Am'MiTi9Pi5arm'}        Volumetric  Solutions  of  US.P.  207 
volumetric  solutions  on  this  method,  we  would  obtain  not  only  con- 
venience and  general  ease  of  manipulation  but  also  a  very  high 
degree  of  accuracy. 
As  to  the  comparatively  great  ease  of  obtaining  pure  metallic 
silver  we  need  only  mention  the  following  facts.  In  his  revision  of 
Stas'  atomic  weight  for  chlorine,  one  of  the  samples  of  silver  which 
Richards  18  used  was  sent  to  him  from  the  Colorado  Smelting 
Works  at  Argo.  Colorado.  This  specimen  was  stated  to  be  the 
product  of  the  treatment  of  ores  in  which  it  occurs  associated  with 
quartz,  barite,  calcite,  pyrite,  sphalerite,  with  more  or  less  copper 
in  the  form  of  chalcopyrite,  together  with  small  amounts  of  arsenic, 
antimony,  lead,  bismuth,  and  tellurium.  To  recover  the  silver,  the  ore 
is  roasted,  mixed  with  other  ores  which  are  chiefly  siliceous,  and  the 
mixture  is  so  arranged  that  when  smelted  it  yields  a  slag,  containing 
about  40  per  cent,  of  silica,  and  a  first  matte  of  fusible  sulphide, 
which  assays  about  40  per  cent,  of  copper,  10  per  cent,  of  lead,  and 
400  ounces  of  silver  and  6  ounces  of  gold  per  ton.  The  next  stage 
in  the  process  includes  the  roasting  and  concentration  of  the  ore- 
metal,  or  first  matte,  to  "  white  metal  "  containing  about  60  per 
cent,  of  copper.  The  silver  is  then  extracted  from  this  white  metal 
by  the  following  operations :  Rough  roasting,  fine  grinding,  fine 
roasting  for  sulphate  of  silver  by  Ziervogel's  process,  leaching  and 
the  precipitation  of  the  silver  on  plates  of  copper.  In  the  precipi- 
tation of  the  silver  a  certain  amount  of  copper  is  found  mixed  with 
the  silver  in  the  form  of  cuprous  oxide  and  of  small  scales  and 
scraps  of  metallic  copper.  This  copper  is  removed  by  prolonged 
boiling  with  water  containing  a  small  quantity  of  sulphuric  acid, 
into  which  air  is  injected  by  means  of  a  small  jet  of  steam.  Sulphate 
of  copper  is  thus  formed,  which  is  carefully  washed  out  of  the  silver. 
The  silver  is  then  dried  and  melted  into  bars  of  an  average  fineness 
of  99.9  per  cent.  The  specimen  of  silver  actually  obtained  by 
Richards,  however,  the  latter  found  to  be  even  purer  than  is 
indicated  by  the  above  figures.  We  thus  see  that  although  starting 
with  a  very  complex  natural  mixture,  containing  many  different 
elements,  only  a  very  small  fraction  of  which  is  the  desired  silver, 
this  metal  is  actually  obtained  on  a  large  commercial  scale  of  a 
degree  of  purity  almost  approaching  absolute  purity.  Can  this  be 
said  of  any  of  the  other  numerous  substances  which  have  been 
proposed  as  standards  in  volumetric  analysis? 
lour.  Amer.  Chain.  Soc,  27,  pp.  474-475  (1905). 
