254  VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC. 
will  be  chilled,  and  even  become  opaque,  and  the  experiment 
fail ;  so  that  attention  to  temperature  is  important. 
Another  precaution  refers  to  extent  of  surface.  By  increas- 
ing this  we  increase  the  adhesive  force,  and  the  figures  may  be 
poor  and  thin,  and  even  torn  up,  before  their  characters  can  be 
studied.  I  have  found  a  surface  of  four  or  four  and  a  half 
inches  in  diameter  well  adapted  to  this  inquiry. 
Now  comes  the  question  of  admixture  of  oils.  Castor  oil  is 
so  abundant  and  so  cheap  that  there  is  no  great  inducement,  so 
far  as  I  know,  to  mix  it  with  an  inferior  oil,  supposing  one  very 
much  cheaper  could  be  found.  But  let  us  imagine  that  lard  oil, 
for  example,  were  wanting  employment  in  the  market,  and  that 
as  a  secondary  product  it  could  be  obtained  cheaply, — could  the 
admixture  be  detected  by  this  method  of  cohesion  figures  ?  My 
answer  is,  that  in  all  the  cases  of  admixture  which  I  have  ex- 
amined, the  features  of  the  two  oils  are  manifest  in  the  cohe- 
sion figure ;  and  these  are,  in  general,  very  distinct,  if  the 
standard  figure  be  examined  by  the  side  of  the  suspected 
figure.  My  memory  is  sufficiently  retentive  of  these  figures  to 
enable  me  to  tell  immediately  whether  a  figure  varies  from  my 
standard.  Not  that  my  standard  is  necessarily  the  correct  one  ; 
because  in  all  cases  I  have  had  to  depend  upon  others  for  the 
integrity  of  my  specimens  ;  and,  in  like  manner,  the  gentlemen 
who  have  supplied  me,  may  in  their  turn  have  to  depend  upon 
others.  So  that  the  production  of  an  undoubted  specimen  is 
not  always  easy,  unless  prepared  at  home,  which  is  also  not  al- 
ways easy.  At  the  International  Exhibition  I  often  watched 
the  process  of  crushing  linseed  and  expressing  the  oil ;  and  I 
obtained  specimens  of  the  product,  including  the  seeds.  Now, 
thought  I,  I  have  an  undoubted  specimen  of  linseed  oil ;  but  on 
examining  the  seeds  they  were  found  to  be  mixed  with  about 
one-fifth  of  other  seeds,  so  that  my  specimen  of  linseed  oil 
from  this  source  could  not  be  relied  on.  Other  circumstances 
may  modify  the  cohesion  figure,  such  as  differences  in  the  cli- 
mate, or  in  the  season  under  which  a  crop  is  gathered.  Thus, 
oil  of  lavender  varies  somewhat  in  specific  gravity  in  different 
years  from  the  same  farm,  and  I  have  observed  that  the  beauti- 
ful carrageen-moss  pattern  produced  by  this  oil  is  more  minute 
in  some  specimens  than  in  others  ;  but  the  pattern  is  still  the 
