This curious fact was first explained by Dr. Black, and gave rise to the knowledge of the latent heat or caloric of fluidity of bodie3. Dr. Black suspended two glass globes, of the same size, near each other; the one filled with ice at 32°, the other with water at 33°: in half an hour the water had risen to 40°, but it took ten hours and a half to liquefy the ice, and heat the resulting temperature to 40°. In this experiment we observe that 7° of heat entered the globe in 1 half-hour, but 21 half-hours were required to melt the ice and raise its temperature to 40°. If from the product 7 X 21 = 147°, we subtract 7°, which the water was above 33°, we have 140° as the measure of a quantity of heat that has entered the substance without being appreciable by the thermometer. Another simple experiment, showing the same result, is made by mixing a pound of pulverized ice at 32°, with a pound of water heated to 172°; the ice is instantly melted, but the temperature is only 32°. Here, then, 140° of heat have disappeared without raising the temperature. From this circumstance, the quantity of heat that always disappears when a body assumes a fluid state, is called latent heat, or caloric of fluidity.

The latent heat of different substances varies, as may be seen in the following table by Dr. Irvine: -

The quantity of heat that disappears in the liquefaction of solids, explains the origin of that cold which always accompanies solution, and enables us to apply the process of artificial cooling by what are termed freezing mixtures. Snow and salt, when rapidly mixed, dissolve and produce a reduction of temperature equal to 38°. The more rapid the liquefaction the greater the cold; hence, if the snow and salt be placed in a pan over the fire, and a glass tube containing water be immersed in it, the water in a few seconds will be found frozen. The solution of all crystallized salts is attended with a depression of temperature, which in general increases with the solubility of the salt; the like occurs with certain metals. If a solid amalgam of bismuth be mingled with a solid amalgam of lead, they become fluid, and the thermometer sinks. A variety of experiments have been made on the frigorific effects of different mixtures, some of which are stated in the following table, abridged from Mr. Walker's Experiments on Frigorific Mixtures.

Similar phenomena to those that take place in liquefaction, occur in vaporizing any liquid. If a vessel of water be placed over the fire, a sound is produced by the successive vaporization and condensation of the particles in contact with the bottom of the vessel. As the liquid increases in heat, the •sound becomes louder till it terminates in ebullition. At this point the temperature ceases to rise, and remains stationary till the whole of the liquid is evaporated. To ascertain the quantity of heat consumed in vapourizing a given quantity of water, Dr. Black set a tin cup full of water at 50° on a red hot iron plate; in 4 minutes it reached the boiling point, and in 20 minutes more it was all boiled off. From 50° to 212° the rise is 162°, which was gained in 4 minutes; but it took five times as long to be converted into vapour; hence 162 X 5=810° is the quantity of heat that disappears, or is rendered latent in the conversion of water into steam. By subsequent experiments, the latent heat of steam is found to be 967°, or 1,000°.

The point at which liquids emit vapour of equal tension with the atmosphere, which is their true boiling point, differs in different liquids, as may be seen in the following table: -

The boiling point of the same liquid varies with the atmospheric pressure, and also with the vessel the liquid is boiled in. Thus, in silver, the boiling point was found to be 211.775°, in common earthenware, 213.8°, at the mean pressure of the atmosphere. If the whole of the pressure be removed, liquors will boil and assume the vaporous state at 124° below their ordinary boiling points. Thus water will boil in vacuo at 88°, instead of 212°; and alcohol, at 49°. On this principle Dr. Wollaston constructed his thermometric barometer, for measuring heights. He found that a difference of 1° in the boiling point of water is occasioned by a difference of pressure equal to 0.589 of an inch on the barometer. If water be heated in a close vessel, or under extraordinary pressure, its temperature may considerably exceed 212°; and as the steam will be always of the same temperature as the liquid, and will have its elasticity increased by heat, the vapour produced will considerably exceed the atmosphere in elasticity, giving rise to what is called high pressure steam.

At the freezing point of water, the vapour that rises will have sufficient elasticity to balance two-tenths of an inch of mercury in the barometer; at 212° it equals the atmospheric pressure (about 30 inches). Its elasticity at some other temperatures is stated in the following table: -

A pint of water at 40°, on being converted into steam, forms 1,694 pints; or in round numbers, 1 cubic inch of water will form 1,728 inches, or 1 cubic foot of steam. We have already observed that the latent heat of steam is about 1,000°. This may be ascertained by evaporating a given weight of water, and condensing it into a known weight of cold water. This may be illustrated by an apparatus similar to the annexed cut. A given weight of water may be evaporated from the vessel a, the vapour of which will pass along the pipe c, and be condensed in the water in b. It will be found that the steam will raise the temperature of the water in b six or seven times more than an equal weight of boiling water would do. By experimenting in this manner, Dr. Ure has ascertained the latent heat of several vapours, as in the annexed table: -