CALORIMETRY 505 the different effects of heat on materia] substances. The ture of the calorimeter at the moment. A more serious objection most important of these effects are, (a) rise of temperature, 6 0 ggln 0f thib moisturp i Atrace of; damp bind isin the moisture. 1T,he least the danger lagging,oforitsofabsorbing moisture (6) change of state, (c) transformation of energy. § 1. The rise of temperature of a body, when heat is loA ofT A the surfac? of the calorimeter, mayDproduce serious imparted to it, is found to be in general nearly proportional ford A, AS h/ e7ap01ratl0n; This is another objection to Rumcooling the calorimeter below the surrounding to the quantity of heat added. The thermal capacity of a tp,^ method of re n0ng minor bp Stafrting difficulties of the body is measured by the quantity of heat required to raise its Sod Savbb 11611i A UIlce camcitvo7thp 1 “^A SA rtainty the thermal capacity of the calorimeter and stirrer, and of theofimmersed nortemperature one degree, and is necessarily proportional to This the mass of the body for bodies of the same substance under values Ss foAhpTAAfor the specific heats of the generally materials calculated obtainedbyby assurning experisimilar conditions. The specific heat of a substance is some- ment between 100° C and 20° C. Since the specify heatsAf most times defined as the thermal capacity of unit mass, but more metals increase rapidly with rise of temperature, the values so often as the ratio of the thermal capacity of unit mass of obtainecl are generally too high. It is best to make tlds correction as small as possible by using a large calorimeter so that the the substance to that of unit mass of water at some d-ffiSs of water is large m proportion to that of metal.’ Analogous standard temperature. The two definitions are identical, difficuities arise m the application of other calorimetric methods provided that the thermal capacity of unit mass of water’ T he accuracy of the work in each case depends principally on tlm 1 1 of at a standard temperature, is taken as the unit of heat! skill and ingenuity of the experimentalist in devising methods as eliminating the various sources of error. or But the specific heat of water is often stated in terms of 3 Method of Cooling.—A common example of this methoda other units. In any case, it is necessary to specify the is• A the* determination of the specific heat of a liquid by filling temperature, and sometimes also the pressure, since the small calorimeter with the liquid, raising it to a conveffifnt specific heat of a substance generally depends to some temperature, and then setting it to cool in an enclosure at a temperature and observing the time taken to fall through extent on the external conditions. The methods of asteady 0iven range when the conditions have become fairly steady The measurement, founded on rise of temperature, may be same calorimeter is afterwards filled with a known liquid such classed as thermometric methods, since they depend on the as water and the time of cooling is observed through the same range of temperature, in the same enclosure, under the same observation of change of temperature with a thermometer. The ratio of the times of cooling is equal to the The most familiar of these are the method of mixture and conditions. ratio of the thermal capacities of the calorimeter and its contents the method of cooling. m the two cases. The advantage of the method is that there is no transference or mixture ; the defect is that the whole measure 2 Method ' Pfi °f Mixture consists in of imparting quantitv ofAheat to be measured to a known mass water, orthe some other ment depends on the assumption that the rate of loss of heat is standard substance, contained in a vessel or calorimeter of known the same in the two cases, and that any variation in the conthermal capacity, and in observing the rise of temperature pro- ditions, or uncertainty in the rate of loss, produces its full effect duced, from which data the quantity of heat may be found as m the result, whereas in the previous case it would only affect a explained in all elementary text-books. This method is the most small correction. Other sources of uncertainty are, thaifthe rate generally convenient and most readily applicable of calorimetric heat gCUerall de e ffilAnf y that P nds extent on theaccurate rate of of °/ temperature, and it to is some difficult to take methods, but it is not always the most accurate, for various fall reasons. Some heat is generally lost in transferring the heated observations on a rapidly falling thermometer. As the method body to the calorimeter ; this loss may be minimized by perform- is usually practised, the calorimeter is made very small and the ing the.transference rapidly, but it cannot be accurately calculated surface is highly polished to diminish radiation. It is ’better to or eliminated. Some heat is lost when the calorimeter is raised use a fairly large calorimeter to diminish the rate of coofing Ld above the temperature of its enclosure, and before the