Calorimetry

By calorimetry we mean the measurement of quantities of heat There are three different units of heat which are employed to express the results : (1) the amount of heat required to raise the temperature of unit mass of water from 0°C. to 1°C.; (2) the amount of heat required to melt unit mass of ice; (3) the amount of heat required to convert unit mass of water at 100° into steam at the same temperature. Experiments will be detailed below ( §39) by which the last two units may be expressed in terms of the first, which is generally regarded as the normal standard. Calo-rimetric measurements are deduced generally from one of the following observations : (1) the range of temperature through which a known quantity of water is raised, (2) the quantity of ice melted, (3) the quantity of water evaporated or condensed; or from combinations of these. The results obtained from the first observation are usually expressed in terms of the normal unit on the assumption that the quantity of heat required to raise a quantity of water through one degree is the same, whatever be the position of the degree in the thermometric scale. This assumption is very nearly justified by experiment As a matter of fact, the quantity of heat required to raise unit mass of water from 99°C to 100°C is 1.016 normal units.

The results of the second and third observations mentioned above give the quantities of heat directly in terms of the second and third units respectively, and may therefore be expressed in terms of normal units when the relations between the various units have once been established.