RADIATION AND ABSORPTION. 55 
Thus the solution of the question before us now depends only 
on two elements which, without doubt, will remain for ever un- 
known to us, namely, the conductibility of the mass of the sun, 
and its capacity for heat; and we have just seen how, by means 
of these two elements, should we arrive at a knowledge of them, 
the question might be resolved in an exact manner. With re- 
gard to the hypotheses I have made respecting them, their only 
object is to show the extent of the uncertainties to which science 
is subjected on this point. 
11. With the same object we shall proceed to examine an- 
other question, which however has the advantage over the pre- 
ceding one of being more accessible to science, namely, whether 
the temperature of the sun may have some analogy with the tem- 
peratures which we are able to produce by chemical or electrical 
actions. 
We shall see, in the following article, that the total quantity 
of heat emitted in 1’ by a square centimetre of surface is always 
expressed by 
Li46,, f£. a’, 
J being the emissive power of that surface, ¢ its temperature, and 
a the number 1:0077 determined with great exactness by MM. 
Dulong and Petit. 
We have found elsewhere that for the sun this quantity of 
heat is 84888. Then 
for f= Fy. oF = 12461, 
for f= 4, t= 1761. 
Thus the temperature of the sun depends on the law of radia- 
tion of heat and of the emissive power of the sun or of its atmo- 
sphere. In a former memoir* I have described an air pyro- 
meter, by means of which I determined all the high temperatures 
up to the fusion of iron; I have since verified the fact that the 
law of radiation applies to temperatures which exceed 1000°; 
these experiments will soon enable me to ascertain whether the 
law in question extends in fact to temperatures of 1400° or 
1500°; but we may even now regard such extension as very 
probable. With respect to the emissive power of the sun, it is 
unknown, but we cannot suppose it greater than unity. It fol- 
lows therefore that the temperature of the sun is at least 1461°, 
that is to say nearly that of the melting point of iron, and that 
* Comptes Rendus de l’ Académie des Sciences, t. iii. p. 782. 
