BOILING OF FLUIDS. When fluids are heated to such a degree as to be strongly agi tated and produce much vapour, they are said to boil, or be in a state of ebullition, Under similar circumstances, the temperature at which this occurs is always the same in the same fluid, and is called its bailing point, being the heat which the fluid is capable of acquiring. When the vapour which arises from a boiling fluid is condensed, the result ing liquid is perfectly similar to that from which its vapour was produced, having suf fered no chemical change. Most metals, though rendered fluid by melting, never boil, and many oils become decomposed before reaching a temperature at which they would otherwise boil.
When water is heated, there is a point, just before it has acquired it highest temperature, at which a slight noise, or rather a succession of noises, is heard, usually called simmering. This is occasioned by the formation of minute bubbles of vapour, at the bottom of the vessel, and nearest the source of heat, which, being specifically lighter than the water in which they are formed, rise into the upper and cooler part of it, and are then condensed. Soon after this and when the whole of the water has ac quired its highest temperature, the bubbles of vapour rise to the surface, and there bursting constitute steam, which, being transparent and colourless, is consequently invisible, but when it comes into contact with the cold air it undergoes partial condensation, and is then visible and appears as a mist. This tempera ture, under average circumstances, is about 212° Fah.; but, with variations in the density of the air, it varies as follows : Barometer. Boiling point.
26 inches . . . . 201.01 20.5 205 79 27 206.07 27.5 207 55 28 208.43 28.5 209.31 29 210.19 29.5 211.07
30 212.00 30.5 212.88 213.70On ascending mountains, by the consequent diminution of atmospheric pressure, and in proportion to it, water is found to boil at a lower temperature. Thus, on the summit of Mont Blanc, which is about 15,000 feet above the level of the sea, Saussure found water to boil at 178° of Fah., or 34° below its usual temperature. Fluids boil at less in vacuo than under ordinary atmospheric pressure ; so that water in vacuo boils at about 72°. On the other hand, if the pressure of 'air be in creased, the boiling point becomes higher.
Thus, Southern established the following relation between the barometric pressure and the boiling point of water :— Atmospheres. Inches of Mercury. Temperature.
1 . 29.8 . . 2 . . 50.6 . 250.3 3 . . 89.4 . . 275 I . . 119.2 . . 293.4 5 . . 119 . . 309.2 6 . . 178.8 . .
7 . . 208.0 . . 331.1 8 . . 238.4 . . 343.6 It is to be observed, that the temperature of the steam is always equal to that of the water from which it is generated. The boiling point of any one liquid, as above stated, de pends mainly on the pressure to which it is exposed ; hut it is also slightly affected by the substance of the vessel containing it, and the contiguity or immersion of other bodies.
The following are the boiling points of a few liquids under the same atmospheric pres sure: Boiling point.
Muriatic rather . . . . 52°Sulphuric mther( sp. gr. 0.7365 at 48°) 113 Bisulphuret of carbon . . . 113 Acetic tether . . . . . 160Nitric acid (sp. gr. 1.5) . . . 210 Oil of turpentine . . . . 314 Naphtha . . . . . 320 Phosphorus . . . 554 Sulphur 570Sulphuric acid (sp. gr. 1.848) 600 Mercury . . . .