Page:Elementary Text-book of Physics (Anthony, 1897).djvu/234

220 will soon fall below the limit necessary to maintain the spheroidal state, the liquid will moisten the capsule, and there will be a rapid ebullition with disengagement of vapor. If a liquid of very low boiling-point, as liquid nitrous oxide, which boils at -88°, be poured into a red-hot capsule, it will assume the spheroidal state; and, since its temperature cannot rise above its boiling-point, water, or even mercury, plunged into it, will be frozen.

207. Production of Vapor in a Limited Space.— When a liquid is heated in a limited space the vapor generated accumulates, increasing the pressure, and the temperature rises above the ordinary boiling-point. Cagniard-Latour experimented upon liquids in spaces but little larger than their own volumes. He found that, at a certain temperature, the liquid suddenly disappeared; that is, it was converted into vapor in a space but little larger than its own volume. It is supposed that above the temperature at which this occurs, which is called the critical temperature, the substance cannot exist in the liquid state (§ 223).

208. Liquefaction.—Only a certain amount of vapor can exist at a given temperature in a given space. If the temperature of a space saturated with vapor be lowered, some of the vapor must condense into the liquid state. It is not necessary that the temperature of the whole space be lowered; for when the vapor in the cooled portion is condensed, its pressure is diminished, the vapor from the warmer portion flows in, to be in its turn condensed, and this continues until the whole is brought to the density and pressure due to the cooled portion. Any diminution of the space occupied by a saturated vapor at constant temperature will cause some of the vapor to become liquid, for, if it do not condense, its pressure must increase; but a saturated vapor is already at its maximum pressure.

If the vapor in a given space be not at its maximum pressure, its pressure will increase when its volume is diminished, until the maximum pressure is reached; when, if the temperature remain constant, further reduction of volume causes condensation into the liquid state, without further increase of pressure or density. This statement is true of several of the gases at ordinary temperatures.