LIQUID  DIFFUSION  APPLIED  TO  ANALYSIS.  513 
LIQUID  DIFFUSION  APPLIED  TO  ANALYSIS. 
By  Thomas  Graham,  Esq.,  F.  R.  S.,  Master  of  the  Mint. 
The  unequal  dilFusibility  of  different  substances  in  water  ap- 
pears to  present  means  of  separation  not  unlike  those  long 
derived  from  unequal  volatility.  For  as  regards  diffusion,  there 
exists  a  "  volatile  "  and  also  a  "  fixed  "  class  of  substances  ;  and 
these  distinctions  appear  to  correspond  with  differences  in  mole- 
cular constitution  of  a  fundamental  nature.  Much  value  is 
attached  to  diffusion,  as  affording  the  means  of  bringing  out 
clearly,  and  subjecting  to  numerical  expression,  the  distinctive 
properties  of  what  appear  to  be  two  great  divisions  of  chemical 
substances. 
The  first,  or  diffusive  class  of  substances,  are  marked  by  their 
tendency  to  crystallize,  either  alone  or  in  combination  with 
water. 
When  in  a  state  of  solution  they  are  held  by  the  solvent  with 
a  certain  force,  so  as  to  affect  the  volatility  of  water  by  their 
presence.  The  solution  is  generally  free  from  viscosity,  and  is 
always  sapid.  Their  reactions  are  energetic  and  quickly  affect- 
ed.   This  is  the  class  of  crystalloids. 
The  other  class,  of  low  diffusibility,  may  be  named  colloids^ 
as  they  appear  to  be  typified  by  animal  gelatine.  They  have 
little,  if  any,  tendency  to  crystallize,  and  they  affect  a  vitreous 
structure.  The  planes  of  the  crystal^  with  its  hardness  and 
brittleness,  are  replaced  in  the  colloid  by  rounded  outlines  with 
more  or  less  softness  and  toughness  of  texture.  Water  of  crys- 
tallization is  represented  by  water  of  gelatination.  Colloids 
are  held  in  solution  by  a  feeble  power,  and  have  little  effect  on 
the  volatility  of  the  solvent.  They  are  also  precipitated  from 
their  solution  by  the  addition  of  crystalloids.  The  solution  of 
colloids  has  always  a  certain  degree  of  viscosity  or  gumminess, 
when  concentrated.  They  appear  to  be  insipid  or  wholely 
tasteless,  unless  when  they  undergo  decomposition  upon  the 
palate  and  give  rise  to  sapid  crystalloids.  Their  solid  hydrates 
are  gelatinous  bodies.  They  are  united  to  water  with  a  force 
of  low  intensity;  and  such  is  the  character  of  the  combinations 
in  general  between  a  colloid  and  a  crystalloid,  even  although 
the  latter  may  be  a  powerful  reagent  in  its  own  class,  such  as 
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