CAUSES  OF  FAILURE  IN  PHOTOGRAPHY. 
441 
silver  to  the  collodion  in  order  to  remove  free  iodine,  is  not  al- 
ways attended  with  success,  and  after  some  time,  silver  decom- 
poses the  collodion,  in  the  presence  of  iodine  or  iodide  of  potas- 
sium, rendering  it  thin  and  useless,  a  precipitate  also  being 
formed,  containing,  besides  organic  substance,  iodate  of  silver. 
Again,  the  neutralization  of  the  free  acid  by  means  of  alkali,  is 
very  difficult  of  execution. 
Babo  finds  that  different  compounds  of  iodine  differ  in  the 
decomposing  influence  exercised  by  them  upon  collodion,  and 
that  this  influence  varies  for  the  same  compound,  according  to 
the  proportions  of  ether,  alcohol,  and  water,  in  the  collodion. 
The  greater  the  amount  of  the  two  former  substances,  the  more 
durable  is  the  collodion.  He  finds  that  for  this  purpose  the  most 
stable  of  the  iodine  compounds  is  the  iodide  of  tetnethylamine. 
This  is  probably  owing  to  the  fact  that,  in  the  decomposition  of 
this  substance,  iodic  acid  is  not  formed,  but  triodide  of  tetrsethy. 
iamine,  which  is  less  readily  decomposed.  The  collodion  pre- 
pared with  iodide  of  potassium  or  of  ammonium,  is  rendered 
much  more  stable  by  diluting  it  with  ether  and  alcohol,  adding  a 
little  iodide,  and  then  boiling,  previously  to  the  addition  of  the 
whole  of  the  iodide,  with  some  pure  urea,  in  an  apparatus  that 
admits  of  the  ether  being  condensed  and  flowing  back.  By  this 
means,  any  trace  of  nitric  acid  is  removed.  The  collodion,  which 
may  in  this  operation  have  become  slightly  yellow,  is  then  to  be 
agitated  with  silver,  and,  when  decolorized,  decanted  or  filtered 
and  mixed  with  the  proper  quantity  of  perfectly  neutral  iodide 
of  potassium.  A  very  sensitive  preparation  is  thus  obtained. 
It  might,  indeed,  be  advantageous  before  dissolving  the  cotton 
to  boil  it  with  a  solution  of  urea. 
The  absence  of  any  reducing  substance  is  quite  as  essential 
as  the  neutrality  of  the  collodion  ;  aldehyde,  sulphurous,  pyro- 
gallic,  and  formic  acids,  sulphuretted  hydrogen,  alloxantine,  pro- 
toxide of  iron,  retard  or  neutralise  the  action  of  light.  This 
would  appear  to  account  for  the  fact  that  collodion  loses  sensi- 
bility when  kept  long,  for  by  the  liberation  of  iodine,  aldehyde 
is  formed.  It  follows,  moreover,  that  the  alcohol  and  ether 
used  in  the  preparation  of  collodion  should  be  freshly  distilled 
over  caustic  potash. 
The  presence  of  iodate  of  silver  in  collodion  almost  entirely 
