A  jOTUMyfwos."1'}     Presence  of  Arsenic  in  Chemicals.  33 
practically  impossible  to  obtain  many  articles  at  present  absolutely 
free  from  arsenic. 
There  is  always  more  or  less  trouble  with  the  metallic  zinc  to  be 
employed  in  the  Marsh-Berzelius  method.  The  Joint  Arsenic  Com- 
mittee of  the  Societies  of  Chemical  Industry  and  Public  Analysts1  in 
connection  with  the  preparation  of  standard  mirrors,  states :  "  It  is 
important  to  note  that  some  <  pure '  zinc  is,  from  a  cause  at  present 
unknown,  not  sufficiently  sensitive  ;  that  is  to  say,  the  addition  of 
minute  quantities  of  arsenic  produces  no  mirror."  Various  methods 
have  been  suggested  for  overcoming  this  difficulty ;  for  example^ 
platinum  chloride  is  added;  A.  H.  Allen2  recommends  that  a  trace 
of  iron  be  always  present ;  W.  Thompson 3  uses  copper  sulphate  and 
says  that  nickel  is  better. 
O.  Hehner  4  claims  that  platinum  chloride  makes  the  reaction  less 
delicate.  Headden  and  Sadler5  get  much  lower  results  on  the 
addition  of  copper  sulphate  or  platinum  chloride.  Investigations 
are  needed  along  this  line. 
The  zinc  employed  in  this  work  was  prepared  electrolytically,  and 
a  careful  test  of  the  same  showed  that  it  was  free  from  arsenic,  but 
contained  a  minute  trace  ot  iron,  the  latter  probably  coming  from 
the  vessels  in  which  the  zinc  was  molten  for  granulation.  The  influ- 
ence of  iron  on  arsenic  will  be  discussed  under  the  iron  compounds. 
O.  Hehner6  prepares  arsenic-free  zinc  as  follows:  melt  ordinary 
block  zinc  in  a  clay  crucible,  when  quite  fluid,  add,  for  each  pound 
of  zinc,  about  1  gramme  of  metallic  sodium,  and  stir  well  with  a 
glass  rod.  A  black  scum  forms  immediately.  Remove  scum  from 
time  to  time  as  formed  by  means  of  a  china  spoon  or  crucible  cover 
held  in  tongs.  When  the  metallic  sodium  appears  to  be  oxidized, 
add  more  sodium,  again  stir  vigorously  and  remove  scum  formed. 
The  above  operation  takes  about  ten  minutes.  Now  pour  the 
molten  zinc  into  a  second  clean  clay  crucible  and  treat  with  metallic 
sodium  as  above  ;  finally  allow  the  molten  metal  to  cool  consider- 
ably, then  granulate  in  the  usual  way. 
1 1902,  Jour.  Soc.  Chem.  Ind.y  21,  95. 
2 1902,  Jour.  Soc.  Chem  fnd.,  21,  94. 
3 1902,  British  Food  Jour.,  4,  193  ;  Chem.  News,  86,  179. 
4 1901,  Jour.  Soc.  Chem.  fnd.,  20,  194. 
5  1885,  Am.  Chem.  Jouy.,  7,  338. 
B 1902,  Jour.  Soc.  Chem.  Ind.,  21,  675. 
