AND OF THE SECOND LAW 53 



for the two are connected by TdS=dQ. By answering one of 

 these questions we at the same time settle the other." 



In the earlier days interest was naturally centered in the 

 directly measurable magnitude temperature and entropy appeared 

 as a more complicated idea which was to be derived from the 

 former. Nowadays this relation is rather reversed and the prime 

 question is to first explain entropy mechanically and this will 

 then define temperature. The reason for this change of attitude 

 is that in all such explanatory efforts to present Thermodynamics 

 mechanically and give temperature a complete mechanical definition 

 it is necessary to come back to the peculiarities of " thermal equi- 

 librium." But the full significance of this equilibrium conception 

 is only to be reached from the standpoint of irreversibility. For 

 thermal equilibrium can only be defined as the final state toward 

 which all irreversible processes strive. In this way the question 

 as to temperature leads necessarily to the nature of irreversibility 

 and this in turn is solely founded on the existence of the entropy 

 function. This magnitude is therefore the primary, general 

 conception which is significant for all kinds of states and changes 

 of state, while temperature emerges from this with the help of 

 the special condition of thermal equilibrium, in which condition 

 the entropy attains its maximum. 



SECTION B 



SIMPLE ANALYTICAL EXPRESSION FOR DEPENDENCE OF ENTROPY 



ON PROBABLITY 



Here also we will dispense with a full proof and content our- 

 selves with the main steps which lead to the desired expression. 

 We will follow PLANCK'S elegant presentation on pp. 136-148 of 

 his Warmestrahlung. On p. 22 we have dwelt on the usefulness 

 and the necessity for the probability idea in general physics and 

 in this particular case. We can start, therefore, with PLANCK'S 

 theorem : 



