440 
CAUSES  OF  FAILURE  IN  PHOTOGRAPHY. 
easily  taken  up  by  a  little  sulphuret  of  carbon,  and  the  operation 
may  be  carried  on  in  a  closed  vessel. 
The  process  is  as  follows  : — A  little  sulphuret  of  carbon  is 
put  into  the  retort  in  which  the  conversion  has  been  effected  ;  if 
the  substance,  which  usually  adheres  strongly,  is  not  detached, 
the  bottom  of  the  retort  is  dipped  into  luke-warm  water,  when 
the  disaggregation  of  the  substance  takes  place  immediately  with 
a  slight  noise.  When  the  phosphorus  is  detached,  the  saline 
solution  is  added,  the  vessel  is  closed  and  shaken,  and  the  sul- 
phuret of  carbon  containing  the  ordinary  phosphorus  floats  on 
the  surface.  If  the  proportion  of  the  latter  is  not  more  than 
one-fourth,  it  may  be  completely  got  rid  of  by  a  single  washing, 
but  it  is  safer  to  decant  the  phosphorized  sulphuret  of  carbon 
and  replace  it  by  a  fresh  quantity  of  pure  sulphuret.  This  is 
necessary  when  the  two  forms  are  mixed  in  equal  proportions, 
but  the  author  says  that  three  washings  are  always  sufficient  to 
remove  every  trace  of  ordinary  phosphorus. 
After  separating  the  two  liquids  by  decantation,  the  saline  so- 
lution containing  the  amorphous  phosphorus  is  poured  upon  a 
cloth,  when  the  purity  of  the  product  is  so  perfect  that  there 
is  no  occasion  to  boil  it  with  a  solution  of  caustic  potash.  The 
whole  operation  may  be  completed  in  half  an  hour  and  without 
the  least  danger,  so  that  it  may  be  carried  on  even  by  inexpe- 
rienced hands,  which  is  of  no  small  importance  now  that  red 
phosphorus  is  an  article  of  commerce. — Chemical  Gazette,  May 
1856,  from  Comptes  Mendus,  April  1856. 
CONDITIONS  OF  SUCCESS  AND  CAUSES  OF  FAILURE  IN  PHOTOG- 
RAPHY. 
Babo*  has  made  a  number  of  experiments  relating  to  this 
subject.  His  results  lead  him  to  the  opinion  that  absolute  neu- 
trality of  the  substances  employed  in  producing  the  film  of 
iodide  of  silver,  is  an  essential  condition.  The  collodion  may  be 
defective  in  this  respect,  a  trace  of  free  acid  causing  a  decompo- 
sition of  the  iodine  compound,  and  thus  affecting  the  sensibility 
of  the  film  of  iodide  of  silver.    The  addition  of  finely  divided 
*  Berichten  u,  d.  Verhandl.  d.  Gesell.  fur,  Reford.  d.  Naturwiss.  zu 
Freiburg,  1855.  and  Ann.  d.  Phys.  u.  Chem. 
