ON  EMULSIONS. 
481 
ity.  These  floating  thoughts,  not  systematized,  but  not  forgot- 
ten, were  occasionally  turned  over,  in  the  hope  that  they  would 
at  last  germinate  and  give  rise  to  a  crop  of  facts  which  should 
prove  of  practical  value. 
Questions  spring  up  like  weeds  in  uncultivated  ground  ;  they 
are  abundant  in  the  field  I  have  entered  upon.  I  propose  now 
to  show  you  the  most  conspicuous  of  these  spontaneous  growths  ; 
some  time,  probably  a  long  time,  must  elapse,  before  I  can  hope 
to  present  you  with  the  answers  which  should  be  the  produce  of 
cultivation. 
The  first  question  is,  what  is  an  emulsion  ?  Is  it  essentially  a 
watery  fluid,  holding  in  suspension  an  oily  or  resinous  body  ? 
The  term  is  a  very  indefinite  one  ;  all  emulsions  separate  into 
two  parts  in  the  course  of  time,  and  we  cannot  say  exactly  how 
slow  the  separation  should  be  to  entitle  the  mixture  to  be  con- 
sidered an  emulsion.  Nor  is  there  any  evident  reason  why  other 
fluids,  or  fluids  and  solids,  mixing  in  a  similar  way,  should  not 
be  considered  emulsions.  Milk  and  chyle  may  be  considered 
animal  emulsions,  and  the  juices  of  Taraxacum,  CheMdonium,  or 
Ficus  elastica,  may  be  regarded  as  natural  emulsions  from  the 
vegetable  kingdom;  and  we  would  not  wait  till  we  had  ascer- 
tained the  length  of  time  required  for  the  separation  of  their 
parts,  not  till  analysis  had  proved  the  nature  of  their  proximate 
constituents,  before  pronouncing  them  members  of  the  emulsion 
family.  My  first  queries  were  in  reference  to  the  state  or 
mechanical  nature  of  the  mixture.  I  asked  myself,  are  the  sus- 
pended particles  always  spheroidal  ?  If  not,  are  emulsions  more 
perfect  when  they  are  so  ? 
Then  a  series  of  chemical  questions  arose.  Do  alkalies  pro- 
mote and  acids  hinder  emulsifaction  ?  Are  the  naturally  emulsi- 
ble  gum  resins  neutral  ?  Then  a  group  of  physical  problems, 
such  as  the  relation  between  colloid  and  crystalloid  conditions 
and  emulsibility,  and  the  questions,  do  substances  which  promote 
frothing,  the  mechanical  division  of  air,  also  promote  emulsifac- 
tion, the  mechanical  division  of  oil,  etc.  ?  These,  with  many 
other  questions,  appeared  to  occupy  an  ample  field  for  both  in- 
ductive and  deductive  work,  in  which  I  had  hoped  to  establish  a 
few  interesting  principles,  and,  by  the  application  of  these  princi- 
31 
