Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/590

556 HEAT the measurement of temperature ( 10-67). As long as the truth or falsity of the materialistic hypothesis seemed an open question, the word caloric was held to imply the materiality of heat. Thus Davy, after discussing some of the fundamental dogmas of the " Calorists," as he called them, and describing his own experiments, which proved beyond all doubt the falsity of their fundamental hypothesis that heat is matter, varied the statement of his conclusion by saying, "or caloric does not exist." While accepting Davy s conclusion, however, we need not accept this way of stating it ; and as most of our best modern writers still use the word calorimetry, and as French writers have, in com paratively recent times, introduced the word " calorie " to designate a unit quantity of heat, it is decidedly convenient still to retain the name caloric to denote definitely the measurable essence of heat. This is convenient scienti fically as tending to give precision to language and ideas respecting the two classes of measurement, calorimetry and thermometry ; and it has the advantage of leaving the more popular word heat available for that somewhat lax general usage, from which we cannot altogether displace it, in which it may sometimes mean high temperature, as when we speak of great heat, or summer heat, or blood heat, sometimes a measurable quantity of heat, as in the term latent heat, and sometimes a branch of study or science dealing with the transference of heat by conduction and radiation, as in the title of Fourier s great work Theorie analytique de la Chaleur, or the whole province of science concerned with heat, including calorimetry and thermometry, and conduc tion and radiation of heat, and generation of heat, and dynamical relations of heat, as in English titles of separate books such as Dixon s, Balfour Stewart s, and Maxwell s, or of chapters or divisions of larger treatises, such as even the present article. P 4. Calorimetry ly Melting of Ice. Calorimetry was first practised by means of the melting of ice as explained above, and the first thermal unit, or unit quantity of heat, or " calorie," although not then called calorie, was the quantity of heat required to melt unit weight of ice. This, for example, is the unit on which Fourier founds his reckoning illustratively when he explains the fundamental principles of his theory of the conduction of heat. Ice seems to have been first used for calorimetry by Wilcke, a Swede. For the systematic application of this method for the measure ment of quantities of heat in various physical inquiries Laplace and Lavoisier constructed an instrument, the first - ^ w hich the name of calorimeter was applied, and described s -j. - n j.j ie memo { rs O f t] ie French Academy of Sciences for 1780. 1 Though in the hands of Laplace and Lavoisier it gave good results, it had a great inconvenience, which with less careful and less scientific expsrimenters might lead to great inaccuracies, on account of the water adhering by capillary attraction to the broken ice, instead of draining away from it completely and showing exactly how much ice had been melted. To avoid this evil Sir John Herschel suggested that, instead of draining away the water from the ice, the water and ice should all be kept together, and the whole bulk measured. The diminution of bulk of the whole thus gives an accurate measurement of the quantity of ice melted, because ice melting into water comas to occupy just 91 675 per cent, of its original volume. This suggestion is admirably carried out by s Bunsen 2 in his ice-calorimeter, an instrument possessing also other novel features of remarkable beauty and scientific interest. It is particularly valuable for the measurement 1 The instrument itself is preserved in tlie Con.tfrrntoire des Artset Mftlers in Paris. It is described and explained in Maxwell s Theory of If eat, chap. iii. 4 Pogrj. Ann., Sept. 1870, and Phil. May., 871; Maxwell s Theory of Heat, p. 81. of small quantities of heat. Its inventor, for example, by means of it succeeded in making satisfactory determinations of the specific heats of some of those rarer metals, such as indium, of which only a few grammes have been obtained. 5. Calorimetry by the Evaporation of Water. By another application of Black s doctrine of latent heat, the evaporation of water may be used for calorimetry with great advantage in many scientific investigations. It is used generally in engineering practice, particularly for testing the heating power of different qualities of coal asd the economy of various forms of furnaces. The thermal unit, which presents itself naturally in this system, is the quantity of heat required to evaporate unit weight of water when the pressure of the atmosphere as measured by the baro meter is of some conventional standard amount, such as that called one atmosphere, or one atmo, being that for which the barometer, with its mercury column at zero centigrade (or the temperature at which ice melts), stands at 76 centimetres in the latitude of Paris, 48 50 , 3 or at 76 x (1 + -00531 sin 2 4850 ) ~1 + -00531 sin H in any latitude /. This thermal unit (see THERMODY NAMICS) is, according to Regnault s observations, equal to 6 8 times the ice-calorimetric unit. 6. Thermometric Calorimetry. The most prevalent mode of calorimetry in scientific investigation has been hitherto, however, neither that by tlie melting of ice, nor that by evaporation of water, nor indeed anything founded on the doctrine of latent heat at all. It has been founded on the elevation of temperature produced in water by the com munication to it of the heat to be measured ; and, for the sake of distinction from calorimetry by latent heat or otherwise, it may be called thermometric calorimetry. We can only consider it now in anticipation, as we have not yet reached the foundation of any thermometric scale ; but even now we can see that, if in any way we fix upon any two particular determinate temperatures, the quantity of water warmed from the lower to the higher of them by the heat to be measured is a perfectly definite measure for the quantity of this heat. The two temperatures chosen for thermometric calorimetry are those marked and 1 on the centigrade scale. The first of these we can under stand at present, being the temperature at which ice melts under ordinary atmospheric pressure. The second is fully defined in 35, 37, 51, 67. The quantity of heat required to raise unit mass of water (1 kilogramme, or 1 gramme, or 1 milligramme, or 1 K, as the case may be) from zero to 1 C. is called the thermal unit centigrade, and some times, especially by French writers, the " calorie." 7. Comparison of Calorimetric Units. Observations by Prevostaye and Desains, and by Regnault, on the latent heat of fusion of ice, show it to be 79 25 thermal units centigrade, a result differing but little from Black s original determination, which made it 142 thermal units Falir., this being equal to 78 9 thermal units centigrade. Thus if one kilogramme of ice be put into 79-] kilogrammes of water at 1 C., and left till the whole is melted (the pro cess may be accelerated by not too violent stirring, 9), the result will be 80]- kilogrammes of water at C. Ftegnault s experiments on the latent heat of steam show that the quantity of heat required to convert into steam unit 3 This is chosen because all the most accurate experimental deter minations depending on a conventional standard for atmospheric pres sure, such as measurements of thermal expansions and specific heats of gases, of latent heat of melting solids in terms of a calorimetric unit depending on the centigrade thermometric scale, of latent heats of vapours, and thermal expansions of mercury and glass, and comparisons of mercury and air thermometers, are those of Regnault, and were made in Paris and calculated and given to the world according to an arbi trary standard atmosphere corresponding to 70 centimetres of mercury

there.