376  Valuation  of  Crude  Carbolic  Acid.  {Amiu|ust,Sym* 
writer's  judgment,  better  suited  than  any  other  single  test,  to  indi- 
cate approximately  the  value  of  a  sample  of  crude  carbolic  acid, 
and  it  furnishes  information  that  cannot  be  obtained  by  the  methods 
in  general  use,  or  by  a  fractional  distillation.  It  may  also  be  safely 
assumed  that  its  results  will  be  less  liable  to  wrong  interpretation. 
The  bromine  consumed  by  a  definite  amount  of  the  sample  and  the 
character  of  the  precipitate  formed  will  give  a  very  fair  indication 
of  quality  and  strength.  Tri-brom-phenol  separates  in  the  assay  as 
a  very  voluminous,  white,  flocculent  precipitate,  leaving  the  solution 
perfectly  clear.  The  tri-brom-cresols  separate  imperfectly,  a  large 
proportion  remaining  suspended,  forming  a  liquid  of  milky  appear- 
ance. A  portion  of  the  precipitate  settles  to  the  bottom  in  form  of 
small  brownish-yellow  globules,  which  occupy  very  little  space  as 
compared  with  the  precipitate  from  the  same  quantity  of  phenol. 
It  is  easy  thus  to  determine  whether  a  sample  in  hand  consists 
mainly  of  cresols  or  of  phenol,  without  determining  the  boiling- 
points.  After  making  control -experiments  with  mixtures  of  known 
quantities  of  phenol  and  cresol,  it  is  possible  to  estimate  approxi- 
mately their  relative  quantities  in  samples  of  unknown  composition. 
It  was  found,  however,  that  the  appearance  of  the  precipitate  of  the 
one  substance  is  but  little  changed  by  the  presence  of  the  other  in 
quantities  less  than-fourth. 
As  the  Pharmacopoeia  places  no  restriction  on  the  proportions  of 
phenol  and  the  cresols,  but  merely  requires  the  absence  of  more 
than  10  per  cent,  of  the  "  less  soluble  constituents  of  coal  tar,"  it 
may  be  assumed  that  a  standard  of  not  less  than  90  per  cent,  of  the 
phenol  series  was  intended,  irrespective  of  the  relative  quantities  of 
these.  The  possible  presence  of  large  quantities  of  water  was  evi- 
dently lost  sight  of,  and  as  this  may  be  legitimately  present  up  to 
5  per  cent.,  it  would  seem  that  a  further  allowance  to  that  extent 
should  be  made  in  interpreting  the  pharmacopceial  intentions,  mak- 
ing a  requirement  of  at  least  85  per  cent,  of  phenols.  The  quantity 
of  volumetric  bromine  solution  required  for  a  given  weight  of  an 
85  per  cent,  acid  must  necessarily  vary  with  the  proportions  of  the 
different  phenols,  as  their  combining  weights  for  bromine  differ,  and 
it  is  therefore  hardly  practicable  to  set  an  arbitrary  limit  of  the 
volume  of  decinormal  bromine  solution  that  a  definite  weight  of  an 
85  per  cent,  acid  shall  require.  It  will  be  necessary  to  judge  from 
the  character  of  the  precipitate  the  approximate  ratio  of  phenol  and 
