The temperature at which water boils is commonly said to be 212°, which is called _the boiling-point_ of the thermometer; but, strictly speaking, this is only true when the barometer stands at 30 inches. If it be lower, water will boil at a lower temperature, because the atmospheric pressure is less; and if it be higher, as at 31, water will not boil until it receives a higher temperature, the pressure being greater. According as the cohesive forces of the particles of liquids are more or less active, they boil at greater or less temperatures. In general the lighter liquids, such as _alcohol_ and _ether_, boil at lower temperatures. These fluids, in fact, would boil by merely removing the atmospheric pressure, as may be proved by placing them under the receiver of an air-pump, and withdrawing the air. From this we may conclude that these and similar substances would never exist in the liquid state at all, but for the atmospheric pressure. (18.) The elasticity of vapour raised from a boiling liquid, is equal to the pressure under which it is produced. Thus, steam raised from water at 212°, and, therefore, under a pressure of 15lb. on the square inch, is endued with an elastic force which would exert a pressure on the sides of any vessel which confines it, also equal to 15lb. on the square inch. Since an increased pressure infers an increased temperature in boiling, it follows, that where steam of a higher pressure than the atmosphere is required, it is necessary that the water should be boiled at a higher temperature. (19.) We have already stated that there is a certain point at which the temperature of a liquid will cease to rise, and that all the heat communicated to it beyond this is consumed in the formation of vapour. It has been ascertained, that when water boils at 212°, under a pressure equal to 30 inches of mercury, a cubic inch of water forms a cubic foot[4] of steam, the elastic force of which is equal to the atmospheric pressure, and the temperature of which is 212°. Since there are 1728 cubic inches in a cubic foot, it follows, that when water at this temperature passes from the liquid to the vaporous state, it is dilated into 1728 times its bulk. [Footnote 4: The terms _cubic inch_ and _cubic foot_ are easily explained. A common die, used in games of chance, has the figure which is called a cube. It is a solid having twelve straight edges equal to one another. It has six sides, each of which is square, and which are also equal to one another. If its edges be each one inch in length, it is called a _cubic inch_, if one foot, a _cubic foot_, if one yard, a _cubic yard_, &c. This figure is represented in perspective, in fig. 6.]