ON  CRYSTALLIZED  HYDRATE  OF  BARYTES. 
181 
By  this  means  large  quantities  of  caustic  barytes  can  be  pre- 
pared at  a  cost  not  much  greater  than  that  of  a  common  salt  of 
barytes,  and  this  substance  may  therefore  be  applied  for  purposes 
for  which  it  was  previously  inapplicable  on  account  of  its  high 
price.  * 
I  may  be  permited  here  to  allude  to  the  preparation  of  caustic 
potash  for  mineral  analysis.  It  is  very  difficult  to  obtain  caustic 
potash  from  the  carbonate,  so  as  to  be  free  from  silica,  chlorine, 
and  sulphuric  acid,  but  sulphate  of  potash  being  free  from  silica 
and  chlorine,  and  one  of  the  purest  of  the  salts  of  potash,  it  is 
very  easy  to  prepare  the  caustic  alkali  from  the  sulphate  by 
means  of  caustic  barytes.  If  the  barytes  has  been  made  from 
the  nitrate,  it  will  contain  no  chlorine ;  silica  cannot  be  present, 
and  sulphuric  acid  will  be  completely  precipitated. 
Common  caustic  potash,  which  has  been  prepared  with  car- 
bonate of  potash  and  lime,  can  be  deprived  of  the  last  traces  of 
carbonic  and  sulphuric  acids,  by  adding  the  requisite  quantity  of 
caustic  barytes  and  leaving  the  mixture  for  the  precipitates  to 
subside.  This  cannot  be  affected  with  caustic  lime,  as  that  sub- 
stance is  too  little  soluble  in  water,  and  is,  moreover,  incapable 
of  decomposing  sulphate  of  potash. 
A  solution  of  caustic  barytes  is  used  for  absorbing  carbonic 
acid  from  gases  ;  but  in  this  case,  as  the  absorption  proceeds,  the 
solution  becomes  progressively  weaker.  I  have  suggested  the 
use  of  a  mixture  of  caustic  potash  and  barytes  for  the  same  pur- 
pose, with  complete  success.  The  caustic  potash  absorbs  the 
carbonic  acid,  and  the  barytes  precipitates  it.  In  this  way  the 
absorbing  liquid — the  potash — is  kept  always  of  the  same  strength 
and  completely  caustic,  while  the  carbonic  acid,  as  it  is  absorbed, 
is  eliminated  by  precipitation. 
Carbonic  acid  is  more  difficultly  absorbed  in  proportion  to  the 
extent  of  its  admixture  with  permanent  gases  which  are  not  sub- 
ject to  absorption.  This  result  is  experienced  in  the  analysis  of 
atmospheric  air,  where  there  are  only  four  or  live  volumes  of 
carbonic  acid  in  ten  thousand  volumes  of  the  air.  Hence  the 
determination  of  the  quantity  of  carbonic  acid  in  air  has  been 
attended  with  much  uncertainty  and  difficulty.  The  simple  ab- 
sorption by  solid  substances  in  tubes,  and  the  determination  of 
the  increment  of  weight,  as  proposed  by  Brunner  and  other 
