ON  EMULSIONS. 
483 
A  careful  examination  of  two  mixtures,  each  containing  equal 
parts  of  oil  and  water,  but  in  the  one  case,  the  oil  being  added 
drachm  by  drachm  to  the  water,  with  agitation  between  each  addi- 
tion, and,  in  the  other,  the  water  being  added  in  like  manner  to 
the  oil,  showed,  that  in  the  former,  the  oil  was  suspended  in  the 
form  of  globules,  the  water  occupying  the  interspaces  ;  and,  in 
the  latter  case,  the  water  was  globular,  the  oil  of  course  occupy- 
ing the  interspaces.  As  in  the  ordinary  cases  of  emulsion,  the 
oil  is  globular,  we  will,  for  convenience,  call  this  condition  of 
positive  emulsion  (+),  and  when  the  water  is  globular,  name  it  a 
negative  emulsion  ( — ).  From  one  or  two  observations,  (-f-) 
emulsions  mix  freely  with  water,  but  not  with  oil,  that  is,  if 
poured  into  it,  they  settle  to  the  bottom  as  a  distinct  layer,  while 
a  negative  emulsion  poured  into  oil  diffuses  through  it,  but  poured 
into  water  refuses  to  mix  ;  if  agitated  with  a  large  bulk  of  water, 
it  separates  into  globules,  but  each  globule  is  not  a  globule  of  oil, 
but  of  ( — )  emulsion. 
Two  drops  of  *960  ammonia  being  added  to  J  oz.  of  one  of 
these  ( — )  emulsions  caused  speedy  separation  of  great  part  of  the 
water,  the  oily  portion  floating  like  cream  on  its  surface,  but  did 
not  mix  upon  agitation  ;  a  further  addition  of  ammonia  converted 
the  whole  into  an  ordinary  emulsion. 
1  have  not  yet  sought  for  an  agent  that  would  convert  a  (-f) 
emulsion  into  a  ( — )  form,  though  such  will  probably  be  found  in 
some  lead  compounds,  judging  from  a  casual  observation.  The 
( — )  emulsion  being  so  thick,  so  nearly  solid,  I  put  some  shot  into 
one  of  them  to  facilitate  the  further  mixture  of  water,  which  it 
did  in  so  marked  a  manner  that  I  judged  it  not  a  purely  mechani- 
cal effect ;  the  more  so  when  I  ascertained  that  fine  gravel  did 
not  act'in  a  like  manner,  but  each  little  pebble  attracted  to  its 
surface  a  film  of  water.  The  shot  did  not  specially  attract  either 
water  or  oil. 
2  drachms  of  oil  and  4  of  water,  being  converted  into  a  ( — ) 
emulsion  by  ten  minutes'  agitation  with  clean  shot,  showed 
scarcely  any  appearance  of  separation  after  standing  at  rest  for 
four  days. 
Castor  oil  and  almond  oil  both  showed  much  less  willingness 
to  mix  with  water  in  anything  like  equal  proportions,  only  a  small 
