HEAT. 683 HEAT. immediate cause of the phenomenon of heat is motion, and "the hiws of its communication are precisely the same as the hiws of the communica- tion of motion." Kuniford and Davy were also assisted by Thomas Young (1773-1820) in their attempts to displace the theory of caloric, but, in spite of their eti'orts, the old theory continued to be accepted for many years, and, in fact, Sadi Carnot (1796-1832) in his famous treatise, Keflexions sur la puissance motrice de feu, rea- soned on this basis, although later he became finally convinced of the truth of the dynamical theory. It is to the experiments of Joule, how- ever, that we owe the most complete evidence in favor of the idea that heat-effects are due to en- ergy. ( It should be borne in mind that no ex- periments can p'l-ove this relation between cause and effect; they can simply be shown to be in accord with the theory.) The modern theory that heat-effects are due to energy alone is found- ed largely on the work of Carnot and Mayer. (See Thermodynamics.) The researches on heat are more or less closely connected with the development of the thermome- ter first made and used by Galileo (156-1-1642), as described under Thermometer. There was also the stimulus of the invention of the steam- engine, and from the time of the construction of the first practical steam-engine (1711) many of the inventors who improved that machine carried on investigations in heat. Joseph Black (1728- t)9), who was among the foremost of these, was an assistant of James ^'att, the great Scotch inventor, and later a professor of chemistry at Edinburgh. He derived the idea of latent and specific heats, and was the first to use proper methods for calorimetry (q.v.) ; Lavoisier (1743- 94) and Laplace (1749-1827), working along lines somewhat similar, determined specific heats of a number of substances and devised calorim- eters. Natire of Heat-Effects. One of the sim- plest modes of producing heat-effects is by means of friction. If two pieces of ice are rubbed to- gether they will melt; if two pieces of metal are rubbed violently their temperature rises — as de- tected by the "temperature-sense' — they also in- crease in volume; if a paddle is turned rapidly enough in a vessel of water the water will boil. It requires work to produce friction ; and the energy thus lost by the agency doing the work is gained by the bodies on which the work is done and which manifest the heat-effects. Simi- larly, in every known case, if such heat-effects as rise in temperature, expansion, boiling, melting, etc., are produced by any external agency, the latter can be proved to have lost energy; and the 'amount of the heat-effect' is proportional to the energy received. If, on the other hand, a body experiences the reverse changes, such as fall in temperature, contraction, condensation, freezing, etc., it may be proved that it loses energy. This energy which the body gains or loses is not ki- netic energy of the whole body or of its visible parts, nor potential energy of strains of the entire body; it is energy associated with the minute portions of the body — its moleciiles and atoms. The energy of the body considered by itself — omitting any kinetic and potential energy of the whole body that it may have — is called its 'in- trinsic' energy. This energy of the molecules and atoms is partly the kinetic energy of their irreg- ular motions and vibrations, and partly potential Vol, IX.— m. energy in case it requires work to change their relative positions. In ordinary language a flame is called a 'source of heat;' wlien a body is brought near it is said to receive 'heat;' and the ell'ccls produced are said to be due to heat. These expressions are most unscientific and misleading. . fiame is a source of encrg)'; when a body is brought near, energy passes from the flame to it; part of this energy is spent in increasing tlic in- trinsic energy of the body, and part in enabling the body to do external work, e.g. if it expands it pushes back the atmosphere or whatever rests against it, thus doing work. It is proper to speak of the energy which the body receives as 'heat- energy;' and in accordance with the conservation of energy one may say that 'heat-energy received' ^ 'increase in intrinsic energy' + 'external work done.' One can speak of the intrinsic energy in a body (although no idea can be formed of its amount or even nature) ; but it is as improper to speak of the amount of 'heat' in a body as it is to refer to the amount of sound in a horn or the amount of light in a candle. Energy of a Gas. One of the most im- portant facts in regard to a gas is that, if it is allowed to expand under such conditions that it does no external work, there is no sensible change of temperature, showing that it has required no work to separate the molecules, and that the molecules themselves have not lost energj'. In other words, there are no sensible forces either of attraction or of repulsion between the mole- cules of a gas, and so the energy of the molecules is entirely kinetic. This fact was first shown by Gay-Lussac and later by Joule; and, although the more elaborate experiments of Thomson and Joule show'ed that there were minute changes in temperature when a gas expanded freel}', they indicate that the molecular forces are extremely small. If a gas is allowed to expand in such a manner as to do external work, the energy re- quired for the work is taken from the kinetic energy of the molecules; a fall in temperature is always observed under these conditions when external work is done ; and therefore the tempera- ture of a gas depends upon the kinetic energy of its molecules. Temperature, The units to be used in the measurement of the quantities involved in heat- effects are in most eases self-evident: the 'heat- energy' itself should be expressed in ergs or joules (q.v.) ; changes in volume, in cubic cen- timeters; melting or boiling, in number of grams experiencing the change, etc. The difficulty comes in giving a numerical value to 'temperature,' Primarily this is a question of sensation; and. although our senses give us a rough idea as to hot and cold bodies, they do not enable us to give numbers to the property of these bodies which corresponds to these sensations. Recourse must be had to the changes which some natural object undergoes under the conditions when the senses recognize dift'erences in temiternture. changes which may be measured. One of the simplest of these changes is alteration in vohime. Two standard thermal states must be chosen arbi- trarily, e.g. the thermal state of a mixture of pure water and ice at normal barometric pressure, and that of the vapor rising from pure water boiling under normal barometric pressure, because experiments have shown that tbesp conditions are perfectly definite and unvarying. Let r, and v, be the volumes of any definite body, e,g, a piece of
