44 
PRECIPITATION  OF  VARIOUS  SALTS,  ETC. 
the  sulphates  of  soda  and  potash  are  decomposed  into  insoluble 
chlorides,  and  free  sulphuric  acid  eliminated  in  the  liquid.  This 
decomposition  may  go  very  far ;  thus  muriatic  acid  gas,  when 
passed  until  rejection  into  a  solution  of  sulphate  of  potash,  con- 
verts nearly  70  per  cent,  of  the  latter  into  chloride  of  potassium, 
setting  free  a  corresponding  amount  of  sulphuric  acid. 
The  double  sulphate  of  potash  and  magnesia  undergoes  a  simi- 
lar decomposition.  Sulphate  of  magnesia  does  not  participate 
in  this  reaction,  and,  like  the  chloride  of  magnesium,  it  is  not 
precipitated  by  muriatic  acid  unless  under  particular  conditions 
of  concentration. 
Sulphate  of  soda,  under  the  same  circumstances,  is  decom- 
posed in  a  more  complete  manner,  in  consequence  of  the  greater 
insolubility  of  chloride  of  sodium  in  muriatic  acid.  The  precipi- 
tation of  the  chlorides  of  sodium  and  potassium  in  this  way  ap- 
pears capable  of  industrial  application — 
1.  In  the  preparation  of  a  quality  of  salt  for  special  purposes  ; 
2.  In  the  production  of  crude  salt ;  and 
3.  In  the  separation  of  chloride  of  potassium  from  the  mother- 
liquors  of  salt  marshes. 
The  peculiar  state  of  the  salt  precipitated  by  muriatic  acid,  its 
extreme  division,  its  whiteness  and  brilliancy,  its  perfect  purity, 
and  the  simplicity  of  its  preparation,  render  it  preferable  for  the 
purposes  of  luxury  to  every  kind  of  salt,  whether  obtained  by 
trituration  or  by  evaporation  with  heat.  It  is  obtained  perfectly 
pure  by  operating  upon  common  salt-water,  and  still  better  with 
a  solution  of  common  salt.  The  acid  liquid  is  decanted,  and  the 
precipitate  dried  upon  the  sole  of  a  furnace  heated  in  a  suitable 
manner,  by  which  the  salt  is  purified  from  the  muriatic  acid  with 
which  it  is  impregnated.  Before  drying  the  precipitate,  it  may 
be  washed  with  the  salt  solution,  which  is  afterwards  to  be  sub- 
mitted to  precipitation.  This  washing  removes  nearly  the  whole 
of  the  muriatic  acid.  Lastly,  the  excess  of  acid  may  be  saturated 
with  carbonate  of  soda,  the  cost  of  which  would  not  be  greater 
than  that  of  heating  the  drying  furnace,  and  this  would  allow  it 
to  be  spread  out,  and  dryed  in  the  open  air. 
The  passage  of  the  gas  keeping  the  liquid  in  a  state  of  con- 
tinual agitation  causes  the  precipitation  of  the  salt  in  extremely 
fine  grains  ;  but  it  may  be  obtained  in  large  crystals  when  the 
