28 CROONIAN LECTURES 



more consistent with that which views them as 

 motion. 



" Heat, which we are at present considering, 

 cannot be insulated. We cannot remove the 

 heat from a substance, and retain it as 

 heat. We can only transmit it to another 

 substance, either as heat, or as some other 

 mode of force. We only know certain changes 

 of matter, for which changes heat is a generic 

 name. The thing heat is unknown." 



In 1863, Professor Tyndall says : " The dy- 

 namical theory, or, as it is sometimes called, 

 the mechanical theory of heat, discards the 

 idea of materiality as applied to heat. The 

 supporters of this theory do not believe heat to 

 be matter, but to be an accident or condition 

 of matter namely, a motion of its ultimate 

 particles. From the direct contemplation of 

 some of the phenomena of heat, a profound 

 mind is led almost instinctively to conclude 

 that heat is a kind of motion." 



The theory, then, which Rumford so power- 

 fully advocated, and Davy so ably supported, 

 was, that heat is a kind of motion ; and that by 

 friction, percussion, and compression, this motion 

 may be generated, as well as by combustion. 



