PHYSICAL THEORIES OF HEAT. 583 



On looking back at our view of this science, it 

 will be seen that it may be distinguished into two 

 parts ; the Doctrines of Conduction and Radiation, 

 which we call Thermotics proper; and the Doc- 

 trines respecting the relation of Heat, Airs, and 

 Moisture, which we have termed Atmology. These 

 two subjects differ in their bearing on our hypo- 

 thetical views. 



Thermotical Theories. The phenomena of ra- 

 diant heat, like those of radiant light, obviously 

 admit of general explanation in two different ways ; 

 by the emission of material particles, or by the 

 propagation of undulations. Both these opinions 

 have found supporters. Probably most persons, in 

 adopting Prevost's theory of exchanges, conceive 

 the radiation of heat to be the radiation of matter. 

 The undulation hypothesis, on the other hand, ap- 

 pears to be suggested by the production of heat by 

 friction, and was accordingly maintained by Rum- 

 ford and others. Leslie 1 appears, in a great part of 

 his Inquiry, to be a supporter of some undulatory 

 doctrine, but it is extremely difficult to make out 

 what his undulating medium is; or rather, his 

 opinions 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, 



1 An Experimental Inquiry into the Nature and 

 of Heat. 1804. 



