VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC.  251 
essential  oil  into  that  of  another  by  dissolving  camphor  in  one 
of  them,  but  in  such  case  other  characters  were  introduced 
which  disturbed  the  comparison. 
As  the  cohesion  figure  of  a  liquid  depends  ?o  much  on  the 
adhesion  of  the  surface,  it  is  quite  necessary  that  that  surface 
be  chemically  clean.  The  water  need  not  be  distilled.  I  have 
found  the  New  River  Company's  water  well  adapted  to  the  pur- 
pose, but  the  vessel  must  be  specially  prepared.  All  vessels 
exposed  to  the  air  contract  an  organic  film  from  their  con- 
densing on  their  surfaces  the  breath  of  animals,  etc.,  and  also 
other  impurities  from  the  products  of  combustion,  dust,  etc.  If 
we  attempt  to  clean  the  glass  with  a  duster,  however  well  we 
may  satisfy  the  eye,  we  do  not  remove  this  organic  film,  or  if 
we  remove  one,  we  substitute  another  from  the  cloth  held  in 
the  hands  ;  so  that  when  the  glass  is  filled  with  water,  the  film 
in  question  is  detached,  and  spread  over  the  liquid  surface, 
effectually  preventing  adhesion.  The  plan  I  recommend  is  to 
appropriate  certain  glasses,  about  four  inches  in  diameter  at  the 
mouth,  to  the  purpose  ;  to  wash  them  out  occasionally  with  com- 
mercial sulphuric  acid,  to  rinse  with  water,  and  after  every 
experiment  to  wash  out  the  glass  with  a  solution  of  caustic  pot- 
ash, and  to  rinse  with  water  before  filling  up  again.  The  water 
used  for  the  experiment  must  be  allowed  to  come  to  rest  before 
the  drop  is  deposited.  The  glass  rods  kept  for  the  purpose 
should  be  of  the  same  size,  and  these  may  for  convenience  be 
kept  in  the  caustic  potash  vessel.  When  one  is  taken  out,  it 
should  be  shaken  in  water  and  wiped  dry  on  a  clean  cloth.  On 
dipping  it  into  the  oil,  etc.,  it  may  be  stirred  round  to  mix  the 
layers,  if  any,  and  then  allowed  to  drain  until  the  drops  fall 
slowly ;  and  the  eye  must  determine  when  the  rod  is  to  be 
carried  over  the  water,  so  as  to  deposit  one,  and  only  one  drop, 
neatly  and  gently  without  any  disturbance.  In  this,  as  in  all 
other  matters,  doubtless  each  operator  will  have  his  personal 
equation,  so  that  one  man's  result  may  not  be  precisely  the 
same  as  that  of  another ;  but  if  the  foregoing  directions  be 
attended  to,  sufficiently  good  cohesion-figures  will  be  produced. 
Gentlemen  have  come  to  me,  and  have  complained  of  their  ina- 
bility to  get  consistent  figures  ;  but  on  inquiry  I  have  found 
their  glasses  not  clean,  or  the  mode  of  depositing  the  drop 
