4o8  Natural  Salicylates.  {^pfiS^fiff ' 
creosotic  acids  as  its  chief  impurities,  and  to  these  a  great  deal  of 
the  ill  effects  is  undoubtedly  due. 
It  can  be  readily  seen  that  the  detection  of  admixtures  is  exceed- 
ingly difficult  and  several  methods  are  here  considered. 
(a)  By  means  of  the  differences  in  optical  rotation.  It  is  very  im- 
probable that  any  instrument  could  positively  identify  even  50  per 
cent,  of  oil  of  birch  or  methyl  salicylate  in  oil  of  gaultheria.  The 
very  slight  optical  rotation  could  easily  be  adjusted  by  the  addition 
of  a  very  small  amount  of  a  strongly  laevo-rotatory  product. 
(b)  The  bead  test  is  of  some  importance  in  quickly  forming  an 
opinion  as  to  whether  sample  is  synthetic  methyl  salicylate.  This 
test  consists  of  violently  shaking  the  container  and  noting  the  rapid- 
ity with  which  the  foam  disappears.  Synthetic  methyl  salicylate 
produces  a  foam  which  rapidly  disappears,  while  the  foam  from  a 
natural  oil  has  much  more  permanency.  This  test  is  not  reliable, 
as  small  amounts  of  certain  mixtures  can  be  added  which  will  pro- 
duce the  proper  bead. 
(c)  Color  Reactions.  Many  color  reactions  have  been  tried  with 
more  or  less  success.  One  which  gave  us  distinguishing  colors  on 
samples  known  to  be  authentic,  consists  in  treating  one  drop  of  the 
oil  with  two  drops  of  hydrochloric  acid  and  rapidly  rotating  in  a 
small  evaporating  dish,  add  one  drop  of  nitric  acid  and  again  rapidly 
rotate,  then  two  drops  of  sulphuric  acid  and  again  rotate.  Pure  oils 
gave  a  yellow  final  color,  while  synthetic  methyl  salicylate  gave  a 
pink. 
Another  reaction  which  may  prove  advantageous  with  some  modi- 
fication, is  the  play  of  colors  seen  when  a  drop  of  the  oil  is  treated 
with  sulphuric  acid  containing  one  per  cent,  of  formaldehyde. 
(d)  Physical  Constants.  The  physical  properties,  such  as  specific 
gravity  and  boiling  point,  are  not  of  much  value  in  detecting  methyl 
salicylate  in  oil  of  birch  or  gaultheria,  although  the  United  States 
Pharmacopoeia  requires  synthetic  methyl  salicylate  to  have  a  higher 
specific  gravity. 
(e)  Odor  Tests.  One  of  the  best  ways  of  identifying  these 
products,  and  even  their  admixtures,  is  to  educate  the  sense  of  smell. 
The  three  products  each  have  a  characteristic  odor,  which,  while 
different  in  various  samples,  is  yet  quite  prominent  for  each  kind. 
Pure  oil  of  gaultheria  has  a  very  heavy,  not  particularly  strong, 
odor,  but  one  which  is  quite  persistent.    Oil  of  birch  has  a  sort  of 
