AND OF THE SECOND LAW 29 



SECTION C 



(i) Existence, Definition, Measure, Relations, Properties, and 

 Scope of Irreversibitity and Reversibility. 



In establishing the existence of irreversibility, we can use 

 one or both of the two general methods of approaching any physical 

 problems (see Introduction, pp. 2, 3) we can approach by way 

 of the atomic theory or by considering the behavior of aggregates 

 in Nature. Enough has already been said in this presentation 

 of atomic behavior and arrangements to justify the statement 

 that irreversibility is not inherent in the elementary procedures 

 themselves but in their irregular arrangement. The motion 

 of each atom is by itself reversible, but their combined mean 

 effect is to produce something irreversible. 1 



This has been rigorously demonstrated by BOLTZMANN'S H- 

 theorem for molecular physics, and when sufficiently general 

 co-ordinates are substituted it is also available for the other domains 

 of natural events. When we consider the behavior of aggregates 

 we recognize at once a general, empirical law, which has also 

 been called the one physical axiom, namely, that all natural 

 processes are essentially irreversible. When we use this method 

 of approach we confessedly rest entirely on experience, and then 

 it does not make any logical difference whether we start with 

 one particular fact or another, whether we start with a fact itself 

 or its necessary consequence: For instance we may recognize 

 that the universe is permanently different after a frictional event 

 from what it was before, or we may start, as PLANCK does, by 

 putting forward the following proposition : 



" // is impossible to construct an engine which will work in 



1 This would seem to imply the existence of a broader principle, the properties 

 of systems as a whole are not necessarily found in their parts. 



