182 
ON  CRYSTALLIZED  HYDRATE  OF  BARYTES. 
chemists,  has  given  most  erroneous  results.  In  that  process  the 
air  must  be  completely  dried  by  sulphuric  acid  or  chloride  of  cal- 
cium. Now,  it  has  been  found  that  sulphuric  acid  absorbs  no 
inconsiderable  quantity  of  carbonic  acid,  which  will  not  be  got 
in  the  tube  in  which  that  body  is  proposed  to  be  estimated. 
Then  chloride  of  calcium  is  decomposed  by  the  ozone  of  the  at- 
mospheric air,  and  loses  an  atom  of  chlorine  for  every  atom  of 
active  oxygen  or  ozone.  The  chlorine  thus  disengaged  is  ab- 
sorbed by  the  caustic  potash  or  lime,  and  causes  an  increment  of 
weight,  which  would  be  erroneously  ascribed  to  carbonic  acid. 
Thus,  in  some  cases  in  which  the  air  has  been  very  dry,  the 
chloride  of  calcium  has  lost  rather  than  gained  weight  by  the 
passage  of  the  air  through  it. 
It  has,  therefore,  been  found  necessary  to  recur  to  the  old 
process  of  Saussure.  But  the  trouble  of  exhausting  large  balloons 
by  an  air-pump,  the  costiliness  of  the  whole  apparatus,  and  the 
impossibility  of  transferring  it  from  place  to  place,  have  prevented 
Chemists  from  generally  adopting  this  process.  Hence  Saus- 
sure's  results  stand  alone,  being  unsupported  by  parallel  experi- 
ments. It  has,  however,  been  found  practicable  to  effect  the 
complete  absorption  of  carbonic  acid  from  atmospheric  air  with  a 
mixture  of  caustic  potash  and  barytes,  by  causing  the  air  to  pass 
through  a  glass  tube  of  from  sixteen  to  eighteen  inches  in  length, 
and  three-quarters  of  an  inch  in  diameter,  placed  in  an  oblique 
direction.  The  current  of  air  is  produced  by  the  dropping  of 
water  from  an  aspirator  in  connexion  with  the  absorbing  ap- 
paratus. Single  bubbles  of  air  rise  in  slow  succession  through 
the  obliquely-placed  tube,  and  are  thus  kept  for  about  ten  seconds 
in  contact  with  the  absorbing  liquid.  The  air  passes  into  the 
apparatus  in  its  ordinary  state,  deprived  neither  of  moisture  nor 
ozone,  as  these  substances  do  not  interfere  with  the  result.  The 
volume  of  air  is  ascertained  by  measuring  the  water,  and  reduc- 
ing it  for  pressure  and  temperature.  The  carbonate  of  barytes 
from  forty  or  fifty  litres  of  air  is  so  small  a  quantity,  that  it 
cannot  be  well  determined  by  weighing  it  upon  a  filter,  and  I 
have  adopted  the  method  of  determining  this  quantity  by  a  solu- 
tion of  nitrate  of  silver,  which  gives  uniform  results  and  is  very 
accurate.  This  process  is  described  in  my  work  on  Volumetri- 
cal  Analysis. 
