The reason water boiled in an open vessel does not rise to a higher temperature than 212° is because all the excess of heat is carried off by the steam, and is said to be rendered latent in the vapour. The fixation of caloric in water by its conversion into steam may be shown by the following experiment. Let a pound of water at 212° and eight pounds of iron filings at 300° be suddenly mixed together. A large quantity of steam is instantly generated, but the temperature of the water and escaping steam are still only 212°; hence the steam must therefore contain all the degrees of heat between 212° and 300°, or eight times 88. When the water is heated in the hydro-electric machine or other boiler, to 322.7°, it very quickly drops to 212° when the steam is allowed to blow off; yet if the latter is collected, it represents but a very small quantity of water which constituted the steam, and it has carried off and rendered latent the excess of heat in the boiler—viz., the difference between 212° and 322.7°, or 110.7°
If steam can carry off heat, of course it may be compelled, as it were, to surrender it again; and this important elementary truth is shown by adapting a tube, bent at right angles, and a cork, to a flask containing a few ounces of water, and when it boils, the steam issuing from the end of the pipe may now be directed into and below the surface of some water contained in a beaker glass; in a very short time the water in the latter will be raised to the boiling point by the condensation of the steam and the latent heat arising from it. (Fig. 390.) The amount of latent heat is enormous, when it is remembered that water by conversion into steam has its bulk prodigiously enlarged—viz., 1698 times, so that a cubic inch of water converted into steam of a temperature of 212°, with the barometer at thirty inches, occupies a space of one cubic foot, and its latent heat amounts, according to Hall, to 950°; Southeron, 945°; Dr. Ure, 967°. When we come to the consideration of the steam-engine, it will be noticed that the question of the latent heat of steam is one of the greatest importance.
Fig. 390.
a. Flask for generating steam. b. Glass pipe bent at right angles to convey the steam into the fluid containing some cold water.
| Temperature of Steam. | Elasticity in inches of Mercury. | Latent Heat. |
| 229° | 40" | 942° |
| 270 | 80 | 942 |
| 295 | 120 | 950 |
The same weight of steam contains, whatever may be its density, the same quantity of caloric, its latent heat being increased in proportion as its sensible heat is diminished; and the reverse. In consequence of the enormous amount of latent heat contained in steam, it is advantageously employed for the purpose of imparting warmth either for heating rooms or drying goods in certain manufacturing processes. The wet rag-pulp pressed and shaken into form on a wire-gauze frame or deckle, passes gradually to cylinders containing steam, and is thoroughly dried before the guillotine knife descends at the end of the paper machine, and cuts it into lengths. In calico stiffening and glazing, also in calico printing, steam-heated cylinders are of great value, because they impart heat without the chance of setting the goods on fire. The elementary principles already described with reference to heat, will prepare the youthful reader for the application of the expansion of water into steam, as the most valuable motive power ever employed to assist the labour of man.