The four irreversible processes just discussed are all spontaneous ones, i.e., they occur without the help of agencies external to the bodies directly engaged in the transformations.
It is evident that the foregoing statements are really identical, expressing the same thought in different ways.
SECTION C
NEGATIVE CHANGE OF ENTROPY; SOME OF ITS PHYSICAL FEATURES OR NECESSARY ACCOMPANIMENTS
A negative transformation in any part of a system is the diminution of entropy which it experiences, and this we know means a diminution in the number of complexions of the part considered. But there are some features of such negative transformations which, while they do not in themselves constitute any additional principle, deserve special mention.
Before we make such mention, however, we will anticipate a little, and state the Second Law in forms which will make said features obvious:
In an irreversible cycle the sum of the changes of entropies experienced by all the bodies concerned is greater than zero. When the cycle is reversible in all of its parts, then said sum of entropy changes is equal to zero.
A corollary from this theorem is that, in a cycle,
All the negative transformations present
all the positive transformations that occur.