CALDRINVETRY (from Lat. calor, heat + Gk. pirpov, metron, measure), The science of the measurement of quantities of energy when mani fested by heat effects. By the name 'heat effects' is meant the changes produced in material bodies when they are exposed to what is called a 'source of heat,' e.g. a flame or the rays of the sun. Among these changes which may take place are expansion, fusion, evaporation, alteration in elec trical and magnetic properties, etc. It is now believed that these changes are occasioned by increase in the energy of the smallest portions of the bodies. When a body is 'heated' or 'warmed,' we mean that its minute parts gain energy; and opposite ehanges, e.g. freezing. condensation, cooling, etc., take place when these parts lose energy. It is the province of calorimetry to measure these amounts of energy gained or lost.
The erg (see VFcitexiC. L LTNITs) is the unit of energy and work, and therefore all quantities of energy should be measured in terms of it; but it rarely happens that heat effects are due direct ly to mechanical work except in ease of friction. Consequently the erg is not a convenient unit. Heat effects and the energy required to produce them are almost invariably compared with one definite heat effect, viz. rise in temperature of water; and the practical unit employed for measuring thermal energy may be defined as the quantity of energy required to raise the tempera ture of one gram of water from 15' to 16' C. on the thermometric scale of the constant-pres sure hydrogen thermometer. (tither definitions of a practical unit have been proposed. e.g. by the substitution of 20' to 21° in place of 15' to 16° C.: or the one-hundredth portion of the quan tity of energy required to raise the temperature of one grain of water from the freezing-point to the boiling-point tinder normal pressure.) This practical unit is called the calorie, and its value is very nearly 4.187 joules, or 4.1S7 X 10' ergs. See HEAT.
By the 'specific heat' of a substance at a given temperature and under definite conditions is meant the number of calories required to raise the temperature of one gram of the substance one degree by the hydrogen scale (see TIIERMOME at that temperature. and under those con ditions. By the 'latent heat' of a substance for a definite change of state (e.g. fusion. evapora tion. sublimation, dissociation), under definite conditions. is meant the number of calories re quired to produce the particular change of state in one grant of the substance under the specified conditions. Thus we speak of the 'specific heat of air at constant pressure,' or the 'latent heat of evaporation of water at normal atmospheric pressure.' In general, however, we can learn simply the arcrage specific heat, i.e. the number of calories required to raise the temperature of one gram through t degrees, divided by t. Cabo
rimetry is. then, chiefly, the science of measuring specific and latent heats.
There are two general methods for the meas urement of specific heats, which may be regarded as satisfactory—the method of mixtures and the use of an ice or a steam calorimeter. In the method of mixtures a known quantity of the substance at a known temperature is mixed with a known quantity of some liquid at a different known temperature and the temperature of the mixture is observed. The specific heat of the liquid for the given range of temperature being known, and allowance being made for losses by radiation and conduction, and for the calories spent in changing the temperature of the vessel containing the liquid, the specific heat of the substance may be at once deduced. The most improved form of apparatus fur use in this method is that of Prof. T. A. Waterman, a full description of which is given in the Physical Jecvietc, Vol. IV.. p. 161 (1896).
in the ice-calorimeter, the substance whose specific heat is desired is introduced into an apparatus which allows the heat energy with drawn from the body to be spent entirely in melt ing ice. The change in temperature of the sub stance and the quantity of ice melted may be observed; and thus, assuming that the latent heat of ice is known, the specific heat of the sub stance may be calculated. This method is due to Black; and the most improved apparatus is that designed by the late Professor Bunsen, of Heidelberg. The toast accurate method of using the 'Bunsen calorimeter' is that of Dr. Dieterici, of Hanover. ( See ann's n n alen der Physik mid der Chemic, Vol. XXXVII.. p. 494, ISS9.) Fairly satisfactory descriptions are given in almost all general text-hooks on physics. In the 'steam-calorimeter' the substance whose spe cific heat is desired is suspended on one pan of a chemical balance, which is inclosed in a box eomiceted with a steam-boiler: if the steam is suddenly admitted, some of it will continue to condense on the pan and the substance until their temperature is raised to that of the steam. The quantity of steam thus condensed may be weighed by placing weights in the other pan of the bal ance, and, if the latent heat of condensation of steam is known, the specific heat may easily be calculated. • This method is due to Professor Joly, of Dublin, and a full description of the latest improvements may be found in the Philo sophical Transactions of the Royal Society of London (1894). In all these methods it should be noted that what is measured is the arerage specific heat of the substance over a given range of temperature. For other methods of measure ment of specific heat. reference may be made to the larger treatises on physics.