It  will  be  seen  from  what  has  gone  before  that,  in  addition
to  the  normal  colloidal  solutions  produced  by  dissolving  typi-
cal colloids  such  as  glue,  gelatine,  or  albumen  in  water,  there
are  a  number  of  artificial  colloidal  solutions,  some  of  which  are
produced  from  normally  insoluble  substances,  such  as  silicic
acid,  gold,  silver,  and  other  inorganic  substances,  and  some  of
which  are  produced  from  crystalloids  such  as  sodium  chloride.
With  such  widely  different  solutions  it  is  not  surprising  to
find  a  very  considerable  difference  in  stability,  and,  as  a  matter
of  fact,  it  is  found  that  all  colloidal  solutions  undergo  with
greater  or  less  readiness  a  change  of  state  resulting  in  their
precipitation  in  the  form  of  a  gel.  The  resulting  gel  may  be
one  which  cannot  be  got  into  solution  again,  in  which  case  the
change  is  said  to  be  irreversible  ;  if,  on  the  other  hand,  the
change  of  state  is  only  temporary,  so  that  a  slight  change  of
conditions  causes  the  re-solution  of  the  gel,  the  change  is  said
to  be  reversible.
The  various  types  of  reversible  or  irreversible  change  of
state  which  colloidal  solutions  may  undergo  can  be  classified
as  follows  : —

ia)  Spontaneous  Precipitation,  {U)  Gelatinization,  {c)  Heat
Coagulation,  {d)  Coagulation  by  Enzymes,  {e)  Precipitation  by
Electrolytes,  (/)  Precipitation  by  other  Colloids.
(«)  Spontaneous  Precipitation. — The  decomposition  of  a  col-
loidal solution  of  silicic  acid  after  keeping  for  a  few  days  may
be  quoted  as  an  illustration  of  spontaneous  precipitation.
