156 
Photography. 
(  Am.  Jour.  Pharm. 
\      March,  1889. 
But  at  the  same  time  the  albumen  is  also  coagulated,  forming  an  insoluble 
compound,  said  to  be  silver  albuminate,  and  thus  the  silver  chloride  sim- 
ultaneously formed  is  prevented  from  sinking  into  or  being  absorbed  by  the 
texture  of  the  paper. 
Placing  the  negative  in  a  printing  frame,  constructed  for  the  purpose,  with 
the  film  side  up,  lay  on  it  a  piece  of  the  sensitized  paper,  cut  to  the  proper 
shape  and  size  with  the  albumenized  surface  down,  and  having  super-im- 
posed a  few  pieces  of  blotting  paper,  to  ensure  close  contact  between  the  print- 
ing paper  and  the  negative ;  fasten  on  the  back.  Next  expose  it  to  a  good  even 
light,  examining  the  print  by  means  of  the  hinged  back  of  the  frame,  from 
time  to  time.  It  will  be  found  necessary  to  print  a  little  deeper  than  the 
required  result,  the  subsequent  processes  of  "  toning  "  and  "  fixing  "  always 
tend  to  make  the  finished  photograph  a  little  lighter  in  color. 
The  chemistry  of  silver  printing  is  still  obscure.  We  have,  as  just  shown, 
a  sheet  of  paper  made  sensitive  to  light,  by  coating  it  with  a  mixture  of  silver 
chloride  and  silver  albuminate ;  ammonium  nitrate,  a  trace  of  free  silver 
nitrate  and  a  little  citric  acid  usually  added  as  a  preservative  being  also 
present.  On  exposure  to  the  light  as  beneath  the  negative,  it  is  darkened  in 
color,  caused  of  course  by  the  action  of  the  light  on  the  silver  chloride  and 
albuminate,  the  former  being  reduced  to  the  theoretical  subchloride,  Ag2Cl, 
and  the  latter,  as  an  organic  salt  of  silver,  to  a  sort  of  sub-oxide.  But  our 
actual  knowledge  of  what  really  takes  place  is  very  slight,  and  little  progress 
in  the  matter  has  been  made  since  the  days  of  Scheele. 
Having  then  obtained  several  prints  from  the  negative,  they  must  next  be 
washed  in  two  or  three  changes  of  water,  allowing  a  few  minutes  for  each 
immersion,  and  the  last  wash-water,  should  be  only  slightly  milky.  They 
are  then  ready  for  the  process  of  "  toning,"  which  really  consists  in  gilding 
the  silver  image,  thereby  making  the  finished  photograph  much  more 
pleasing  in  appearance  and  greatly  increasing  its  durability  and  permanency. 
Of  the  numerous  toning  solutions,  I  recommend  the  "  acetate,"  which  is 
made  as  follows  : — 
Auri  chlorid   grs.  viiss. 
Sodii  acetat   Siv. 
Aq.  dest   ad  Oij. 
This  solution,  like  good  wine,  improves  with  keeping,  and  is  better  pre- 
pared at  least  two  days  before  required  for  use.  Its  action  on  the  washed 
prints  would  appear  to  be,  according  to  Captain  Abney,  expressed  by  the 
equation  :— 2  AuCl3+NaC2H302=Au2-f  NaC2C]302+3HCl. 
The  washed  prints  are  then  put  into  it,  one  by  one,  face  downwards,  and 
kept  in  constant  motion  to  prevent .  overlapping  and  consequent  uneven 
toning.  When  their  foxy  red  color  has  gradually  turned  to  a  purple  tint 
the  operation  is  complete,  and  they  are  at  once  taken  out  and  immersed  in 
a  weak  solution  of  sodium  chloride,  which  immediately  arrests  any  further 
action  of  the  toning  bath,  then  finally  transferred  to  the  "fixing"  bath, 
which,  as  before,  consists  of  a  solution  of  hypo,  4  ounces  to  the  pint,  and 
after  having  been  allowed  to  remain  in  it  for  about  ten  minutes  they  must 
