DISSIPATION OF ENERGY, IRREVERSIBILITY, EN TROPY. If a body is changed from one state to an other by a series of operations, and then, when subjected to these operations in an opposite sense and in exactly the reverse order, the body returns to its first state, the whole cycle of operations is called a reversible process. When the trans formations cannot be repeated in the reverse or der, or if, when reversed, the agencies are not equal in magnitude and opposite in sign to those which occur at the same points in the direct process, the transformation is said to be irrever sible. According to Planck, the most immediate enterion of irreversibility of a process consists in the proof of a function completely determined by the instantaneous condition of the system, which possesses the peculiarity that it changes (hiring the whole process always in the same sense—perhaps increases. Suppose a body in a state A as to its temperature and quantity of heat, to be changed by any small quantity of heat k at the temperature t, the ratio of change in heat to the temperature at which the change is effected and let a succession of such changes bring the body to a state B. If now changes be effected in the opposite sense in ly the reverse order, and when the sum of the re versed quantities equals the sum of the direct ones, the body has regained the state A, the cycle through which it has passed is a reversible one. Then "the limit which the sum of the h.
quantities from the state A to the state B approaches is a constant quantity which depends only on the pressure, volume. and temperature at the state A and the state B, and not all upon the intermediate stages; there is a distinct and measurable physical property of a body, \ is characterized by the peculiarity that it in creases or diminishes as heat enters or leaves the body, but remains constant when there is no communication of heat. This quantity is
h called tIte entropy,' being the measure of change in it. (Hastings and Beach.) Planck shows that electro-magnetic radiation, which may HIM be considered the form in which energy is transmitted through the ether of space, under conditions termed by him 'natural radiation,' is attended by a constant increase of entropy in the system of bodies emitting and receiving the rays. By the above criterion, then, such radiation is not a reversible process. Suppose a quantity of heat h to pass from a body at tem perature t„ to another at the temperature the entropy of the former is diminished that of the latter increased by and since exceeds in order that heat shall so pass, the en tropy of the two is greater than at first by h ) Therefore all transfers of heat by radia t2 tion and conduction in a system of bodies in creases the entropy of the system, and hence, "the entropy of the universe tends to a maximum." (Clausius). In any irreversible process, then, a portion of the energy expended in some of the operations cannot be regained for a similar purpose; they are available only for purposes requiring energy of a lower order. No known natural process is exactly reversible, a certain portion of the energy being always dissipated as heat. It follows that, as energy is constantly undergoing transformation, there is a constant degradation of energy to the final unavailable form of uniformly diffused heat.