The term generally employed to designate water in its elastic form, at or above the temperature of 212°. It is at present applied to many economical purposes, as well as in various manufactures, independent of its important office in the steam-engine. In order to make water boil, the fire must be applied to the bottom or sides of the vessel which contains it: if the heat be applied at the surface of the water, it will waste away without boiling, because the superficial particles, by imbibing the heat necessary to render them elastic, fly off without agitating the rest; but when applied to the lower surface of the water, the bubbles which are formed at the bottom rise, and give off their heat to the incumbent mass, and then disappear by collapsing: the distances which they reach before collapsing increase as the water continues to warm further up the mass, till it breaks out into boiling on the surface. If the handle of a tea-kettle be grasped with the hand, a tremor will be felt for some little time before boiling, arising from the little succussions which are produced by the collapsing of the bubbles of vapour.

This is much more violent, and is really a remarkable phenomenon, if we suddenly plunge a lump of red hot iron into a vessel of cold water, when, if the hand be applied to the side of the vessel, a most violent tremor is felt, and sometimes strong thumps; these arise from the collapsing of very large bubbles. The great resemblance of this tremor to the feeling experienced during the shock of an earthquake has led many to suppose that these last are produced in the same way; and the hypothesis is by no means unfeasible.

The following propositions have been generally assumed by certain authorities as correct data: -

1. A cubic inch of water forms a cubic foot of steam, when its elasticity is equal to 30 inches of mercury.

2. One pound of Newcastle coal will convert seven pounds of boiling water into steam.

3. The time required to convert a given quantity of boiling water into steam is six times that required to raise it from the freezing to the boiling point, or from 32° to 212°, supposing the supply of heat to be uniform.

4. When a quantity of water is exposed to a given temperature, the quantity of steam formed in a given time will be as the surface, all other things being equal. The quantity will also be jointly as the force of vapour answering to each degree of heat, and the surface. The depth of water evaporated in a given time will be as the force of vapour, whatever the surface, if the mass be uniformly of the same temperature. When the force of vapour is 30 inches, and the temperature at 212°, this degree being just preserved only, the depth evaporated is 1.3 inch in one hour.

5. When a quantity of water is raised to the boiling point, or 212°, it requires as much heat to give it the elastic form as would raise the same water 900o higher if its volume were not changed by the heat; that is, if it could be prevented from expanding, its temperature would become 1112° with the same quantity of caloric; thus, agreeably to fact 3, the heat required to convert water of 212° in to steam is six times that required to raise the temperature from 32° to 212°. See also Steam-Engine, power of