116 JAMKS CLEUK MAXWELL 



physical properties of gases enumerated abovo in the 

 third objection to Newton's hypothesis. [The paper 

 from which the quotation is taken is on ' T ho Theory 

 of Sound/] In the Archives of the Royal Society for 

 1S45-4G there is a paper on 'The Physics of Media 

 that consist of perfectly " Elastic Molecules in a 

 State of Motion," ' which contains the synthetical 

 reasoning on which the demonstration of these 

 matters rests. . . . This theory does not take 

 account of the size of the molecules. It assumes 

 that no time is lost at the impact, and that if the 

 impacts produce rotatory motion, the vis viva thus 

 invested bears a constant ratio to the rectilineal vis 

 viva, so as not to require separate consideration. It 

 does, also, not take account of the probable internal 

 motion of composite molecules : yet the results so 

 closely accord with observation in every part of the 

 subject avs to leave no doubt that Mr. Herapath's idea 

 of the physical constitution of g;tses approximates 

 closely to the truth." 



In his introduction to Waterston's paper (Phil. 

 Trans., 1892) Lord Rayleigh writes: "Impressed 

 with the above passage, and with the general in- 

 genuity and soundness of Waterston's views, I took 

 the first opportunity of consulting the Archives, and 

 saw at once that the memoir justified the large claims 

 made for it, and that it marks an immense advance 

 in the direction of the now generally received theory." 



In the first section of the paper Waterston's great 

 advance consisted in the statement that the mean 

 square of the kinetic energy of each molecule 

 measures the temperature. 



