179 



essential, but it was found to be convenient mainly for 

 didactic purposes to elaborate such analogies, explaining 

 or describing the less known by that which is more 

 familiar. Eegarding the value of such attempts there 

 have always existed two opinions. I have had occasion 

 to refer to them when explaining the atomic theory. 

 There were those who looked upon that theory merely 

 as a convenient symbolism, and there were those who 

 looked upon atoms and molecules as really existing 

 things. The latter view has gained force and importance 

 through the necessity of more and more elaborating the 

 atomic hypothesis in order to represent not merely the 

 chemical constitution of compounds, but likewise their 

 manifold physical differences, some of which, in fact, 

 could only be described by geometrical conceptions. I 

 need only refer to what I said above on the kinetic 

 theory of gases, and on the property termed chirality 

 manifested by some chemical substances in solution, as 

 well as on the phenomena of isomerism. In the last 



state of motion of a hot body, and 

 that he liad arrived at a conception 

 which he had already before his 

 first publication (in 1850) used for 

 various investigations and calcu- 

 lations." He further states that 

 hearing through William Siemens 

 that Joule had expressed a similar 

 idea (Manchester Phil. Soc. , 1848 

 and 1857), and more especially after 

 the publication of Kronig (1856), 

 he resolved to publish his views. 

 It is interesting for our present 

 purpose to see how Clausius, like 

 Maxwell in a different domain of 

 research, was originally guided by 

 definite mechanical representations. 

 It is equally noteworthy that Lord 

 Kelvin's original researches on the 



subject of heat were quite free 

 from this element, though we 

 owe to him in other departments 

 some of the most suggestive kin- 

 etic illustrations ; and that he has 

 quite recently offered valuable 

 criticisms on the attempted me- 

 chanical interpretation of the second 

 law of thermo-dynamics (see p. 112 

 of Bryan's Report, quoted above, p. 

 176, note). Also the first English 

 treatise on thermo-dynamics writ- 

 ten for didactic purposes (Tait's 

 Sketch, 1868) contains no reference 

 to molecular theory, and Him, one 

 of the most active workers in the 

 region of experimental proofs, kept 

 clear of it. 



