24 



BULLETIN 98, U. S. DEPABTMENT OF AGBICULTUBE. 



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- 



