RESEARCHES  ON  COCHINEAL. 
261 
through  a  charcoal  air-filter,  the  hydrogen  from  either  malleable 
or  cast  iron  is  rendered  perfectly  odorless,  the  whole  of  the  im- 
purities present  being  absorbed  and  retained  by  the  charcoal. 
To  effect  this,  I  take  a  tube,  either  of  glass  or  tin-plate,  about 
an  inch  and  a  quarter  in  diameter  and  from  eight  to  ten  inches 
in  length ;  this  I  fill  with  bits  of  wood  charcoal  about  the  size  of 
peas  or  beans  :  the  charcoal  is  packed  loosely  into  the  tube,  so  as 
to  present  no  perceptible  obstruction  to  the  passage  of  the  gas : 
the  charcoal  is  placed  between  the  wash-bottle  and  the  eduction 
tube.  The  charcoal  column  just  described  is  large  enough  for 
laboratory  use  ;  but  when  hydrogen  is  manufactured  on  the  great 
scale,  a  larger  amount  of  charcoal  becomes  necessary.  In  this 
way  I  have  obtained,  as  already  stated,  a  pure  and  perfectly 
odorless  hydrogen  from  both  malleable  and  cast  iron  ;  and  the 
charcoal  has  remained  as  effective  as  at  first,  though  in  daily 
use  for  several  weeks.  The  efficiency  of  the  charcoal  can  always 
be  restored  by  simply  heating  it  to  redness  in  close  vessels. 
Carbonic  acid,  as  is  well  known,  is  usually  prepared  for  the 
manufacture  of  aerated  waters,  by  acting  upon  chalk  or  powder- 
ed limestone  by  means  of  dilute  sulphuric  acid.  The  gas  ob- 
tained in  this  way  has  invariably  an  offensive  odor,  which 
communicates  a  disagreeable  taste  to  the  aerated  waters.  So 
much  is  this  the  case,  that  many  manufacturers  obtain  the  gas, 
for  the  superior  kinds  of  soda  water,  by  decomposing  bicarbonate 
of  soda  by  sulphuric  acid.  This  expensive  process,  however,  is 
altogether  unnecessary,  for,  by  passing  the  impure  carbonic  acid 
through  such  a  column  of  charcoal  as  that  described  above,  I 
have  obtained  a  perfectly  inodorous  carbonic  acid,  both  from 
chalk  and  impure  limestones,  containing  much  bituminous  matter. 
RESEARCHES  UPON  COCHINEAL. 
By  M.  SCHUTZENBERGER. 
Manufacturers  of  printed  cottons  have  long  known  that 
cochineal  when  left  for  several  days  in  contract  with  an  aqueous 
solution  of  ammonia,  undergoes  an  interesting  modification  which 
has  not  yet  attracted  the  attention  of  chemists.  The  red  color- 
ing matter  (carminic  acid)  passes  to  the  state  of  a  matter  of  a 
