78        Preparation  of  Biniodide  of  Mercury,  etc.  {AFeJb°ui,'  K"1 
boiling  water  is  unnecessary,  as  the  quantity  ordered  would,  if  cold, 
dissolve  the  salts  readily.  The  precipitate  from  a  hot  solution  is  more 
granular  than  that  from  one  which  is  cold,  and  for  the  preparation  of 
ointments  it  will  be  conceded  that  the  finer  and  softer  salt  is  to  be- 
preferred. 
In  the  process  of  the  U.  S.  Pharmacopoeia  the  excess  of  iodide  is 
the  same  as  that  of  the  B.  P.,  but  cold  water  is  applied  for  solution. 
The  most  satisfactory  and  economical  results  I  have  obtained  from 
the  decomposition  of  the  salts  alluded  to,  have  been  by  employing  4 
parts  of  the  mercuric  salt,  in  powder,  dissolved  in  64  parts  of  cold 
water,  adding  a  sufficient  quantity  of  solution  of  iodide  of  potassium 
(4*9  parts  in  10  parts  of  water).  The  yield  will  be  6 '7  parts  of  a 
salt  sufficiently  dry  to  be  pulverulent.  This  is  very  little  short  of  the 
theoretical  yield  ;  271  parts  HgCl2  require  for  decomposition  332 
parts  KI,  and  should  produce  454  parts  (6*701)  Hgl2. 
Another  method  which  may  be  easily  and  economically  pursued  is. 
that  in  which  iodide  of  iron  is  used  instead  of  iodide  of  potassium. 
The  relative  prices  of  iodine  and  iodide  of  potassium  are  generally  as. 
21  to  1(<>,  and  254  parts  iodine  are  equal  to  332  of  iodide  of  potas- 
sium. If,  therefore,  we  mix  3*75  parts  of  iodine  with  4  parts  of  cold 
water  and  sufficient  iron  wire  to  saturate,  allowing  the  mixture  to 
stand  for  several  hours,  heating  towards  the  close  of  the  reaction,  we 
shall  obtain  a  solution  of  iodide  of  iron  equivalent  in  iodine  strength 
to  the  quantity  of  iodide  of  potassium  required  to  decompore  4  parts 
of  perchloride  of  mercury.  The  yield  will  be  about  6*7  parts,  and 
the  quality  of  the  product  is  equal  to  that  produced  in  any  other  way, 
but  the  precautions  of  using  the  iron  solution  as  soon  as  possible,  and 
of  washing  the  precipitate  as  soon  as  deposited,  must  be  observed ;.. 
otherwise  the  product  might  be  contaminated  with  a  basic  ferric  chlo- 
ride, which  in  time  is  thrown  down.  It  will  be  seen  that  this  method 
is  as  economical  as  could  well  be  devised ;  the  operator  getting  the 
profits  of  the  manufacturer  of  iodide  of  potassium,  minus  the  labor 
of  the  former  in  making  the  iodide  of  iron. 
The  last  process  which  we  shall  notice  is  that  devised  by  Mr.  Wil- 
liams, described  in  the  "  Chicago  Pharmacist."  In  this,  the  use  of  a 
large  quantity  of  water,  for  the  solution  of  the  mercuric  salt,  is  obvi- 
ated by  employing  a  concentrated  solution  of  chloride  of  ammonium,, 
in  which  the  mercuric  salt  dissolves  readily.  Four  parts  of  perchlo- 
ride of  mercury  are  dissolved  in  four  parts  of  water  to  which  2  parts. 
