250  VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC. 
Hence  a  lecturer  on  physics  may  address  chemists  without  be- 
ing accused  of  temerity,  and  there  are  numerous  precedents  for 
substituting  a  physical  test  for  a  chemical  one,  where  the  latter 
is  defective,  or  not  sufficiently  expeditious  for  ordinary  purposes. 
Such  is  the  test  that  I  venture  to  propose  to-night,  and  I  ap- 
ply it  to  castor  oil  and  balsam  of  copaiba,  not  because  it  is 
peculiarly  adapted  to  those  substances,  but  because  they  are  so 
well  known  and  so  largely  used  in  medical  practice.  My  test 
is  of  a  far  more  general  kind.  It  is  applicable  to  every  inde- 
pendent liquid,  by  which  I  mean,  not  a  solution,  although  it  is 
possible  that  hereafter  solutions  may  be  examined  by  its  means. 
The  test,  as  I  now  submit  it  to  your  critical  judgment,  depends 
on  the  forces  of  cohesion,  adhesion,  and  diffusion.  For  exam- 
ple, if  I  gently  deposit  a  drop  of  an  oil  hanging  from  the  end 
of  a  glass  rod,  upon  the  surface  of  chemically  clean  water,  con- 
tained in  a  chemically  clean  glass,  a  contest  takes  place  between 
the  forces  in  question  the  moment  the  drop  flattens  down  by  its 
gravity  upon  the  surface  of  the  water.  The  adhesion  of  the 
liquid  surface  tends  to  spread  out  the  drop  into  a  film,  the 
cohesive  force  of  the  particles  of  the  drop  strives  to  prevent 
that  extension,  and  the  resultant  of  these  two  forces  is  a  figure 
which  I  believe  to  be  definite  for  every  independent  liquid.* 
The  figure  thus  produced  I  name  the  cohesion  figure  of  the- 
liquid  in  question.  Doubtless  if  there  be  two  or  more  liquids 
in  nature  of  different  chemical  composition,  but  precisely  alike 
in  their  physical  characters,  such  as  their  density  and  molecu- 
lar attraction,  and  relations  to  heat,  whereby  at  a  given  tem- 
perature they  are  equally  fluid,  or  limpid,  or  viscid,  then  doubt- 
less the  cohesion  figures  of  those  two  liquids  would  be  identical. 
I  have  succeeded  in  converting  the  cohesion  figure  of  one 
*  Strictly,  the  figure  is  a  function  of  adhesion,  and  diffusion  :  or — ■ 
F=/(CAJ) 
in  which  F  is  the  figure,  C  the  cohesive,  A  the  adhesive,  and  J"  the  diffu- 
sive forces.  The  formula  may  also  be  represented  in  other  ways.  For  if 
S  be  the  solubility,  d  the  density,  and  a  the  molecular  attraction,  then— 
F=/(SJ) 
or  F=/(Sda) 
or  F  =/(CAda) 
all  these  being  identical. 
