198  a-  and  $-Eucaine.  {Kx&'l^\\\^m' 
REACTIONS   CHARACTERISTIC   OF   a-EUCAINE  SALTS. 
Potassium  iodide  (i  :  10)  gives,  in  even  moderately  dilute  solu- 
tions of  a-eucaine  hydrochloride,  a  white  silky  and  glistening  pre- 
cipitate. This  precipitate  has  much  the  same  appearance  as  the 
one  obtained  when  stannous  chloride  is  added  to  a  cold  dilute  solu- 
tion of  mercuric  chloride.    /3-Eucaine  and  cocaine  give  no  reaction. 
Ammonia,  even  in  dilute  solution,  precipitates  the  bases  a-  or 
^-eucaine  or  cocaine,  but  a-eucaine  is  almost  insoluble  in  excess,  in 
I  per  cent,  solution  the  white  precipitate  is  at  once  thrown  down, 
and  in  the  case  of  /3-eucaine  or  cocaine  dissolves  immediately  on 
addition  of  about  their  own  volume  otlstrong  ammonia.  a-Eucaine, 
so  precipitated,  can  be  diluted  at  least  ten  times  with  strong 
ammonia  without  solution.  In  stronger  solutions  the  difference 
still  exists  but  is  not  so  easily  recognized.  A  3  per  cent,  solution 
of  /3-eucaine  or  cocaine  requires  about  five  times  its  own  volume  of 
ammonia  to  be  dissolved,  and  stronger  solutions  much  in  propor- 
tion to  the  per  cent,  present.  In  other  words  a  strong  solution  of 
ammonia  will  dissolve  about  one-half  of  one  per  cent,  of  the  bases 
/9-eucaine  or  cocaine,  while  it  will  dissolve  but  a  very  small  fraction 
of  a  per  cent,  of  a-eucaine.  In  dilute  solutions  this  is  a  very  char- 
acteristic reaction  for  a-eucaine  and  strong  solutions  are,  of  course, 
very  easily  rendered  dilute  for  the  test. 
Potassium  dichromate,  in  strong  solution,  added  drop  by  drop  to 
a  0'5  per  cent,  solution  of  a-eucaine,  begins  to  throw  down  a  fine 
lemon-yellow  precipitate  after  addition  of  one  or  two  drops.  The 
precipitate  is  then  much  increased  by  one  or  two  drops  of  strong 
hydrochloric  acid,  and  is  then  quite  insoluble,  dissolving  only  after 
several  times  diluting  the  volume  of  the  solution.  With  stronger 
solutions  the  precipitation  takes  place  at  once,  the  first  drop  giving 
a  more  and  more  permanent  precipitate  as  the  solution  grows 
stronger.  The  precipitate  is  notably  insoluble  in  either  water  or 
hydrochloric  acid.  More  dilute  solutions  either  show  no  precipitate 
or  only  after  addition  of  hydrochloric  acid.  Cocaine,  1  per  cent, 
solution,  is  not  precipitated  by  potassium  dichromate,  but  the  addi- 
tion of  one  or  two  drops  of  concentrated  hydrochloric  acid  throws 
down  a  yellow  precipitate  easily  soluble  in  very  slight  excess  of 
hydrochloric  acid  or  on  dilution  ot  the  solution  with  water.  Weaker 
solutions  do  not  precipitate,  while  stronger  solutions  precipitate  at 
once.    The  precipitate  is,  however,  easily  soluble  as  before.  ,9-Eu- 
