﻿AKTIFICIAL 
  COLD 
  WILKES 
  231 
  

  

  below 
  that 
  of 
  the 
  surrounding 
  air. 
  This 
  simple 
  primitive 
  expedient, 
  

   strangely 
  enough, 
  contains 
  the 
  germ 
  of 
  the 
  principle 
  upon 
  which 
  are 
  

   based 
  all 
  of 
  the 
  mechanical 
  refrigeration 
  systems 
  now 
  in 
  domestic 
  

   use. 
  The 
  principle 
  is 
  this: 
  that 
  evaporation 
  — 
  or 
  what 
  is 
  the 
  same 
  

   thing, 
  the 
  transition 
  from 
  the 
  liquid 
  to 
  the 
  vapor 
  state— 
  requires 
  a 
  

   large 
  amount 
  of 
  heat 
  energy, 
  which 
  must 
  be 
  supplied 
  by 
  the 
  liquid 
  

   itself 
  or 
  the 
  immediate 
  surroundings. 
  If 
  one 
  is 
  boiling 
  water, 
  most 
  

   of 
  the 
  heat 
  energy 
  comes 
  from 
  the 
  heated 
  air 
  around 
  the 
  vessel 
  and 
  

   the 
  air 
  is 
  thereby 
  cooled. 
  If 
  water 
  is 
  evaporating 
  from 
  the 
  surface 
  of 
  

   an 
  earthenware 
  water 
  jar, 
  the 
  heat 
  comes 
  from 
  the 
  vessel 
  and 
  the 
  

   surrounding 
  air, 
  both 
  of 
  which 
  are 
  cooled 
  in 
  the 
  process. 
  

  

  One 
  must 
  also 
  recognize 
  the 
  fact 
  that 
  the 
  temperature 
  at 
  which 
  

   a 
  liquid 
  boils 
  (its 
  "boiling 
  point") 
  depends 
  upon 
  the 
  pressure. 
  With 
  

   the 
  atmospheric 
  pressure 
  as 
  it 
  is 
  at 
  sea 
  level, 
  water 
  boils 
  at 
  approxi- 
  

   mately 
  212° 
  F., 
  but 
  if 
  the 
  pressure 
  be 
  increased 
  twenty 
  times, 
  the 
  

   boiling 
  point 
  is 
  increased 
  to 
  about 
  417° 
  F. 
  If 
  the 
  pressure 
  be 
  suffici- 
  

   ently 
  lowered, 
  one 
  can 
  make 
  water 
  boil 
  at 
  room 
  temperature 
  or 
  even 
  

   at 
  32°, 
  the 
  ordinary 
  freezing 
  point. 
  

  

  This 
  we 
  can 
  readily 
  demonstrate 
  on 
  the 
  lecture 
  table 
  by 
  repeating 
  

   what 
  is 
  known 
  as 
  Leslie's 
  Experiment. 
  If 
  we 
  place 
  some 
  water 
  at 
  

   room 
  temperature 
  in 
  a 
  thermos 
  bottle 
  and 
  reduce 
  the 
  pressure 
  untO 
  

   the 
  water 
  boils, 
  heat 
  will 
  be 
  drawn 
  from 
  the 
  remaining 
  water 
  (since 
  

   little 
  can 
  come 
  from 
  the 
  surroundings) 
  and 
  it 
  will 
  become 
  cooler. 
  

   Then 
  if 
  we 
  continue 
  to 
  reduce 
  the 
  pressure 
  in 
  order 
  to 
  keep 
  the 
  

   water 
  boiling, 
  it 
  will 
  soon 
  reach 
  a 
  temperature 
  of 
  32° 
  F. 
  and 
  some 
  of 
  

   the 
  water 
  will 
  be 
  converted 
  into 
  ice, 
  inasmuch 
  as 
  water 
  does 
  not 
  nor- 
  

   mally 
  exist 
  in 
  the 
  liquid 
  state 
  at 
  a 
  temperature 
  below 
  32° 
  F. 
  

  

  The 
  boiling 
  points 
  of 
  all 
  other 
  liquids 
  vary 
  with 
  the 
  pressure 
  and 
  

   consequently 
  all 
  that 
  has 
  been 
  said 
  in 
  regard 
  to 
  water 
  applies 
  equally 
  

   well 
  to 
  ammonia, 
  sulphur 
  dioxide, 
  carbon 
  dioxide, 
  and 
  so 
  on; 
  only, 
  

   of 
  course, 
  the 
  temperature-pressure 
  conditions 
  may 
  be 
  very 
  difTerent 
  

   from 
  those 
  of 
  water. 
  This 
  principle 
  of 
  cooling 
  by 
  evaporation 
  or 
  

   boiling 
  of 
  various 
  liquids 
  is, 
  as 
  I 
  have 
  already 
  mentioned, 
  the 
  founda- 
  

   tion 
  upon 
  which 
  nearly 
  all 
  of 
  our 
  refrigerating 
  machines 
  are 
  con- 
  

   structed. 
  

  

  Refrigerating 
  units 
  for 
  home 
  use 
  are, 
  in 
  general, 
  of 
  two 
  different 
  

   types 
  — 
  those 
  using 
  a 
  small 
  electrically 
  driven 
  pump, 
  the 
  compression 
  

   type, 
  and 
  those 
  using 
  heat 
  generated 
  by 
  a 
  gas 
  or 
  kerosene 
  oil 
  burner, 
  

   the 
  absorption 
  type. 
  The 
  operating 
  principle 
  of 
  each 
  is 
  simple, 
  the 
  

   former 
  particularly 
  so. 
  A 
  suitable 
  liquid 
  (called 
  the 
  refrigerant) 
  

   such 
  as 
  ammonia, 
  sulphur 
  dioxide, 
  carbon 
  dioxide, 
  methyl 
  chloride, 
  

   or 
  ethyl 
  chloride, 
  is 
  placed 
  in 
  the 
  cooling 
  coil 
  inside 
  the 
  refrigerator 
  

   cabinet, 
  where 
  it 
  is 
  made 
  to 
  "boil" 
  by 
  having 
  the 
  pressure 
  upon 
  it 
  

   reduced 
  with 
  the 
  motor-driven 
  pump. 
  This 
  pump 
  receives 
  the 
  vapor 
  

   from 
  the 
  coil 
  at 
  low 
  pressure, 
  compresses 
  it, 
  and 
  passes 
  it 
  along 
  to 
  

  

  