56 SCIENTIFIC THOUGHT. 



the supposed static properties of matter could be ex- 

 plained by different modes of motion, translational, 

 periodic, or rotational. The mathematical and experi- 

 mental investigations connected with the theory of 

 radiations and vibrations had thus an influence 1 on 

 our general views of the nature of physical processes 

 which far exceeded the aims for which they were origi- 

 nally undertaken. That a substance so attenuated as the 

 ether should have the properties of a solid ; that brittle 

 substances like pitch should flow like liquids, if only 

 sufficient time were given ; that towards very rapid 

 impulses gases and liquids might behave as solids all 

 these observations resulted in a complete revolution of 

 our scientific notions as well as of our vocabulary. The 

 great turning-point, indeed, lay in the kinetic theory of 

 gases, which about the middle of the century had intro- 

 duced quite novel considerations by showing how the 

 dead pressure of gases and vapours could be explained on 

 the hypothesis of a very rapid but disorderly transla- 

 tional movement of the smallest particles in every 

 possible direction. Pressure of gases having been ex- 

 plained by a very rapid motion of the minute par- 

 ticles of matter, heat was immediately conceived to be 

 merely a " mode of motion." As no event did more to 

 spread modern views in the theory of light, and to 

 popularise modern scientific methods, than Kirchhoff's 



1 It has been asserted that the ] an elastic medium. To this view of 



theory of elasticity received a great ' the origin of the modern theory of 



impulse when Fresnel was forced to , elasticity Prof. Karl Pearson takes 



make assumptions as to the mode of I exception, as Navier's memoir of 



vibrations of the ether which were ; 1827 was not suggested by optical 



quite incompatible with the then investigations (Todhunter- Pearson, 



accepted laws of the vibrations of vol. ii. 2. p. 5>. 



