* 
A  COMPOUND  OF  IODIDE  AND  NITRATE  OF  SILVER.  457 
amorphous  form,  as  it  appears  on  the  surface  of  the  collodion 
plate  ;  and,  thirdly,  it  may  be  obtained  in  the  crystalline  form 
by  adding  recently  precipitated  iodide  of  silver  to  a  nearly  satu- 
rated solution  of  silver  nitrate,  when  it  will  be  found  that  much 
of  the  iodide,  which  at  first  dissolves,  reprecipitates  as  a  crystal- 
line  iodo-nitrate,  and  that  even  the  excess  of  iodide  which  re- 
mains undissolved  slowly  becomes  converted  into  a  similar  crys- 
talline deposit.  This  crystalline  iodo-nitrate  is  sensitive  to 
light,  like  the  amorphous  deposit  on  the  sensitized  plate,  and  it 
may  be  dried  without  losing  its  crystalline  form ;  but  as  soon  as 
it  is  touched  by  water  it  decomposes,  loses  its  silver  nitrate,  and 
by  sufficiently  prolonged  washing,  becomes  reconverted  into  sil- 
ver iodide,  as  insensible  to  light  as  it  was  before  being  placed  in 
contact  with  the  silver  nitrate.  The  amorphous  deposit  on  the 
plate  is  subject  to  exactly  the  same  laws,  only  it  is  rather  more 
sensitive  than  the  crystalline  iodo-nitrate,  a  difference  probably 
attributable  to  the  crystalline  form  of  the  latter;  but  by  washing, 
it  loses,  like  it,  its  combined  nitrate,  and  becomes  equally  in- 
sensible to  light,  while  at  the  same  time  it  passes  from  a  straw- 
yellow  to  a  pale  primrose. 
"  jSTow,  what  results  from  the  consideration  of  the  above 
facts?*  Simply,  that  by  their  means  we  are  enabled  to  explain 
several  of  the  rules  which  practice  has  from  time  to  time  given 
to  photography.  As  the  iodide  of  silver,  when  placed  in  a  con- 
centrated solution  of  silver  nitrate,  first  dissolves  and  then  re- 
precipitates  as  crystalline  iodo-nitrate,  we  can  understand  why 
it  is  that  in  practice  it  is  found  inconvenient  to  employ  a  bath 
of  nitrate  of  silver  over  a  certain  strength,  as  it  infallibly  de- 
stroys the  sensitive  film  ;  and  for  a  similar  reason  we  should  not 
allow  the  nitrate  bath  to  concentrate  by  evaporation  and  dry  on 
the  plate.  Again,  we  destroy  the  sensibility  of  the  plate  by 
prolonged  washing,  since  we  thus  decompose  the  iodo-nitrate 
upon  its  surface,  as  above  described  ;  and,  lastly,  when  we  wish 
to  preserve  the  sensibility  of  a  collodion  plate,  we  wash  it  with  a 
*  "  It  is  also  curious  to  observe  the  change  of  color  which  takes  place  when 
iodide  of  silver,  which  ha3  been  precipitated  in  presence  of  an  excess  of  iodide 
of  potassium,  is  added,  after  being  well  washed,  to  a  solution  of  silver  nitrate  . 
when  its  color  will  be  seen  to  change  at  once  from  pale  primrose  to  a  deeper 
and  more  brilliant  yellow." 
