514 
LIQUID  DIFFUSION  APPLIED  TO  ANALYSIS. 
an  acid  or  an  alkali.  In  their  chemical  reactions,  the  crystal- 
loidal  appears  the  energetic  form,  and  the  colloidal  the  inert 
form  of  matter.  The  combining  equivalent  of  the  colloid  appears 
always  to  be  high,  and  it  has  a  heavy  molecule.  Among  the 
colloids  rank  hydrated  silicic  acid,  and  a  number  of  soluble  hy- 
drated  metallic  peroxites,  of  which  little  has  hitherto  been  known  ; 
also  starch,  the  vegetable  gums  and  dextrin,  caramel,  tannin, 
albumen,  and  vegetable  and  animal  extractive  matters.  The 
peculiar  structure  and  chemical  indifference  of  colloids  appear 
to  adapt  them  for  the  animal  organization,  of  which  they  become 
the  plastic  elements. 
Although  the  two  classes  are  widely  separated  in  their  proper- 
ties, a  complete  parallelism  appears  to  hold  between  them.  Their 
existence  in  nature  appears  to  call  for  a  corresponding  division 
of  chemistry  into  a  crystalloid  and  a  colloid  department. 
Although  chemically  inert  in  the  ordinary  sense,  colloids 
possess  a  comparative  activity  of  their  own,  arising  out  of  their 
physical  properties.  While  the  rigidity  of  the  crystalline  struc- 
ture shuts  out  external  impressions,  the  softness  of  the  gelatinous 
colloid  partakes  of  fluidity,  and  enables  the  colloid  to_  become  a 
medium  for  liquid  diffusion,  like  water  itself.  The  same  pene- 
trability appears  to  take  the  form  of  a  capacity  for  cementation 
in  such  colloids  as  can  exist  at  a  high  temperature.  Hence  a 
wide  sensibility  on  the  part  of  colloids  to  external  agents. 
Another  eminently  characteristic  quality  of  colloids,  is  their 
mutability.  Their  existence  is  a  continued  metastasis.  A  col- 
loid may  be  compared  in  this  respect  to  water  while  existing 
liquid  at  a  temperature  below  its  usual  freezing  point,  or  to  a 
supersaturated  saline  solution.  The  solution  of  hydrated  silicic 
acid,  for  instance,  is  easily  obtained  in  a  state  of  purity,  but 
cannot  be  preserved.  It  may  remain  fluid  for  days  or  weeks  in 
a  sealed  tube,  but  is  sure  to  gelatinize  at  last.  Nor  does  the 
change  of  this  colloid  appear  to  stop  at  that  point.  For  the 
mineral  forms  of  silicic  acid  deposited  from  water,  such  as  flint, 
are  found  to  have  passed,  during  the  geological  ages  of  their 
existence,  from  the  vitreous  or  colloidal  into  the  crystalline  con- 
dition (H.  Rose).  The  colloidal  is  in  fact  a  dynamical  state  of 
matter ;  the  crystalloidal  being  the  statical  condition.  The 
colloid  possesses  enerqia.    It  may  be  looked  upon  as  the 
