Am'septuri8P84arm'}         Precipitates  in  Fluid  Extracts.  451 
Take  an  ordinary  porous  blotting-paper,  and  drop  into  its  centre, 
drop  after  drop,  some  writing  fluid.  The  spot  will  spread,  but  it  will 
not  present  the  same  appearance  from  the  center  outward.  There  is 
usually  a  dark  center,  and  then  a  dark  line  of  demarkation,  after  which 
another  shade  appears,  which,  after  spreading  to  a  certain  distance, 
will  perhaps  suddenly  give  place  to  a  nearly  colorless  liquid.  Con- 
tinue to  add  the  fluid  slowly  to  the  center  of  the  blot,  and  the  shades 
of  color  will  expand  and  preserve  their  individuality,  but  the  outer 
will  usually  grow  more  rapidly  than  the  one  immediately  within. 
Sometimes  several  shades  will  be  formed,  but  their  individual  charac- 
teristics will  be  maintained.  If  the  ink  be  one  of  the  purple  or  other 
colors  of  aniline,  or  a  carmine,  it  will  be  generally  found  that  the  outer 
liquid  will  be  colorless.  The  striking  feature  is  the  abrupt  change 
from  one  shade  to  the  other.  It  is  not  a  gradual  grading  off,  for  a 
distinct  line  of  demarkation  usually  separates  each  shade.  We  have 
introduced  this  experiment  because  it  can  be  so  readily  performed,  and 
because,  upon  second  thought,  every  person  must  even  now  admit  its 
familiarity.  Mix  two  colors  of  ink,  say  red  and  blue,  and  try  the  ex- 
periment again.  Very  likely  it  will  be  observed  that,  under  the  same 
conditions,  one  color  will  leave  the  other  after  both  have  passed  to- 
gether for  a  certain  distance,  and  leave  it  completely,  and  by  a  distinct 
line  of  demarkation.  Then  perhaps  this  second  color  will  cease  to 
spread,  and  a  colorless  liquid  will  pass 
out,  and  form  a  ring  encircling  the  ink 
spot.    (Fig.  1.) 
These  experiments  may  be  easily 
made,  and  will  illustrate  the  phenome- 
non ;  but  since  there  are  so  many  kinds 
of  ink,  it  is  impossible  to  predict  a  cer- 
tain result.  Therefore,  to  enter  into  \ 
the  subject  more  systematically,  we  will  \ 
bring  forward  the  following  experi- 
ment, in  order  to  illustrate  a  natural 
phenomenon  that  we  have  not  been  able  A' dark  purp!e  B' bright  retL  c>  c<)lorless" 
to  find  recorded  in  any  work,  and  upon  which  those  we  have  consulted 
can  furnish  us  no  information  : 
Dilute  one  part,  by  measure,  of  officinal  solution  of  tersulphate  of 
iron  with  thirty-two  parts  of  water;  then  place  a  strip  of  blotting- 
paper,  of  loose  texture,  so  that  the  lower  end  is  immersed  in  the  liquid. 
