530  ON  THE  SOLIDIFICATION  OF  CARBONIC  ACID. 
mine  the  liquefaction  of  carbonic  acid  under  atmospheric  pres- 
sure. We  have,  in  fact,  proved  that  carbonic  gas  liquefies  by 
passing  a  current  of  the  dry  carbonic  acid  gas  into  a  small  U 
shaped  tube,  immersed  in  ammonia;  but  as  the  temperature  ob- 
tained is  only  a  few  degrees  below  that  of  saturation,  we  get 
only  a  small  quantity  liquefied.  If,  on  the  contrary,  a  slight 
elevation  of  pressure  is  employed,  the  experiment  becomes  easy, 
and  yields  in  a  short  time  notable  quantities  of  solid  carbonic 
acid.  The  following  is  the  manner  of  operating  :  Introduce 
about  150  cubic  centimetres  of  liquid  ammonia  into  a  reversed 
glass  receiver,  the  sides  of  which  are  cemented  to  a  plate  with 
two  holes.  In  the  central  opening  lit  a  glass  tube,  closed  in- 
ternally^ and  reaching  the  bottom  of  the  receiver,  the  other 
opening  serving  to  place  the  interior  of  the  receiver  in  commu- 
nication with  the  pneumatic  machine.  Carbonic  acid  is  pro- 
duced by  heating  previously  dried  bicarbonate  of  soda  in  a 
copper  retort,  the  neck  containing  fragments  of  chloride  of 
calcium.  One  part  of  this  retort  communicates  by  a  leaden 
tube  on  one  hand  with  the  tube  which  is  immersed  in  liquid 
ammonia,  on  the  other  hand  with  a  small  manometer  of  com- 
pressed air.  The  air  being  previously  expelled  from  the  appa- 
ratus, and  the  temperature  of  the  ammonia  lowered  to  about 
the  point  of  solidification,  the  retort  is  heated,  noting  meanwhile 
carefully  the  pressure.  The  pressure  is  thus  maintained  be- 
tween three  and  four  atmospheres.  Rapidly  augmenting  trans- 
parent crystals  soon  appear  on  the  sides  of  the  interior  tube,  so 
that  in  about  half  an  hour  all  that  portion  of  the  tube  which  is 
plunged  in  ammonia  becomes  covered  with  a  thick  stratum  of 
crystals  (about  25  grammes.)  The  experiment  may  then  be 
concluded  and  the  apparatus  dismounted. 
Solid  carbonic  acid,  obtained  under  the  above-mentioned  con- 
ditions, appears  a  colorless  mass  as  transparent  as  ice.  It  is 
easily  detached  from  the  sides  of  the  condensing  tube  by  means 
of  a  glass  rod  ;  it  then  divides  into  large  cubic  crystals,  each 
side  about  three  to  four  millimetres.  Exposed  to  the  air,  these 
crystals  slowly  return  to  their  gaseous  state  ;  they  evaporate, 
leaving  no  residue.  Placed  on  the  hand,  they  communicate  no 
immediate  sensation  of  heat  or  cold  ;  they  are  with  difficulty 
held  in  the  fingers,  and  with  a  slight  pressure  escape  as  if  cov- 
