ions seem to show that the intensities
of the radiating rays are proportional (it is necessary, Gentlemen, that
I employ the scientific expression) to the sines of the angles which
these rays form with the heated surface. But the quantities upon which
the experimenter had to operate were too feeble; the uncertainties of
the thermometric estimations compared with the total effect were, on the
contrary, too great not to inspire a strong degree of distrust: well,
Gentlemen, a problem before which all the processes, all the instruments
of modern physics have remained powerless, Fourier has completely solved
without the necessity of having recourse to any new experiment. He has
traced the law of the emission of caloric sought for, with a perspicuity
which one cannot sufficiently admire, in the most ordinary phenomena of
temperature, in the phenomena which at first sight appeared to be
entirely independent of it.

Such is the privilege of genius; it perceives, it seizes relations where
vulgar eyes see only isolated facts.

Nobody doubts, and besides experiment has confirmed the fact, that in
all the points of a space terminated by any envelop maintained at a
constant temperature, we ought also to experience a constant
temperature, and precisely that of the envelop. Now Fourier has
established, that if the calorific rays emitted were equally intense in
all directions, if the intensity did not vary proportionally to the sine
of the angle of emission, the temperature of a body situated in the
enclosure would depend on the place which it would occupy there: _that
the temperature of boiling water or of melting iron, for example, would
exist in certain points of a hollow envelop of glass!_ In all the vast
domain of the physical sciences, we should be unable to find a more
striking application of the celebrated method of the _reductio ad
absurdum_ of which the ancient mathematicians made use, in order to
demonstrate the abstract truths of geometry.

I shall not quit this first part of the labours of Fourier without
adding, that he has not contented himself with demonstrating with so
much felicity the remarkable law which connects the comparative
intensities of the calorific rays, emanating under all angles from
heated bodies; he has sought, moreover, the physical cause of this law,
and he has found it in a circumstance which his predecessors had
entirely neglected. Let us suppose, says he, that bodies emit heat not
only from the