406 
IMPURITIES  OF  COMMERCIAL  ZINC. 
plausibility  to  command  ready  and  general  belief.    Using  such  acid  and 
two  hundred  grammes  of  Silesian  zinc,  we  obtained  at  the  end  of  an  hour, 
during  which  the  reduction  tube  was  heated,  a  deposit  of  arsenic  perfectly 
distinct,  though  not  bright  enough  to  be  called  a  mirror.    Our  next  experi- 
ment was  made  with  the  same  acid  upon  two  hundred  grammes  of  a 
sample  of  Vieille  Montagne  zinc,  perfectly  clean  and  carefully  granulated. 
At  the  end  of  the  hour  during  which  the  gas  was  passed  through  the  re- 
duction tube,  a  brownish,  volatile  coating  was  found  in  that  part  of  the 
tube  where  the  bore  was  smallest.    These  experiments  on  Silesian  and 
Vieille  Montagne  zinc  were  several  times  repeated  and  always  with  the 
same  result ;  the  deposit  in  the  reduction  tube  was  often  too  thin  and 
slight  to  be  positively  identified  as  arsenic  ;  but  it  could  not  be  distinguish- 
ed from  the  deposit  of  that  metal  and  would  have  been  perfectly  fatal  in 
a  medico-legal  investigation,  or  in  any  case  in  which  absolute  purity  of  the 
materials  was  desired.    Not  convinced  that  the  zincs  were  the  source  of 
the  arsenic,  we  desired  to  prepare  a  quantity  of  sulphuric  acid  in  which 
the  presence  of  arsenic  could  not  possibly  be  suspected.    To  attain  this 
object  we  subjected  a  specimen  of  American  sulphuric  acid  to  the  follow- 
ing process  :    The  acid  was  first  boiled  with  a  little  flowers  of  sulphur, 
as  proposed  by  Barreul,  in  order  to  free  it  from  the  nitrous  fumes  which 
the  common  sulphuric  acid  almost  always  contains;  a  small  quantity  of 
pure  chlorohydric  acid  was  then  stirred  into  the  cooled  acid,  which  had 
been  carefully  decanted  from  the  free  sulphur,  and  the  whole  again  boiled  ; 
to  the  acid  again  cooled,  a  second  addition  of  chlorohydric  acid  was  made, 
and  again  the  acid  was  heated  till  dense  white  fumes  had  been  escaping 
for  upwards  of  half  an  hour.     During  this  process  the  volatile  chloride  of 
arsenic  is  completely  driven  off,  the  second  addition  of  chlorohydric  acid 
being  made,  as  has  been  recommended  by  H.  Rose,  in  order  to  insure  this 
result.    Lastly,  a  portion  of  chlorine-water  was  added  to  the  cooled  acid  to 
oxidize  any  sulphurous  acid  which  might  be  contained  in  it  ;  and  after  a 
third  boiling,  the  acid,  cooled  and  diluted  with  three  parts  of  water,  was 
ready  for  use.    This  method  of  purifying  sulphuric  acid  is  a  combination 
and  modification  of  several  well  known  processes."    "  With  the  acid  thus 
prepared,  we  tested  two  hundred  grammes  of  Vieille  Montagne  zinc,  and 
after  passing,  during  more  than  an  hour,  a  continuous  gentle  stream  of 
gas  through  a  reduction  tube,  of  which  about  four  centimetres  were  main- 
tained at  a  bright  red  heat,  we  found  that  there  was  absolutely  no  deposit 
whatever  in  the  cool  and  narrow  part  of  the  reduction  tube.    With  the 
same  acid  and  apparatus,  two  hundred  grammes  of  Pennsylvania  zinc 
(which  has  proved  to  be  altogether  the  purest  zinc  in  our  possession)  gave 
absolutely  no  deposit  of  any  kind  in  the  fine  reduction  tube  at  the  end  of 
one  hour,  the  time  during  which,  in  all  our  examinations  for  arsenic,  we 
maintained  a  steady  flow  of  hydrogen  through  the  red  hot  reduction  tube." 
"  In  order  to  satisfactorily  establish  these  conclusions  it  was  necessary  to 
prove  by  frequent  repetition,  that  the  same  result  might  always  be  expect- 
