﻿ARTIFICIAL 
  COLD 
  — 
  WILKES 
  233 
  

  

  cooled, 
  the 
  ammonia 
  would 
  be 
  reabsorbed, 
  thus 
  reducing 
  the 
  pressure 
  

   on 
  the 
  other 
  chamber 
  so 
  that 
  the 
  ammonia 
  liquid 
  boiled, 
  producing 
  

   a 
  considerable 
  cooling 
  effect. 
  This 
  type 
  of 
  machine 
  is 
  still 
  being 
  sold 
  

   and 
  if 
  one 
  is 
  willing 
  to 
  heat 
  the 
  unit 
  for 
  about 
  an 
  hour 
  each 
  day 
  with 
  

   kerosene 
  or 
  gas, 
  it 
  will 
  keep 
  a 
  refrigerator 
  at 
  a 
  useful 
  temperature 
  for 
  

   the 
  preservation 
  of 
  food. 
  

  

  This 
  same 
  principle 
  has 
  been 
  developed 
  by 
  many 
  manufacturers 
  

   so 
  that 
  there 
  are 
  large 
  commercial 
  machines 
  using 
  this 
  process 
  as 
  a 
  

   means 
  of 
  continuous 
  refrigeration. 
  There 
  are 
  also 
  several 
  domestic 
  

   refrigerators 
  using 
  this 
  principle 
  with 
  gas 
  as 
  the 
  form 
  of 
  fuel 
  for 
  

   heating; 
  one 
  of 
  these 
  machines, 
  in 
  fact, 
  operates 
  without 
  a 
  single 
  

   moving 
  part, 
  except 
  for 
  the 
  thermostat 
  control 
  of 
  the 
  gas 
  valve. 
  

  

  Another 
  important 
  principle 
  of 
  cooling 
  is 
  the 
  Joule-Thompson 
  

   Effect 
  or 
  the 
  cooling 
  of 
  a 
  gas 
  by 
  expansion 
  from 
  a 
  high 
  to 
  a 
  low 
  

   pressure. 
  If 
  the 
  gas 
  can 
  be 
  made 
  to 
  do 
  work 
  while 
  expanding, 
  a 
  

   still 
  greater 
  cooling 
  effect 
  will 
  be 
  produced 
  and 
  the 
  economy 
  of 
  

   operation 
  will 
  be 
  increased, 
  since 
  the 
  gas 
  in 
  expanding 
  can 
  be 
  made 
  

   to 
  help 
  compress 
  the 
  incoming 
  gas. 
  There 
  are 
  some 
  refrigerating 
  

   machines 
  based 
  upon 
  this 
  principle, 
  using 
  air 
  as 
  the 
  refrigerant. 
  

  

  Carbon 
  dioxide 
  is 
  a 
  by-product 
  of 
  many 
  industries 
  and 
  conse- 
  

   quently 
  is 
  an 
  inexpensive 
  gas. 
  It 
  is 
  usually 
  sold 
  in 
  the 
  liquid 
  form 
  

   under 
  a 
  pressure 
  of 
  about 
  850 
  pounds 
  per 
  square 
  inch. 
  Nearly 
  all 
  

   substances 
  can 
  exist 
  in 
  three 
  different 
  states, 
  solid, 
  liquid, 
  or 
  gaseous, 
  

   provided 
  the 
  temperature 
  and 
  pressure 
  conditions 
  are 
  suitable. 
  If 
  

   sufficient 
  heat 
  is 
  added 
  to 
  solid 
  iron, 
  it 
  can 
  be 
  converted 
  into 
  the 
  

   liquid 
  state, 
  and 
  if 
  the 
  temperature 
  could 
  be 
  raised 
  still 
  higher 
  a 
  point 
  

   would 
  be 
  reached 
  where 
  it 
  would 
  be 
  converted 
  into 
  iron 
  vapor. 
  We 
  

   are 
  all 
  familiar 
  with 
  the 
  three 
  states 
  of 
  water, 
  such 
  as 
  steam, 
  water, 
  

   and 
  ice; 
  and 
  within 
  the 
  past 
  50 
  years 
  we 
  have 
  learned 
  that 
  the 
  so- 
  

   called 
  permanent 
  gases 
  can 
  be 
  converted 
  to 
  liquids 
  or 
  even 
  solids, 
  

   provided 
  the 
  temperature 
  is 
  sufficiently 
  low 
  and 
  the 
  pressure 
  is 
  

   suitable. 
  Now, 
  if 
  a 
  cylinder 
  of 
  liquid 
  carbon 
  dioxide 
  under 
  850 
  

   pounds 
  per 
  square 
  inch 
  pressure 
  is 
  inverted 
  so 
  that 
  when 
  the 
  valve 
  

   is 
  opened 
  only 
  liquid 
  will 
  escape, 
  you 
  will 
  find 
  that 
  there 
  is 
  a 
  great 
  

   cooling 
  effect, 
  due 
  to 
  the 
  vaporization 
  of 
  the 
  liquid, 
  since 
  carbon 
  

   dioxide 
  can 
  not 
  exist 
  as 
  a 
  liquid 
  at 
  room 
  temperature 
  and 
  under 
  

   atmospheric 
  pressure. 
  This 
  cooling 
  effect 
  is 
  so 
  great 
  that 
  some 
  of 
  

   the 
  escaping 
  liquid 
  is 
  cooled 
  to 
  such 
  a 
  temperature 
  that 
  it 
  can 
  no 
  

   longer 
  exist 
  as 
  a 
  liquid, 
  but 
  is 
  converted 
  to 
  the 
  solid 
  state 
  or 
  carbon 
  

   dioxide 
  snow. 
  If 
  a 
  strong 
  cloth 
  bag 
  is 
  tied 
  over 
  the 
  outlet 
  from 
  the 
  

   tank, 
  the 
  gas 
  will 
  pass 
  through 
  the 
  bag 
  but 
  the 
  sohd 
  or 
  snow 
  can 
  be 
  

   collected. 
  This 
  snow 
  is 
  at 
  a 
  temperature 
  of 
  109° 
  F. 
  below 
  zero 
  and 
  

   for 
  many 
  years 
  has 
  been 
  used 
  as 
  a 
  cooling 
  agent 
  in 
  laboratory 
  work. 
  

   The 
  snow 
  itself 
  sublimes 
  or 
  goes 
  directly 
  from 
  the 
  solid 
  to 
  the 
  gaseous 
  

   state 
  under 
  atmospheric 
  pressure 
  and 
  does 
  not 
  make 
  very 
  good 
  ther- 
  

  

  