256  VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC. 
flashes  out  on  the  surface  of  water  into  a  large  well-defined  film, 
the  edge  is  marked  by  a  double  row  of  small  bosses,  the  outer 
being  the  smaller ;  in  the  course  of  a  few  minutes  irregular 
patches  of  iridescent  color  appear  on  the  surface,  the  film  then 
becomes  perforated  by  a  multitude  of  minute  holes,  and  the 
fourth  and  final  stage  is  an  exceedingly  delicate  resinous  net- 
work, which  is  very  permanent.  Oil  of  turpentine  is,  I  am  told, 
sometimes  mixed  with  castor  oil  as  a  vermifuge.  It  is  perfectly 
easy  to  detect  the  admixture  by  means  of  the  cohesion  figure. 
I  fear  I  have  left  myself  but  very  little  time  for  any  remarks 
on  balsam  of  copaiba.  The  readiness  with  which  this  substance 
unites  with  the  oils,  both  fixed  and  volatile,  renders  an  easy  and 
reliable  test  desirable.  A  drop  of  the  pure  balsam  spreading 
out  with  its  cohesion  figure  on  the  surface  of  water  is  a  magnifi- 
cient  sight.  It  consists  of  a  succession  of  sharply  cut,  expand- 
ing, superposed  disks,  glowing  with  the  most  brilliant  iridescent 
colors,  of  a  lustre  all  but  metallic.  These  disappear  when  the 
adhesion  of  the  surface  is  satisfied,  and  a  clean,  sharply  defined, 
colorless  disk  is  left  on  the  surface. 
Now  I  am  told  that  balsam  of  copaiba  is  often  mixed  with 
castor  oil  in  various  proportions,  to  the  great  embarrassment  of 
the  medical  practitioner.  He  is  taught  by  the  older  Pharma- 
copoeias to  shake  up  the  balsam  with  alcohol ;  but  as  castor  oil 
is  also  soluble  in  alcohol,  the  test  is  worthless.  The  magnesia 
test  is  troublesome,  and  a  drop  on  writing  paper  almost  equally 
so ;  but  a  drop  on  the  surface  of  water,  so  far  as  I  have  ex- 
amined it,  instantly  detects  the  admixture.  The  castor-oil 
figure  has  narrow  iridescent  rings,  and  a  wide  and  beautiful 
lace  border  ;  the  balsam  figure  has  wide  rings,  a  clean  cut  edge, 
and  no  border ;  but  curiously  enough,  the  mixture  of  the  balsam 
with  the  castor  oil  gives  a  figure  with  no  color,  and  a  scanty  per- 
forated border. 
I  have  not  tried  this  experiment  with  various  proportions  of 
castor  oil.  Indeed,  my  object  to-night  is  not  to  lay  before  you 
a  finished  work,  but  rather  to  invite  attention  to  the  subject, 
and  to  court  inquiry  and  examination.  When  I  brought  this 
subject  before  the  British  Association  at  Manchester  two  years 
and  a  half  ago,  my  object  was  rather  scientific  than  practical. 
I  wished  to  establish  the  principle  that  cohesion  asserts  itself  in 
