Page:Elementary Text-book of Physics (Anthony, 1897).djvu/233

§ 306] matter how intense the source of heat. This fixed temperature is called the boiling-point of the liquid. It differs for different liquids, and for the same liquid under different pressures. That the boiling-point must depend upon the pressure is evident from the explanation of the phenomenon of ebullition above given.

Substances in solution, if less volatile than the liquid, raise the boiling-point. While pure water boils at 100°, water saturated with common salt boils at 109°. The material of the containing vessel also influences the boiling-point. In a glass vessel the temperature of boiling water is higher than in one of metal. If water be deprived of air by long boiling, and then cooled, its temperature may afterwards be raised considerably above the boiling-point before ebullition commences. Under these conditions the first bubbles of vapor will form with explosive violence. The air dissolved in water separates from it at a high temperature in minute bubbles. Into these the water evaporates, and, whenever the elastic force of the vapor is sufficient to overcome the superincumbent pressure, it enlarges them, and causes the commotion that marks the phenomenon of ebullition. If no such openings in the mass of the fiuid exist, the cohesion of the fiuid, or its adhesion to the vessel, as well as the pressure, must be overcome by the vapor. This explains the higher temperature at which ebullition commences when the liquid has been deprived of air.

206. Spheroidal State.—If a liquid be introduced into a highly heated capsule, or poured upon a very hot plate, it does not wet the heated surface, but forms a fiattened spheroid, which presents no appearance of boiling, and evaporates only very slowly. Boutigny has carefully studied these phenomena, and made known the following facts: The temperature of the spheroid is below the boiling-point of the liquid. The spheroid does not touch the heated plate, but is separated from it by a non-conducting layer of vapor. This accounts for the slowness of the evaporation. To maintain the liquid in this condition the temperature of the capsule must be much above the boiling-point of the liquid; for water it must be at least 200° C. If the capsule be allowed to cool, the temperature