Boiling, Boiling-point.

Boiling, Boiling-point. When heat is applied to a vessel containing a liquid (say water) at ordinary temperatures, the temperature of the liquid gradually rises. Vapour may be seen rising from its surface; and, after a time, bubbles of vapour form in the interior and move upwards, but disappear before they reach the surface. The disappearance of the bubbles is caused by the condensation of the contained vapour, and is accompanied by the so-called singing of the water. This noise is produced by the falling in of the walls of the bubbles when the vapour is condensed, and increases in intensity as the number of the bubbles increases. If heat be still applied, the temperature rises to such an amount that the bubbles no longer condense, but pass freely to the surface of the liquid. The liquid is then said to boil. When this occurs the temperature remains constant on the whole until all the water is boiled off. This temperature is called the boiling-point. It may be defined as the temperature at which vapour comes off freely from the surface of the liquid. Different liquids boil at different temperatures; and, for each liquid, this temperature increases with increase of the atmospheric pressure. The following numbers, extracted from Regnault's tables, show how the boiling-point of water is affected by pressure:

Thus, if the pressure be 0.023 atmospheres, water will boil at $20^\{\backslash circ\}\backslash text\{\; C.\}$ or $68^\{\backslash circ\}\backslash text\{\; F.\}$ And if water at a temperature much below its ordinary boiling-point be placed under the receiver of an air-pump and the pressure be reduced, it may be made to boil.

It will give off vapour under these conditions until the pressure rises to that corresponding (as in the table above) to the given temperature. When this condition of equilibrium is reached, the vapour is said to be saturated. Thus we obtain another definition—the boiling-point of a liquid is the temperature at which its saturated vapour has a pressure equal to that to which its surface is subjected.

Abnormal boiling-points may be obtained artificially. Thus, if water be carefully heated in a very smooth and clean glass vessel, its temperature may be raised considerably above the ordinary point before it boils. But in this case the boiling takes place explosively when it does begin.

Since the atmospheric pressure depends upon the height above the earth's surface, the boiling-point of water varies with the height. This furnishes the principle of the hypsometric thermometer (see BAROMETER, and HEIGHTS, MEASUREMENT OF). Under the surface of a liquid the pressure increases with the depth. Hence the boiling-point increases with the depth. This explains the phenomena of the Geysers (q.v.). See also PAPIN'S DIGESTER, and SPHEROIDAL STATE OF LIQUIDS.