520 Sir William Thomson [March 4, 



WEEKLY EVENING MEETING, 



Friday, March 4, 1881. 



William Bowman, Esq. F.E.S. Vice-President, in the Chair. 



Sir William Thomson, LL.D. F.R.S. etc. 



Elasticity vieived as possibly a Mode of Motion. 



With reference to the title of his discourse the speaker said : " The 

 mere title of Dr. Tyndall's beautiful book, ' Heat, a Mode of Motion,' 

 is a lesson of truth which has manifested far and wide through 

 the world one of the greatest discoveries of modern philosophy. 

 I have always admired it; I have long coveted it for Elasticity; 

 and now, by kind permission of its inventor, I have borrowed it for 

 this evening's discourse. 



" A century and a half ago Daniel Bernouilli shadowed forth the 

 kinetic theory of the elasticity of gases, which has been accepted as 

 truth by Joule, splendidly developed by Clausius and Maxwell, raised 

 from statistics of the swayings of a crowd to observation and measure- 

 ment 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 experi- 

 ments ; 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 elasti- 

 city 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 motion ; and showed experiments with gyrostats in which 

 upright positions, utterly unstable without rotation, were maintained 

 with a firmness and strength and elasticity as might be by bands of 

 steel. A flexible endless chain seemed rigid when caused to run 

 rapidly round a pulley, and when caused to jump off the pulley, and 

 let fall to the floor, stood stiffly upright for a time till its motion 

 was lost by impact and friction of its links on the floor. A limp disc 

 of indiarubber caused to rotate rapidly seemed to acquire the stiffaess 

 of a gigantic Rubens' hat-brim. A little wooden ball which when 

 thrust down under still water jumped up again in a moment, remained 

 down as if embedded in jelly when the water was caused to rotate 



