364  Acids  with  Oil  of  Peppermint.      { ^JJ^SP* 
addition  of  ammonia  it  quickly  forms  red  crystals,  which  are  proba- 
bly picramate  of  ammonia,  and  some  crystals  of  picrate,  whilst  in 
the  midst  of  them  is  disseminated  an  amorphous  powder  of  a  beau- 
tiful red  color.  This  red  powder  is  soluble  in  water,  insoluble  in  ben- 
zin  and  oil  of  turpentine,  and  very  slightly  soluble  in  ether  and  al- 
cohol. 
That  oil  of  peppermint  is  a  reducing  agent  is  shown  by  its  produc- 
ing the  characteristic  color  of  Prussian  Hue  in  paper  saturated  with 
a  solution  of  ferric  sulphate  and  ferricyanide  of  potassium.  At  the 
boiling  temperature  it  partially  transforms  the  perchloride  of  iron  to 
the  state  of  protochloride,  and  the  perchloride  of  mercury  is  also  par- 
tially reduced  to  calomel.  Submitted  to  dialysis  in  alcoholic  solution, 
the  picric  acid  diffuses  through  from  the  green  product,  together,  no 
doubt,  with  a  little  menthene,  the  solid  and  crystallizable  portion  of 
the  essential  oil.  This  solution  has  a  bitter,  sweetish,  and  slightly 
cool  taste.  Distilled  in  a  water -bath  it  leaves  a  yellow  residue,  which, 
treated  with  sulphuretted  hydrogen  and  ammonia,  gives  the  intensely 
red  coloration  of  picramate  of  ammonia.  The  alcoholic  distillate  has 
the  cool  taste  of  oil  of  peppermint  due  to  the  menthene. 
It  was  thought  interesting  to  investigate  whether  the  reaction  took 
place  between  the  crystallizable  portion  or  the  hydrocarbide  of  the 
essential  oil  and  the  picric  acid  ;  whether  the  green  reaction  was  also 
produced  with  oil  of  pennyroyal,  which  contains  no  stearopten.  The 
phenomenon  of  coloration  was  found  to  be  limited  to  the  oil  of  pep- 
permint, except  that  it  occurred  also  with  oil  of  chamomile  ;  but  in 
that  case  there  is  no  combination,  the  green  being  produced  by  the 
mixture  of  blue  and  yellow,  and  there  is  no  red  fluorescence. 
The  reaction  between  oil  of  peppermint  and  picric  acid  is  so  clear, 
that  the  author  thought  that  picric  acid  might  be  used  in  testing  for  the 
presence  of  that  essence  in  a  mixture  of  essential  oils,  and  vice  versa 
that  oil  of  peppermint  would  be  a  suitable  test  for  the  presence  of  picric 
acid.  For  this  purpose  he  made  a  mixture  of  several  essences,  about  two 
grams,  to  which  he  added  two  drops  of  oil  of  peppermint.  This  was 
shaken  with  a  solution  of  ten  centigrams  of  picric  acid  in  about  fifty 
grams  of  water.  At  the  end  of  twenty-four  hours  the  essential  oils 
collected  on  the  surface,  and  presented  a  very  perceptible  green  tint. 
In  a  second  experiment,  a  decoction  of  barley  and  hops  was  made,  to 
which  an  extremely  small  quantity  of  picric  acid  was  added.  The 
