:I hem prui-. and posses in'' energy in POIP•e• fluent, f its train. equal to the work done in it. if it i permitted to itself ,m in part, therehs movin• moo other body, it •oe• work on Chi body. and if the only effect is to give it a certain velocity, the kinetic energy the body acquires will exactly equal the work Clone by the spring. But the spring will now possess less energy, less capacity tor doing work, by the amount it has done; the energy it loses equals that gained by the other. the total energy of the two, however, is the same as be fore. If the two bodies are viewed as comprising a system, the total energy of the system is not altered by any exchange of energy between its parts. We may extend this consideration in definitely. Given a system of bodies arranged in a definite configuration, and with certain stresses between them: if in obedience to these stresses a rearrangement of time bodies takes place. a change of configuration ensues, the energy of the parts may be altered, but the total is not changed. The energy is conserved, and such a system is called a conservative system. So far as known, all material systems are eon servative. The energy of such a system is a quantity that can neither be increased nor dimin ished by any action between the bodies them selves, though the form of energy may be changed If the total energy is to be increased, it can only be done by work expended upon the system by some external agent, and then the agent loses energy by the amount it expends upon the sys tem. By further extending the same considera tions we reach the view that energy cannot be created or destroyed, and that the total energy of the universe is a constant quantity. This is the doctrine of the conservation of energy. Kinetic energy is constantly being changed into potential, and viea versa; but besides the forms ill which energy has been mentioned, it exists in a variety of other forms which are not so obviously of a mechanical nature. Both potential and kinetic energy may be classified, as follows: Potential. Kinetie.
Strain (extension, compress motion (translation or ro sion ,,r distortion ). tation).
Electrification. vibration.
Magnet ization. Electricity in motion.
Chemical separation. Heat.
tirarit stir, separatiim. Radiation.
In all wave-motions, i.e. in radiation, enemy is transferred from one point to another; and at any instant part of the energy of the medium carrying the waves is kinetic and part potential. In heat-phenomena, all beat-efTeets are due to the addition or withdrawal of energy front the molecules or small portions of the bodies expe riencing the effects; the energy of the minute parts is both kinetie and potential in general The energy of t he parts of a gas is nImost. entirely Motion and strain are the obvious me chanical forms of energy, and their l•quivalence wasp:tidy reeognized,hut energet ies today involves the statements (a) that energy in any form may he ehanged into energy of any other form, which is a declaration of the correlation and transfor vm lion of tmergy; (Il) that when energy in any form disappears. an exact equivalent of sonic other form or forms takes its place, which is a declaration of the coaxers/lion of energy, and (e) that when energy undergoes transformation, nr transferenee from one body to another, the prowess is not eionph•tely reversible, but that if some of the energy is recovered in its original form a n...11111:11 portion reappears in what is called a form. This is the degradation and dissipotion of energy. Both (a) and (b) are commonly implied in the principle of con servation of IlisTonicAL S K n. Although the concept of force as the effort made in doing work, or that of energy as the capability of a body to do work, might either have been made the starting-point »Ar it system of dynamics, the former, which was the Newtonian, was first and most completely developed. This seems to have been owing to the fact that force appeals to our so-called `muscular sense' or sense of muscular effort, whereas there is no distinct sense-pereeption of work or energy. When branches of physical science other than mechanics were found to he related to mechanical work with a definiteness that had not before been suspected, the field of energy was widened and the Buygenian concep tion became a more familiar one. The first great step of this kind was the recognition of an identity equivalence of mechanical work and of heat-effects produced by such work. Experiments by Count Rumford in 1798, on beat produced by the boring of cannon, and by Sir Humphry Davy, in 1799, ou melting of ice by friction, introduced the idea that heat is a form of energy for which there is an exact mechanical equivalent. This proposition, con troverting the then accepted theory that heat is material, was too radical to meet with wide acceptance, and the subject received little further development for nearly half a century. It was reasserted by Julius Robert Mayer in a philosophical discussion in May, 1842, but his determination of a definite numerical value for the mechanical equivalent of heat was not made public until 1345. To measure the unit chosen was the 'quantity of heat' necessary to raise a unit mass of water one degree in temperature Taking for the unit mass one gram, and the Centigrade scale for temperatures, the heat unit is called a calorie (or sometimes a degree). The unit of work in gravitation meas
