506

CALORIMETRY

•uimratus per unit of heat added. This can be determined by a of being frequently the most accurate in practical applicadirect calibration, by inserting a known quantity of water at a tion, since energy can be more accurately measured m known temperature, and observing the contraction, or weighing other forms than in that of heat The two most imthe mercury drawn into the apparatus. In order to be inde- portant varieties of the method are (a) mechanical, and (b) pendent of the accuracy of the thermometer employed for electrical. These methods have reached their highest observing the initial temperature of the water introduced, it development in connexion with the determination of the has been usual to employ water at 100 C., adopting as unit of heat the “ mean calorie,” which is one-hundredth part of the heat mechanical equivalent of heat, but they may be applied oiven up by one gramme of water in cooling from 100 to 0 O. with great advantage in connexion with other problems, The weight of mercury corresponding to the mean calorie has such as the measurement of the variation of specific heat, been determined with considerable care by a number of observers or of latent heats of fusion or vaporization. well skilled in the use of the instrument. The following are some 15 47 8 7 Mechanical Equivalent of Heat. — The phrase of their results Bunsen, 15-41 mgm.; ' “^-j. Zakrevski, 15 -57 mgm.; Staub, 15-26 mgm. The explanation of “mechanical equivalent of heat” is somewhat vague but has these discrepancies in the fundamental constant is not at all been sanctioned by long usage. It is generally employed to clear but they may be taken as an illustration of the difficulties denote the number of units of mechanical work or energy of manipulation attending the use of this instrument, to which reference has already been made. It is not possible to deduce a which when completely converted into heat without loss, more satisfactory value from the latent beat and the change o would be required to produce one heat unit. The numeridensity because these constants are very difficult to determin . cal value of the mechanical equivalent necessarily depends The following are some of the values deduced by well-known on the particular units of heat and work employed m the experimentalists for the latent heat of fusion :-Regnault, 79 06 to 79-24 calories, corrected by Person to 79 43 Person, /y yy comparison. The British engineer prefers to state results calories • Hess, 80-34 calories; Bunsen, 80-025 calories. Regnault, in terms of foot-pounds of work in any convenient latitude Person! and5 Hess employed the method of mi, ure whrch per pound-degree-Fahrenheit of heat. The Continental probably the most accurate for the purpose. Person and Hess engineer prefers kilogrammetres per kilogramme-degreeavoided the error of water sticking to the ice by using dry ice at centigrade For scientific use the C.G.S. system of exvarious temperatures below 0° C and determining the specific pression in ergs per gramme - degree- Centigrade or Pcnf of ice as well as the latent heat of fusion. These disc pancies might, no doubt, be partly expiained by d^erences in the “ calorie ” is the most appropriate, as being independent units employed, which are somewhat uncertain as the specinc of the value of gravity. A more convenient unit of work heat of water changes rapidly in the neighbourhood of 0 C but or energy, in practice, on account of the smallness ot the making all due allowance for this, it remains evident tliat tne 7 method of ice-calorimetry, in spite of its theoretical simplicity, era, is 'the joule, which is equal to 10 ergs, or one wattpresents "rave difficulties in its practical application. second of electrical energy. On account of its practical § 5. The Method of Condensation was first successfully aPP1^ convenience, and its close relation to the international by Joly in the construction of his steam caionmeter a i tion of which will be found m several recent text-books. Hie electrical units, the joule has been recommended by the bodv to be tested is placed in a special scale-pan, suspended by a British Association for adoption as the absolute unit ot fine^vire from the a^m of a balance inside an enclosure which ^ heat. Other convenient practical units of the same kind be filled with steam at atmospheric pressure. The temperature would be the watt-hour, 3600 joules, which is of the same Tffie^wefght'of steam^ondensed^cm1^^ bodygives ameans of cal- order of magnitude as the kilo-calorie, and the kilowatt-hour, which is the ordinary commercial unit of electrical energy. S 8 Joule.—The earlier work of Joule is now chiefly of historical interest but his later measurements in 1878, which were underon a larger scale, adopting Hirn’s method ot measuring the sufficiently rapid, since the walls of the enclosure are taken work expended in terms of the torque and the number of resomaintained at 100° C., very nearly. The thermal capacity of the lutions still possess value as experimental evidence. In these scale-pan, &c., can be determined by a separate experiment, or, experiments the paddles were revolved by hand at such a speed still better, eliminated by the differential method of counter- asPto produce a constant torque on the calorimeter, which was poising with an exactly similar arrangement on the other aim ot supported so as to be free to turn, but was kept at rest by the couple the balance. The method requires very delicate weighing, as one dueto a pair of equal weights suspended from Jne calorie corresponds to less than two milligrammes of steam con- round the circumference of a horizontal wheel attacl densed but the successful application of the method to the very calorimeter. Each experiment lasted about forty minutes, and difficult problem of measuring the specific heat of a gas at constant the rise of temperature produced was nearly 3 C The caioa volume shows that these and other difficulties have been very meter contained about 5 kilogrammes of water, so that the rate o skilfully overcome. The application of the method appears to be heat-supply was about 6 calories per second Joule s gal result practically limited to the measurements of specific heat between was772-55 foot-pounds at Manchester per pound-degree-Fahrenheit the aimosUeTic temperature and 100“ C. The results depend on at a temnerature of 62° E., but individual experiments differed b) the value assumed for the latent heat of fX^al himself7deter! fs mSTl per cent. This result in C G S. measure ts equ,as 536-7 calories, following Regnault. Joly has himselt deter valent to 4-177 ioules per calorie at 16 5 C., on the scale mined the mean specific heat ol water between 12 and 100 U Joule’s mercury thermometer. His thermometers were subseby this method, in terms of the latent heat of steam as above quently corrected to the Pans scale by Schuster in 1895, which gfven and finds the result -9952. Assuming that the mean had the effect of reducing the above figure to 4-1/3. specific heat of water between 12° and 100 is really 1 0011 in 8 9 Rowland. — About the same time Rowland {Pwc. Amcr. terms of the calorie at 20° C. (see table below), the value of the Acad xv p 75 1880) repeated the experiment, employing the latent heat of steam at 100° C., as determined by Joly, would be same method, hut using aW»r ‘^‘rTof Em) 640-2 in terms of the same unit. The calorie employed by and a petroleum motor, so as to obtain a greater rate ol heating Rocmault is to some extent uncertain, but the difference is hardly (about 84 calories per second), and to reduce the miputance^oftl^ beyond the probable errors of experiment, since it appears from uncertain correction for. external loss o < d rrnometrv ^nd the results of recent experiments that Regnault made an error of the same order in his determination of the specific heat of extended^ M^researches ove?°a much wider range of^temperature, water at 100° 0. 8 6 Energy Methods. — The third general method of calorimetry,' that based on the transformation of some ffis^therimnneters haveI1been 1tc^.el^'l^0^1raredjW)i^^ other kind of energy into the form of heat, rests on the thermometer standardized in Pans, and wRh a platinum *1 general principle of the conservation of energy, and on the meter standardized by Griffiths. The result has been to i ed experimental fact that all other forms of energy are readily the coefficient of diminution of specific urfchanged. half, but the absolute value at 20 G. is practically g and completely convertible into the form of heat. It is one therefore often possible to measure quantities of heat in- Thus corrected his values are .as tollows . directly, by measuring the energy m some other form and ^ ^ ^ then converting it into heat. In addition to its great These are expressed in terms of the hydrogen scale but theoretical interest, this method possesses the advantage