ON  TESTING  ARSENIURETTED  HYDROGEN  SPOTS. 
61 
better  than  pieces  of  crystalline  zinc.  The  evolution  of  gas,  how- 
ever, easily  becomes  so  violent  that  it  must  be  moderated  by  the 
addition  of  water.  When  it  has  gradually  become  weak,  it  may 
be  remarkably  enlivened,  frequently  even  to  a  most  violent  degree, 
by  the  addition  of  a  small  quantity  of  a  solution  of  arsenious  or 
arsenic  acid.  This  circumstance  must  therefore  not  be  neglected 
in  the  employment  of  Marsh's  apparatus  in  judicial  investiga- 
tions. 
1.  Arsenical  Spots. — When  sufficient  hydrogen  gas  has  been 
generated  to  expel  the  atmospheric  air  from  the  bottle,  the  fluid  or 
substance  (such,  for  instance,  as  a  piece  of  green  paper)  which  is 
to  be  examined  for  arsenic,  is  introduced  into  it.  If  there  be  a 
funnel-tube  to  the  flask,  the  addition  of  small  quantities  of  the 
liquid  to  be  examined  to  the  sulphuric  acid  employed  to  generate 
the  gas  is  readily  effected  by  stopping  the  gas-tube  with  the  finger, 
so  that  as  much  acid  rises  into  the  funnel  as  it  can  hold.  The 
arsenical  spots  produced  on  the  porcelain  are  rapidly  dissolved  by 
pouring  over  them  hypochlorite  of  soda,  and  the  porcelain  becomes 
perfectly  clean  ;  on  the  other  hand,  the  spots  do  not  disappear  on 
being  poured  over  with  dilute  muriatic  acid.  If  the  spots  of  arse- 
nic be  shining,  and  consequently  proportionately  thick,  the  hypo- 
chlorite of  soda  is  somewhat  longer  in  effecting  their  solution,  which 
however  it  always  does  in  a  period  of  not  more  than  a  few  se- 
conds. 
2.  Antimonial  Spots. — If  a  little  tartar-emetic  (or  any  other 
compound  of  antimony)  be  added  to  the  mixture  of  zinc  and  sul- 
phuric acid  in  the  flask,  antimoniuretted  hydrogen  is  immediately 
given  off  with  the  hydrogen.  Whilst  the  spots  on  the  porcelain 
are  still  faint  and  of  a  dull  appearance,  they  are  acted  upon,  after 
some  time,  by  hypochlorite  of  soda,  in  such  a  manner  that  they 
become  gradually  indistinct,  rather  by  being  washed  off  by  the 
alkaline  fluid,  than  in  consequence  of  becoming  dissolved  in  it. 
But  if  the  antimonial  spots  be  black  and  shining,  and  consequently 
more  firmly  fixed  on  the  porcelain,  they  completely  withstand  the 
action  of  both  hypochlorite  of  soda  and  diluted  muriatic  acid,  but 
are  quickly  dissolved  by  a  mixture  of  the  two  liquids. 
3.  Arsenical  Spots  containing  Antimony ,  and  Antimonial  Spots 
containing  Arsenic. — If  any  arsenical  fluid  containing  antimony 
6 
