IMPURITIES  OF  COMMERCIAL  ZINC. 
405 
in  1817,  most  probably  confounded  the  sulphides  of  these 
metals,  the  latter  of  which  being  a  common  admixture.  The 
delicate  indications  of  Marsh's  apparatus,  which  so  easily  detects 
arsenic  in  zinc,  when  present,  added  to  this  belief  before  atten- 
tion was  turned  to  the  contamination  of  acids  by  the  same 
metal,  and  there  is  reason  to  believe,  at  times,  that  this  has  been 
the  source  of  the  metal  when  it  has  been  attributed  to  the  zinc. 
"  The  conclusions  at  which  we  have  arrived,  after  a  long  course  of 
experiments  with  many  different  zincs,  and  various  acids,  are  these  : — 
first,  that  much  of  the  zinc  of  commerce  is  free  from  arsenic,  or  at  least 
contains  no  arsenic  that  can  be  detected  by  the  most  delicate  tests  for  that 
metal ;  secondly,  that  the  sulphuric  and  chlorohydric  acids  found  in  com- 
merce do  very  often  contain  arsenic,  and  are  always  so  liable  to  contain 
it  as  to  be  utterly  unfit  for  use  in  Marsh's  process  without  special  purifi- 
cation for  that  purpose.  The  steps  by  which  we  are  led  to  these  results, 
and  the  evidence  on  which  they  are  founded,  we  proceed  to  describe.  We 
have  used  exclusively  Marsh's  process  for  the  detection  of  arsenic,  applied 
with  the  apparatus  and  with  all  the  precautions  recommended  by  Otto.* 
Our  apparatus  consisted  of  a  flask  provided  with  a  funnel  tube,  and  a  tube 
bent  at  right  angles,  with  which  were  connected  by  connectors  of  sheet 
india  rubber,  first,  a  tube  of  the  form  of  a  chloride  of  calcium  tube,  filled 
with  asbestos  ;  secondly,  a  similar  tube  filled  with  pumice  stone  soaked  in 
caustic  potassa  ;  and  thirdly,  one  filled  with  chloride  of  calcium.  Through 
these  three  tubes,  in  the  order  in  which  they  are  named,  the  gas  generated 
in  the  flask  was  obliged  to  pass  before  it  arrived  at  the  reduction  tube, 
which  was  of  hard  German  glass,  and  about  one  centimetre  in  diameter. 
The  reduction  tube  was  drawn  down  to  a  fine  bore,  and  during  the  pro- 
gress of  an  experiment  was.  heated  by  one  of  Buns.en's  triple  gas-burners. 
To  prevent  any  elevation  of  the  temperature  in  the  flask  during  an  experi- 
ment, it  was  immersed  in  cold  water,  and  the  dilute  acid  used  was  always 
cold  and  added  in  small  quantities.  With  this  apparatus,  (which  for  con- 
venience we  shall  designate  as  Otto's  apparatus),  taking  every  possible 
precaution  to  insure  its  perfect  cleanness,  we  made  several  experiments 
upon  Silesian  zinc.  Two  hundred  grammes  of  this  spelter,,  carefully 
granulated,  were  used  in  each  experiment,  and  the  sulphuric  acid  employ- 
ed was  a  commercial  acid  made  in  this  country  from  Sicily  sulphur.  We 
Were  not  unaware  of  the  fact  that  arsenic  is  almost  invariably  found  in  the 
foreign  sulphuric  acid  made  from  various  impure  sulphurs  of  unknown 
origin,  or  from  pyrites  ;  but  it  is  a  common  impression  that  the  American 
acid  manufactured  directly  from  Sicily  sulphur  is  free  from  arsenic. 
Positive  statements  to  this  effect  have  been  made  by  chemists  who  have 
had  mainly  in  view  the  common  use  of  sulphuric  acid  in  the  preparation 
of  chemical  compounds  used  in  Pharmacy,  and  the  assertion  has  enough 
*  Manuel  of  the  Detection  of  Poisons.    New  York,  Baillierc,  1857. 
