It  may  be  occasionally  necessary  to  deter-
mine whether  the  stains  on  the  paper  moist-
ened by  the  silver  solution  arc  due  to  arsenic
or  antimony.  It  is  then  best  to  digest  the
paper-stain  in  sulphide  of  ammonium,  the
metal  present  being  converted  into  u  sulphide,
and  dissolving  in  the  excess  of  the  alkaline
salt,  leaving  the  silver  sulphide  undissolved;
the  alkaline  solution  when  evaporated  will,  in
the  case  of  arsenic,  leave  a  bright  yellow  re-
sidue, almost  insoluble  in  hydrochloric  acid ;
whereas  in  the  case  of  antimony  an  orange-
coloured  residue  will  remain  soluble  in  that
acid.  Dr  Russell  observes  that  hydrogen  alone
is  capable  of  reducing  silver  solution  to  the
metallic  state,  but  acknowledges  that  this
action  is  exceedingly  slow.  Pellet,  on  the
other  hand,  maintains  that  pure  hydrogen
when  passed  throngh  solutions  of  soda  and
nitrate  of  silver  has  no  action  at  the  ordinary
temperature ;  but  he  states  that  the  silver  salt
which  has  been  fused  possesses  an  alkaline
reaction  in  solution,  and  hydrogen  thus  pro-
duces a  slight  precipitate,  which  can  be  pre-
vented by  adding  a  drop  or  two  of  nitric  acid.
Davy,  however,  found  in  his  experiments
only  the  faintest  possible  eflect  of  the  reduc-
ing action  of  pure  hydrogen  in  solutions  of
caustic  soda  and  nitrate  of  silver.
