ENERGETICS. 73 measure work and energy. In the C. *'<■ S. system the absolute unit of work is the erg, which is the work of a force of one dyne, exerted along a ]>alh one centimeter in Length. In the foot- pound-second system the absolute unit, is the i'oot-poundal. In gravitation measure the cor- responding units are the gram-centimeter and (lie foot-pound. When expressed in the funda- mental quantities of length, mass, and time, (he dimension of energy in mechanical measure is al- ways of I he order of the square of a length, mul- tiplied by the first power of a mass, and divided by the square of a time, or L'-'MT - -. (It should be noted that, the two lengths which enter this expression are in the same direction. A moment of a force has the dimensions LrMT -2 , but the two lengths are at right angles to each other.) By this we may see whether physical quantities represent energy if we observe whether they are expressible in these dimensions. For example, in a gas the pressure per unit area, p, is of the, dimen- sions L"'MT" ! , and the volume, v, is of the dimen- sions L s ; then the product pv has the dimensions leMT"*, and it may be shown that the work done in compressing a gas is the product of the pressure and the change in volume. In heat the unit is the calorie, which equals 4.2 X 10" ergs. The abso- lute unit of energy of an electric current is also the erg, but the practical unit is the work of transferring a coulomb of electricity over one volt difference of potential, and is called a volt- coulomb, or one joule ; its mechanical value is 10 7 ergs. The calorie therefore equals 4.2 joules, or one joule equals .24 calorie. Dissipation of Energy. Irreversibility, En- tropy'. If a bodj' 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 criterion 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 during 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 h at the temperature *, the ratio of change in heat to the temperature at which the change is effected being — and let a succession of such changes bring the body to a state B. If now changes be effected in the opposite sense in exact- 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 ENERGETICS. the intermediate stages; there is a distinct and measurable physical property ol a body, which is characterized bj the peculiarity thai ii in creases or diminishes as heal enters or lea the body, but remains constant when there is no i munication of heat. This quantity is called the entropy," _ being the measure of change in it. ( Hastings and Beach.) Planck shows that electro-magnetic radiation, which may now 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 /,, to another at the temperature t % ; the entropy of the former is diminished by — h ' ' that of the latter increased by—, and since t t exceeds t', in order that heat shall so pass, the en- tropy of the two is greater than at first by h I — — — Therefore all transfers of heat by radia- 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. Sources of Energy; Matter; Ether. Of the ultimate nature of energy, like that of matter, we are ignorant; nor do we know of energy by direct observation, except as associated with mat- ter. It is the change of energy, either as to quantity or form, that is scrutinized and meas- ured, and such change is continually occurring as energy passes from body to body ; but the condition of a body totally devoid of energy is a subject of speculation only. Matter, on the other hand, is to the physicist only a vehicle for energy; to hold or to convey energy is its sole physical function. Any substance which performs this function, whatever else it may do, is, in so far, of the nature of matter, and this function is performed by the luminifcrous ether through which radiant energy is conveyed. In a radiating body of ordinary matter and ether the total energy must be shared by both. Terrestrial energy available to man may be classi- fied as energy of fuel, food, head of water, wind, tides, chemical action, solar radiation. These. with a partial exception in the ease of the fifth and sixth, are all traceable to the sun. The earth receives ^T^Winr P art of the tntal solar radiation, and the rate at which radiant energy from the sun is expended upon a unit area of the earth's surface is called the 'solar constant.' From tests conducted above the clouds, notably at Mont Blanc and at Mount Whitney, this constant has been determined.
