348       REMARKS  ON  THE  COHESION  FIGURES  OF  LIQUIDS. 
I  may,  however,  remark  in  conclusion  that  the  greatest  pre- 
caution must  be  taken  in  ensuring  the  purity  of  the  glasses  used 
in  obtaining  cohesion-figures.  I  have  repeatedly  found  that 
after  glasses  have  been  used  with  fatty  oils  they  produce  nothing 
but  failures,  until  they  have  been  well  washed  with  caustic  pot- 
ash or  sulphuric  acid.  There  may  be  cases  where  spirits  of  wine 
will  be  found  more  convenient  than  potash,  especially  in  the 
case  of  some  of  the  essential  oils  which  do  not  readily  saponify. 
P.  S. — Since  writing  the  above,  I  have  made  some  inquiries 
respecting  the  commercial  treatment  of  one  or  two  of  the  oils 
operated  on,  and  I  have  also  visited  the  laboratory  of  a  whole- 
sale druggist,  where  castor  oil,  as  imported,  is  purified  by  filtra- 
tion through  bags  at  a  steam  heat. 
What  is  called  "  English  croton  oil  "  is  pressed  in  this  country 
from  imported  seeds.  Very  little  oil  is  so  made  at  present,  on 
account  of  the  trouble  in  preparing  it.  There  is  much  difference 
in  the  appearance  and  character  of  samples,  even  from  the  same 
case  or  package.  One  will  remain  fluid  at  a  low  temperature, 
and  another  will  set  speedily  into  a  pasty  mass.  This  may  be 
partly  due  to  the  mode  of  pressing,  whether  cold  or  hot,  as  some 
oils  pressed  hot  from  the  seeds  or  nuts  will  be  harder  than  oil 
pressed  cold  from  similar  seeds.  This  has  been  proved  by  care- 
ful experiments  on  cocoa-nut  oil.  It  is  believed  also  that  some 
croton  is  made  by  pressure  and  other  samples  by  boiling  the 
seeds  which  have  been  first  stripped  of  their  outer  skin  or  husk. 
This  may  affect  considerably  both  the  melting-point  of  the  oil, 
and  also  its  chemical  character,  since  resinous  substances  may 
be  brought  out  in  one  case  and  not  in  another.  Similar  differ- 
ences as  to  melting-points  are  found  in  castor  oils,  and  it  is  not 
always  easy  to  equallize  them  even  by  filtration  at  the  same 
temperature,  as  some  oils  will  carry  with  them  more  stearine 
than  others.  This,  my  informant  knows  to  be  the  case,  from 
his  own  experience  in  oils.  If  two  oils  were  cooled  and  filtered 
alike,  one  of  which  is  quite  neutral,  and  the  other  partly  acidi- 
fied, the  latter  would  be  more  fluid  after  filtration  than  the 
other.  The  neutral  stearines  do  not  crystallize  freely  like  the 
acid  hard  fats.  They  usually  form  masses  of  fine  hair-like 
crystals,  which  do  not  separate  readily  from  the  oily  part,  so 
that  a  certain  proportion  will  pass  through  with  the  oil.  The 
