Am.  Jour,  Pharm.  \ 
March,  1889.  J 
Photography, 
157 
be  taken  out  and  thoroughly  washed,  because  the  least  trace  of  "  hypo  "  left 
in  the  prints  will  cause  them,  sooner  or  later,  to  fade.  For  convenience  in 
washing  I  have  found  the  "  Godstohe  "  rocking  tray  answer  admirably,  and 
it  is  also  one  of  the  cheapest  in  the  market. 
The  addition  of  a  little  sol.  hydrogen  peroxide  has  been  recommended  for 
the  elimination  of  the  last  traces  of  "  hypo,"  which  it  oxidizes  into  innocuous 
acid  sulphate,  thus  :-  Na2S203+4  H202=2NaHS04+3  H20. 
The  last  wash-water  may  also  be  tested  by  the  "  iodide  of  starch  "  reac- 
tion for  traces  of  "  hypo." 
The  end  of  the  various  chemical  processes  has  now  been  reached;  it  only 
remains  to  trim  the  prints  to  the  exact  size  required,  by  means  of  the  cut- 
ting shapes  sold  for  the  purpose,  and  mount  them  on  the  usual  pieces  of 
pasteboard.  For  this  purpose  I  find  nothing  succeeds  like  starch  paste  made 
sec.  artem. 
A  burnisher  supplies  the  finishing  touch,  greatly  improving  their  appear- 
ance by  imparting  a  glossy  surface,  but  this,  of  course,  is  by  no  means 
necessary. 
The  intending  beginner  will  find  his  greatest  difficulty  at  the  commence- 
ment to  be  correct  exposure.  For  his  under-exposed  plates  there  is  no 
reniedy,  but  those  a  little  "  over  "  maybe  sometimes  turned  to  good  account 
by  resorting  to  intensification.  This  is  effected  by  placing  the  negative  in  a 
saturated  aqueous  solution  of  mercuric  chloride,  until  the  requisite  amount 
of  density  is  obtained.    The  action  of  the  intensifier  being : — 
Ag+HgCl2      =  AgCl'HgCl. 
Silver  forming  Double  chloride  of  mercury 
the  image.  and  silver. 
The  plate  is  then  well  washed  under  the  tap,  and  put  into  a  dilute  solution 
of  NH4HO,  when  the  bleached  appearance  of  the  film  is  instantly  turned  to 
a  jet  black,  caused  by  the  action  of  the  ammonia  on  the  mercurous  chloride, 
which  it  converts  into  mercurous  ammonium  chloride,  the  silver  chloride 
being  for  the  most  part  dissolved  away, 
2HgCl+2NH4OH=-NH2Hg2Cl+NH4Cl+2H20, 
and 
AgCl+2NH4OH==AgNH4NH2Cl+2H20. 
In  contradistinction  to  the  above,  we  have,  also,  what  is  termed  "  thin- 
ning "  or  "  reducing  "  the  negative. 
This  is  requisite  when,  during  the  necessarily  prolonged  development  of  a 
slightly  under-exposed  plate,  with  the  aim  of  bringing  out  some  more  detail 
in  the  shadows,  the  high  lights  have  become  too  dense  for  printing.  A 
method  recommended  by  H.  P.  Kobinson  answers  fairly  well.  It  consists  in 
first  wetting  the  refractory  negative  in  water  to  swell  the  gelatine,  then  soak- 
ing it  in  a  weak  solution  of  calx,  chlorinata,  until  the  requisite  amount  of 
reduction  has  taken  place. 
It  is  also  a  good  practice  to  varnish  the  negative  before  taking  any  prints 
from  it,  as,  unless  the  film  and  the  printing  paper  are  both  in  a  perfectly  dry 
state,  some  free  silver  nitrate  from  the  latter  is  liable  to  cause  stains  on  the 
