PHARMACEUTICAL  GLEANINGS  AND  NOTICES.  321 
cantharides  is  sufficient  not  only  to  prevent,  in  great  measure,  the 
contractile  force  of  the  film,  but  to  so  alter  its  consistence  as 
to  meet  the  conditions  required  by  Mr.  Tichborne.  We  will 
first  give  his  remarks  and  formula,  and  then  state  in  outline  that 
adopted  in  the  forthcoming  United  States  Pharmacopoeia.  "  First, 
then,  let  us  examine  the  texture  of  a  collodion  film,  to  mark  its 
applicability  in  the  present  case.  If  we  pour  upon  a  glass 
slide  some  recently  prepared  collodion,  and  then  examine  it  by 
the  microscope,  it  will  present  the  following  appearance : — a 
pretty  homogeneous  and  smooth  ground,  but  running  through 
which  are  slight  ridges,  which  produce  large  honeycomb  mark- 
ings. These  ridges  are  caused  by  the  quick  evaporation  of  the 
ether  ;  the  whole  is  interspersed  with  filaments  of  partially  dis- 
organized cotton  in  a  semi-gelatinous  state.  However  carefully 
the  cotton  is  prepared,  it  is  next  to  impossible  to  get  rid  of 
these  fibres,  some  portion  always  escaping  the  perfect  action  of 
the  acids.  If  we  can  add  a  small  quantity  of  glacial  acetic 
acid  to  the  collodion,  we  shall  find  the  character  of  the  film 
greatly  changed.  From  the  slower  evaporation,  the  honey- 
comb ridges  are  no  longer  palpable,  whilst  the  solubility  is  so 
much  increased  that  the  filaments  are  found  to  have  disappeared. 
This  film  is  perfectly  uniform,  but  it  presents  this  peculiarity, 
that  it  gradually  dries  into  a  mass  of  jelly-like  globules,  which, 
however,  possess  but  little  cohesion  ;  when  dry,  it  is  very  short, 
for  if  the  finger  be  run  up  the  glass,  instead  of  leaving  it  as  a 
tough  skin,  it  collects  as  a  moist  crumbled  mass.  Having  so 
far  seen  that  the  glacial  acid,  besides  destroying  the  contract- 
ility, gives  it  the  properties  of  porosity  and  slowness  in  drying, 
it  follows  that  such  a  collodion  is  particularly  suited  for  the 
application  of  any  vesicant  which  we  may  intend  to  apply,  in- 
stead of  being  a  varnish  which  hermetically  seals  up  the  active 
matters.  Glacial  acetic  acid  is  one  of  the  best  direct  solvents 
of  cantharidin  with  which  we  are  acquainted.  Pure  cantharidin 
was  found  to  be  very  soluble  in  that  acid,  a  saturated  solution 
depositing  it  unchanged  on  evaporation  in  hard  mica-like  crys- 
tals. The  principle,  then,  we  propose  is,  to  exhaust  cantharides 
by  a  mixture  of  ether  and  acetic  acid,  and  to  convert  these 
into  collodion  by  the  addition  of  gun  cotton. 
21 
