62 PRESERVATION OF FOOD BY REFRIGERATION ; 
followed, and many experiments were made on the subject with 
more or less success until] the time of MM. Dulong and Petit, 
who made a very complete and successful investigation of the 
velocity of cooling of a thermometer both in vacuo and in 
air.’’—Balfour-Stewart, op. cit. 
Without going into the details, which are very difficult 
and complicated, and which may be found it Stewart’s 
work on heat, Messrs. Dulong and Petit found that— 
“rst. The loss of heat varies with the adsolu/e temperature of 
the absorbent, so that, for example, if the temperature of WW in 
fig. 1 had been 212°, and that of S = 213”, thedosseot heat 
would have been -nearly double the amount with the respective 
temperatures 59° and 60°. 
“and. That the loss of heat increases also in a much more 
rapid ratio than the difference of temperature; thus, with 432° 
difference, and with the absorbent at 212° (the radiant being in 
this case 644°), the loss is six times greater than at the lower 
temperature first given.”’—Box, op. cit. 
By putting Dulong and Petit’s rules into such a form as 
to give a ratio, these calculations may be expressed as 
follows :— 
124572 %- 110077") KA(1 Co77= Tee R” 
Where t = the temperature of the absorbent of the radiant heat 
in degrees centigrade; 'T =the excess of temperature of the 
radiating body in degrees centigrade, and R” =the ra/zo of the 
loss of heat under the given temperature. The constant 124°72 
is given by Peclét, who found the rule to agree perfectly with his 
own experiments. As an example, if the temperature of the 
absorbent be 59° F. or 15° C., and the excess of temperature of 
the radiant above the absorbent be 180° F., or 100° C., then— 
R’‘=124°72 X 10077" x (10077 
100 
=124°7 x £22 % (2153: 
100 
= 1°613 = ratio of loss of heat. 
If the radiant, for example, be cast iron, whose radiating power 
is ‘6480 by table No. 1, the rate at which it loses heat, for a 
difference of 180° F., will be *6480 x 1°613 = 1°045 units per 
square foot per hour for 1° difference in temperature between the 
walls of the enclosure and the radiant; and as in this case the 
difference in the temperature between them is 100° C., or 180° F., 
