388  Report  of  Research  Committee  B.  {AmA^!iPmlm' 
Pharmacopoeia,  the  greatest  intensity  of  color  obtained  after  an 
hour's  reaction  occurred  when  I  cc.  of  the  arsenical  solution  was 
mixed  with  2-5  cc.  of  the  reagent.  Between  3  cc.  and  2  cc.  of  the 
German  solution  hardly  any  difference  could  be  found ;  but  with 
the  pale  U.  S.  P.  solution  a  very  slight  difference  appeared  in  favor 
of  2  cc.  as  against  3  cc.  Heat  did  not  appear  to  affect  the  propor- 
tion needed. 
When  metallic  tin  was  added  it  seemed  to  be  best  to  use  equal 
volumes  of  specimen  and  reagent,  although  the  variable  amount  of 
dark  coatings  of  reduced  arsenic  adhering  to  the  tin  prevented  an 
accurate  comparison. 
3-  INFLUENCE  OF  THE  USE  OF  METALLIC  TIN  TOGETHER  WITH 
STANNOUS  CHLORID. 
When  metallic  tin  is  heated  with  pure  concentrated  hydrochloric 
acid,  a  copious  evolution  of  hydrogen  results.  When  heated  with 
the  saturated  solution  of  stannous  chlorid  in  hydrochloric  acid,  the 
evolution  of  hydrogen  is  but  scant.  When  arsenic  is  mixed  with 
the  solution,  the  evolution  of  gas  is  perceptible,  but  very  feeble. 
Very  little  arsin  appears  to  be  evolved,  for  a  paper  cap  with  a  drop 
-of  acidulated  silver  nitrate  does  not  show  any  evidence  of  reduction 
by  AsH3  for  over  fifteen  minutes,  and  even  at  the  end  of  an  hour 
but  a  very  faint  arsenic  reaction  is  seen.  To  ascertain  whether  any 
of  the  arsin  formed,  would  react  with  the  stannous  chlorid  and  thus 
hasten  the  reduction  of  As,  I  arranged  a  small  apparatus  in  which 
a  copious  current  of  arsin,  mixed  with  hydrogen,  was  generated  and, 
after  passing  through  a  bottle  filled  with  dry  cotton,  was  permitted 
to  bubble  through  stannous  chlorid  solution  of  the  U.  S.  P.  After 
more  than  an  hour's  time,  not  a  trace  of  coloration  could  be 
detected. 
So  that  after  arsin  has  once  been  formed,  it  is  not  again  decom- 
,  posed  by  stannous  chlorid.    Hence  the  prompt  action  of  metallic 
tin  must  depend  upon  the  nascent  hydrogen,  which  aids  the 
stannous  chlorid  in  reducing  the  trioxid  and  pentoxid. 
But  however  satisfactory  the  action  of  metallic  tin  in  accelerating 
and  intensifying  the  reduction  of  arsenic,  it  cannot  be  employed  to 
detect  arsenic  in  preparations  of  bismuth  or  antimony ;  for  the 
-  metallic  tin  reduces  both  of  those  metals  and  precipitates  them  from 
their  solution  as  black  flocculi  which  aggregate  into  small  granular 
