5 60  Preparation  of  Dialysed  Iron.        { %^%p7h7arm- 
name,  but  a  paper  made  less  pervious,  and  strengthened  by  being  dipped 
in  sulphuric  acid.  Some  of  the  strong  and  well-sized  papers,  as  those 
used  for  legal  documents,  may  be  made  to  answer.  It  is  absolutely 
necessary  that  there  be  no  holes  in  the  septum,  and  to  ascertain  this  it 
is  best  to  sponge  with  water  the  upper  side  of  the  paper,  and  then  care- 
fully examine  the  other  side.  If  any  drops  appear  the  places  should  be 
marked  and  a  little  white  of  an  egg  may  be  applied,  and  coagulated  by 
heat,  or  a  drop  of  collodion  or  shellac  varnish  may  be  put  upon  the  spot. 
Bladder,  previously  washed,  may  be  used,  and  will  be  found  to  work 
well,  especially  if  divested  of  its  outer  coat. 
The  septum  should  be  two  or  three  inches  larger  than  the  hoop,  and 
should  be  secured  around  it  with  twine,  not  bound  tightly,  and  the  edge 
should  be  allowed  to  stand  up  around  the  hoop,  so  that  if  any  liquid 
escapes  through  the  joint  or  hoop  it  will  be  retained  by  the  paper.  The 
dialyser  will  now  resemble  a  drum  or  sieve,  and  into  this  the  liquid  to 
be  dialysed  is  poured  to  a  depth  of,  at  most,  half  an  inch.  It  is  then 
floated  on  the  surface  of  some  distilled  water  contained  in  a  suitable 
vessel.  If  the  hoop  be  of  some  heavy  material  it  must  be  supported  so 
that  the  septum  is  but  barely  below  the  level  of  the  water. 
The  time  required  for  dialysing  either  of  the  solutions  whose  pre- 
paration has  been  described  will  vary  with  the  nature  of  the  septum,  its 
extent  of  surface,  the  depth  of  liquid,  the  frequency  of  changing  the 
water  beneath,  temperature,  and  other  conditions  which  need  not  be 
enumerated.  If  everything  works  well,  and  the  water  is  changed  daily, 
the  process  will  be  finished  in  one  or  two  weeks.  Distilled  water  is 
always  preferable,  and  indeed  necessary,  especially  for  the  first  two  or 
three  days.  Clean  rain-water  is  the  best  substitute.  The  process  may 
be  said  to  be  complete  when  the  water  no  longer  shows  traces  of 
chlorides,  and  the  preparation  becomes  nearly  tasteless,  or  at  least  not 
ferruginous. 
A  pig's  bladder,  completely  filled  with  the  iron  solution,  securely 
tied,  and  immersed  in  water,  frequently  changed,  answers  well  for  mak- 
ing this  preparation.  The  process  requires  a  longer  time  than  with  a 
carefully-regulated  and  properly-conducted  dialysis,  but  it  entails  con- 
siderable less  trouble.  When  I  first  tried  this  plan  I  was  not  aware 
that  Professor  DragendorfF,  of  Russia,  had,  some  five  years  ago,  sug- 
gested its  application  to  dialysed  iron.  I  can,  however,  corroborate  all 
that  he  says.    I  may  also  mention  that  I  think  it  an  advantage  to  pro- 
