Page:Elementary Text-book of Physics (Anthony, 1897).djvu/230

216 Other calorimetric methods may be employed. The best experiments give, for the heat equivalent of fusion of ice, very nearly eighty calories.

200. The Gaseous State.—A gas may be defined as a highly compressible fluid. A given mass of gas has no definite volume. Its volume varies with every change in the external pressure to which it is exposed. A vapor is the gaseous state of a substance which at ordinary temperatures exists as a solid or a liquid.

201. Vaporization is the process of formation of vapor. There are two phases of the process: evaporation, in which vapor is formed at the free surface of the liquid; and ebullition, in which the vapor is formed in bubbles in the mass of the liquid, or at the heated surface with which it is in contact.

202. Evaporation.—If a liquid be enclosed in a vessel which it does not entirely fill, the space above the liquid begins at once to be occupied by the vapor of the liquid. The presence of the vapor can be detected in many ways, some of which are applicable only in special cases. Those which are always applicable are the measurement of the increased pressure due to the vapor and the condensation of the vapor into the liquid state after isolating it from the mass of liquid beneath it. The process of forming vapor in this way is evaporation. Evaporation goes on continually from the free surfaces of many liquids, and even of solids. It increases in rapidity as the temperature increases, and ceases when the vapor has reached a certain density, always the same for the same temperature, but greater for a higher temperature. It goes on very rapidly in a vacuum; but it is found that the final density of the vapor is no greater, or but little greater, than when some other gas is present. While evaporation is going on, heat must be supplied to the liquid to keep its temperature constant.

Evaporation may be readily explained on the kinetic theory (§ 184) on the supposition that, in the interaction of the molecules, the motion of any one may be more or less violent, as it receives