148    Action  of  Sulphuric  Acid  upon  Woo  d-C liar  coal.  { AmMJa°rc£  im rm" 
If  the  samples  examined  can  be  taken  as  similar  to  the  glycerin 
generally  met  with,  it  would  appear  that  in  regard  to  the  amount  of 
arsenic  there  has  been  a  great  improvement  since  1890,  when  Mr. 
Siebold's  paper  was  published.  He  then  found  1  part  in  6,000  or 
4,000,  and  even  more,  whereas  the  arsenic  in  samples  Nos.  2  and  8 
amounts  to  only  one  part  in  one  hundred  thousand,  and  that  in  the 
samples  Nos.  6  and  7  to  only  one  part  in  a  million. 
The  circumstance  that  zinc  frequently  contains  a  trace  of  sulphur 
which  would  be  eliminated,  by  the  action  of  acid,  in  the  state  of 
sulphuretted  hydrogen,  may  exercise  a  disturbing  influence  in  the 
application  of  the  test.  In  such  a  case  the  quantity  of  sulphuretted 
hydrogen  will  generally  be  so  small  that  it  does  not  produce  a  black 
spot  on  the  filter  paper  impregnated  with  mercuric  chloride,  but  a 
spot  having  almost  the  same  color  as  that  produced  by  arsenietted 
hydrogen.  In  this  way  a  sample  of  glycerin  that  is  perfectly  free 
from  arsenic  may  appear  to  be  contaminated. 
For  this  reason  it  is  always  advisable  to  repeat  the  experiment, 
adding  to  the  mixture  before  the  zinc  is  put  in,  some  starch  mucil- 
age and  a  small  quantity  of  iodine  solution  until  a  blue  tinge  is  pro- 
duced. In  this  way  the  formation  of  sulphuretted  hydrogen  is  coun- 
teracted. Mr.  Siebold1  has,  however,  pointed  out  that  the  use  of 
iodine  for  this  purpose  requires  caution,  for  if  too  much  iodine  is 
added  for  the  destruction  of  sulphur  compounds  it  interferes  with 
the  reaction,  and  may  lead  to  erroneous  conclusions. 
In  every  instance  it  is,  of  course,  desirable  in  testing  glycerin  for 
arsenic  to  make  a  blank  experiment,  so  as  to  be  absolutely  certain 
that  neither  the  hydrochloric  acid  nor  the  zinc  employed  contains 
any  trace  of  arsenic. 
ACTION  OF  SULPHURIC  ACID  UPON  WOOD- 
CHARCOAL.2 
By  A.  Verneuil. 
The  secondary  reactions  which  accompany  the  formation  of  sul- 
phurous and  carbonic  anhydrides  in  the  reaction  of  sulphuric  acid 
1  Pharm.Journ.,  [3],  xx,  p.  682. 
2  Comptes  Rendus,  118,  195,  through  Chemical  News. 
