ON  THE  ADULTERATION  OF  TIN  FOIL. 
259 
would  be  gained.  On  the  contrary,  such  an  alloy  is  more  easily 
acted  upon  than  either  of  its  components  would  be  alone.  Some 
time  back  I  noticed  on  some  foil,  which  had  been  in  contact  for 
some  time  with  a  packet  of  acid  paper  for  Seidlitz  powders,  evi- 
dence of  corrosion  or  chemical  action,  and  the  presence  of  lead 
was  shown  by  the  resulting  compound  blackening  in  the  pre- 
sence of  HS. 
For  simply  detecting  the  presence  of  the  lead,  I  found  the 
readiest  method  to  consist  in  oxidizing  the  foil  with  nitric  acid, 
and  to  dissolve  out  the  nitrate  of  lead  from  the  insoluble  per- 
oxide of  tin  with  hot  water.  Crystals  of  the  salt  were  generally 
deposited  from  the  solution  on  standing.  For  the  quantitative 
estimation  of  the  two  metals,  I  have  tried  several  processes,  with 
varied  degrees  of  success. 
First,  dry  chlorine  gas  was  passed  over  the  foil,  heated  in  a 
bulb  tube,  when  the  volatile  bichloride  of  tin  distilled  over,  leav- 
ing the  fixed  chloride  of  lead  in  the  bulb.  This  process  was  not, 
however,  adopted,  for  two  reasons:  1st,  because  some  difficulty 
was  experienced  in  condensing  the  whole  of  the  distillate;  2dly, 
and  most  important,  that  some  of  the  foil  at  the  termination  of 
the  process  fused  and  escaped  the  action  of  the  gas. 
Secondly,  the  foil  was  treated  with  nitric  acid,  sp,  gr.  1-37, 
whereby  the  tin  became  converted  into  the  insoluble  binoxide, 
and  the  lead  into  nitrate;  the  latter  was  dissolved  out  by  water, 
the  lead  precipitated  as  sulphate,  which  was  washed,  dried,  and 
calcined,  and  from  which  was  calculated  the  amount  of  the 
metallic  lead ;  the  binoxide  of  tin  was  also  washed,  dried,  and 
calcined,  and  the  amount  of  metallic  tin  calculated  from  it;  this 
process  is  simple,  expeditious,  and  appears  to  give  very  good 
results. 
Thirdly,  the  foil  was  treated  with  nitric  acid,  and  evaporated 
nearly  to  dryness,  with  a  small  excess  of  sulphuric  acid,  where- 
by the  two  metals  were  converted  into  sulphates ;  the  nearly  dry 
residue  was  then  washed  with  water,  and  digested  repeatedly, 
with  solution  of  acetate  of  ammonia  of  sp.  gr.  1-06  or  above, 
which  dissolved  out  the  whole  of  the  sulphate  of  lead.  Sulphide 
of  ammonium  was  added  to  the  solution,  and  the  precipitated 
sulphide  of  lead  washed,  and  reconverted  into  sulphate  by  heat- 
ing with  a  mixture  of  nitric  and  sulphuric  acids,  and  from  this 
