58 PRESERVATION OF FOOD BY REFRIGERATION ; 
pendent of the substance and temperature of the bodies that 
surround them. 
“ard. Consequently, when a body is kept at a uniform 
temperature, it receives back just as much heat as it gives out.”’— 
B. Stewart, op. cit. 
Different substances radiate and absorb very different 
amounts of heat, and it has been a work of some difficulty 
to ascertain correctly the variation in their absorbing and 
radiating powers. Mr. Peclét, who studied the question 
with great care, gives a table of the radiating and absorbing 
power of the following substances in units of heat emitted 
or absorbed per square foot per hour, for a difference in 
temperature of 1° Fahr. between the substance and the 
surrounding bodies. Thus, if a cubic foot of polished silver 
in vacuo be heated, say, to 60° Fahr., and kept uniformly at 
that temperature, and the temperature of the surrounding 
bodies be 59°, there will radiate from every square foot of 
surface of the silver as much heat as would warm 1 lb. of 
water, at 59° Fah. 0'02657 degrees; or, to speak more 
accurately, the difference in the amount of heat radiated 
by the square foot of silver, and the amount absorbed from 
the surrounding objects will be sufficient to heat 1 lb. of 
water 0.02657 degrees. 
The following table, taken from Mr. Box’s work 
previously cited, gives the value of R, or the radiating 
and absorbing power of substances in ordinary use, and 
denotes units of heat emitted or absorbed per square foot 
per hour, for a difference in temperature of 1° Fah., from 
the experiments of Mr. Peclét :— 
TABLE 1.—VALUE OF R. 
Value of R. 
Silver, polished ie ee rer be 02657 
Copper sts fe. rs kee sie ‘03270 
Pinay. ty! Lice ag sia 04395 
Zine and Brass, polished ode oar yop 704906 
Tinned Iron ... “i se aa 08585 
sheet Iron” —.,. ae sind ah sas "09220 
Bead vic: wo) si y. ve 4 1328 
Iron, ordinary — sah s 4a$ Js 5662 
Glass ... ak! _ si igh a "5948 
Cast Iron, new ve ke aoe oda "6480 
Chalk .., eee ee “rh S. he "6786 
