558 HEAT Joule s great experiments from 1840 to 1849, l creating new provinces of science in the thermodynamics of electricity, and magnetism, and electro-chemistry, recalled attention to Davy and Kumford s doctrine regarding the nature of heat, and supplied several fresh proofs, each like Davy s absolutely in itself complete and cogent, that heat is not a material sub stance, and each advancing with exact dynamical measure ment on the way pointed out by Rumford in his measure ment of the quantity of heat generated in a certain time by the action of two horses not urged to overwork them selves. The full conversion of the scientific world to the kinetic theory of heat took place about the middle of this century, and was no doubt an immediate consequence of Joule s work, although Rumford and Davy s demonstrative experiments, and the ingenious and penetrating speculations of Mohr, and Sdguin, and Mayer, and the experimental thermodynamic measurements of Colding, all no doubt contributed to the result. Each of the several subjects of thermodynamic measurement undertaken by Joule gave him a means of estimating the quantity of work required to generate a certain quantity of heat ; but after several years of trials he was led to prefer to all others the direct method of simply stirring a quantity of water by a paddle, and measuring the quantity of heat produced by a measured quantity of work ; and this method he has accordingly used in all his experiments for the purpose of determining the "dynamical equivalent of heat" from the year 1845 to the present time. By this he found his final result of 1849, 2 which was 772 Manchester foot-pounds for the quantity of work required to warm by 1 Fahr., at any temperature between 55 and 61 Fahr., 1 Ib of water weighed in vacuum. In 1870 he commenced work for a fresh deter mination of the dynamical equivalent of heat at the request of the British Association, and the result was communicated to the Royal Society 3 about the end of 1877, with the following preface : "The, committee of the British Association on standards of electri cal resistance having judged it desirable that a fresh determination of the mechanical equivalent of heat should be made by observing the thermal effects due to the transmission of electrical currents 1 List of titles of, and references to, papers by Dr James Prescott Joule, F.R.S. : "Description of an Electromagnetic Engine," Sturgeon Ann. Electr., ii., 1838, pp. 122-123. "Description of an Electromagnetic Engine," Sturgeon Arm. Elect/-., 1839, pp. 437-439. " On the use of Electromagnets made of Iron Wire for the Electromagnetic Engine," Sturgeon Ann. Electr., iv., 1839-40, pp. 58-62. " Investigations in Magnetism and Electromagnetism," Sturgeon Ann. Electr., iv., 1839-40, pp. 131-137. " Description of an Electromagnetic Engine," Sturgeon Ann. Electr., iv., 1839-40, pp. 203-205. " On Electromagnetic Forces," Sturgeon Ann. Electr-, iv., 1839-40, pp. 474-481 ; v., 1840, pp. 187-198, 470-472. " On the Production of Heat by Voltaic Electricity," Roy. Soc. Proc., iv.. 1840, pp. 280-282. " On the Heat evolved by Metallic Conductors of Electricity, and in the Cells of a Battery during Electrolysis," Phil. Mag., xix., 1841, pp. 200-277; Archives de VElectr.,i., 1842, pp. 54-79; Sturgeon Ann. Electr.,m., 1842, pp. 287-301. "On the Electric Origin of the Heat of Combustion," Brit. Assoc. Report. 1842 (pt. 2), p. 31 ; Archives de I Electr., ii., 1842, pp. 80-102 ; Phil. Mag., xx., 1842, pp. 98-113 ; Sturgeon Ann. Electr., viii., 1842. pp. 302-315. "On the Electrical Origin of Chemical Heat," Phil. Mag., xxii., 1843, pp. 204-208; Annul, de Chimie, xvi., 1846, pp. 474-430; Napoli Rendiconto, iii., 1844, pp. 226-233. "On the Calorific Effects of Magneto-electricity, and on the Mechanical Value of Heat," Phil. Mag., xxiii., 1843, pp. 263-276, 347-355, 435-443; Anna!, de Chimie, xxxiv., 1852. pp. 504-508; Brit. Assoc. Report, 1843 (pt. 2), p. 33. "On the Intermittent Char acter of the Voltaic Current in certain cases of Electrolysis, and on the Intensities of various Voltaic Arrangements," Phil. Mag., xxiv., 1844, pp. 106-115 ; Archives de V Electr., iv., 1814, pp. 209-275; "On Specific Heat," Phil. Mag., xxv., 1844, pp. 334-337. " On the Changes of Temperature produced by the Rarefaction and Condensation of Air," Roy. Soc. Proc., v., 1844, pp. 517-518; Anna!, de Chimie, xxxv., 1852. pp. 118-120; Phil. Mag., xxvi., 1845, pp. 369-383. "On the Mechanical Equivalent of Heat," Brit. Assoc. Report, 1845 (pt. 2), p. 31. " On the Existence of an Equivalent Relation between Heat and the ordinary forms of Mechanical Power," Phil. Mag., xxvii., 1845, pp. 205-207. " On the Heat evolved during the Electrolysis of Water (1843), Manchester Phil. Soc. Mem., vii , 1846, pp. 87-113. " On a new Theory of Heat," Manchester Phil. Soc. Mem , vii., 1846. pp. 111-112. " On a neur Method of ascertaining the Specific Heat of Bodies (1845)," Manchester Phil. Soc. Mem., vii., 1846, pp. 559-573. " Expediences sur I ldentite" entre le caloriqne et la fores me caniqnc," Paris Comptes Rendus, xxv., 1847, pp. 309-324. "On the Theoretical Velocity of Sound," Phil. Mag., xxxi., 1847, pp. 114-115. " On th-s Mechanical Equivalent of Heat as determined by the Friction of Fluids," Phil. Mag. , xxxi. , 1847, pp. 173-176 ; Brit. Assoc. Report, 1S47 (pt. 2), p. 55; Poggen. Anna.!.. Ixxiii., 1848, pp. 479-484. "On the Mechanical Equivalent of Heat-, and on the Constitution of Elastic Fluids," Brit. Assoc. Report, 1848 (pt, 2), pp. 21-22. " On Shooting Stars," Phil. Mag., xxxii., 1848, pp. 349-351. " Snr 1 equiv- alent mdcanique du calorique," Paris Comptes Rendus, xxviii., 1849, pp. 132-135. 2 Joule "On the Mechanical Equivalent of Heat," Philosophical Transactions of 1850. 3 " New Determination of the Mechanical Equivalent of Heat," by James Prescott Joule, Phil. Trans, for 1878, pp. 365-383. through resistances measured by the unit they had issued, I under took experiments with that view, resulting in a larger figure (782 5, Brit. Assoc. Report, Dundee, 1867, p. 522) than that which I had obtained by the friction of fluids (772 6, Phil. Trails., 1850, p. 82). "The only way to account for this discrepancy was to admit the existence of error either in my thermal experiments or in the unit of resistance. A committee, consisting of Sir William Thomson, Professor P. G. Tait, Professor Clerk Maxwell, Professor B. Stewart, and myself, were appointed at the meeting of the British Association in 1870 ; and with the funds thus placed at my disposal I was charged with the present investigation, for the pur pose of giving greater accuracy to the results of the direct method." The result of this final investigation of Joule s is 772 - 43 Manchester foot-pounds for the quantity of heat required to warm from 60 to 61 Fahr. a pound of water weighed in vacuum, which is about ^Vth per cent, greater than tho final result of 1849 expressed in the same terms. According to Regnault s measurements 4 of the thermal capacity of water at different temperatures from to 230 C., it niusfc be about % 08 per cent, greater at 60 Fahr. than at 32. According to this, Joule s thermodynamic result would be 771 81 Manchester foot-pounds, for the work required to warm a pound of water from 32 to 33 Fahr., or 1389 26 to warm a pound of water from to 1 C. Eeducing 1389-26 feet to metres, we have 423 437 metres. At Faris the force of gravity is about -j-^ per cent, less than in Manchester. Hence for about the middle of France and the southern latitudes of Germany, Joule s result, according to the ordinary reckoning of French and German engineers, may be stated as 423 - 5 kilogramme-metres for the amount of work required to warm 1 kilogramme of water from to 1 C. The force of gravity at Manchester is 981 - 34 dynes (centimetres per second per second). Multiplying 423 437 by this, we find accordingly 41,553,000 centi metre-dynes, or "ergs," for the amount of work in C.G.S. measure required to warm 1 gramme of water from to 1 C. THERMOMETRY. 10. Preliminary for Thermometry. Sense of Heat (resumed from 1). The sense of heat and cold is not simply dependent on the temperature of the body touched. If a person takes a piece of iron, or a stone, or a piece of wood, or a ball of worsted, or a quantity of finely carded cotton-wool, or of eider down, in his hand, or touches an iron column, or a stone wall, or a wooden beam, or a mass of wool or of down, he will perceive the iron cold, the stone cold, but less cold than the iron, the wood but slightly cold much less cold than the stone, the wool or down decidedly warm. We now know that if all the bodies before being touched were near one another in similar exposure, they must have been at the same temperature, and from the iron and stone being felt cold we know that this mean temperature is lower than the temperature of the hand. Each of the bodies touched must at the first instant have taken some heat from the hand, and therefore, if the perception were quick enough, all at the very instant of being touched would have seemed cold to the sense. The iron by its high the-rmal conductivity ( 76, 78, 80) keeps drawing off heat from the hand and lowering its temperature, till after many seconds of time an approximately per manent temperature is reached, which may be consider ably lower than the temperature of the hand before v contact, but somewhat higher than the previous tem perature of the iron, because of the internal furnace (see THERMODYNAMICS) generating heat in the hand. A similar result, but in less time and with less ultimate lowering of temperature of the hand, takes place when stone is touched. When wood is touched its comparatively small conductivity ( 76) allows its surface to be warmed again after the first few seconds, sometimes to a higher tem-
4 Relation des Experiences, vol. i. p. 748, Paris, 1847.