62 
MANUFACTURE  OF  ARSENIC  ACID. 
terior,  but  is  black  within.  The  "  serpent  "  shape  of  course  re- 
sults from  the  salt  being  burnt  in  a  cone  of  tin-foil. 
Both  the  mercurous  and  mercuric  sulphocyanides  decompose 
in  the  same  manner ;  but  the  mercuric  salt,  containing  more  sul- 
phocyanogen,  seems  capable  of  furnishing  a  larger  quantity  of 
mellon,  and  is  the  one  used  in  the  French  serpents.  A  solution 
of  pernitrate  of  mercury  is  readily  precipitated  by  sulphocyanide 
of  ammonium,  and  the  mercuric  sulphocyanide  may  be  easily  so 
prepared.  It  is  best  to  use  the  mercurial  solution  as  strong  as 
possible,  and  to  keep  it  in  excess  throughout  the  precipitation. 
Solution  of  perchloride  of  mercury  is  not  so  easily  precipitated 
as  the  pernitrate,  probably  owing  to  the  solubility  of  the  mercuric 
sulphocyanide  in  the  chlorides. 
Perhaps  I  may  be  excused  for  adding  that  sulphocyanide  of 
ammonium,  suitable  for  the  above  purpose,  may  be  very  easily 
and  economically  prepared,  as  follows  : — One  volume  of  bisul- 
phide of  carbon,  four  volumes  of  liq.  ammon.  fort.,  and  four  vol- 
umes of  methylated  spirit  are  put  into  a  large  bottle,  and  the  mix- 
ture frequently  shaken.  In  the  course  of  one  or  two  hours  the 
sulphide  of  carbon  will  have  entirely  dissolved  in  the  ammoniacal 
liquid,  forming  a  deep  red  solution.  When  this  result  is  attained, 
the  liquid  is  boiled  until  the  red  color  disappears  and  is  replaced 
by  light  yellow.  The  solution  is  then  evaporated  at  a  very  gentle 
heat  (about  80°  or  90°  F.)  until  it  crystallizes,  or  just  to  dryness. 
The  product  is  sulphocyanide  of  ammonium,  sufficiently  pure  for 
the  above  purpose.  One  crystallization  from  alcohol  will  render 
it  quite  white. 
One  ounce  of  bisulphide  of  carbon  yields,  by  this  process,  ex- 
actly one  ounce  of  sulphocyanide  of  ammonium. 
I  am,  etc., 
C.  H.  Wood,  F  C.  S. 
— London  Pliarm.  Journ.,  Oct.  1,  1865. 
MANUFACTURE  OF  ARSENIC  ACID. 
Grirardin  suspends  powdered  arsenious  acid  in  water,  and 
passes  chlorine  into  the  mixture,  by  which  he  soon  obtains  a 
clear  solution  of  arsenic  acid  in  hydrochloric  acid.    By  evapo- 
