484 
ON  EMULSIONS. 
portion  of  oil  remaining  suspended  in  the  water,  and  a  small  por- 
tion of  water  in  the  oil.  Balsam  of  copaiba  very  readily  yielded 
a  negative  emulsion  when  agitated  with  successive  small  portions 
of  water  till  the  hulk  of  water  equalled  or  exceeded  that  of  the 
balsam ;  it  was  a  thick  creamy  yellowish  semifluid  :  the  addition 
of  a  little  solution  of  potassa  made  it  much  whiter  and  mobile, 
by  converting  it  into  a  positive  emulsion. 
Solution  of  bicarbonate  of  magnesia  promotes  the  formation  of 
negative  emulsion  between  water  and  almond  oil ;  solution  of 
diacetate  of  lead  has  the  same  effect  with  water  and  olive  oil ; 
the  addition  of  acetic  acid  to  the  latter  till  it  had  an  acid  reac- 
tion did  not  alter  its  character. 
Two  or  three  years  ago,  while  experimenting  upon  the  inter- 
solubility  of  liquids,  I  observed  that  strong  liq.  ammonise,  agita- 
ted with  an  equal  bulk  of  ether,  boils  violently.  They  become 
viscid  and  opalescent  if  agitated  in  a  stoppered  bottle,  and  if  the 
stopper  be  removed  while  they  are  mixed  the  disengagement  of 
gas  throws  the  fluids  out  of  the  bottle  ;  but  if  allowed  to  separate 
under  pressure  they  are  not  disturbed  by  the  removal  of  the 
stopper.  From  the  viscidity  of  the  mixture  of  two  liquids  usually 
so  mobile,  I  conjectured  they  were  in  the  condition  of  a  negative 
emulsion.  • 
The  relation  of  emulsibility  to  intersolubility  is  a  branch  of 
the  subject  suggestive  of  interesting  speculations. 
There  is  also  an  interesting  group  of  questions  to  be  investi- 
gated, relating  to  the  influence  of  the  relative  density  of  the  sus- 
pended matter  and  its  medium,  the  state  of  division  to  which  the 
former  is  reduced,  and  the  viscosity  or  mobility  of  the  latter  and 
their  effects  upon  emulsifaction.  That  density  alone  is»not  suffi- 
cient to  ensure  rapid  subsidence  we  have  illustrated  in  the  ob- 
servations recorded  by  Faraday  (on  gold  in  relation  to  light),  in 
which  he  found  gold  in  a  fine  state  of  division  remained  suspended 
in  water  for  months. 
I  have  added  one  or  two  observations  which  bear  also  upon  the 
mobility  of  the  liquid, — 1  grain  of  heavy  carb.  magnesia  was 
added  to  each  of  the  following  liquids  in  tubes  of  similar  size  and 
shape  : — water,  rectified  spirit,  ether,  chloroform.  In  the  two 
latter,  the  magnesia  took  a  somewhat  clotty  condition,  and  settled 
