56 PRESERVATION OF FOOD BY REFRIGERATION ; 
scientists, and is now universally accepted as correct, and 
may be stated as follows :— 
‘All bodies are constantly giving out radiant heat at a rate 
depending upon their substances and temperature, but indepen- 
dent of the substances and temperature of the bodies that surround 
them. Take, for instance, a thermometer hung in an enclosure ; 
this, according to Prevost’s Theory of Exchanges, as it is called, is 
constantly giving out heat ata rate depending upon the temperature 
of the bulb, and independent of that of the surrounding enclosure. 
On the other hand it is receiving heat from this enclosure at a rate 
depending upon the temperature of the enclosure, and independent 
of that of the bulb. Thus its heat expenditure depends upon 
its own temperature, its heat receipts upon that of the enclosure, 
and there is equilibrium when its expenditure is exactly balanced 
by its receipts. 
“ Radiation takes place in vacuo as well as in air, for it takes 
place between the sun and the earth, and between the fixed stars 
and the earth, and we have no reason to think that all space is 
filled with some kind of air. Radiation takes place equally on all 
sides. Ifa sphere be heated and very delicate thermometers be 
placed on different sides of such a sphere at equal distances, they 
will always give the same indications.’’—Stewart, op. cit. 
‘“‘ Radiant heat is capable of passing through certain substances 
without sensibly heating them. Atmospheric air and rock-salt 
may be quoted as two examples of bodies very slightly affected by 
radiant heat. 
‘Radiant heat, like light, is capable of being reflected, the 
metals being the best reflectors. The following table shows the 
proportion of reflected rays in every 100 from some of the metals 
in ordinary use. For example, out of every 100 rays falling upon 
the surface of a plate of polished silver at an angle of 50°, 97 of 
them are reflected ; from brass, 93 ; from steel, 82 ; from zinc, 81 ; 
from iron, 77.’—Ganot, op. cit. 
“Tt has been found that the absorbing power of a body varies 
inversely to its reflecting power, and thus a good reflector is a bad 
absorber and zce versa. The absorbing power of a body is its 
property of allowing a greater or less quantity of the heat which 
falls upon it to pass into its mass. The radiating power of a body 
is its capability of emitting at the same temperature and with the 
same extent of surface greater or less quantities of heat.’—Ganot, 
op. cit. 
Although the identity of the absorbing and radiating 
powers of a body cannot be accurately deduced from its 
