Amju°nUe"'Sarm-}  Photographic  Properties  of  Cerium  Salts.  291 
that  although  the  separation  of  the  components  of  the  first  two  sub- 
stances in  a  given  time  is  more  effectively  accomplished  with  the 
aid  of  membranes  than  with  bibulous  paper,  the  contrary  is  true  in 
the  case  of  the  mercuric  sodium  chloride. 
THE  PHOTOGRAPHIC  PROPERTIES  OF  CERIUM  SALTS. 
By  MM.  Auguste  and  Louis  Lumiere. 
We  know  that  cerium  yields  two  principal  series  of  salts.  The 
former  are  very  stable,  while  the  eerie  salts  are  brought  back  to  *he 
lower  stage  of  oxidation  even  by  feeble  reducing  agents.  Some 
among  them,  more  especially  the  organic  salts,  are  even  reduced 
spontaneously  as  soon  as  formed,  so  that  hitherto  it  has  not  been 
found  possible  to  isolate  them. 
The  easy  reductibility  of  the  eerie  salts  has  led  us  to  study  the 
action  of  light  upon  these  substances,  and  we  have  been  able  to 
observe  that  this  action  effects  a  rapid  reduction  which  may  serve 
as  a  basis  for  the  establishment  of  interesting  photographic  pro- 
cedures. 
Among  the  mineral  salts  which  have  yielded  us  the  best  results 
we  may  mention  eerie  sulphate  and  nitrate  obtained  by  dissolving 
eerie  hydroxide  in  sulphuric  or  nitric  acids.  The  aqueous  solutions 
of  these  salts  have  served  to  saturate  sheets  of  paper,  suitably  sized 
and  coated  with  a  thin  layer  of  gelatin,  which  the  cerium  salt  colors 
an  intense  yellow.  After  drying  in  the  dark,  the  papers  were 
exposed  to  light  under  a  positive  proof.  In  all  the  transparent 
parts  the  luminous  rays  reduce  the  eerie  salt  to  the  cerous  state,  and 
the  paper  is  decolorized  at  these  parts.  This  progressive  decolora- 
tion enables  us  to  follow  the  action  of  the  light  and  to  stop  the 
impression  at  the  proper  moment. 
The  proof  when  thus  obtained  must  be  treated  with  a  reagent 
capable  of  differentiating  the  cerous  from  the  eerie  salt,  so  as  to 
accentuate  and  fix  the  image.  In  an  analogous  process  with  the 
manganic  salts,  which  we  have  formerly  published  {Bulletin  de  la 
Soc.  Francaise  de  Photographie,  p.  218,  1892),  we  used  the  striking 
oxidizing  properties  of  the  manganic  salts  to  form  insoluble  color- 
ing matters  with  a  great  number  of  substances  of  the  aromatic 
series.  In  the  same  manner,  if  we  treat  the  proofs  with  cerium 
salts,  with  these  reagents  we  form  and  fix  coloring  matters  at  the 
