2 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
heat is used to destroy the bacteria, or at least to reduce their num- 
ber to such an extent as to prevent their producing disease; but pas- 
teurized milk as well as unpasteurized market milk should be cooled 
to a temperature of 50° F. or below and held at this lower tempera- 
ture until used. At a temperature below 50° F. bacteria multiply 
less rapidly, but between 50° and 100° F. the increase is very fast; 
hence the necessity for thorough cooling and the maintenance of low 
temperatures until used. 
DEFINITIONS OF TERMS. 
A knowledge of the terms used in refrigeration is necessary in order 
to better understand the matter given in the following pages. There- 
fore definitions of the principal terms and units employed are given 
for the benefit of those not already familiar with them. 
British thermal unit. — A British thermal unit (B. T. U.) is the quan- 
tity of heat required to raise 1 pound of pure water 1 degree Fahren- 
heit, at or near its maximum density, 39.1° F. Some authorities 
consider a British thermal unit as the heat required to raise 1 pound 
of pure water from 61° to 62° F. For practical purposes, however, 
it may be considered the heat required to raise the temperature of 
1 pound of water 1 degree Fahrenheit. 
Sensible heat. — Sensible heat is the heat that may be felt by the 
hand or measured by a thermometer. 
Latent heat. — Latent or "hidden" heat is the heat which is expended 
in molecular work of separating the molecules of the substance and 
can not be measured by a thermometer. Every substance has a 
latent heat of fusion, required to convert it from a solid to a liquid, and 
another, latent heat of vaporization, required to convert it from a 
liquid to a gas or vapor. Thus, if heat is applied to a pound of ice 
at 32° F. it will begin to melt, and no matter how much heat is applied 
the ice will not get any hotter. After every particle of ice has melted, 
we will have 1 pound of water at 32° F., the same temperature as the 
ice before heat was applied. Experiments have shown that it requires 
144 British thermal units to melt 1 pound of ice at 32° F. into water 
at 32° F.: hence the latent heat of fusion of ice is said to be 144. 
If heat is applied to 1 pound of water at 212° F., the water will 
remain at 212° F. under atmospheric pressure until all of it has been 
evaporated into steam at 212° F. This has been found to require 
970.4 British thermal units; hence the latent heat of vaporization of 
steam at atmospheric pressure is said to be 970.4 B. T. U. 
Specific heat. — The specific heat of a substance may be defined as 
the ability of that substance to absorb heat compared to that of water. 
Water being one of the hardest of all substances to heat, its specific 
heat is taken at unity. Therefore the specific heat of other substances 
is usually less than unity. A better understanding of latent and spe- 
