ON  COLLODION  AND  CANTHARIDAL  COLLODION.  115 
ture  is  reduced  to  70°  or  80°  Fahr.,  when  the  cotton  may  be  im- 
mersed and  allowed  to  remain  from  one  to  twelve  hours,  or  even 
longer.  At  the  temperature  of  110°  Fahr.  a  good  cotton  may 
be  made  in  one  hour,  but  above  that  temperature  much  waste 
and  sometimes  damage  to  the  cotton  ensues. 
A  cause  of  failure  in  preparing  collodion  cotton  is  probably  in 
using  too  large  a  quantity  of  cotton  to  the  amount  of  acid  mix- 
ture used.  Sixty  grains  of  cotton  cannot  be  properly  immersed 
in  two  fluid-ounces,  but  I  have  succeeded  in  every  instance  when 
only  thirty  grains  were  used.  After  the  cotton  is  immersed  in 
the  acid  mixture  there  should  be  a  sufficient  excess  to  allow  it  to 
flow  freely  over  and  through  the  fibres,  so  as  to  insure  contact 
with  every  particle.  When  the  larger  quantity  of  cotton  is  used 
it  is  almost  impossible  to  immerse  it  properly,  hence  there  is 
always  a  portion  not  thoroughly  acted  on  by  the  acid,  and  which 
is  necessarily  to  the  same  extent  insoluble. 
Washing  the  cotton  is  another  important  part  of  the  process. 
When  the  quantity  operated  upon  is  small  this  may  be  accom- 
plished without  difficulty,  by  simply  throwing  the  whole  into  a 
large  bulk  of  water  and  keeping  the  temperature  down  by  agita- 
tion with  a  glass  rod  ;  but  with  larger  quantities  it  is  necessary 
to  proceed  more  cautiously,  to  avoid  a  great  and  sudden  elevation 
of  temperature,  to  the  waste  and  damage  of  the  cotton.  A  good 
plan  is  to  take  up  a  small  quantity  at  a  time  on  the  end  of  a 
glass  rod  and  immerse  it  in  cold  water,  keeping  it  in  motion 
until  a  large  portion  of  the  acid  is  removed,  and  so  on  until  the 
whole  is  cooled.  It  may  then  be  washed  in  the  usual  manner  to 
free  it  entirely  from  acid. 
Collodion  cotton  prepared  as  above  retains  its  toughness  of 
fibre,  and  is  readily  and  entirely  soluble  in  equal  measures  of 
stronger  ether  and  stronger  alcohol,  with  the  exception  of  a 
minute  amount  of  flocculent  substance,  which  is  deposited  on 
standing.  When  the  cotton  is  prepared  with  the  acid  mixture 
at  a  temperature  considerable  above  110°  Fahr.  the  fibre  is  short 
and  weak,  and  the  cotton  will  be  found  to  have  increased  but 
little  or  none  in  weight,  owing  to  its  solubility  in  the  acid  at  an 
elevated  temperature.  This  cotton,  which  is  also  very  soluble, 
is  said  to  be  preferred  by  photographers,  but  is  not  so  good  for 
