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876  VARIETIES. 
On  a  new  Method  of  obtaining  Carbonate  of  Potash  from  Felspar  and 
similar  Minerals.  By  Dr.  E.  Meyer. — The  author's  process  consists  essen- 
tially in  decomposing  the  mineral  by  calcination  with  lime,  and  then  treat- 
ing it  with  water  under  a  pressure  of  7  to  8  atmospheres.  With  felspar  14 
to  19  equivs.  of  lime  are  used  to  1  equiv.  of  felspar,  or  to  100  parts  of  fel- 
spar 139  to  188  parts  of  lime. 
The  lime  is  employed  either  as  hydrate  or  in  the  form  of  chalk ;  it  is 
intimately  mixed  with  the  felspar  to  a  plastic  mass,  which  is  made  into 
round  balls  of  3  to  4  inches  in  diameter,  slowly  dried,  and  then  exposed  to 
a  temperature  between  a  bright  red  and  a  white  heat.  The  temperature 
must  be  so  high,  that  the  mass,  after  burning,  may  contain  neither  carbo- 
nate of  lime  nor  uncombined  caustic  lime.  It  should  therefore  exhibit  a 
very  inconsiderable  elevation  of  temperature  with  water.  It  is'  usually 
caked  together.  Of  course,  for  such  a  decomposition  a  very  intimate  mix- 
ture of  the  felspar  and  lime  is  requisite.  The  more  lime  employed,  the 
shorter  the  time  necessary.  After  burning,  the  mass  is  powdered  and 
heated  with  water  in  a  vessel  capable  of  bearing  a  pressure  of  8  atmos- 
pheres, in  which  the  decomposition  is  completed  in  2  to  4  hours.  The  solu- 
tion above  the  powder  (which  is  never  firmly  solidified,  as  the  formation  of 
steam  probably  prevents  cohesion)  is  caustic  to  the  touch,  is  free  from  hy- 
drate of  lime,  and  always  contains  all  the  soda,  and  potash  to  the  amount 
of  about  9  to  11  per  cent,  of  the  weight  of  the  felspar  employed. 
A  second  extraction  of  the  powder  freed  from  the  solution  of  potash  is 
of  no  great  use  ;  little  potash,  but  plenty  of  lime  is  dissolved  ;  the  latter 
cannot  be  taken  up  by  the  solution  in  the  first  instance.  It  is  of  no  great 
advantage  to  continue  the  extraction  longer  than  4  hours. 
If  the  alkaline  solution,  after  saturation  with  carbonic  acid,  be  evapo- 
rated to  dryness,  a  little  alumina  and  silica  separate  first  of  all  ;  the  car- 
bonate of  soda  then  crystallizes,  and  at  last  carbonate  of  potash  remains, 
which,  when  pure  minerals  are  employed,  is  perfectly  free  from  other 
acids. 
As  regards  the  mass  remaining  insoluble  in  water,  the  very  intimate 
mixture  of  its  constituents  renders  it  peculiarly  suitable  for  the  prepara- 
tion of  a  Portland  cement,  the  composition  of  which  varies  within  the  same 
limits.  These  cements,  however,  sometimes  contain  more  alumina.  This 
want  of  alumina,  if  it  be  a  defect  at  all,  is  easily  supplied  by  the  addition 
of  a  little  clay,  with  which  the  residue  need  only  be  mixed.  The  author 
has  found,  however,  that  the  powder  taken  out  of  the  kettle,  and  again  * 
strongly  calcined,  sets  very  rapidly  and  firmly  under  water,  so  that  the 
addition  of  clay  is  unnecessary. 
As  a  matter  of  course  this  mode  of  preparation  will  not  be  applied  ex- 
clusively to  pure  felspar,  as  other  felspar  or  minerals  containing  potash, 
must  also  be  adapted  for  this  purpose.  Thus,  for  example,  there  are  many 
granites  which  contain  about  7  per  cent,  of  potash^  and  from  which  the 
