72 
Colloids  and  Crystals. 
j  Am.  Jour.  Pharm. 
(   February,  1914. 
absence  of  interaction,  as  when  rosin  is  shaken  up  with  water  or  chalk 
with  alcohol.  Or,  as  when  sugar  is  agitated  with  water,  the  solid 
may  disappear,  entering  into  solution  in  the  liquid.  The  study  of 
sugar  solution  shows  quite  clearly  that  the  connection  of  the  sugar 
molecules  with  each  other  has  been  completely  destroyed.  They  are 
dispersed  through  the  water  very  much  as  the  molecules  of  a  gas 
distribute  themselves  uniformly  in  a  vacant  space,  and  in  both  cases 
the  permanence  of  the  uniform  dispersion  is  due  to  the  incessant 
motion  of  the  molecules.  Were  the  molecules  at  rest,  both  the  sugar 
and  the  gas  would  settle  and  form  a  layer  on  the  bottom  of  the 
containing  vessel. 
However,  the  molecules  of  the  sugar  retain  their  structure  intact, 
the  action  being  limited  to  their  dispersion.  When  salt,  on  the  other 
hand,  is  dissolved  in  water,  a  further  breakdown  occurs,  the  molecule 
is  separated  and  ions  of  sodium  and  of  chlorine  move  about  in  the 
liquid.  Both  solutions  freeze  below  o°  C.  and  boil  above  ioo°  C. 
The  most  important  difference  between  them  is  that  the  salt  solution 
conducts  the  electric  current,  while  the  sugar  solution  is  as  poor 
a  conductor  as  water  itself. 
A  fourth  possibility  presents  itself  when  glue  or  gelatin  is  treated 
with  water.  The  gelatin  absorbs  water,  swells  up  and,  under  the 
influence  of  heat,  dissolves,  but  the  liquid  freezes  and  boils  at  prac- 
tically the  same  temperatures  as  pure  water.  The  study  of  the  solu- 
tion shows  that  the  dispersion  is  not  molecular.  The  particles  of 
gelatin  in  it  are  composed  of  variable  and  rather  large  numbers  of 
molecules.  A  system  like  this  gelatin  solution  which  presents  a  case 
of  very  fine  but  not  molecular  subdivision  is  called  a  colloidal  solution. 
There  are  certain  solids  such  as  gelatin  and  dextrin  (with  water), 
and  rubber  (with  benzene  and  carbon  disulphide),  which,  when  they 
dissolve  in  liquids,  are  invariably  dispersed  in  this  way.  Such  solids 
may  properly  be  referred  to  as  colloids.  They  are  all  amorphous. 
Crystallized  substances  never  yield  colloidal  solutions  by  mere  spon- 
taneous solution  in  a  liquid.  They  always  produce  molecular  or 
ionic  dispersions.  However,  the  phenomenon  of  colloidal  solution  is 
perfectly  general,  and  crystallized  substances  can  also  be  obtained 
in  this  condition,  but  not  by  mere  solution. 
It  is  an  interesting  fact  that  a  substance  which  yields  a  colloidal 
solution  with  one  solvent  may  form  an  ordinary  molecular  solution 
with  another.    Soap  is  an  example.    Its  concentrated  solution  in 
