172       USE  OF  PLATINUM  INSTEAD  OF  PORCELAIN  OR  GLASS. 
ON  THE  USE  OF  PLATINUM  INSTEAD  OF  PORCELAIN  OR  GLASS 
IN  MARSH'S  METHOD  OF  DETECTING  ARSENIC. 
By  Edmund  Dayt,  F.R.S.,  M.R.I.A..  F.C.S.L.,  &c. 
Professor  of  Agricultural  Chemistry  to  the  Royal  Dublin  Society. 
Marsh's  method,  as  is  well  known,  is  founded  on  the  facility 
with  which  hydrogen  gas,  generated  in  the  usual  way,,  reduces 
arsenious  or  arsenic  acid,  if  present,  forming  arseniuretted  hy- 
drogen, which  being  inflamed  in  contact  with  air,  deposits  on 
particular  surfaces  the  arsenic,  either  in  the  state  of  metal  or 
oxide,  or  a  mixture  of  both,  according  to  circumstances. 
Porcelain  or  glass  surfaces  have  been  generally  recommended 
by  chemists.  I  have  seen  no  mention  ma'de  of  platinum.  On 
lately  trying  surfaces  of  this  metal,  it  appeared  to  me  to  offer 
peculiar  advantages,  and  to  be  decidedly  superior  to  either  por- 
celain or  glass.  In  my  first  experiment  I  used  a  slip  of  thin 
platinum  foil,  but  there  was  no  deposition  on  it ;  the  flame, 
though  small,  produced  sufficient  heat  to  carry  off  all  the  arsenic 
as  arsenious  acid  ;  but  on  trying  the  blade  of  a  platinum  spatula, 
a  spot  of  arsenic  was  at  once  deposited  on  its  surface  (closely 
resembling  that  produced  by  my  electro-chemical  method  of  de- 
tecting arsenic  by  the  agency  of  zinc  and  muriatic  acid  on 
platinum.)  A  number  of  similar  spots  may  be  produced  at 
pleasure,  by  holding  the  spatula  for  successive  instants  very  near 
the  orifice  of  the  tube  delivering  the  gas,  the  flame  being  small ; 
then  the  spots  of  arsenic  appear  to  be  in  the  metallic  state,  or 
only  partially  oxidated,  and  they  adhere  strongly  to  the  spatula; 
but  if  the  spatula  be  held  a  little  further  from  the  orifice,  then 
only  arsenious  acid  will  be  deposited  on  it.  The  heat  of  a  candle 
is  sufficient  to  volatilise  the  arsenic  as  arsenious  acid  from  the 
spatula,  and  this  effect  is  accompanied  by  the  alliaceous  odor, 
even  when  the  quantity  of  arsenic  is  less  than  one  thousandth 
part  of  a  grain. 
In  cases  when  antimony  is  present  in  solution  under  similar 
circumstances,  antimoniuretted  hydrogen  is  produced,  the  flame 
of  which  occasions  the  depositions  of  blackish  spots  of  antimony, 
which  are  not  removed  by  the  flame  of  a  candle. 
The  advantages  of  the  platinum  surface  over  porcelain  or 
glass  appear  to  be  pretty  obvious.  The  former  is  more  manageable 
for  subsequent  experiments.    The  arsenic  strongly  adheres  to 
