VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC.  255 
same,  in  the  sense  that  the  Queen's  head  is  the  same,  whether 
seen  on  a  sovereign  or  on  a  half-sovereign. 
In  genera],  when  there  is  an  admixture  of  oils  the  charac- 
ters of  the  two  figures  appear  at  once.  Thus,  olive  oil  has  its 
cohesion  figure ;  oil  of  sesame,  with  which  it  is  often  mixed, 
has  quite  another  figure ;  a  mixture  of  the  two  in  various  pro- 
portions will  give  a  figure  which  is  neither  that  of  olive  oil  nor 
of  sesame,  hut  giving  the  characters  of  both,  leaning  of  course 
to  the  side  of  that  which  is  in  excess.  In  some  cases,  how- 
ever, the  characters  of  the  added  oil  do  not  at  once  appear, 
and  this  is  the  case  when  lard  oil  is  added  to  castor.  The 
figure  opens  in  the  same  beautiful  manner,  and  goes  through  its 
lovely  phases,  with  a  difference,  it  is  certain,  that  requires  a 
practised  eye  to  detect;  but  there  is  no  difficulty  in  detecting 
the  lard  oil  among  the  residual  phenomena  of  the  experiment. 
Outside  and  beyond  the  castor-oil  figure,  we  have  numerous 
small  blotches  of  lard  oil,  which  are  entirely  absent  from  the 
residual  phenomena  of  the  pure  castor-oil  figure.  I  have  se- 
lected lard  oil  as  being  the  most  common  source  of  admixture  ; 
I  suppose  as  much  as  ten  per  cent,  might  be  added  without 
greatly  reducing  the  viscidity  of  castor  oil.  Doubtless  any 
large  admixture,  if  it  were  worth  while,  of  olive,  nut,  etc.,  oils, 
would  be  at  once  seen  by  the  increased  fluidity  of  the  castor 
oil ;  but  I  do  not  imagine  any  difficulty  in  detecting  these  oils 
in  the  mixture,  their  nature,  and  even  amount  (roughly)  by 
means  of  their  cohesion  figures. 
I  do  not  give  diagrams  of  the  figures  produced  by  these 
various  admixtures.  The  subject  requires  an  education  of  the 
eye,  which  is  soon  completed,  as  far  as  the  figures  are  concerned, 
for  any  one  who  works  at  the  subject  ;  and  no  one  can  work 
at  a  subject  long  without  getting  that  technical  kind  of  famili- 
arity which  no  books  or  graphic  illustration  can  supply. 
Croton  oil  furnishes  a  magnificent  cohesion  figure  which  may 
be  represented  as  a  very  enlarged  pattern  of  that  of  castor  oil. 
Whether  a  small  admixture  of  croton  oil,  such  as  one  per  cent, 
with  castor  oil,  could  be  detected  by  means  of  th*e  resulting 
figure,  I  am  not  yet  prepared  to  say.  I  think  there  is  no  diffi- 
culty in  detecting  five  per  cent. 
Oil  of  turpentine  forms  a  very  characteristic  figure.  It 
