VESICATING  ACTIOisr  OF  CANTHARIDATE  OF  POTASSA.  241 
water,  and  which  makes  the  formula  of  cantharidic  acid  C^^H^O*, 
2H0.  This  acid  does  not  exist  in  a  free  state,  but  is  described 
as  forming  compounds  with  the  metals. 
The  cantharidates  of  potassa,  soda,  and  ammonia  are  soluble 
in  water,  whilst  the  cantharidates  of  the  common  metals  are  in- 
soluble, and  may  be  obtained  by  double  decomposition. 
Cantharidic  acid  is  considered  bi-basic.  Solutions  of  the  alka- 
line cantharidates,  treated  with  acetic  acid,  precipitate,  not 
cantharidic  acid,  but  its  anhydride  cantharidin.  This  form  of 
cantharidin  is  more  soluble  than  the  ordinary,  due  probably  to 
its  pulverulent  state.  The  author  has  not  directed  his  researches 
to  the  constitution  of  this  acid,  nor  has  he  examined  the  theory 
of  Messrs.  Massing  and  Dragendorff,  which  he  thinks  is  not  sup- 
ported by  sufficient  evidence. 
Some  particles  of  cantharidate  of  potassa  placed  on  the  arm 
caused  vesication  in  a  rapid  manner,  without  the  intervention  of 
a  solvent.  A  morsel  of  filtering  paper  moistened  with  a  cold 
watery  solution  of  cantharidate  of  potassa  has,  after  drying, 
caused  a  vesication  perfectly  defined.  This  paper  after  fifteen 
days  had  lost  none  of  its  energy,  from  which  the  author  infers 
that  it  is  perfectly  fixed  and  stable.  It  is  also  as  vesicant  as 
cantharidin.  Three  blisters  were  prepared,  and  applied  simulta- 
neously ;  one  dry,  the  second  moistened  with  vinegar,  and  the 
third  with  water.    The  first  took  seven  hours,  the  other  two  five. 
Cantharidates  are  prepared  by  the  direct  action  of  the  alkali 
on  cantharidin  in  the  presence  of  water,  and  by  the  aid  of  heat. 
The  solution  is  evaporated  and  crystallized.  It  presents  the  form 
of  fine  scales.  The  ammonia  salt  loses  its  base  at  212°  F. ;  it  is 
acid  to  litmus.  The  cantharidate  of  potassa,  on  the  contrary,  is 
very  stable,  and  has  an  alkaline  reaction  with  litmus.  The  soda 
salt  has  the  same  characters. 
The  author  has  found  another  process  for  the  preparation  of 
the  potassa  salt.  Two  grammes  of  cantharidin  are  dissolved  in 
150  grammes  of  alcohol.  Then  add  1-6  gramme  of  caustic  po- 
tassa dissolved  in  a  very  little  water,  and  mix  them,  when  the 
whole  becomes  a  soft  crystalline  mass,  from  which  the  alcohol  is 
separated  by  pressure. 
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