Much has been argued about the separate existence of cold, from such facts as these: A piece of ice held before the thermoelectric pile, produces an opposite deflection of the galvanometer to that due to a hot ball. If a freezing mixture be placed at one focus of a spheroidal mirror, and a thermometer with a blackened bulb at the conjugate focus, the latter will fall speedily, though very far off 'from the mixture. low, the real explanation of such observations is to be found in what is called the "theory of exchanges," first enunciated by Prevost, and since greatly extended and carefully verified by Stewart, which is to this effect: " Every body is continually radiating beat in all directions, the amount radiated being (nearly) proportional to its own temperature." Hence the apparent radiation of cold in the experiments above mentioned is due to the fact of the pile or thermometer radiating of more heat than it receives, as its temperature is higher than that of the freezing mixture to which it is opposed. From this it is evident that any number of bodies left near each other, tend gradually to assume a common temperature. By this theory of exchanges, we explain the cold felt in sitting opposite a window in a frosty day, even when there is no draught.
hot body cools faster in a current of air than in a still atmosphere of the same temperature, evidently because fresh supplies of the colder air are continually brought into contact with it. It is by convection mainly that heat is conveyed from particle to particle in liquids and gases. Thus, when a lamp is applied to the bottom of a vessel of water, the heat does not diffuse itself in the water as it would (by con duction) in a mass of metal, but the expansion of the heated water at the bottom render ing it lighter, bulk for bulk, than the superincumbent fluid, causes it to rise to the sur face; and thus, by convection, the heat is diffused throught he mass. Conduction, properly so-called, can scarcely be shown, even if it really exist, in liquids or gases, on this account. The trem thous appearance of any object as seen by light which passes near a hot surface, as that of a boiler or a red-hot poker, is due to the convection of heat in the air, the warm current refracting light less than the cold air. See VENTILATION.
For the mechanical applications of heat, see STEAM-ENGINE, CALORIC-ENGINE, Cie.
Sources of may be, so far as we know, ultimately reduced to two chemical combination, and mechanical force; and, indeed, in all probability, the former is only a variety of the immensely different forms in which the latter is manifested. A more full examination of this point, and a general statement of the ultimate nature of the various sources of heat, will be found in the article FORCE above referred to. See also CONII3USTION, FUEL.
HEAT, (ante.). The history of thcrmotics, the science of heat, from the earliest speculations to the most recent investigations, is full of interest. The ancients held notions in regard to it, which, although they lacked the precision which attends modern scientific investigation, were remarkable examples of the power of human reason to advance in the direction of truth unaided by anything except the inward light vouchsafed by the Creator. The medium of light and heat radiation, the cosmic or interstellar ether, which within the last century has been demonstrated to have an existence, was believed in by many of them. It was often the divine personification of cosmic material and also of force. In the Orphic hymns Miler is the soul of the uni verse and the author of all life. Anaxagoras, who taught that the sun was an inanimate fiery mass and not a deity, considered /Ether to be the principle of fire or beat, and a half a century later Democritus, born about 460 B.C., taught that beat was produced by the efflux of extremely minute particles of matter, and moving with such velocity as to penetrate solid bodies; that some of these particles were infinitely small and from them the soul was formed. Plato had similar ideas, which may be found in his writings.
Aristotle believed heat to be a condition of matter, and not a material substance, and his ideas were probably the first suggestions of a purely mechanical theory of heat. Twenty centuries later, Francis Bacon, 1561-1626, said in his .Norum Organum, " lice 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, striving and struggling, and irritated by repercussion, whence springs the fury and fire of heat." Three-quarters of a century later, John Locke made a statement which approached still nearer the modern ideas on the subject. He sacs: " Heat is a very brisk agitation of the insensible parts of an object which produces in us that sepsation from whence we denominate, the object hot; so that what in our sensation is heat, in tfiq object is nothing but motion:" About the same time, Huygens, in his Tractatus de Lumine, brought forward the undulatory theory of light, which contains many passages approaching much nearer than anything previously written, or for a century afterwards, to what may be called a scientific exposition of the laws of heat and light, lie says: "It appears that light when gathered in the focus of a concave mirror, has the property of burning like lire, that is to say, it dissociates the particles of bodies, and this most certainly indicates motion, at least according to that philosophy wherein the causes of all natural effects are conceived by means of mechanical reasons." As a general statement of the doctrines of heat as a mode of motion this has not been put in much better words since his time. The doctrine of the actual converti bility or heat into mechanical force, which should stand as an equivalent, cannot be said to have been fairly started previous to the experiments of the American count Rumford. When at Munich superintending the manufacture of ordnance for the Bavarian government his mind was impressed with the great production of heat in boring cannon. By the use of a borer of an inch in diameter, applied with a pressure of 10,000 lbs. and a velocity of 32 revolutions per minute, sufficient heat was produced to raise 18 lbs. of water from 60' to 212' F. in two hours and a half. The capacity of heat in the turnings having not changed, he concluded that the heat, whose source seemed to be inexhausti ble, was the result of motion. Quantitative determinations, however, were necessary to demonstrate the correlations of heat and mechanical force, or heat and motion. These were furnished by the experiments of Dr. J. P. Joule of Manchester, England, and by Dr. J. H. Mayer of Hielbroun, Germany, which established what has been called the 'mechanical equivalent of heat. Joule's experiments were made in various ways. In one he employed paddle-wheels, which were made to revolve with a measured power in various liquids, whose specific gravities being known, the mechanical force could be compared with the amount of heat generated. Disks of metal were also revolved and forced against each other, the result in the evolution of heat being the same in all cases. He established the law which goes by the name of Joule's equivalent, or the dynamical unit of heat, viz.: that the fall of 772 lbs. through the space of one vertical foot affords a force sufficient to raise the temperature of water 1° F. In other words, the force given by the fall of 772 lbs. through one foot is equal to that generated by the elevation of 1 lb. of water 1° F. Dr. J. R. Mayer arrived at the same conclusions a year or two earlier by investigating the effects of the expansion and compression of gases, (see Mechanical Equivalent of Heat in Correlation and Conservation of Forces, N. Y., 1876. BOILING OF LIQUIDS, DIATHERMANCY, FORCE, RIIMFORD,) (ante.).