AND OF THE SECOND LAW 3 



less distinct modes of attack, namely, (a) a mode of attack in 

 which the effort is made to develop conceptions of the physical 

 processes of nature, and (b) a mode of attack in which the attempt 

 is made to correlate phenomena on the basis of sensible things, 

 things that can be seen and measured. In the theory of heat the 

 first mode is represented by the application of the atomic theory 

 to the study of heat phenomena, and the second mode is represented 

 by what is called thermodynamics." In solving the special 

 problem before us, as to the physical meaning of entropy and of 

 the Second Law, our main dependence must be on the first mode 

 of attack. 



The second mode will furnish checks and confirmations of the 

 results developed by the first, or we may say that the combination 

 of the two modes will give the well-established characteristic 

 equations and relations of bodies and their physical elements. 



The whole discussion will now be taken up in a non-mathe- 

 matical way, without the full proof required by a complete presenta- 

 tion, and about in this order: 



(a) The definitions, general preliminaries and current state- 

 ments of the matters considered. 



(b) More or less precise statement of the primary relations 

 and theorems. 



(c) The physical interpretations. 



(d) Summary of the connection between probability, irreversi- 

 bility, entropy and the Second Law. 



(e) Reach or scope of the Second Law. 



On account of the difficulty which every student experience sin 

 realizing the physical nature of entropy we will in the main con- 

 fine our attention here to gases and indeed to their simplest case, 

 the monatomic gas, and will as usual assume that the dimen- 

 sions of an atom or particle are very small in comparison with the 

 average distance between two adjacent particles, that for the atoms 

 approaching collision the distance within which they exert a signifi- 

 cant influence on each other is very small as compared with the 



