BOILING

, or Ebullition, the bubbling up of any fluid, by the application of heat. This is, in general, occasioned by the discharge of an elastic vapour through the fluid that boils; whether that be common air, fixed air, or steam, &c. It is proved by Dr. Hamilton of Dublin, in his Essay on the ascent of vapour, that the boiling of water is occasioned by the lowermost particles of it being heated and rarefied into vapour, or steam; in consequence of this diminution of their specific gravity, they ascend through the surrounding heavier fluid with great velocity, lacerating and throwing up the body of water in the ascent, and so giving it the tumultuous motion called boiling.

That this is occasioned by elastic steam, and not by particles of fire or air, as some have imagined, is easily proved by the following fimple experiment: Take a common drinking glass filled with hot water, and invert it into a vessel of the same: then, as soon as the water in the vessel begins to boil, large bubbles will be seen to ascend in the glass, by which the water in it will be displaced, and there will soon be a continued bubbling from under its edge: but if the glass be then drawn up, so that its mouth may just touch the water, and a cloth wetted in cold water be applied to the outside, the elastic steam within it will be instantly condensed, upon which the water will ascend so as nearly to sill it again. Some small parts of air &c, that may happen to be lodged in the fluid, may also perhaps be expelled, as well as the rarefied steam. And this is particularly recommended as a method of purifying quicksilver, for making more accurately barometers and thermometers.

We commonly annex the idea of a certain very great degree of heat to the boiling of liquids, though often without reason; for different liquids boil with different degrees of heat; and any one given liquid also, under different pressures of the atmosphere. Thus, a vessel of tar being set over the fire till it boils, it is said a person may then put his hand into it without | injury: and by putting water under the receiver of an air-pump, and applying the flame of a candle or lamp under it, by gradually exhausting, the water is made to boil with always less and less degrees of heat; and without applying any heat at all, the water, or even the moisture about the bottom or edges of the receiver, will rise in an elastic vapour up into it, when the exhaustion is near completed.

Spirit of wine boils still sooner in vacuo than water. And Dr. Freind gives a table of the different times required to make several fluids boil by the same heat. See also Philos. Trans. N° 122.