the effect produced on liquids by the application of heat sufficient to convert them rapidly into vapour, with the phenomena of ebullition. The boiling point varies greatly in different liquids, and is also affected by the degree of atmospheric pressure, being higher when the pressure is increased, and lower when it is diminished. When the barometer stands at 30 inches, the boiling point of water is 212° Fahr., and it is found to vary 0.88° of a degree for each half inch of barometric variation; consequently, for every variation of one-tenth of an inch in the barometer, the boiling point of water is altered 0.175° of a degree. Hence water will boil sooner at the top of a mountain than at its base; and thus a method is afforded for determining its height. Water boils in vacuo at 98° Fahr. Under a uniform barometric pressure of 30 inches, mercury boils at 662°; alcohol at 176°; ether at 96°; and oil of turpentine at 316° Fahr.

**Boiling Springs.** See Iceland; Physical Geography.