BOILING (of liquids)—BOILING-POINT. •When heat is applied to a vessel containing water, the temperature gradually rises, and vapor comes silently off the surface; but at a certain degree of heat, steam (q.v.) begins to be formed in small explosive bursts at the bottom, and rising through the liquid in considerable bubbles, throws it into com motion. If, after this, the steam is allowed freely to escape, the temperature of the water rises no higher, however great the heat of the fire. The water is then said to boa, and the temperature at which it remains permanent is its boiling-point. The boiling point of water is ordinarily 212'; but every liquid has n point of its own. Thus, sul phuric ether boils at 96'; alcohol, at 176'; oil of turpentine, at 316'; sulphuric acid, at 620'• and mercury, at 662°. The boiling-point of liquids is constant, tinder the saute conditions, but is liable to be altered by various circumstances: • -Water with common salt in it, e.g., requires greater heat to make it boil than pure water. The nature of the vessel, too. exerts an influence; in a glass vessel, the boiling-point of water is a degree or two higher than in one of metal, owing to the greater attraction that there is between water and glass than between water and a metal. Ilia what most affects the boiling point is variation of pressure. It is ouly When the barometer stands at 30 in., showing an atmospheric pressure of 15 lbs. on the sq. in., that the boiling-point of water is 212'. When the barometer falls, or Whet] part of the pressure is in any other way removed, it boils before coming to 212°, and when the pressure is increased, the boiling-point rises. —Thus, in elevated positions, where there is less air above the liquid to press on its sur face. the boiling point is lower than at the level of the sea. An elevation of 510 ft. above the sett-level. makes a diminution of a degreot• at .higher levels, the difference of elevation corresponding to a degree of temperature in •the boiling-point increases; hitt the rate of variation once ascertained. 0 method is thus furnished of measuring the heights of mountains. See IImours, :MEASUREMENT OF. At the city of 3Iexieo, 7000
ft. above the sea, water boils at 200'; at Quito. 9000 fL, at 104'; and on Donkia monn tain, in the Himalaya, at the height of 18,000 ft., Dr. Hooker found it to boil at 180'. Boiling water is thus not always equally 1ml. and tit elevated places, many substances cannot be cooked by boiling. Under the receiver of an air-pump, the saute effect is still more strikingly seen; water may be made to boil at the temperature of summer, and ether when colder than ice. In complete vaeno, liquids, in general, boil at It tempera ture 140' lower than in the open air. The knowledge of this effect of diminished pres sure is now largely turned to account in suga•-boiling, in distilling vegetable essences, and in other processes where the substances are apt to be injured by a high temperature. —By increasing the pressure, again. water may he heated to any degree without lioil in Digester (q.v.) is formed on this principle. Under a pressure of t•ontmos• pheres, the hoiling-point rises to 234'; of four atmospheres, it is' 94'; of ten atmospheres, 359'; of fifty atmospheres, 510°.
In a deep vessel, the water at the bottom has to sustain the pressure not only of the atmosphere, hut also of the water above it. AL a depth of 34 ft., the pressure of the water above is equal to an atmosphre, or 15 lbs. on the sq. in.; and thus. at the bottom of a vessel of that depth, the water must he heated to 234' before it is at its boiling point. This principle has been successfully applied to explain the phenomena of the Geysers (q.v.).
If a small quantity of water be poured into n silver basin, heated above the boiling point, but below redness, it will begin to boil violently, or perhaps burst into steam at once. But if the basin is heated to redness, the water will gather itself into a globule, and roll about on the hot surface, without becoming heated to the boiling-point. For the explanation of this and other interesting phenomena connected with it, see SPIIE. 120IDAI. CONDITION OP LIQUIDS.