452 
Precipitates  in  Fluid  Extracts. 
Am.  Jour.  Pharm. 
Sept.,  1884. 
A  liquid  is  absorbed,  and  passes  rapidly  into  the  paper,  reaching  to  a 
height  of  about  half  an  inch  at  once.  Then  it  ceases  to  extend  upward 
as  solution  of  tersulphate  of  iron,  but  not  as  a  liquid.1  A  line  of  cle- 
roarkation  appears  as  distinct  as  though  drawn  by  a  pencil,  and  above 
this  line  a  colorless  solution  passes ;  and  this  liquid  is  absolutely  free 
from  any  salt  of  iron.  If  a  piece  of  ferrocyanide  of  potassium  be  drawn 
over  this  paper,  it  refuses  to  strike  a  blue  color  until  the  dividing  line 
is  struck.  Other  reagents  demonstrate  conclusively  the  absence  of 
even  traces  of  iron  above  this  line.  (Fig.  2 — for  explanation  see  Fig. 
3.)  Here  we  have  presented  a  reaction  in  which  a  substance  in  solu- 
fig.  ii.  tion  nas  separated  from  the  solvent,  without  evaporation  of 
the  liquid,  apparent  precipitation  of  the  solid  in  insoluble 
form,  or  change  of  solvent  power  of  the  liquid. 
In  considering  this  question  from  the  experiment  presented, 
a  doubt  must  arise  in  our  minds  regarding  the  subject.  Is  it 
really  a  separation  of  a  soluble  iron  salt  from  a  solvent  capa- 
ble of  dissolving  it?  This  query  naturally  occurs  when  we 
notice  that  the  upper  edge  of  the  iron  solution,  as  it  is  ab- 
sorbed by  the  paper,  has  a  red  color,  which  deepens  as  it 
passes  upward,  until  finally  the  colorless  liquid  shoots  above 
it.  May  it  not  be  that  an  insoluble  basic  salt  of  iron  is  formed 
by  oxidation  of  the  iron,  in  the  very  thin  layer  of  liquid  ? 
We  thus  questioned  the  matter,  and  found  that  the  line  of 
division  formed  as  readily  and  the  same  in  an  atmosphere  of 
carbonic  acid  gas.  Again,  a  piece  of  paper  from  just  beneath 
the  line — indeed,  the  very  edge  of  the  line  of  division — when 
dipped  into  water,  formed  a  solution  that  gave  a  deep  blue 
color  with  solution  of  ferrocyanide  of  potassium. 
This  experiment,  then,  seemed  to  show  that  by  means  of  an  agency 
heretofore  unrecognized  in  this  manner,  and  which  seems  to  be  capil- 
lary attraction,  a  separation  of  solvent  from  substance  dissolved  can  be 
effected,  and  absolutely.  In  analyzing  the  phenomenon,  we  find  that 
there  is  not  a  gradual  shading  off  of  iron  salt  from  below  upward.  It 
might  seem  natural  to  view  the  reaction  as  an  absorption  of  the  iron 
1  The  texture  of  the  paper  influences  the  height  to  which  the  solution 
passes  before  the  separation.  The  line  of  separation  is  soonest  formed  when 
the  paper  is  porous.  Very  firm,  compact  paper  will  carry  the  entire  solu- 
tion to  a  considerable  height.  Common  Swedish  filter  paper  will  answer, 
but  not  so  well  as  blotting-paper. 
