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 l 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 

 theory of elasticity received a great 

 impulse when Fresnel was forced to 

 make assumptions as to the mode of 

 vibrations of the ether which were 

 quite incompatible with the then 

 accepted laws of the vibrations of 



an elastic medium. To this view of 

 the origin of the modern theory of 

 elasticity Prof. Karl Pearson takes 

 exception, as Navier's memoir of 

 1827 was not suggested by optical 

 investigations (Todhunter-Pearson, 

 vol. ii. 2, p. 5). 



