Am.  Jour.  Pharm.I 
June  1,  1872.  j 
Caniharidal  Plaster. 
273 
An  inferior  sample,  on  the  other  hand,  may  contain: — 
Iodine,           .           .           .           .           .  76-21 
Chlorine,   0-88 
Ash  .          .          .          .          .          .  1-11 
Water,   21-80 
100-00 
— American  Chemist,  April,  1872,  from  Mech.  Mag. 
PREPARATION  OP  A  VERY  ACTIVE  CANTHARIDAL  PLASTER, 
By  Professor  Dr.  G.  Dragendorff. 
Apothecaries  frequently  complain  that  some  cantharides  do  not  fur- 
nish an  active  blistering  plaster  ;  that  the  same  furnish,  even  when 
treated  with  acetic  ether,  an  extract  so  poor  in  cantharidin,  that  with 
its  aid  no  good  Drouott's  blistering  tissue  can  be  produced.  In  most 
cases  the  opinion  is  expressed  that  the  flies  contain  too  small  a  per- 
centage of  cantharidin.  My  experience  teaches  me  to  discredit  the 
latter  opinion.  It  is  possible  to  obtain  good  preparations  even  from 
such  apparently  poor  cantharides,  it  being  only  necessary  to  thor- 
oughly extract  the  cantharidin  they  contain. 
A  few  observations  show  how  poorly  this  is  commonly  accomplished. 
According  to  my  experience  the  amount  of  cantharidin  in  Spanish  flies 
varies  from  0-27  to  0-5  per  cent.  The  coating  of  a  vesicating  tissue 
20  c.  m.  long  and  twelve  wide  requires  about  25  grm.  plaster  sub- 
stance, containing  usually  about  6  grm.  powdered  Spanish  flies,  fur- 
nishing at  least  0-016  cantharidin.  0-00002  grm.  cantharidin  suffice 
for  a  blistering  surface  of  a  square  centimetre,  or  0*0048  grm.  for 
240  square  centimetres,  or  less  than  one-third  of  the  smallest  quan- 
tity that  may  be  considered  present  in  the  plaster.  Mechanical 
causes  may  partly  be  found  to  be  the  ones  that  prevent  a  thorough 
action  of  the  plaster.  A  plaster  of  poor  adhesiveness,  not  being  in 
close  contact  with  the  epidermis,  does  not  act  because  that  close  con- 
tact is  wanting,  which  is  necessary  for  the  absorption  of  the  canthari- 
din. It  is  also  a  mistake  of  several  pharmacopoeias  to  permit  the  use 
of  coarsely-powdered  cantharides,  the  quantity  of  cantharidin  in  which 
is  not  uniformly  distributed  in  the  plaster,  even  if  the  powder  is  heated 
for  a  long  time  with  the  oil. 
Other  causes,  unnoticed  heretofore,  also  weigh  heavily  in  this  di- 
rection.   The  cantharidin  is  present  in  the  Spanish  flies  in  several 
18 
