AND OF THE SECOND LAW 87 



would demand a co-operation and concert of action on the part of 

 the elementary constituents which is felt to be quite impossible. 

 It will not be so general and scientific, but perhaps more easily 

 apprehended, if we put this result in terms of human effort, 

 namely, " by asserting that any process is irreversible we assert 

 that by no means within our present or future power can we 

 reverse it, i.e., we cannot control the individual molecules." 



SECTION C 



ENTROPY 



We have seen above that the inevitable growth in the number 

 of complexions is the mark of irreversibility; the number of 

 complexions at any stage can also in a certain sense be regarded 

 as the measure, index or determinant of that stage or state of the 

 system of elements under consideration. Any function of the 

 number of complexions can be regarded as such measure, index 

 or determinant. Now it has been shown by BOLTZMANN that 

 the expression found thermodynamically for the quantity called 

 entropy differs only by a physically insignificant constant from 

 the logarithm of said number of complexions. But the latter 

 may properly be regarded as a true measure of the probability 

 of the system being in the state considered. BOLTZMANN has 

 defined the entropy of a physical system as the logarithm of the 

 probability of the mechanical condition of the system and PLANCK 

 has cast it into the numerical form, 



5=1.35 log e ("probability")io- 16 +constant K 

 = 1.35 log e (number of complexions) io- 16 + const. K; 



where S is the entropy of any natural state of the body and K is 

 an arbitrary constant, the numerical value of the first term of the 

 second member is the quotient of the energy (expressed in ergs) 



