Am.  Jour.  Pharm.  ) 
February,  1914.  / 
Colloids  and  Crystals. 
77 
insolubility  of  the  product  is  as  complete  as  possible.  Thus,  in  mixing 
very  dilute  solutions  of  sodium  sulphate  and  barium  chloride,  a  crys- 
talline precipitate  is  usually  obtained.  The  reason  is  that  barium 
sulphate  possesses  a  very  slight  but  real  solubility  in  water.  Hence 
the  liquid  in  contact  with  the  particles  first  formed  contains  enough 
barium  sulphate  to  nourish  their  growth  and  allow  them  to  develop 
to  crystals.  If  alcohol  is  added  to  the  sulphate,  before  the  barium 
chloride  is  introduced,  the  solubility  of  the  barium  sulphate  is  greatly 
reduced,  and  it  is  obtained  in  colloidal  solution  without  difficulty. 
In  the  same  way,  if  we  mix  water  solutions  of  sodium  hydroxide 
and  of  hydrochloric  acid  we  obtain  merely  an  ordinary  solution  of 
common  salt.  But  if  salt  is  produced  by  a  reaction  between  organic 
compounds  in  a  liquid  in  which  the  sodium  chloride  is  insoluble,  then 
a  colloidal  solution  is  obtained.  For  instance,  when  chlor-acetic  ester 
interacts  with  sodio-malonic  ester  a  grayish  opalescent  sol  of  sodium 
chloride  in  ethenyl  tri-carboxylic  ester  results :  CHX1  COOC2H5  -f- 
CHNa(COOC2H5)2  =  CH2(COOC2B5)  —  CH(COOC2H,)2  + 
NaCl.  At  low  temperatures,  in  such  liquids  as  toluene  and  chloro- 
form, even  ice  has  been  obtained  in  colloidal  solution. 
V. 
The  most  striking  property  of  the  reversible  colloids  is  that  they 
are  able  to  communicate  their  reversibility  to  the  irreversible  ones. 
Thus,  if  a  trace  of  gelatin  is  added  to  a  gold  solution,  the  gold 
becomes  much  more  difficult  to  coagulate  by  electrolytes,  and  when 
coagulated  it  can  be  dispersed  again  by  merely  warming  with  water. 
This  curious  protective  action  is  exerted,  in  greatly  varying  degree, 
by  most  reversible  colloids.  Direct  study  of  the  phenomenon  with 
the  ultra-microscope  shows  that  the  view  frequently  expressed  that 
the  gelatin  envelops  or  forms  a  film  around  the  gold  particles  is 
incorrect.  What  actually  happens  seems  to  be  a  direct  combination 
between  gelatin  particles  and  gold  particles,  which  then  pass  through 
the  reversible  changes  together. 
Protective  colloids  enjoy  a  wide  practical  application.  In  the 
manufacture  of  photographic  films  the  gelatin  retards  the  crystalliza- 
tion of  the  silver  bromide.  Ink  often  contains  a  colloid  which  pre- 
vents the  pigment  from  settling.  The  lubricant  "  aqua  dag  "  put  in 
the  market  by  the  Acheson  Company  consists  of  finely-divided  arti- 
ficial graphite,  held  up  by  a  protective  colloid.    Clay  is  made  plastic 
