HEAT 567 HEAT, the natural force or principle which is known by its effects upon matter, causing it to expand, or to assume a solid, a liquid, or a gaseous condition, according to the degree with which it acts and the nature of the body. It is also known by its effects upon the sense of feeling, but only in a comparative manner ; because a body at the same temperature may produce a sensation of heat at one time, and at another a sensation of what is called cold, in consequence of a variation of temperature in the organs of touch ; and a body may at the same time seem cold to one hand and warm to the other. The science which treats of the phenomena and properties of heat is called thermotics. Two general theories of the na- ture of heat have been held from the earliest times : one regarding it as a kind of subtile matter which insinuates itself into the sub- stance of bodies, and resides there with a greater or less manifestation of its presence ; the other as simply a condition of matter, a force, or a molecular motion. Some of the ancients called it the fourth elenient, which by its levity rose to the highest place in the heav- ens and spread itself in ethereal lam bent flames over the universe ; and the ancient philoso- phers of all nations generally regarded it as a subtile efflux, an attribute or manifestation of creative power, or as the creative power itself, the vital spirit of the universe ; and thus the sun was by many nations regarded with pecu- liar veneration, and adopted as the chief object of worship, forming the basis of religions which have been preserved by some peoples till the present time. Democritus (born 460 B. 0.), regarded as the originator of the doctrine of atoms which in the hands of John Dalton twenty-two centuries later was elaborated to a highly philosophical theory, and which since his day has been placed upon a basis of al- most mathematical precision, conceived heat to be an efflux of minute spherical particles, hav- ing a rapid motion by which they penetrated the densest substances. He believed that the finest of those particles formed the substance of the soul, and Lucretius held similar views. Aristotle considered it to be a condition of matter rather than a substance, and was prob- ably the first to suggest an immaterial or pure- ly mechanical theory. In later times Francis Bacon advocated the doctrine of its immateri- ality, and some passages in his Novum Orga- num are remarkable for the hints they contain of the dynamical theory of heat; he says: " Heat is a motion of expansion, not uniformly of the body together, but in the smaller parts of it ; and at the same time checked, repelled, and beaten back, so that the body acquires a motion alternate, perpetually quivering, stri- ving, and struggling, and irritated by repercus- sion, whence springs the fury of fire and heat." Descartes also, in his Principia Philosophica, has some observations foreshadowing the vi- bratory theory, in which he speaks of heat as being the motion of the insensibly small par- ticles of matter, and upon this theory explains why bodies get hot under concussion. Locke, a half century later, places the theory in a still clearer light. " Heat," he says, " is a very brisk agitation of the insensible parts of an object which produces in us that sensation from whence we denominate the object hot ; so that what in our sensation is heat, in the object is nothing but motion." (Works, vol. iii., p. 327, London, 1823.) The ideas of the old philosophers on the subject of heat possessed a good deal of vagueness, and were derived mainly from speculation, and not from actual experiment. They were not even put to the test of inquiry as to their adequacy to account for phenom- ena ; and although they contained germs of truth, they cannot be regarded as much more than the remarkable opinions of great minds, who lacked the advantages of accurate chemi- cal and physical investigations. Between the time of Descartes and Locke, Becher, a Ger- man chemist living in England, proposed a the- ory, more fully elaborated soon after Locke's time by Stahl under the name of the phlogis- tic theory, which held that phlogiston is the principle of heat, and that combustible mat- ter is a union of this principle with ordina- ry matter, and that when this is burned the phlogiston is expelled. To account for the in- crease in weight of metals after calcination, it was held that the combination with phlogis- ton, in consequence of its buoyancy, rendered them lighter. The discovery of oxygen by Priestley, and the establishment of the oxygen theory of combustion by Lavoisier, overturned the phlogistic theory, but left in its place an equally material theory which regarded ca- loric as the imponderable element which con- stitutes heat. Lavoisier and Black were the great promulgators of the material doctrine, holding that caloric is an actual substance hav- ing the power of combining with ponderable matter and of passing from one body to anoth- er. The caloric theory lasted a long time, and perhaps did not obstruct the progress of sci- ence as much as is often thought, for many im- portant results were obtained by experiments which were made under a belief in its truth. It was easier to conceive of definite quanti- ties of a substance susceptible, as heat was, of measurement, than of quantities of motion which had not been demonstrated, and of which no definite conception had been formed. The doctrine that heat could not be produced, but was an original and indestructible element, passing from one body to another, was also a consequence of these views, and any experi- ments which seemed to demonstrate that heat could be generated by mechanical motion were calculated to overturn it. Such experiments were made by Count Rumford in 1796-'8, soon after by Sir Humphry Davy, and more recent- ly by Mr. Joule of Manchester, by which it was demonstrated that mechanical power and heat were mutually convertible forces. A de- scription of these experiments, and a discussion
