An irreversible process is a passage from a less probable to a more probable state of the system.
An irreversible process is a passage from a less stable to a more stable state of the system.
An irreversible process is essentially a spontaneous one, inasmuch as once started it will proceed without the help of any external agency.
We have in a general way reached the conclusion that entropy is both the criterion and the measure of irreversibility. But now let us become more specific and go more into certain details, namely, the common features in all irreversibility. The property of irreversibility is not inherent in the elementary occurrences themselves, but only in their irregular arrangement. Irreversibility depends only on the statistical property of a system possessing many degrees of freedom, and is therefore essentially based on mean values; in this connection we may repeat an earlier statement, the individual motions of atoms are in themselves reversible, but their result in the aggregate is not.
(3) All the Irreversible Processes Stand or Fall Together
This is proved with the help of the theorem ([p. 30]) which denies the possibility of perpetual motion of the second kind.[15] The argument is this: take any case in any one of the four classes of irreversible processes given on [p. 31]. Now if this selected case is in reality reversible, i.e., suppose a method were discovered of completely reversing this process and thus leave no other change whatsoever, then combining the direct course of the process with this latter reversed process, they would together constitute a cyclical process, which would effect nothing but the production of work and the absorption of an equivalent amount of heat. But this would be perpetual motion of the second kind, which to be sure is denied by the empirical theorem on [p. 30]. But for the sake of the argument we may just now waive said impossibility; then we would have an engine which, co-operating with any second (so-called), irreversible process, would completely restore the initial state of the whole system without leaving any other change whatsoever. Then under our definition on [p. 30] this second process ceases to be irreversible. The same result will obtain for any third, fourth, etc. So that the above proposition is established. "All the irreversible processes stand or fall together." If any one of them is reversible all are reversible.[16]
[15]At this stage we appreciate that any irreversible process is a passage from a state
of low entropy to a state
