Page:Elementary Text-book of Physics (Anthony, 1897).djvu/242

228 ELEMENTARY PHYSICS. . By promoting evaporation, intense cold may be produced. In a vacuum, water may be frozen by its own evaporation. If a liquid be heated to a temperature above its ordinary boiling-point under pressure, relief of the pressure is followed by a very rapid evolution of vapor and a rapid cooling of the liquid. Liquid nitrous oxide at a temperature of zero is still far above its boiling-point, and its vapor exerts a pressure of about thirty atmospheres. If the liquid be drawn off into an open vessel, it at first boils with extreme violence, but is soon cooled to its boiling-point for the atmospheric pressure, about -88°, and then boils away slowly, while its temperature remains at that low point. By liquefying nitrogen and then allowing it to evaporate under low pressure, Olszewski obtained the temperature of -220° C, and by allowing liquid hydrogen to boil under atmospheric pressure, -243.5° C. was reached.

218. Heat Equivalent of Vaporization.—It is plain that the formation of vapor is work requiring the expenditure of energy for its accomplishment. Each molecule that is shot off into space obtains the motion which projected it beyond the reach of the molecular attraction, at the expense of the energy of the molecules that remain behind. A quantity of heat disappears when a liquid evaporates; and experiment demonstrates, that to evaporate a kilogram of a liquid at a given temperature always requires the same amount of heat. This is the heat equivalent of vaporization. When a vapor condenses into the liquid state, the same amount of heat is generated as disappears when the liquid assumes the state of vapor. The heat equivalent of vaporization is determined by passing the vapor at a known temperature into a calorimeter, there condensing it into the liquid state, and noting the rise of temperature in the calorimeter. This, it will be seen, is essentially the method of mixtures. Many experimenters have given attention to this determination; but here, again, the best experiments are those of Regnault. He determined what he called the total heat of steam at various pressures. By this was meant the heat required to raise the temperature of a kilogram of water from zero to the temperature