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 CORRELATION OF FORCES 377 viour, shows his command over the stronger contrasts of light and shadow, as well as his utter carelessness in drawing the subordinate parts of his pictures; while in his "Mercury instructing Cupid," the gradations are so fine that the shadows seem mutable and aerial, as if between the eye and the colors. The read- ing Magdalen, in the Dresden gallery, has per- haps been multiplied through engravings and copies beyond any other picture. Among other celebrated pictures are the St. Catharine in the Louvre, the St. Jerome in Parma, the St. George in Berlin, and the " Zingarella " Ma- donna in Naples. Correggio probably had no pupils, unless the persons who assisted him in executing his frescoes can be called such ; but his imitators, who formed what was called the school of Parma, were numerous. Among the latter, the most celebrated was Parmigianino. His son, Pomponio Allegri, followed his father's style, but acquired no great reputation. CORRELATION OF FORCES, and Conservation of Force, terms now used to express certain rela- tions among the forces of nature which have been discovered by modern physical investiga- tion. The view is that the various forms of force are mutually convertible into each other, while at the same time power, like matter, is indestructible, its total amount in the uni- verse being conserved, or remaining perpetu- ally unchanged. The principle is described by Helmholtz as " a new and universal natural law," and by Faraday as "the highest law in physical science that our faculties permit us to perceive." Two profound errors long pre- vailed in regard to the workings of nature. The first was that the material elements were transmutable into each other, which led to the search for the philosopher's stone and the art of making gold. The second was that force could be spent, destroyed, or annihilated, and could be created and come again into existence out of nothing ; and this led to the pursuit of the perpetual motion. Before science arose with its rigorous investigations, these were far from being irrational beliefs. "When but four elements were known, fire, air, earth, and wa- ter, the changes of nature seemed to be little else than transmutations of substances. Such facts as combustion, by which solid fuel dis- solved in smoke and left ashes ; the rusting of metals, by which they seemed to become new substances ; the absorption of water by quick- lime, by which it was apparently changed into earth ; and the extraction of metals from ore by heat all of which were familiar before chemical science arose were only explicable by the idea of the transmutation of material ele- ments. To produce gold from baser metals seemed therefore to be in accordance with the possibilities of nature, and was long an object of experiment by the alchemists. On the other hand, force, or that which produces the movements of matter, was constantly seen to be expended and to disappear. Bodies set in motion always came to rest, the motion ap- parently ceasing. Beasts and men were in action all their lives without being wound up or set in motion, as food was not understood to be a source of power, and the development of force out of itself or out of nothing ap- peared to be the essence of organic life. The ever-revolving planets, besides, were an ex- ample of perpetual motion, and it was there- fore thought to be within the compass of nat- ural operations to construct a machine that should go on for ever creating its own force. The first great steps toward the establishment of the modern scientific philosophy of nature were due to the perfection of the instruments of investigation and the gradual development of alchemy into chemistry. The introduction of the balance by Lavoisier and the art of ex- act weighing put an end to phlogiston; and with the discovery of oxygen chemistry was planted upon its firm experimental basis, with the establishment of the doctrine of the sta- bility of the chemical elements. This ascen- dancy of chemical ideas favored the view that the forces are also of the nature of subtle elements. They were hence regarded as en- tities, imponderable material substances, which were supposed to be no more convertible into each other than metals or gases. The effects of heat were ascribed to the substantive prin- ciple of caloric ; light to the emanation of ma- terial corpuscles; electricity and magnetism to ethereal fluids ; and even sound was attrib- uted to a peculiar resonant ether before it was explained by atmospheric vibrations. There was also a tendency to explain chemical effects by a peculiar entity called affinity, and the ac- tions of living beings were held to be due to the agency of vitality. The various effects of forces were ascribed to the properties of these subtle agents ; and when the chemical elements were proved to be not transmutable, it was con- sidered that the same thing must be true of the imponderable elements. It has been main- tained that the principle of the conservation of force is involved in the old mechanical prop- osition that action and reaction are equal, and that cause must equal effect ; and that there- fore the doctrine is as old as the writings of Galileo, Newton, Bernoulli, and Laplace. How- ever this may be, it is certain that these phi- losophers had no conception of the law as it is now established, and which is purely the re- sult of modern experimental research. It has grown out of investigations into the properties and effects of heat. There are indeed remark- able intimations of the doctrine now estab- lished in relation to heat in writers of the 17th and 18th centuries. Lord Bacon in his Novum Organon says : " When I say of motion that it is the genus of which heat is the species, I would be understood to mean, not that 'heat generates motion, or that motion generates heat (though both are true in certain cases), but that heat itself, its essence and quiddity, is motion and nothing else." Locke has the following remarkable passage : " Heat is a very