EBULLITION. When the bottom of an open vessel containing water is ex posed to heat, the lowest stratum of find immediately expands, beeom, therefore specifically 'lighter, and is forced upwards by the superior gravity of the superin cumbent colder and heavier particles. The heat is in this way diffused through the whole liquid mass, not by simple com munication of that power from particle to particle as in solids, called the conduction, of calorie, but by a translation of the several particles from the bottom to the top, and the top to the bottom, in alter nate succession. This is denominated the carruimg, power of fluids, being com mon to both liquid and gaseous bodies. These internal movements may be render ed very conspicuous and instructive, by mingling a little powdered amber with water contained in a tall glass cylinder, standing upon a sand-bath. A column of the heated and lighter particles will be seen ascending near the axis of the cylin der, surrounded by a hollow column of the cooler ones descending near the sides. That this molecular translation or loco motion is almost the sole mode in which fluids get heated, may be demonstrated by placing the middle of a pretty long glass tube, nearly filled with water, ob liquely over an argand flame. The upper half of the liquid will soon boil, but the portion under the middle will continue cool, so that a lump of ice may remain for a considerable time at the bottom. When the heat is rapidly applied the liquid is thrown into agitation, in consequence of elastic vapor being suddenly generated at the bottom of the vessel, and being as suddenly condensed at a little distance above iebythe surrounding cold columns. These alternate expansions and contrac tions of volume become more manifest as the liquid becomes hotter, and constitute the simmering vibratory sound which is the prelude of ebullition. The whole mass being now heated to a pitch com patible with its permanent elasticity, be comes turbulent and explosive under the continued influence of fire, and emitting more or less copious volumes of vapor, is said to boil. The further elevation of
temperature by the influence of caloric, becomes impossible in these circumstan ces with almost all liquids, because the vapor carries off from them as much heat in a latent state as they are capable of re ceiving from the fire.
The temperature at which liquids boil in the open air varies with the degree of atmospheric pressure, being higher as that is increased, and lower as it is di minished. Hence boiling water is colder by some degrees in bad weather, or in an elevated situation, with a depressed bar ometer, than in fine weather, or at the bottom of a coal-pit, when the barometer is elevated. A high column of liquid, also, by resisting the discharge of steam, raises the boiling point. In cacao, all li quids boil at a temperature about 124° F. lower than under the average atmospheric pressure.
The following is a table of the boiling points of a few substances, on Fahren heit's scale : Ether, 100° Alcohol, 173° Nitric Acid, 210° Water, 212° Solution of Salt, 224° Chloride of Calci Muriatic Acid, um, Oil of Turpentine, 315° Sulphuric Acid, 600° Phosphorus, 554° Sulphur, Linseed Oil, 640° Mercury, 662° M. Marcet has shown, that whatever the nature of the boiler, the temperature of the steam is invariably lower than that of the water from which the steam is generated. In glass vessels, this differ ence amounts, on an average, to 1,908 de grees,—in metal vessels, only to between 0.27 and 0.36 of a degree. There is but one exception to this rule, viz: where the inside of the boiler is coated with a thin layer of sulphur, gum lac, or any other matter possessing an adhesion for water. In that ease the boiling water and the steam have the same temperature. Thus, contrary to the generally received notion, it is not in metal vessels that the boiling point is lower under a stronger pressure, but in glass vessels; if the latter are coated with sulphur, gum lac, &c.