34  Presence  of  Arsentc  in  Chemicals.     {A  January fim™ 
It  is  not  so  very  difficult  to  obtain  sulphuric  acid  comparatively 
free  from  arsenic ;  but  the  first  sample  secured  contained  quite  an 
appreciable  quantity  of  the  arsenic ;  a  second  sample  proved  to  be 
very  good.  For  testing  the  acid,  30  c.c.  were  diluted  to  150  c.c.  and 
poured  through  the  separatory  funnel,  a  little  at  a  time,  so  that  the 
evolution  of  gas  could  be  regulated. 
There  appeared  within  fifteen  minutes  alter  the  hydrogen  current 
became  uniform  a  very  faint  yellowish-brown  spot  in  the  constric- 
tion of  the  tube.  This  did  not  increase  even  after  one  hour  of  con- 
tinuous evolution  of  the  gas.  The  spot  was  probably  due  to  a  mix- 
ture of  sulphur  and  arsenic.  Inasmuch  as  only  12  c.c.  of  the 
sulphuric  acid  were  employed  for  each  subsequent  operation,  and 
the  apparatus  was  tested  in  each  case  for  thirty  minutes  before 
introducing  the  material  to  be  tested,  this  small  coloration  could  be 
entirely  neglected.  As  a  matter  of  fact,  12  c.c.  of  the  concentrated 
sulphuric  acid,  diluted,  did  not  produce  a  coloration  sufficient  to  be 
seen  with  the  naked  eye. 
No  hydrochloric  acid  could  be  obtained  that  was  free  from  arsenic. 
It  was,  however,  prepared  by  distilling  the  purest  obtainable  article 
with  ferrous  chloride,  rejecting  the  first  tenth  that  came  over.  This 
procedure  gave  an  acid  which  was  free  from  arsenic  when  tested 
thoroughly  with  the  three  methods  used. 
The  tin  and  copper  foils,  as  well  as  the  calcium  chloride,  were 
examined  and  found  to  be  free  from  arsenic. 
The  Marsh-Berzelius  apparatus  consisted  of  a  200  c.c.  Erlenmeyer 
flask,  provided  with  a  double  perforated  rubber  stopple,  carrying  a 
50  c.c.  separatory  funnel  and  exit  tube,  which  was  connected  with 
a  straight,  bulbed,  calcium-chloride  tube,  the  latter  being  nearly 
filled  with  pure  anhydrous  calcium  chloride.  At  the  end  of  the  cal- 
cium-chloride tube,  toward  the  Marsh  tube,  there  was  placed  a  wad 
of  cotton  so  as  to  prevent  a  flashing  back  of  the  flame  into  the 
apparatus,  and  thus  avoid  explosions.  The  separatory  funnel  was 
employed  because  the  amounts  to  be  used  could  be  carefully  regu- 
lated and  the  danger  of  introducing  air  was  eliminated. 
The  hard  glass  Marsh  tubes  (free  from  arsenic,  lead  and  anti- 
mony) were  drawn  out  with  two  constrictions  of  the  conventional 
diameter,  and  the  far  end  was  also  drawn  out  fine  and  fused  so  as 
to  leave  only  a  small  orifice.  This  precaution  prevented  any  further 
fusion  of  the  glass  by  the  burning  hydrogen. 
