THERMO-DYNAMICS. 



BY PROFESSOR FORBES. 



IT is a remarkable fact that many truths in physics after 

 they are well known appear to us to be almost axiomatic, 

 although it required ages before they were thoroughly 

 appreciated as scientific facts. For example, nothing would 

 seem simpler and more evidently true to those who have 

 acquired a fundamental knowledge of science than the 

 principle that the planets in revolving round the sun 

 require some force to attract them towards the sun in 

 order to make them complete their path, whereas for ages 

 people could not get it out of their minds that it was 

 necessary to have a force behind the planet driving it 

 round with a sort of vortical movement. Take again, 

 the celebrated facts enunciated by Galileo, such as this, 

 that a heavy body takes as long as a light body to fall to 

 the ground from a given height. This also seems to us 

 with a slight knowledge of physics to be perfectly certain 

 and to require no proof. 



So it is with the dynamical theory of heat. .For many 

 centuries past there have been men who looked upon heat 

 as necessarily caused by the motions of molecules or 

 particles of the body which is hot. But men's minds 

 being firmly occupied with the theory that heat was a sub- 

 stance, it became a matter of difficulty to eradicate this 

 notion, although now it appears to us to be perfectly 

 axiomatic that no effects such as those produced by heat 

 could be produced simply by matter which is not in 

 motion. 



The proofs on which the dynamic theory of heat rests 

 are of various kinds, and I shall commence by simply point- 

 ing out some of those which are less directly included 



