48 
Photography. 
Am.  Jour.  Pharm. 
Jan.,  1889. 
or  some  non-actinic  medium,  of  course,  then  finally  fixed  by  a  solution  of 
thiosulphate  of  sodium. 
Now,  as  regards  the  chemistry  of  this  process,  the  first  question  that  nat- 
urally presents  itself  is,  What  important  change  does  the  light  produce  on 
the  sensitive  surface  of  silver  haloid  salts  when  exposed  to  its  influence  f 
You  will  at  once  remember  that  this  is  the  very  problem  Scheele  tried  so 
hard  to  solve,  and  to-day,  although  a  century  after,  we  have  not  yet  arrived 
at  a  clear  solution.  This  is  possibly  little  matter  for  surprise  when  we  con- 
sider that,  to  begin  with,  we  do  not  know  what  light  itself  really  is.  Some 
theorists  even  argue  that  the  light  does  not  produce  any  chemical  decom- 
position whatever,  but  merely  a  physical  change.  The  accepted  theory, 
however,  appears  to  be,  and  it  receives  the  support  of  our  best  authority — 
Captain  Abney — that  a  subsalt  is  formed  thus  : 
Ag2I2+ligbt=Ag2I+I, 
the  free  iodine  combining  with  excess  of  silver  present  to  form  more  silver 
iodide. 
But  then,  on  the  other  hand,  no  one  has  ever  yet  prepared  a  definite 
chemical  compound  of  silver  with  any  of  the  halogens,  containing  less  of 
the  latter  than  the  ordinary  chloride,  bromide  or  iodide,  and  so  the  matter 
remains. 
Accepting  the  subsalt  theory,  then,  the  development  of  the  latent  or  pho- 
tographic image  is  apparently  due  to  the  attraction  of  the  sub-iodide  for  the 
mercury-vapor,  the  resulting  visible  image  consisting  of  a  white  amalgam  of 
mercury  and  silver,  whilst  in  the  process  of  "  fixing  "  the  thiosulphate  sim- 
ply dissolves  away  the  unaltered  iodide  by  forming  a  soluble  double  salt 
thus : 
2AgI+3Na2S203=Ag2Na4(S203)3+2NaI. 
Turning  to  the  collodion  process  we  find  glass  plates  are  used,  coated  on 
one  side  with  collodion,  containing  in  its  solution  a  bromide  and  iodide, 
usually  ammonium  iodide  and  cadmium  bromide,  and  as  soon  as  it  is"set," 
they  are  sensitized  by  immersion  in  a  bath  of  silver  nitrate,  when  double  de- 
composition of  course  takes  place  in  the  film,  silver  bromide  and  iodide  be- 
ing formed.  Whilst  still  in  this  moist  state,  they  are  exposed  in  the  camera 
and,  after  exposure,  a  solution  of  ferrous  sulphate,  with  the  addition  of  a 
little  acetic  acid  and  alcohol,  is  used  for  their  development ;  the  reactions 
that  take  place  whilst  developing  being  oxidation  of  the  ferrous  salt  to  the 
ferric,  and  consequent  reduction  of  the  silver  salts  to  the  metallic  state,  the 
latter  remaining  insoluble  in  the  film,  and  constituting  the  image,  the  acid 
contained  in  the  developing  solution  merely  acting  as  a  restrainer  to  pre- 
vent the  too  rapid  reduction  of  the  silver  salt,  whilst  the  alcohol  simply  ex- 
ercises a  mechanical  action  in  causing  the  solution  to  flow  more  evenly  over 
the  plate. 
The  reaction  would,  I  think,  be  expressed  by  the  following  equation : 
6Ag2Br+6FeS04=Fe2Br6+2(Fe23S04)+12Ag. 
After  washing  they  are  then  "fixed  "by  means  of  a  solution  of  "  hype," 
which,  as  before,  merely  dissolves  out  the  unaltered  iodide  and  bromide  of 
silver. 
(To  be  continued.) 
