PREPARATION  OF  CALOMEL. 
419 
universally  considered  to  be  the  fuming  principle  of  the  ordinary 
fuming  nitric  acid.  It  is  remarkably  volatile,  and  cannot  well 
be  preserved  at  ordinary  temperatures,  but  may  be  made  use  of 
to  mix  with  the  ordinary  nitric  acid,  so  as  to  convert  it  into  fum- 
ing acid. — London  Chem.  Gaz.  from  Mittheilungen  der  JSaturf. 
Cresellsch.  in  Bern. 
ON  THE  PREPARATION  OF  CALOMEL  IN  THE  HUMID  WAY. 
By  Prof.  Wohler. 
It  has  long  been  known  from  Vogel's  experiments,  that  proto- 
chloride  of  mercury  is  precipitated  from  the  solution  of  the  per- 
chloride  by  sulphurous  acid.  This  behavior  appears  to  me  to  be 
available  in  the  practical  preparation  of  calomel.  It  is  obtained 
in  this  manner  as  a  very  delicate  powder,  of  a  dazzling  white 
color,  which  glitters  in  the  sunlight.  The  difficult  process  of 
sublimation  and  the  tedious  preparation  would  thus  be  avoided ; 
and  its  preparation  in  the  laboratory  would  be  a  very  easy  matter. 
It  would  be  obtained  immediately  in  the  finely  divided  state  in 
which  the  pulverulent  sublimed  calomel  is  produced,  without  any 
necessity  for  an  operation  of  so  much  danger  as  the  preparation 
of  calomel  by  sublimation,  which  moreover  can  only  be  per- 
formed on  a  large  scale.  As  the  calomel  formed  by  sulphurous 
acid  is  crystalline,  and  therefore  in  the  same  condition  as  the 
sublimed,  there  can  also  be  no  doubt  that  it  will  not  differ  from 
this  in  its  medicinal  efficacy.  The  crystals  may  be  distinctly  re- 
cognized, even  with  a  magnifying  power  of  only  100  diameters  ; 
they  are  generally  united,  forming  regular  crosses. 
For  its  preparation  it  is  only  necessary  to  dissolve  commercial 
perchloride  of  mercury  in  water  heated  to  about  122°  F.  until 
this  is  saturated,  and  afterwards  to  pass  sulphurous  acid  gas  into 
the  hot  solution.  The  gas  is  evolved  by  heating  coarse  charcoal 
powder  with  concentrated  sulphuric  acid.  The  separation  of  the 
calomel  commences  immediately.  When  the  solution  is  saturated 
with  gas,  it  is  digested  for  some  time,  then  left  to  get  cold,  and 
filtered  from  the  calomel,  which  is  afterwards  washed.  The  fil- 
trate usually  still  contains  some  unchanged  perchloride,  which 
may  be  converted  into  calomel  either  by  heating  to  boiling,  or  by 
a  fresh  introduction  of  sulphurous  acid  and  heating.    It  still 
