44 
COLORING  MATTERS  FROM  COAL  TAR. 
Before  rectification,  the  oils  are  agitated  for  an  hour  with 
concentrated  sulphuric  acid,  the  light  with  5  and  the  heavy  with 
10  per  cent.  Thej  are  then  allowed  to  rest  for  24  or  36  hours 
for  the  acid  and  impurities  to  deposit.  The  oil  is  then  separa- 
ted and  washed  once  or  twice  with  water  and  afterwards  with 
a  solution  of  caustic  soda  sp.  gr.  1-382.  For  the  lighter,  2  per 
cent,  of  the  soda  solution  will  be  enough,  but  the  heavier  will 
require  6  per  cent.  When  so  purified  the  light  oil  is  rectified 
by  distillation  with  a  current  of  steam.  The  condensed  product 
having  a  mean  density  of  -815  to  '820,  is  the  benzole  of  com- 
merce. 
The  heavy  oil  is  distilled  without  the  assistance  of  a  current 
of  steam.  The  condensed  product  has  a  mean  density  of  -SGO, 
is  of  a  clear  yellowish  color  similar  to  that  of  Madeira  wine, 
and  has  the  disagreeable  odor  of  sulphur  compounds,  formed 
by  the  action  of  the  sulphuric  acid.  This  may  be  destroyed  by 
shaking  the  oil  before  distillation  with  a  solution  of  sulphate  of 
iron,  or  after  distillation  with  the  addition  of  some  caustic  soda 
to  the  sulphate  of  iron.  A  blackish  deposit  of  sulphide  of  iron 
is  formed  and  the  oil  loses  its  bad  odor. 
Parafiine  and  the  heavier  mineral  oils  which  drain  from  the 
paraffine  are  purified  in  the  same  way  by  means  of  sulphuric 
acid,  which  is  sometimes  combined  with  oxidising  agents,  such  as 
bichromate  of  potash,  peroxide  of  manganese  and  manganate  of 
potash,  &c.,  and  subsequently  washing  with  caustic  soda.  After 
the  action  of  the  acid  and  alkali,  parafiine  is  sometimes  recti- 
fied by  a  second  distillation,  but  more  frequently  the  purifica- 
tion is  completed  by  a  second  treatment  with  sulphuric  acid 
followed  by  a  careful  washing,  after  which  the  paraffine  is 
mixed  with  1  per  cent,  of  stearic  acid  and  treated  with  the 
caustic  soda.  The  alkali  by  saponifying  the  stearic  acid  forms 
soapy  flocculi  which  envelope  the  impurities,  and  the  melted 
paraffine  is  rendered  perfectly  limpid. 
The  acid  and  alkaline  residues  of  the  above  purifying  pro- 
cesses are  generally  thrown  away,  but  in  them  are  found  the 
principles  which  may  be  utilised  for  the  production  of  the  color- 
ing matters.  The  sulphuric  acid,  for  example,  must  combine 
with  all  the  alkaline  compounds  such  as  aniline,  quinoline, 
toluidine,  cumidine,  &c.:  while  the  caustic  soda  must  unite  with 
