DETECTION  OF  ARSENIC. 
351 
was  added,  and  the  liquids  then  tested  in  Marsh's  apparatus, 
but  no  arsenical  stains  could  be  procured ;  whereas  ordinarily 
the  one-hundredth  of  a  grain  of  arsenious  acid  in  three  ounces 
of  dilute  muriatic  or  sulphuric  acid,  affords  characteristic  arseni- 
cal stains  with  the  greatest  facility. 
From  these  results  it  appears  that  Marsh's  process  cannot  be 
relied  upon  to  detect  minute  quantities  of  arsenic  in  the  presence 
of  organic  matter,  that  is,  under  conditions  in  which  it  can  be 
most  satisfactorily  detected  by  Reinsch's  process.    The  plan 
recommended  by  Dr.  Odling  for  extracting  the  arsenic  from  an 
animal  tissue,  in  a  state  suitable  for  testing,  is  as  follows  : — 
About  a  quarter  of  a  pound  of  the  tissue,  cut  or  broken  up,  is 
placed  in  a  beaker  glass  with  an  ounce  and  a  half  of  water,  and 
half  an  ounce  of  muriatic  acid.    The  beaker  with  its  contents, 
covered  by  a  glass  plate,  is  then  stood  upon  a  sand  bath,  and 
maintained  at  an  almost  boiling  temperature  for  half  an  hour. 
There  is  no  fear  of  any  arsenic  being  lost  during  this  digestion, 
for  even  the  first  half  of  the  distillate  from  such  a  liquid  does 
not  carry  off  a  trace  thereof,  unless,  indeed,  a  very  large  pro- 
portion be  present.    The  decoction  when  cold  is  filtered,  and  a 
clear  liquid  obtained,  holding  dissolved  all  the  arsenic  originally 
present,  even  when  a  great  portion  of  it  existed  in  the  state  of 
tersulphide,  from  the  action  of  sulphuretted  products  of  decom- 
position.   This  solution  is  then  in  a  suitable  condition  for  being 
tested  by  Reinsch's  process,  but  to  fit  it  for  Marsh's  apparatus 
it  is  distilled  to  dryness,  or  almost  to  dryness.    There  is  no 
frothing  or  any  other  difficulty  until  the  liquid  has  nearly  all 
passed  over,  when  it  is  necessary  to  be  careful.    To  the  residue 
another  half  ounce  of  strong  muriatic  acid  is  added,  and  the 
distillation  continued.    On  first  applying  heat  after  this  addition, 
there  is  usually  considerable  frothing,  lasting,  however,  for  a 
few  minutes  only,  after  which  the  distillation  can  be  carried  to 
dryness  without  any  difficulty.    In  this  way  a  colorless  acid 
liquid,  well  fitted  for  testing  in  Marsh's  apparatus,  is  obtained, 
and  a  small  caput  mortuum  entirely  free  from  arsenic.  This 
residue  may  be  dissolved  or  oxidized  by  the  usual  methods,  and 
examined  for  other  metals.    The  distillate,  instead  of  being 
tested  in  Marsh's  apparatus,  may  be  treated  with  sulphuretted 
hydrogen  gas,  when  the  yellow  sulphide  of  arsenic,  in  a  well 