final the uncertainty of the correction for the water equivalent. g The temperature is reached. This can be roughly estimated by an Af+f6 blackened A lA calorimeter d thetheenclosure should be permaso as to increase loss of heat by radiation as observing the rate of change of temperature before and after the nently experiment, and assuming that the loss of heat is directly proP S1 e a com ar d AnAfrf v A i P ® with the losses by convection and conportional to the duration of the experiment and to the average duction, which are less regular. For accurate work it is essential that excess of temperature. . It can be minimized by making the the liquid in the calorimeter should be continuously stirred, and mixing as rapid as possible, and by using a large calorimeter, so A ? vn JA ®£cl°SUre’ the,lld of which must be water-jacketed that the excess of temperature is always small. The latter and kept at the same steady temperature as the sides. When all these precautions are taken, the method loses most of the sim8 ge b Joule b rT Aad0I)ted y with > ut the rise of temperature is Tn then difficult to measure accuracy, since it is 18 ltS AA advantage- It cannot be satisfactorily necessarily reduced in nearly the same proportion as the correc- annliJAS tmn. there is however, the advantage that the correction is § 4. Method of Fusion.—The methods depending on change of rendered much less uncertain by this procedure, since the assumption that the loss of heat is proportional to the temperature- state are theoretically the simplest, since they do not necessarily excess is only true for small differences of temperature. Rumford involve any reference to thermometry, and ^he co^ ternal loss ol heat and for the thermal capacity of the conproposed to eliminate this correction by starting with the initial temperature of the calorimeter as much below that of its enclosure taining vessel can be completely eliminated. They nevertheless as the final temperature was expected to be above the same limit, present peculiar difficulties and limitations, which render their i ns “ethod has been very generally recommended, but it is practical application more troublesome and more uncertain than really bad because, although it diminishes the absolute magniTbey d nd 0 the LTuaAiKTA " aexperimental ffict S at re(1UlredT roduce ,®f the correction it greatly the uncertainty of it, givenwater change convert one gramme ofAiceP at 0° C. into at 0°ofCstate or and therefore the probable error ofincreases the result. The coefficient of ie g to couvp/A I eating of a calorimeter when it is below the temperature of its r W a ter 1 int K team at 100 C s?mf an(r?W t b ^ S' ° -) is always the 1 S be n ° chan cooW «Athe h i!- S,e drT’ lf ever’ the same as the coefficient of t An’roc l h Ahe Tff;"A ° Se of temperature during dlfficu rnlJAc A lties arise in connexion the deterwwAAm St,hl§b? temperature, since the convection currents, ination of the quantities of ice melted or steam with condensed and in thi i CaSeS ° °fanA moreover beating or cooling, are rarely symmetrical e h Iatent heat f ’ T > the duration of the two stages is otner units for the comiianson of observations. The earlier forms Xr unrL‘ th ° ” vaporization ilTterms^f W n? l A Sanie- ln any case> ifc is desirable to diminish the calorimH-?^ aS ™Uch as Possible by polishing the exterior of the were uselST ^ ^7 °f ?lack’ and of LaPlace and Lavoisier, ^mSlde diminish radiation, and by suspending it by nonaccnSw7 estlmat A°rA0f P71S1°n’ °n account of the impossibility a olishe tbP ff-A A ! “g.tbe quantity of water left adhering to dranvSinS rT P d case, to protect it from 13 als CaSe l n di conK Tt VGry lmP°rtant to keep the surrounding tion nf tb -R s_ fhculty was overcome by the invenBunse ' 5 C0nst ai t as osslble calorimeter, the quantity ice Tim nArV ’ P throughout the experiment. melted is• measured? by observing in thewhich diminution of volume,of but 1 ! amnAA bebut secured by using a of large water-bath the apparatus, in experiments long duration toit surround is necessary nil ooossful employment of this instrument requires considerable skill m mampulation. The sheath of ice surrounding the .mAii8'11caaccu .rate temperature regulator. The method of lag- bulb must be sufficiently continuous to prevent escape of heat, 0nrneter whA i^ ! with eotton-wool, or other non-conductors, put it must not be so solid as to produce risk of strain. The sA AvA wn rec°mmended, diminishes the loss of heat con- ideal condition is difficult to secure. In the practical use of the nev,>r K7, butd ln renders it very uncertain and variable, and should instrument it is not necessary to know both the latent heat of W tJr. l1 astead 70A of Precision. The bad conductors take so usion of ice and the change of volume which occurs on melting • AnA depends A state the rate lossonofthe heat at any it is sufficient to determine the change of volume per calorie or moment on ythe pastthat history moreofthan temperatne quantity of mercury which is drawn into the bulb of the S. II. — 64
