XLVI. NORMAN SELFE. 
that a pound of one medium in evaporating will abstract heat 
enough to bring two pounds of the substance to be refrigerated 
down 100°, while one pound of another medium will under similar 
conditions lower the temperature of ten pounds of the same sub- 
stance but only 50°, then the medium in the later case would, 
other things being equal, be two and a half times as efficient for 
refrigerating as the first one, because it would abstract two and a 
half times as many thermal units in its conversion into vapour. 
If the substance to be cooled is water, then two pounds lowered 
100° represents 200 B.T.U., while ten pounds lowered 50° would 
be 500 B.T.U. {hn order therefore to find the relative efficiency 
of one pound of a given refrigerating medium for abstracting heat 
in the refrigerator, the latent heat of liquefaction must first be 
taken into account, Diagram Fig. 6 shows by curves the latent 
heat of the four principal media before referred to through con- 
siderable ranges of temperature. 
USELESS WORK PERFORMED IN THE REFRIGERATOR. 
When a gas is liquefied under the influence of pressure—whether 
produced by a pump, or through the direct application of heat— 
and the abstraction of heat by cooling it in a condenser, the resul- 
ting liquid is necessarily at a temperature something above that 
of the condensing water and is ready to change its condition again 
when those influences are removed. In actual practice the pres- 
sure is retained in the condenser or liquid receiver by an expansion 
or flash valve, which regulates the passage of thie liquid refrigerant 
into the coils of the refrigerator, releasing its pressure at the same 
time to that of the refrigerator; under these conditions the liquid 
immediately boils or evaporates, and in so doing abstracts heat 
from the metal of the coils and the air or liquid surrounding such 
coils; but it must be particularly noted that it has to be cooled 
itself before it can cool the refrigerator down to any given OF 
required temperature and that therefore a certain amount of its 
actual cooling power is not effective for external refrigeration. 
The amount of heat or the number of thermal units that is thus 
3 pice 6e useful < eosme aire is the product of the 
