ELASTICITY AS A MODE OF MOTION. 143 



Joule, splendidly developed by Clausius and 

 Maxwell, raised from statistics of the swayings 

 of a crowd to observation and measurement of 

 the free path of an individual atom in Tait and 

 Dewar's explanation of Crookes' grand discovery 

 of the radiometer, and in the vivid realisation of 

 the old Lucretian torrents with which Crookes 

 himself has followed up their explanation of his 

 own earlier experiments ; by which, less than two 

 hundred years after its first discovery by Robert 

 Boyle, 'the Spring of Air' is ascertained to be 

 a mere statistical resultant of myriads of molecular 

 collisions." 



" But the molecules or atoms must have 

 elasticity, and this elasticity must be explained 

 by motion before the uncertain sound given forth 

 in the title of the discourse, ' Elasticity viewed as 

 possibly a Mode of Motion,' can be raised to 

 the glorious certainty of ' Heat, a Mode of 

 Motion.' " 



The speaker referred to spinning-tops, the child's 

 rolling hoop, and the bicycle in rapid motion as 

 cases of stiff, elastic-like firmness produced by 



