I02 PRESERVATION OF FOOD BY REFRIGERATION ; 
extra cost of reducing the temperature of the meat from 98° 
to 70° in the freezing room, instead of allowing it to cool in 
the open air, is very small. 
A comparison of tables 10 and 11 will show the reason 
why meat takes so much longer to freeze in a chamber than 
when hung in the open air, when the temperature of the 
external atmosphere and that of the empty chambers are 
about the same. Taking the temperature in both cases to 
be, say, zero, meat hanging in the open air is exposed to 
the contact of air at zero during the entire operation, no 
rise taking place in the air temperature, as the volume of 
air circulating is practically infinite, as compared with the 
limited quantity passing through the chambers. In the 
case of meat hanging in the chambers, the heat given off by 
radiation to the walls, and by contact to the air, necessarily 
warms the limited quantity of air circulating, as shown in 
the table of air temperatures (table 7). In the case where 
a wind is blowing upon meat hanging in the open air, the 
loss by contact becomes very much greater, depending, of 
course, upon the velocity of the wind. The value of Q, in 
the formula given, being increased in proportion. 
In endeavouring to trace the rate at which the emission 
proceeds from the surface of meat hung in a cold atmos- 
phere for the purpose of being chilled, it will be convenient 
to divide the process into three parts. The first part (A) 
of the operation will be that in which the heat is dissipated 
from the external surface, gradually lowering the tempera- 
ture of the whole mass, proportionately to the thickness, 
until the temperature of the internal surface commences to 
fall below 98°; when this occurs the temperature of the 
external surface will, of course, be considerably lower than 
that of the internal surface, but not as low as that of the 
air in which it is hung. The second part (B) will comprise 
the abstraction of the heat until the internal surface is 
reduced to 32°, but not frozen; and the ¢hzrd part (C) of 
the operation will be that in which the whole mass is frozen, 
and which will also necessarily include the fall of the 
external surface considerably below 32°; the heat lost 
during this latter part of the process being given in units 
in table 8, col. 5. 
