RELATION OF VAPOR AND AIE. 181 



On looking back at our view of this science, it will l seen that it 

 may be distinguished into two parts ; the Doctrines of Conduction and 

 Radiation, which we call Thermotics proper ; and the Doctrines res- 

 pecting the relation of Heat, Airs, and Moisture, which we have termed 

 Atmology. These two subjects differ in their bearing on our hypothe- 

 tical views. 



Thcrmotical Theories. The phenomena of radiant heat, like those 

 of radiant light, obviously admit of general explanation in two differ- 

 ent ways ; by the emission of material particles, or by the propaga- 

 tion of undulations. Both these opinions have found supporters. 

 Probably most persons, in adopting Prevost's theory of exchanges, con- 

 ceive the radiation of heat to be the radiation of matter. The undu- 

 lation hypothesis, on the other hand, appears to be suggested by the 

 production of heat by friction, and was accordingly maintained by 

 Rumford and others. Leslie 1 appears, in a great part of his Inquiry 

 to be a supporter of some undulatory doctrine, but it is extremely diffi 

 cult to make out what his undulating medium is ; or rather, his opi- 

 nions wavered during his progress. In page 31, he asks, " What is this 

 calorific and frigorific fluid ?" and after keeping the reader in suspense 

 for a moment, he replies, 



" Quod petis hie est. 



It is merely the ambient AIR." But at page 150, he again asks the 

 question, and, at page 188, he answers, " It is the same subtile matter 

 that, according to its different modes of existence, constitutes either heat 

 or light." A person thus vacillating'between two opinions, one of which 

 is palpably false, and the other laden with exceeding difficulties which 

 he does not even attempt to remove, had little right to protest against" 

 " the sportive freaks of some intangible aura ;" to rank all other hypo- 

 theses than his own with the " occult qualities of the schools ;" and to 

 class the "prejudices" 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 favor with those who had 

 studied mathematical thermoties. As we have said, the laws of con- 



1 An Experimental Inquiry into the Nature and Propagation of Heat, 1804. 



2 Ib. p. 47. 



