406  Salicylic  Acid  and  Sodium  Salicylate.  {^pfeS^Sos?" 
in  assisting  the  analyst  to  determine  adulteration  ;  but  the  most 
accurate  and  convincing  test  is  what  is  known  as  crystallization  test, 
giving  large,  square-ended,  laminated,  opaque  prisms,  while  the 
crystals  of  the  synthetic  acid  are  pointed  and  of  different  appear- 
ance generally.  This  method  enables  a  careful  analyst  to  differen- 
tiate quite  satisfactorily  between  a  pure  and  impure  oil. 
In  making  the  crystallization  test  two  stock  solutions  are  required: 
SOLUTION  NO.  I. 
NaOH  320  grammes. 
Water,  q.  s  4,000  c.c. 
SOLUTION  no.  2. 
HC1  1,280  c.c. 
Water,  q.  s  4,000  c.c. 
Place  6  c.c.  of  the  oil  in  a  500  c.c.  round-bottomed  flask,  and  add 
25  c.c.  of  solution  No.  1  and  25  c.c.  of  water.  Boil  until  clear,  i.e., 
saponified.  Pour  in  about  350  c.c.  of  hot  water  and  bring  to  a  boil. 
Now  add  25  c.c.  of  solution  No.  2  and  boil  for  a  few  moments,  then 
set  aside  to  crystallize  in  a  moderately  warm  place  so  that  crystalli- 
zation will  be  slow.  A  pure  oil  will  give  the  characteristic  large, 
square-ended,  opaque  crystals.  The  specific  gravity  is  also  of  assist- 
ance in  determining  adulteration,  a  pure  oil  running  from  1-84  to  1-83 
at  1 50  C,  boiling-point  from  near  21 8°  to  221 0  C. 
This  .test,  however,  will  not  detect  the  adulteration  of  a  pure  oil 
with  a  synthetic  methyl  salicylate  of  less  than  about  5  per  cent., 
and  to  be  employed  successfully  it  is  necessary,  of  course,  that  the 
operator  be  perfectly  familiar  with  the  crystallization  of  the  synthetic 
oil  and  comparative  characteristics  of  the  crystals. 
This  test  does  not  distinguish  between  the  oil  of  wintergreen  and 
oil  of  sweet  birch,  and  while  these  two  oils  may  possess  different 
physical  and  physiological  characteristics,  the  acid  made  from 
them  is  identical.  But  it  is  essential  that  we  have  the  means  of 
noting  the  presence  of  synthetic  methyl  salicylate  which  acts  physio- 
logically, as  we  have  seen,  in  an  entirely  different  manner. 
Referring  to  the  physical  characteristics  of  these  oils,  an  expert 
can  readily  distinguish  by  the  odor  the  different  oils.  It  is  particu- 
larly marked  between  the  synthetic  oil  and  the  oils  of  vegetable 
origin.  It  is  interesting  to  note  that  distillers  of  oil  of  sweet  birch 
often  add  the  leaves  of  the  wintergreen  plant  in  distilling,  evidently 
with  the  purpose  of  imparting  a  certain  characteristic  odor  to  the 
