RADIATION AND ABSORPTION. 89 
Let p be the weight of this globe, ¢ its capacity for heat; 
for a loss of heat pc, it loses in temperature 1°; and, for the 
loss expressed above, it loses in temperature a number of de- 
grees expressed by 
sBf, t+6 4 sC es: 
ate raph eh igner erate a 
Such is the rapidity of cooling. 
For this result to coincide with that of MM. Dulong and 
Petit, we must have 
pee BI ae 6: 
pe 
Now MM. Dulong and Petit having submitted to experiment 
a silyered thermometer in a blackened inclosure, and the perfect 
accuracy of the law of cooling having been thus verified in the 
most exact manner, we see that the constant of emission is ne- 
cessarily equal to zero in this particular case, and consequently 
it is always null in all possible cases. 
We must therefore, without exception, employ the formula 
e=B.fa', 
at least throughout the extent of the scale for which the law of 
cooling is demonstrated. The constant B is, as I have said, an 
invariable constant, the numerical value of which depends on the 
unities of surface and of time which we select and on the point 
whence we reckon the temperatures, whilst the numerical value 
of a depends on the number of degrees or divisions which we 
establish between the two fixed points which serve as a base to 
the thermometric scale. If, for example, we raised the zero of 
the scale 100°, preserving for the degree the magnitude which it 
has actually in the centigrade scale, a would remain equal to 
1:0077, and the value of B would have to be multiphed by a!™. 
With respect to the radiating power f, we may remark that, 
for athermanous substances, its value only depends absolutely 
upon the state of the surface, and appears to be comprised be- 
tween ;/, and 1 for all known surfaces: but, in diathermanous 
substances, f depends both upon the state of the surface and 
upon various other properties of these substances. It will now 
be an essential point for the theory of heat to determine what are 
the properties which may modify the value of f, and the part of 
the influence which pertains to each of them in these variations. 
Nore II. 
I have made a great number of experiments with the actino- 
meter, exposing it perpendicularly to the solar rays at different 
