442 
CAUSES  OF  FAILURE  IN  PHOTOGRAPHY. 
prevents  the  action  of  a  faint  light,  and  as  this  salt  is  formed 
by  the  action  of  iodine  upon  nitrate  of  silver,  this  is  another 
cause  of  the  slight  sensibility  of  collodion  containing  free  iodine. 
When  nitrate  of  silver  solution  is  mixed  in  the  dark  with 
iodide  of  potassium  in  excess,  then  exposed  to  light  and  mixed 
"with  pyrogallic  acid,  no  immediate  reduction  takes  place.  Fur- 
ther, iodide  of  silver  precipitated  with  excess  of  iodide  of  potas- 
sium, collected  on  a  filter  and  washed  in  the  dark,  is  not  sensitive. 
But  when  the  amount  of  iodide  of  potassium  mixed  with  the 
silver  solution  is  insufficient  to  precipitate  the  whole  of  the  sil- 
ver, there  is  an  immediate  reduction  of  silver  on  the  addition  of 
pyrogallic  acid.  The  same  result  takes  place  when  pure  iodide 
is  mixed  with  some  nitrate  of  silver,  provided  the  amount  is  too 
small  to  determine  the  solution  of  any  iodide,  in  which  case  the 
sensibility  of  the  substance  is  reduced. 
This  circumstance  is  undoubtedly  not  only  a  frequent  cause 
of  failure,  but  also  of  uncertainty  in  the  character  of  the  picture 
produced.  The  quantity  of  silver,  not  converted  into  iodide, 
that  is  taken  up  by  the  collodion  film,  depends  upon  circumstan- 
ces which  are  not  always  to  be  controlled.  Among  these,  the 
strength  of  the  silver  solution,  and  the  amount  of  iodine  com- 
pound in  the  collodion  are  of  prominent  influence,  but  phenom- 
ena of  diffusion  must  likewise  be  concerned  within  certain  limits. 
The  interchange  of  substances  between  the  collodion  and  silver 
solution  must  be  very  complicated,  dependent  upon  the  amounts 
of  alcohol  and  ether.,  indirectly  upon  the  temperature,  and  upon 
the  amount  of  undecomposed  silver  solution  taken  up,  according 
to  the  nature  of  the  iodine  compound  used.  Babo  is  of  opin- 
ion that  in  this  respect  a  more  certain  result  is  obtained  with 
ethylamine  or  urea  salts,  than  with  salts  of  potash  or  ammonia. 
He  found  collodion  prepared  according  to  the  following  formula, 
very  suitable  for  silver  solution,  containing  from  eight  to  nine 
per  cent,  of  silver  salt : — 
For  developing  the  picture. 
Pyroxylin  e  1    .    .    —  Water  100 
Alcohol  (80  per  cent.)  30.0  .    .  40.0         Alcohol  20 
Ether  50.0  .    .  60.9         Acetic  Acid  30 
Iodide  of  tetrsethylamine        0.5  .    .    1.0         Pyrogallic  Acid  1 
Attempts  to  substitute  aldehyde,  ammonia,  alloxantine,  phos- 
phorous acid,  &c.  for  pyrogallic  acid,  proved  unsuccessful.  The 
