424 JOSEPH FOURIER. 
Leibnitz conferred upon this hypothesis the honour of 
appropriating it to himself. He attempted to deduce 
from it the mode of formation of the different solid envel- 
opes of which the earth consists. Buffon, also, imparted 
to it the weight of his eloquent authority. According to 
that great naturalist, the planets of our system are merely 
portions of the sun, which the shock of a comet had de- 
tached from it some tens of thousands of years ago. 
In support of this igneous origin of the earth, Mairan 
and Buffon cited already the high temperature of deep 
mines, and, among others, those of the mines of Giro- 
magny. It appears evident that if the earth was for- 
merly incandescent, we should not fail to meet in the 
interior strata, that is to say, in those which ought to 
have cooled last, traces of their primitive temperature. 
The cbserver who, upon penetrating into the interior of 
the earth, did not find an increasing heat, might then 
consider himself amply authorized to reject the hypo- 
thetical conceptions of Descartes, of Mairan, of Leibnitz, 
and of Buffon. But has the converse proposition the 
same certainty? Would not the torrents of heat, which 
the sun has continued incessantly to launch for so many 
ages, have diffused themselves into the mass of the earth, 
so as to produce there a temperature increasing with the 
depth ? This a question of high importance. Certain 
easily satisfied minds conscientiously supposed that they 
had solved it, when they stated that the idea of a con- 
stant temperature was by far the most natural ; but woe 
to the sciences if they thus included vague considerations 
which escape all criticism, among the motives for admit- 
ting and rejecting facts and theories! Fontenelle, Gen- 
tlemen, would have traced their horoscope in these words, 
so well adapted for humbling our pride, and the truth of 
