298 MARKET DAIRYING 
When a liquid evaporates or changes into the gaseous 
state it absorbs a definite amount of heat called heat of 
vaporization or “‘latent” heat. Thus to change water from 
212° F. to steam at 212° F. requires a considerable 
amount of heat which is apparently lost, hence the term 
latent (hidden) heat. 
Ether changes into its gas at a much lower temperature 
than water which is illustrated by its instant evaporation 
when poured upon the hand. The heat of the hand in this 
case is sufficient to cause vaporization and the sensation 
of cold indicates that a certain amount of heat has been 
abstracted from the hand in the process. 
Manifestly for refrigerating purposes a liquid must be 
used that can be evaporated at a very low temperature; 
for the cold in mechanical refrigeration is produced by 
the evaporation of the liquid in iron pipes, the heat for 
the purpose being absorbed from the room in which the 
pipes are laid. Anhydrous ammonia has thus far proven 
to be the best refrigerant for ordinary refrigeration. 
Anhydrous Ammonia (Refrigerant). This substance 
is a gas at ordinary temperatures but liquifies at 30° F. 
under one atmospheric pressure. In practical refrigera- 
tion the ammonia is liquified at rather high temperatures 
by subjecting it to pressure. The ammonia is alternately 
evaporated and liquified so that it may be used over and 
over again almost indefinitely. 
Circulation of Ammonia. The cycle of operations in 
mechanical refrigeration is as follows: The liquid am- 
monia starts on its course from a liquid receiver, and 
enters the refrigerating coils in which it evaporates, ab- 
sorbing a large amount of heat in the process. By means 
of a compression pump, operated by an engine, the am- 
monia vapors are forced in the condenser coils where the 
