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, where is the entropy of any natural state of the body and is an arbitrary constant, the numerical value of the first term of the second member is the quotient of the energy (expressed in ergs) divided by the temperature (in centigrade degrees). In English units and the F.P.S. system this numerical value is .
From the whole development we see that entropy depends only on the number of complexions; it should not be considered, as is sometimes done, as of the same dimensions of energy or anything that may generally be called a factor of energy.