II.

Temperature

The sensation of warmth or coldness of a substance on contact is determined by the property known as temperature. Although it is easy to compare the relative temperatures of two substances by the sense of touch, it is impossible to evaluate the absolute magnitude of the temperatures by subjective reactions. Adding heat to a substance, however, not only raises its temperature, causing it to impart a more acute sensation of warmth, but also produces alterations in several physical properties, which may be measured with precision. As the temperature varies, a substance expands or contracts, its electrical resistivity (see Resistance) changes, and in the gaseous form, it exerts varying pressure. The variation in a standard property usually serves as a basis for an accurate numerical temperature scale (see below).

Temperature depends on the average kinetic energy of the molecules of a substance, and according to kinetic theory (see Gases; Thermodynamics), energy may exist in rotational, vibrational, and translational motions of the particles of a substance. Temperature, however, depends only on the translational molecular motion. Theoretically, the molecules of a substance would exhibit no activity at the temperature termed absolute zero. See Molecule.