ure may he taken as the work of lifting one grain weight a height of one meter, called] gram-meter. In the various experiments other units were employed. but we give the results, re duced to these. The problem was to determine how many gram-meters can produce one calorie. and are therefore equiralent to it in energy. This number is called the mechanical, or better. the dynamical equivalent of heat. It may of course he expressed finally in absolute units of work. Mayer's value, 365. was obtained by oh serving the heat evolved in compressing air. In January, 1843, James Prescott Joule read a paper before the Philosophical Society of Man chester regarding the thermal and chemical ef fects of an electric current, which was followed by other investigations in rapid succession, and on August 21, 1840, he communicated to the Brit ish Association for the Advancement of Science the result of a most significant investigation, in a paper "On the Calorific Effeels of Magneto-Elec tricity. and on the Mechanical Value of Heat." He obtained for the latter 460 gram-meters. Then, by allowing the work to be done by weights descending under gravity. and the heat to be pro duced by the friction of water forced through narrow tubes, he obtained the value 423 gram meters. In November of the same year a Danish engineer, A. Colding, presented before the Acad emy of Copenhagen the results of experiments upon the heat produced by friction of solid bod ies, and expressed the view that the law of con servation of force was a general one. His result for the D. E. was 370. Although the principle of conservation of energy was suggested almost simultaneously by several physicists, Joule was most indefatigable in the prosecution of his exper iments, and within two years he had determined the dynamical equivalent of heat by a variety of methods, the most celebrated of which was the employment of descending weights to drive pad dles in a vessel of water, the latter being heated by the friction of the currents in the water pro duced by the vanes. His paper giving an account of this determination was published in the Philo sophical Magazine in 1845, and has become elastic. In 1847 appeared a discussion of the subject by Helmholtz, 'entitled "Ueber die Erhal tung der Kraft," which contributed greatly to the establishing of the principle. Between that time and 1860 the whole subject was discussed theo retically and extended experimentally by Helm holtz, Joule, Rankine, Thomson (Lord Kelvin), Clausius, Maxwell, and many others. By 1850 Joule had obtained the value 423.55 gram-meters as his best result, and that number stood as the most acceptable value for more than twenty years. In this interval, however, many experi ments were made to determine this important quantity by transformations of energy through mechanical, electric, magnetic, and chemical pro cesses, and by 1860 the generally accordant re sults had conclusively demonstrated not only that beat is a form of ener;,ty, but they also demonstrated 'the conservation of energy. The effect upon scientific investigation was extraor dinary. "From now on, one was in possession of a principle which, tested in all known realms by careful experiments, offeied now an excellent guide also to wholly unknown and unexplored regions." (Planck.) "It is the one generalized statement which is found to he consistent with fact, not in one physical science only, but in all. When once apprehended, it furnishes to the physical inquirer a principle on which he may hang every known law relating to physical ac tions, and by which he may be put in the way to diteover the relations of such actions in new branches of science." (Maxwell.) So important a constant is the dynamical equivalent of heat that attempts to determine its value have been made in many ways, including among them various indirect methods in which heat is pro duced electrically or otherwise than by mechani cal work directly; but the most elaborate re determination by Joule's method of the friction of water by stirring was made in the years 1S77 to 1879. by Professor H. A. Rowland, in Balti more. 11-Id., U. S. A. His results are, on the whole, the most acceptable, and give not only a highly accurate Value, hut bring out the differ ences in the value owing to differences in the specific heat of water at different temperatures. They range from 429.8 at 5° C. to 425.S at 36°, passing, through a minimum value of 425.5 at 29°. To express these values in ergs they must he multiplied by 100 times the weight of one gram in dynes at the place to which the results apply. This weight is numerically equal to the acceleration of gravity in ems. per sec'. At Baltimore, g = 980.05 and Rowland's mean value of the dynamical equivalent from 20° C. to 35° C. is 425.9 grain-meters, or 4.17 X 10' ergs. The value 4.2 X 10' ergs is usually assumed as the mean value, and this is Rowland's value for water at 10' C. The work of lifting one gram a height of 425.9 meters, or 425.9 grams a height of one meter, against gravity, is exactly sufficient to raise one grain of water one centigrade de gree in temperature.