250  Iodide  and  Bromide  of  Ammonium.  {AljuneRi,i873BM° 
hundred  pounds  during  a  number  of  years  past  by  the  sulphate  of 
ammonia  process,  and  have  never  found  the  least  trace  of  decompo- 
sition, while  the  same  salt  prepared  by  the  other  (now  officinal)  pro- 
cess has  not  unfrequently  liberated  free  bromine  and  turned  acid. 
It  is  a  curious  fact  that  iodides  and  bromides,  especially  the  former, 
prepared  by  the  intervention  of  iron,  are  rather  prone  to  develop  in 
the  course  of  time,  free  hydracids  and  halogens,  unless  the  salt  has 
been  exposed  to  a  high  degree  of  temperature,  which  is,  of  course, 
inadmissible  in  the  case  of  ammonium  salts.  One  explanation  of 
this  fact  is  suggested  by  the  results  observed  in  following  another 
well  known  process,  formerly  much  used,  and  even  yet  practised,  in 
the  preparation  of  iodides  and  bromides,  namely,  to  convert  the  I 
and  Br  into  HI  and  HBr  by  means  of  HS,  either  in  the  presence  of 
the  base  or  its  carbonate,  or  previous  to  coming  in  contact  therewith. 
This  process  yields  a  product,  in  the  case  of  I,  of  very  feeble  sta- 
bility, and  obstinately  retaining  traces  of  sulphur,  which  it  is  next  to 
impossible  to  get  rid  of.  In  the  case  of  bromine,  the  durability  is 
longer,  but  decomposition  frequently  ensues  after  some  time.  The 
presence  of  sulphur,  even  in  most  minute  proportions,  appears  to  lead 
to  such  a  result,  and  traces  of  it,  present  in  the  iron  employed,  are, 
I  believe,  the  cause  of  like  effects  in  the  first-mentioned  process* 
Photographers  can  make  no  use  whatever  of  such  iodides  and  bro- 
mides ;  the  faint  trace  of  sulphur  still  remaining  produces  at  once  a 
peculiar  fogginess  and  spots  upon  the  film,  and  there  is  scarcely  a 
more  sensitive  test  for  the  detection  of  minute  traces  of  sulphur  than 
the  silvered  collodion-film. 
I  annex  a  working  formula  for  bromide  of  ammonium. 
Dissolve  4  troy-ounces  of  bromide  of  potassium  in  6  fluid-ounces  of 
boiling  water,  and  3  troy-ounces  of  sulphate  of  ammonia  in  4J  fluid- 
ounces  of  boiling  water.  Mix  the  solutions  while  hot,  and  allow  to 
cool.  Then  add  H  ounces  of  alcohol,  and  set  it  aside  for  twenty- 
four  hours.  Pour  ofl"  the  clear  liquid,  wash  the  precipitate  with  a 
small  quantity  of  a  mixture  of  1  part  alcohol  and  4  parts  wafer,  and. 
concentrate  to  the  point  of  crystallization.  In  working  upon  a  larger 
scale,  it  is  advisable  to  redissolve  the  first  crop  of  crystals  of  bromide 
of  ammonium  in  a  small  quantity  of  very  cold  water,  and  allowing  as 
short  a  time  as  possible  for  the  solution.  The  greater  part  of  the- 
accompanying  sulphate  of  potassa,  which  has  crystallized  out  at  the 
same  time,  will  remain  undissolved  at  first,  and  may  be  removed,, 
