AffiM0ay?ifo1]frm"}  Liquid  Carbonic  Acid  Gas.  241 
15,000  pounds.  When  the  tanks  are  taken  out  of  the  freezing  mix- 
ture and  come  to  the  temperature  of  surrounding  air,  the  pressure 
of  the  gas  inside  is  about  900  pounds  to  the  inch  in  winter  and  1 100 
in  summer,  and  there  is  also  a  varying  development  of  pressure 
inside  when  the  gas  is  being  drawn  off  for  use.  Under  almost  all 
circumstances,  these  tubes  of  liquid  gas  are  perfectly  safe  to  handle 
and  will  stand  a  great  amount  of  jolting,  yet  there  are  conditions 
when  the  critical  temperature  of  the  liquid  gas  is  passed  and  it 
assumes  the  gaseous  form  inside  the  tube,  and  then  a  seemingly 
slight  cause  or  weakness  in  the  steel  will  cause  a  disastrous  explo- 
sion. It  is  well  to  be  on  the  safe  side  and  to  handle  these  tubes 
carefully  and  not  to  open  the  valve  too  suddenly,  a  gradual  opening 
until  the  pressure  gauge  stands  at  the  desired  pressure  being  safest. 
The  small  size  cylinders  are  about  y%  to  y2  an  inch  in  thickness  of 
their  steel  walls  and  weigh,  when  filled,  from  fifty  to  seventy  pounds. 
It  is  of  course  understood  that  the  process  I  have  just  mentioned 
is  the  particular  one  used  in  the  Philadelphia  plant  I  visited  ;  there 
are  other  methods,  of  later  date,  by  which  greater  economy  of  time 
and  material  are  achieved,  the  method  of  the  Liquid  Gas  Company, 
for  instance ;  but  the  essential  principle  is  the  same,  the  escape 
of  G02  from  fine  orifices  under  pressure.  In  this  plant  I  mention, 
an  average  of  15  horse-power  working  for  10  hours  produces  from 
fifty  to  eighty  tubes  full  of  liquid  gas,  according  to  the  speed  with 
which  the  compressors  are  run.  These  figures  will  differ  greatly 
from  those  of  more  modern  plants. 
It  must  be  borne  in  mind,  when  considering  these  figures,  that 
when  the  gas  is  brought  under  a  pressure  of  540  pounds  at  0° 
Centigrade  and  allowed  to  flow  into  the  cylinders  through  the 
specially  devised  arrangement  described  above  that  it  in  great  part 
liquefies  itself  by  its  expansion. 
This,  of  course,  is  because  the  gas  escaping  suddenly  from  a  great 
pressure  to  that  of  the  atmosphere  requires  a  great  deal  of  heat  in 
its  expansion  and  this  heat  it  takes  from  the  gas  immediately  follow- 
ing it,  thus  bringing  the  temperature  down  low  enough  to  cause  its 
liquefaction  under  the  pressure  it  is  sustaining.  This  principle  is 
now  widely  used  in  the  liquefaction  of  all  gases,  such  as  air,  hydro- 
gen, etc.,  it  being  practicable  to  liquefy  air  by  allowing  it  to  escape 
from  minute  openings  under  high  pressure  into  the  open  atmos- 
phere. 
