VERIFICATION  OF  CASTOR  OIL  AND  BALSAM  COPAIBA,  ETC.  253 
the  characteristic  portions  of  some  of  them  remain  for  a  long 
time. 
The  strong  adhesion  between  water  and  an  oil  may  be  shown 
by  taking  a  slice  of  one  of  the  seeds  of  the  castor-oil  plant  and 
throwing  it  on  the  surface  of  clean  water,  when  it  will  rotate 
feebly,  after  the  manner  of  camphor  on  the  surface  of  water,  in 
consequence  of  the  oil  which  diffuses  from  it  over  the  surface  in 
one  direction  producing  rotation  by  reaction  in  another  direc- 
tion ;  but  after  a  few  turns  the  oil  becomes  equally  diffused  ail 
round,  and  the  motion  ceases.  Castor-oil  seeds  are  heavy,  they 
partly  sink  in  cold  water,  and  slices  sink  rapidly  in  hot. 
The  oil  that  escapes  from  the  seed  upon  the  surface  of  water 
does  not  form  its  cohesion  figure.  To  get  this  we  must  allow  a 
single  drop  to  descend  gently  near  to  the  surface  of  clean  water, 
and  the  result  will  be  a  figure  of  extraordinary  beauty.  The 
drop  will  flatten  into  a  disk,  from  which  there  will  proceed  a 
number  of  perfect  iridescent  rings,  showing  most  of  the  colors 
in  Newton's  seven  series  ;  but  beyond,  and  bounding  these 
rainbows,  is  a  broad  silvery  corona  of  exquisite  delicacy,  which 
almost  immediately  breaks  up  into  a  charming  lace-like  pattern, 
which  I  have  feebly  attempted  to  represent  on  the  diagram  be- 
fore you.  These  effects  last  some  seconds,  but  gradually  the 
color  disappears,  the  lace-like  pattern  follows,  and  a  large 
colorless  disk,  with  a  well-defined  edge,  is  left  on  the  surface  of 
the  water.  It  undergoes  some  change,  due  to  atmospheric  ex- 
posure, but  remains  permanent  for  hours. 
Such  is  the  figure  produced  by  the  various  specimens  of 
castor  oil  that  I  have  examined.  They  are  of  different  growths, 
but  are  I  believe  all  pure,  and  supposing  the  specific  gravity  of 
all  these  specimens  to  be  about  0*969  there  is  no  reason  why 
their  cohesion  figures  should  not  resemble  each  other  at  about 
60°  F.,  which  I  take  to  be  the  mean  temperature  of  an  inhab- 
ited apartment  both  in  winter  and  in  summer.  There  are  obvi- 
ous reasons,  with  a  viscid  oil  like  castor,  and  other  oils  that  are 
sensitive  to  cold,  why  this  test  should  be  applied  at  a  genial 
temperature.  Some  of  the  animal  oleines,  for  example,  are 
solid  at  about  40°.  These  may  be  perfectly  fluid  in  a  warm 
room,  but  if  water  be  brought  in  from  a  cistern  at  40°,  and  be 
poured  into  the  test-glass,  the  drop,  as  soon  as  it  is  deposited, 
