JOSEPPI FOURIER. 159 



of hoillnr/ icater or of mcltlnfi iron, for example, icould exist in eertain 

 points of a hollow envelope 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 rediictio ad ahsurdum of which the 

 ancient matheuiaticians made use in order to demonstrate the abstract 

 truths of geometry. 



I shall not quit this first part of the labors 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 jihysical 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 molecules 

 of their surfaces, but also from the particles in the interior. Let us 

 suppose, moreover, that the heat of these latter particles cannot arrive 

 at the surface by traversing a certain thickness of matter without 

 undergoing some degree of absori^tion. Fourier has reduced these two 

 hypotheses to calculation, and he has hence deduced mathematically the 

 experimental law of the sines. After having resisted so radical a test, 

 the two hypotheses were found to be completely verified ; they have 

 become laws of nature ; they point out latent i)roperties of caloric 

 which could only be discerned by the eye of the intellect. 



In the second question treated by Fourier, heat presents itself under 

 a new form. There is more difficulty in following its movements; but 

 the conclusions deduced from the theory are also more general and more 

 important. 



Heat excited, concentrated into a certain iioint of a solid body, com- 

 municates itself by way of conduction, first to the particles nearest the 

 heated point, then gradually to all the regions of the body. Whence 

 the problem of which the following is the enunciation. 



By what routes, and with what velocities, is the propagation of iifeat 

 effected in bodies of difierent forms and different natures subjected to 

 certain initial conditions ? 



Fundamentally, the Academy of Sciences had already proposed this 

 problem as the subject of a prize as early as the year 1736. Then the 

 terms heat and caloric were not in use ; it demanded the study of nature^ 

 and the propagation of fire ! The word fire, thrown thus into the pro- 

 gramme without any other explanation, gave rise to a mistake of the 

 most singular kind. The majority of philosophers imagined that the 

 question was to explain in what way hurning communicates itself, and 

 increases in a mass of combustible matter. Fifteen competitors pre- 

 sented themselves ; three were crowned. 



This competition was productive of very meager results. However, a 

 singular combination of circumstances and of proper names will render 

 the recollection of it lasting. 



Has not the public a right to be surprised a[wu reading this academic 



