AmDe°cUri8P76frm'}       Basis  for  C ant har ides  Plaster.  545 
formula.  I  chose  the  latter,  for  I  considered  the  presence  of  a  granular 
body  like  mutton  fat  a  quite  unnecessary  and  useless  ingredient,  and  to 
be  the  main  cause  of  the  objections  urged.  After  numerous  trials  with 
mixtures  of  various  proportions  of  wax,  resin,  fats  and  oils,  most  of 
which  answered  my  object  but  indifferently,  I  tried  Canada  balsam,  and 
found  that  when  mixed  with  wax  and  lard  in  the  following  proportions 
it  gave  a  very  good  result  : 
Take  ot 
Cantharides  in  fine  powder  .......  6  ounces. 
Canada  Balsam        ........  8  " 
Yellow  Wax   5  " 
Lard   4  " 
Dissolve  the  wax,  lard  and  balsam  over  a  water-bath,  add  the  can- 
tharides, and  allow  them  to  macerate  at  a  gentle  heat  for  one  hour,  then 
stir  till  cold  ;  during  the  winter  months  the  lard  may  be  doubled  with 
advantage. 
In  deciding  upon  the  above  proportions,  I  have  borne  in  mind  that 
the  plaster  should  be  only  just  sufficiently  adherent  that  on  its  removal 
from  the  patient  the  blister  is  not  torn. 
Plaster  prepared  according  to  this  form  has  been  on  its  trial  for  some 
time,  it  produces  a  good  blister  in  from  four  to  eight  hours,  and  I  have 
had  no  complaints  concerning  it.  For  experiment  I  placed  a  small 
plaster  of  this  formula,  and  one  of  the  B.P.,  upon  my  own  arm,  the 
former  was  seven  hours  and  the  latter  nearly  eight  hours  producing  the 
desired  result. 
Canada  balsam  is,  I  consider,  well  adapted  for  the  purpose  I  have 
chosen  it,  as  from  its  terebinthinate  character  it  has  a  solvent  action 
upon  the  wax,  which  imparts  to  the  mixture  the  desired  qualities  of 
adhesion  and  flexibility  ;  it  is  also  probable  that  it  aids  the  extraction  of 
the  cantharidin  from  the  flies. 
A  German  pharmarcist,  whose  name  I  am  unable  to  call  to  mind, 
has  stated  that  an  advantage  is  gained  by  treating  the  cantharides  by  the 
following  process  before  making  them  into  plaster  :  Digest  the  powdered 
flies  with  caustic  soda,  cantharidate  of  soda  is  formed,  neutralize  this 
with  hydrochloric  acid,  cantharidin  is  liberated  and  chloride  of  sodium 
produced  ;  the  mass  is  then  dried  and  mixed  with  the  basis,  the  chloride 
of  sodium  being  in  no  way  detrimental.  I  tried  the  experiment,  but 
obtained  no  apparent  advantage. 
35 
