Am.  Jour.  Pharm. 
May,  1901. 
Liquid  Carbonic  Acid  Gas. 
239 
the  cost  of  liquefaction,  a  pure  dry  gas  being  liquefied  with  less 
trouble  and  cost  than  a  wet  impure  quality.  Usually,  the  gas  is 
generated  in  large  iron  retorts  or  tanks,  when  made  by  chemical 
action,  or  in  specially  made  tank-like  retorts  when  made  by  the 
action  of  heat  on  carbonates ;  from  these  it  is  pumped  through  coils 
of  pipes  surrounded  by  water  through  the  "  purifiers  "  and  "  driers  " 
to  the  first  compressor.  The  "purifiers"  are  large  tanks  full  of 
water  through  which  the  gas  bubbles  up  just  as  in  the  familiar 
wash-bottle  for  gases  of  our  laboratories,  and  is  pumped  off  as  it 
comes  through  to  the  "  dryer."  The  best  grades  of  liquid  gas  are 
washed  four  times  by  being  passed  through  as  many  separate  tanks 
of  water.  From  the  purifiers,  the  gas  is  made  to  pass  either 
through  sulphuric  acid  or  over  calcium  chloride  to  remove  all 
moisture,  this  interfering  seriously  with  the  compression ;  in  this 
part  of  the  process  there  are  several  trade  secrets  as  to  the  way  and 
materials  used. 
After  having  been  washed  and  dried,  the  gas,  still  in  its  normal 
state,  is  pumped  to  the  first  compressor,  where  it  is  condensed 
under  a  pressure  of  about  200  pounds  to  the  square  inch ;  from  this 
it  passes  through  coils  of  pipe  immersed  in  a  freezing  mixture  of  ice 
and  salt  to  absorb  the  heat  of  compression  and  comes  to  the  second 
compressor  at  a  temperature  little  above  o°  Centigrade.  The 
amount  of  heat  generated  in  the  compression  of  gases  is  amazing  to 
the  unitiated  ;  to  absorb  it  and  cool  the  gas  requires  a  large  quan- 
tity of  ice  daily.  In  the  second  compressor,  the  gas  is  brought  to  a 
compression  of  540  pounds  to  the  square  inch,  the  pipes  of  which 
are  also  surrounded  by  a  freezing  mixture,  and  passes  into  a  coil  of 
pipe  immersed  in  the  same.  The  gas  is  still  in  a  gaseous  form,  but 
now  physical  effects  begin  to  play  their  part  and  cause  it  to  liquefy 
by  its  own  expansion.  The  end  of  the  final  coil  of  pipes  is  con- 
nected directly  with  the  "  tank  "  or  cylinder  in  which  the  liquid  gas 
is  sold  to  the  druggist.  The  process  by  which  these  tanks  are  filled 
is  extremely  interesting  and  simple. 
If  you  will  examine  a  tube  of  liquid  gas  you  will  see  screwed 
into  the  top  a  piece  of  heavy  brass  pipe,  with  a  valve  for  opening 
or  closing  the  tube  at  the  top  (worked  by  a  wrench)  and  a  threaded 
tube  on  one  side.  The  pipe  connecting  with  the  soda  founts  is 
screwed  on  to  this  threaded  bit  of  pipe  on  the  side.  Inside  of  this 
brass  pipe,  the  bore  turns  at  right  angles  to  the  bore  of  the  side 
