34 SCIENTIFIC THOUGHT. 



scale, and the amplitude or height of the wave-motion 



22. of which decided its intensity. There was floating about 



Other 



kinetic the vague idea that heat also was to be interpreted 



theories. 



as a mode of motion ; still vaguer were the kinetic 

 notions as to electricity and magnetism ; whilst some 

 early attempts to explain gravity, not as an inherent 

 property of matter, but as a consequence of the motion 

 of matter itself, which was possessed merely of inertia, 

 had been half forgotten. 



There is no doubt that the successful development of 

 the undulatory theory of light induced many minds to 

 dream of an ultimate kinetic explanation or interpretation 

 of all natural phenomena, when in the course of the 

 third quarter of the century this direction of thought 

 received a great impetus through three independent 

 branches of research of a purely theoretical kind. These 

 have led to a very remarkable development of the kinetic 

 view of nature ; in fact it is mainly through them that 

 this view has become possible not only in special depart- 

 ments, but on a universal scale. They have, each in 

 its own way, led to a great extension of our experi- 

 mental knowledge ; one of them has likewise led to many 

 practical applications. What most interests us here is 

 the peculiar direction which they have given to a great 

 volume of mathematical and physical thought of our day. 



23. The first of these lines of research was connected with, 



Kinetic 



theory of and grew out of. the atomic hypothesis. It culminated 



gases. 



in the kinetic theory of gases, in which the names of 

 Joule, Clausius, and Clerk Maxwell are prominent. Of 

 this I have treated already in the fifth chapter. It 

 rests on a study of the average effect produced by a 



