584 HISTORY OF THERMOTICS. 



he again asks the question, and, at page 188, he 

 answers, "It is the same subtile matter that, ac- 

 cording to its different modes of existence, consti- 

 tutes either heat or light." A person thus vacil- 

 lating between two opinions, one of which is pal- 

 pably false, and the other laden with exceeding 

 difficulties which he does not even attempt to 

 remove, had little right to protest against 2 "the 

 sportive freaks of some intangible aura ;" to rank 

 all other hypotheses than his own with the " occult 

 qualities of the schools;" and to class the "pre- 

 judices" of his opponents with the tenets of those 

 who maintained the fuga vacui in opposition to 

 Torricelli. It is worth while noticing this kind of 

 rhetoric, in order to observe, that it may be used 

 just as easily on the wrong side as on the right. 



Till recently, the theory of material heat, and of 

 its propagation by emission, was probably the one 

 most in favour with those who had studied mathe- 

 matical thermotics. As we have said, the laws of 

 conduction, in their ultimate analytical form, were 

 almost identical with the laws of motion of fluids. 

 Fourier's principle also, that the radiation of heat 

 takes place from points below the surface, and is 

 intercepted by the superficial particles, appears to 

 favour the notion of material emission. 



Accordingly, some of the most eminent modern 

 French mathematicians have accepted and extended 

 the hypothesis of a material caloric. In addition 

 to Fourier's doctrine of molecular extra-radiation, 



2 Jnrjuirif, p. 47- 



