462       PHENIC  ACID,  ITS  MANUFACTURE  AND  PROPERTIES. 
4.  A  solid  porous  body,  known  by  everybody  as  "coke," 
which  remains  in  the  retorts. 
When  tar  is  submitted  to  distillation,  first  water  is  obtained, 
then  products  which  pass  over  with  this  liquid,  but  which,  lighter 
than  it,  floats  on  the  surface,  and  are  therefore  termed  the  light 
coal  oils.  Lastly,  there  is  distilled  a  compound  heavier  than 
water,  and  consequently  called  heavy  oil. 
It  was  about  the  year  1837  that  these  heavy  oils  were  first 
used  for  the  preservation  of  sleepers  according  to  Bethell's  pro- 
cess. M.  Farestier,  engineer-in-chief  of  the  department  of  the 
Vende'e,  conjointly  with  M.  Marin,  engineer,  published  a  very 
remarkable  and  very  complete  work  on  the  creosoting  of  wood 
and  its  preservation  for  twelve,  fifteen,  or  twenty  years  from 
decay  and  the  ravages  of  water  and  the  teredo.  There  remains  in 
the  retort  a  substance  fusible  at  the  high  temperature  attained  after 
the  oils  have  passed  over.  This  is  asphalte  or  bitumen,  which 
hardens  on  cooling.  The  distinguished  lecturer  then  proceeded 
to  "  phenic  acid,"  stating  that  M.  Laurent,  the  great  French 
chemist,  was  the  first  to  indicate  the  method  of  extracting  phenic 
acid  from  tar.  It  consisted  in  submitting  the  light  coal  oils  to 
a  partial  distillation,  and  treating  by  a  concentrated  solution  of 
potash,  the  products  distilling  at  a  temperature  between  160° 
and  200° 
In  1847  Mr.  Mansfield  indicated  another  method  of  treating 
the  heavy  oils  by  caustic  alkalies,  and  towards  1856  M.  Boboeuf 
made  known  his  modified  process  of  M.  Laurent.  This  consists 
chiefly  in  the  use  of  caustic  soda  instead  of  potash,  and  treating 
the  whole  of  the  light -oils,  instead  of  a  portion,  by  Laurent's 
method  ;  but  this  only  gave  an  impure  acid,  yet,  in  a  commercial 
point  of  view,  it  was  a  progress.  Of  a  similar  nature  were  the 
products  manufactured  by  Mr.  John  Bethell,  since  1847,  under 
the  direction  of  Mr.  Calvert.  They  were  used  for  several  pur- 
poses, either  for  the  production  of  picric  acid,  or  for  transforming 
tannic  acid  into  gallic  acid,  or  for  preserving  organic  substances 
from  putrefaction.  M,  Boboeuf  used  it  also  very  extensively  for 
this  purpose. 
In  1859,  M.  Marmas,  of  the  firm  of  Guinon,  Marmas,  and 
Bonnet,  of  Lyons,  came  to  Manchester,  and  requested  Mr.  Cal- 
