24 
BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
It will be noted from figure 9 that to raise the temperature of the 
1 pound of ice from zero to the melting point (32° F.) 16 B. T. U were 
expended; in melting the ice, 144 B. T. U.; in raising the water to 
the boiling point, 180 B. T. U.; and to evaporate the water, 970.4 
B. T. U. If the operation is reversed, the heat being extracted 
instead of being added, the curve will follow backward on itself to the 
starting point. 
The latent heat of fusion and the latent heat of vaporization are 
represented on the diagram by the two lines parallel to the horizontal 
base line, the length of the lines representing to scale the amount of 
heat expended in molecular work in separating the molecules of the 
substances. Starting from the left, the rising lines represent the heat 
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Fig. 9.— Diagram showing the relation of heat to temperature. 
required to raise the temperature, of the ice, water, steam at constant 
volume, and steam at constant pressure, respectively. 
The same law applies to liquified anhydrous ammonia, carbon 
dioxid, and sulphur dioxid, which are the substances most com- 
monly used in commercial refrigerating machines. These liquids 
are extremely volatile, their change of state takes place very rapidly, 
and their latent heat is absorbed at a corresponding rate. Their 
boiling point is sufficiently low under atmospheric or other con- 
veniently produced pressure to give the temperature desired. Al- 
though the same principles underlie the use of all such fluids, their 
physical properties vary, and consequently demand different treat- 
ment in order to produce the best results. 
The theoretical requirements of a good refrigerant are: A low 
boiling point at ordinary pressure, a large latent heat of vaporiza- 
