the  operation  under  uniform  conditions  of  desiccation,  tempera- 
ture and  pressure,  the  reactions  and  the  products  would  be  much 
a  ore  simple.  If,  for  example,  wood  be  heated  very  slowly  in 
close  vessels,  first  to  100^  C.  then  to  200^,  300^,  and  so  on, 
there  is  at  first  disengaged  almost  pure  water,  then  impure  strong 
acetic  acid,  and  afterwards  a  mixture  of  acetone  and  acetate  of 
methylene ;  the  maximum  of  charcoal  is  left  as  residue,  and  the 
least  amount  of  tar  and  gas  is  produced,  the  latter  consisting 
only  of  carbonic  acid  and  carburetted  hydrogen. 
In  practice,  however,  when  wood  is  distilled  in  cylinders  of 
iron  heated  from  the  outside,  the  heat  only  penetrates  to  the 
interior  gradually.  The  outside  layers  are  therefore  the  first 
decomposed  ;  they  at  first  lose  water,  then  furnish  pyroligneous 
acid  and  wood-spirit,  at  the  same  time  giving  off  carbonic  acid 
and  a  little  carburetted  hydrogen.  The  inner  layers  in  turn  are 
similarly  decomposed  ;  but  the  products  as  they  are  given  off  are 
brought  into  contact  with  the  outer  layer,  already  in  a  more 
advanced  state  of  decomposition  and  at  a  much  higher  tempera- 
ture, and  hence  new  reactions  take  place  and  new  products  are 
formed.  Thus,  the  vapor  of  water  in  contact  with  red  hot  char- 
coal is  decomposed,  and  forms  carbonic  acid  and  hydrogen ;  a 
part  of  the  carbonic  acid  is  again  decomposed  by  the  red-hot 
carbon  to  form  some  carbonic  oxide ;  a  part  of  the  nascent 
hydrogen  combines  with  carbon  to  form  various  hydrocarbons; 
one  part  of  the  acetic  acid  is  decomposed  by  the  high  tempera- 
ture to  form  acetone  and  carbonic  acid ;  another  part  reacts  on 
the  wood-spirit  and  forms  methylic  acetate  ;  a  fraction  of  the 
wood-spirit  and  acetone  are  also  decomposed,  producing  tarry 
matters,  pyroxanthrine,  oxyphemic  acid,  dumasine,  &c.  To 
these  must  be  added  the  influence  of  certain  nitrogenised  bodies, 
and  we  can  understand  how  all  these  compounds,  successively 
formed  under  the  most  favorable  circumstances  for  acting  on 
one  another,  since  they  are  in  the  nascent  state,  and  exposed 
to  a  high  temperature,  may  give  rise  to  the  formation  of  a  great 
