FOE DETEEMINING THE EADIATING- POWEES OF SUEFACES IN AIE. 
381 
excess of temperature of the radiating surface above that of the surrounding 
medium, in Centigrade degrees ; 
^=the pressure of the surrounding medium, expressed by the height of the mercurial 
column in metres, which the pressure of the medium supported ; 
c=a numerical value constant for the same surrounding medium at diiferent tempera- 
tures, but different for different media; in the case of atmospheric air it ='45 ; 
5 = 1*233, a numerical value which is the same for all radiating surfaces, surrounding 
media, and temperatures ; 
A and B are constants depending on the area of the radiating surface, and the time 
to which Q is referred. 
When the radiation took place into a vacuum, the pressure j), and therefore the 
second term of the above formula vanished, and the quantity of heat which radiated in 
a given time was expressed by the first term, which exhibits the dependence of Q on 
the temperatm’e of the surrounding space (or of the physical surface which bounds that 
space), as well as on t, the excess of temperature of the radiating surface. This term 
will manifestly vary approximately as the simple power of t when the variations of t and 
0 are not too large. The second term expresses the quantity of heat carried off from 
the surface by the contact of a suiTounding elastic medium, and expresses the law that 
this quantity is independent of the nature of the radiating surface, and depends on the 
surrounding medium only so far as regards the elasticity [p) of that medium, and not 
directly on its temperature, but only so far as the elastic force is affected by that tem- 
perature. 
These conclusions, as applicable to all radiating surfaces, were made to rest, as I have 
already observed, on the somewhat narrow basis of induction from experiments on two 
kinds of surface only, those of glass and silver. Moreover the complete numerical 
values of A and B depend upon that of the area of the radiating surface, which, as 
remarked by MM. Duloxg and Petit, was not easy to determine for the bulbs of their 
thermometers ; nor, in fact, was it their object to determine the numerical value of the 
quantity of heat denoted by Q, but to ascertain the laws according to which the cooling 
of bodies takes place as depending on simple radiation into a vacuum, or on the contact 
of a circumambient medium. 
It should also be remarked that these experiments can only be regarded as establish- 
ing the above formula for those cases in which the surrounding elastic medium is placed 
under the same conditions as in the apparatus with which the experiments were made. 
In another instrument these conditions might be different (as they are in that which I 
am about to describe), and the quantity of heat carried from the heated surface by con- 
vection might be different, while that due to radiation in vacuum must be the same. In 
other words, the relation between A and B might be different for different instruments. 
In adopting this formula I have assumed this relation to be the same for different 
instruments. The accordance which will be found between the quantity of heat cal- 
culated from the formula and that obtained by experiment, proves that the formula 
