Page:Elementary Text-book of Physics (Anthony, 1897).djvu/224

210 apparent expansion of this liquid found exactly as was that of mercury. The absolute coeflScient for the liquid is then the sum of the coefficient of apparent expansion and the coefficient for the glass.

190. Expansion of Water.—The use of water as a standard with which to compare the densities of other substances makes it necessary to know, not merely its mean coefficient of expansion, but its actual expansion, degree by degree. This is the more important since water expands very irregularly. The best determinations of the volumes of water at different temperatures are those of Matthiessen. The method which he employed was to weigh in water a mass of glass of which the coefficient of expansion had been previously determined.

Water contracts, instead of expanding, from 0° to 4°. At 4° it is at its maximum density, and from that temperature to its boiling-point it expands.

191. Effect of Variation of Temperature upon Specific Heat.—It has already been stated (§ 166) that the specific heat of bodies changes with temperature. With most substances the specific heat increases as the temperature rises.

For example, the true specific heat of the diamond

192. Effect of Change of Physical State upon Specific Heat.—The specific heat of a substance is not the same in its different physical states. In the solid or gaseous state of the substance it is generally less than in the liquid. For example: