102 
Photography. 
Am.  Jour.  Pharm. 
Feb.,  1889. 
using  add  to  it  2  grains  pyrogallol ;  for  a  half  plate  use,  of  course,  just  dou- 
ble the  amount. 
The  reason  for  adding  the  "  pyro  "  at  the  last  moment  will  be  obvious,  be- 
ing simply  because  its  alkaline  solution  so  rapidly  absorbs  oxygen  from  the 
air,  that  it  is  soon  rendered  useless  as  a  "  developer."  Sulphites  have  been 
used  to  prevent  this,  but  it  is  really  very  little  trouble  to  add  the  pinch  of 
"  pyro  "  at  the  last  moment. 
Developing  solutions  containing  pyrogallol  have  a  nasty  habit  of  stain- 
ing everything,  linen,  hands,  etc.,  with  which  they  come  in  contact,  said  to 
be  due  to  the  formation  of  metagallic  acid,  and  recently  Capt.  Abney  has 
suggested  the  substitution  of  hydroquinone  for  it.  These  bodies  are  closely 
related  chemically,  the  former  being,  of  course,  trihydroxybenzene  C6H3 
(OH)3  and  the  latter  dihydroxybenzene  C6H4(OH)2.  In  the  reaction  during 
development  the  latter  is  simply  oxidized  to  quinone,  which  crystallizes  in 
light  yellow  crystals,  and  is  thus  devoid  of  this  evil  habit  of  discoloring 
everything. 
C6H4(OH)2+  0=C6H  A+HaO, 
Quinone. 
but  it  does  not  appear  to  answer  the  purpose  quite  so  well  as  the  former. 
Hydroxylamine  hydrochlorate  NH2OH.HCl  has  also  been  pressed  into 
the  service  of  photography,  in  this  capacity,  but  during  the  process  of  de- 
velopment, nitrous  oxide  gas  N20  is  liberated,  causing  little  cracks  in  the 
film,  and  the  potassium,  or  sodium  hydrate,  which  must  necessarily  be 
used  with  it,  has  a  softening  action  on  the  gelatin.  Its  mode  of  action  is 
perhaps  expressed  by  the  following  equation  : 
4Ag2Br+2NH2OH.HCl+6NaHO= 
8Ag+4NaBr+2NaCl+7H20+NA. 
Another  developer,  very  largely  used,  is  one  with  ferrous  oxalate,  made 
by  adding  large  excess  of  neutral  potassium  oxalate  to  ferrous  sulphate, 
K2CA+FeS04=^FeCA+K2S04. 
In  the  process  of  development  the  ferrous  salt  is  raised  to  ferric  at  the  ex- 
pense of  the  silver  salt,  which  is  reduced  to  the  metallic  state,  forming  the 
image,  thus — 
6Ag2Br+6FeCA=12Ag+Fe2Br6+2Fe2(CA):v  * 
And  so  we  might  go  on  enumerating  the  different  developing  solutions, 
each  having  its  own  supporters,  who  laud  the  virtues  of  their  own  partic- 
ular "  developer  "  sky-high  ;  but,  as  I  said  before,  let  the  amateur  stick  to 
the  alkaline  pyro-developer,  and  first  thoroughly  master  the  action  of  that. 
He  can  then  afterwards  try  the  others,  and  decide  the  matter  to  his  own  sat- 
isfaction. In  this  developer  the  ammonia  acts  as  an  accelerator,  the  bro- 
mide as  a  restrainer,  whilst  the  pyrogallol  is  the  reducing  agent;  therefore, 
if  we  have  reason  to  believe,  prior  to  development,  that  the  plate  was 
slightly  over  exposed,  we  must  add  a  little  more  bromide  to  the  developing 
solution,  or  if  under  exposed,  a  drop  or  two  of  the  strong  liquid  ammonia 
•880. 
