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signifying the temperature at which the air is saturated with

vapour.

Experiment 3.-Fill a tumbler one quarter full with cool water, place in it a thermometer, and at short

intervals gradually add ice or cold water, until_Fig. 17. moisture begins to deposit on the outside of the glass. Then observe the degree indicated by the thermometer, which is the dew-point. If much cold water must be added before the glass clouds over, that is, if the dew-point is much lower than the temperature of the air, fair weather may be expected; while, on the contrary, if the difference between the dew-point and the temperature of the air be but slight, rain may soon be expected, as then the air requires but a slight addition of moisture or increase of cold to become saturated. Instruments by means of which the amount of moisture in the air is ascertained are called hygrometers. Many substances readily imbibe moisture from the air, and become damp; such bodies, for instance, as catgut, carbonate of potassium, sulphuric acid, fresh barley-sugar, &c., are called hygroscopic.

Evaporation may be accelerated, not only by heat, but also by a current of air, because by this means the air above the surface of the fluid, which is charged with vapour, is removed and replaced by a drier, and, as it were, more thirsty air, which takes up the vapour more rapidly and abundantly than the former. For this reason, the earth dries rapidly after rain, when followed by a high wind, and hence it is necessary in kilns, laundries, drying-rooms, &c., to arrange them in such a manner that the air, when saturated with moisture, may be constantly replaced by dry air.

That heat disappears during slow as well as rapid evaporation may be readily illustrated by the following experiment. Experiment 4.-Fill a tube half full of water, and fasten securely round the bulb of it a piece of wadding; saturate the wadding with cold water, and then twirl the tube rapidly between the hands; presently the water in the tube will become sensibly colder, and the degree of cold may be accurately determined by the thermometer. Moisten the wadding with ether, a very volatile liquid, and twirl it again in the same manner as before; by which means its contents,

Fig. 18.

even in summer, may be converted into ice. Water evaporates slowly, ether rapidly; and both require heat for their conversion into vapour, and in the above experiment they obtain this heat from the water in the bulb, which is of course the reason of the water becoming cold. On this principle, one feels cool on just leaving the bath, when invested in damp garments, or when the floor of a hot apartment is sprinkled with water. It explains, also, how man is enabled to support the scorching sun of the hottest climates, and even to endure a heat of 212° F. (100° C.), without his blood exceeding the temperature of from 100° F. (38° C.) to 104° F. (40° C.); it is owing to the more copious perspiration, which, by evaporation, renders all the heat above 104° F. (40° C.) latent. If we blow on hot soup, it is also the increased evaporation which cools it more rapidly; but if we blow on the cold hands in winter, they become moist and warm, because the latent heat contained in the vapour of the breath is set free, as the vapour is condensed into water.

Distillation.-If the condensation of watery vapour be carried on in a closed vessel, the water may be collected as it forms. Experiment 5.-A small glass retort is half filled with water,

Fig. 19.

and heated; the steam, as it forms, passes through the neck of the retort into a glass receiver, contained in a vessel filled with cold water, and is there condensed. That the refrigeration may take place more rapidly, the receiver is covered with coarse blotting-paper which is frequently moistened with cold water. This operation is called distillation (from distillare, to drop), and the pure water obtained is said to be distilled. It is purer than spring-water, for this reason, that the non-volatile, earthy, and saline portions contained in all spring-water do not evaporate, but remain in the retort. By this means also

[graphic]

very volatile bodies can easily be separated from less volatile ones; as in the distillation of brandy, where the more volatile spirit is separated from the less volatile water. Copper stills are usually employed for distillation on a large scale, and for condensers vats are constructed, holding serpentine pipes, or worms, which present a greater condensing surface than if the pipe had passed straight through the vat. The cold water with which the vats must be filled is very soon warmed by the heat liberated in the condensation of the steam, and must occasionally be renewed by leading off the hot water from above, and letting in a fresh supply of cold water beneath.

DIFFUSION OF HEAT.

Fig. 20.

Conduction of Heat.-Experiment 1.-A test-tube, nearly filled with water, is held over a spiritlamp, in such a manner as to direct the flame against the upper layers of the water; the water will boil at the top, but remain quite cold below. If mercury is treated in a similar way, its lower layers will gradually become heated. The particles of mercury will communicate the heat to each other, but not so the particles of water. Substances through which, as in mercury, heat rapidly passes, are called good conductors; but bodies which comport themselves like water are called bad conductors of heat. In the former class are included principally the metals, and in the latter, stone, glass, wood, snow, water, and especially soft bodies, as cloth, fur, linen, straw, paper, ashes, &c., and also the gases.

The good conductors are readily heated, and soon become cold again, as is well known to be the case with iron stoves. A piece of iron feels hotter in the sun and colder in the shade than a piece of wood at the same temperature. The explanation of this delusion of the sense of touch is, that the warm iron conducts the heat more rapidly to the hand, while the cold iron withdraws it more rapidly than the wood is capable of doing.

The bad conductors of heat become only slowly heated, and

also slowly cooled; for this reason, stoves constructed of brick (the Russian stove) and those made of Dutch tiles, a preparation of clay, retain their heat longer than iron stoves. Bad conductors are frequently employed both for preventing the quick heating and the quick cooling of bodies. Vessels of glass and porcelain are placed on sand (a sand-bath) or ashes, to heat them gradually, and thus guard against their breaking. If a hot liquid is to be poured into them, it must be done by small portions at a time, twirling the vessels round for some minutes before adding more.

On removing vessels from the fire, the precaution should be taken never to place them while hot on metal or stone, but always on some bad conductor, such as straw (straw rings) wood, paper, cloth, &c.; as they are often cracked by sudden cooling and contraction, which is frequently produced even by a current of cold air. Doors of furnaces, ladles, &c., are provided with wooden handles, to prevent those using them from being burnt. Should a person desire to hold a flask or a test-tube while liquids are boiling in them, he must wrap round them several folds of paper, or tie round them a piece of twine, in order that they may serve as a bad conductor between the glass and his fingers. By inclosing substances in bad conductors, the entrance of cold, or, more correctly, the departure of heat, may be prevented; this principle is illustrated in our method of clothing, in the protection given to our wells and trees by covering them with straw, in the preservation of plants by snow, and in numerous other phenomena of daily occurrence. Hence bad conductors are frequently called preservers of heat.

Fig. 21.

Convection of Heat.-Liquids and gases, though very bad conductors, readily become heated by a process called convection, which is a necessary consequence of the expansion produced in them by heat.

Experiment 2.-Water, to which some sawdust has been added, is heated in a test-tube over a spirit-lamp. The tube is held by the upper part, and rotated for some minutes between the fingers, that the flame may have equal access to all the lower parts of the tube. If the water be carefully observed, it will be

Fig. 22.

seen that the sawdust ascends on the upper surface of the liquid, and descends in the lower strata; the warm water expands, and, becoming lighter, rises upwards, while the colder, and consequently heavier, water sinks; the water circulates. In consequence of this circulation, the heating of fluids takes place more rapidly when the heat is applied beneath. If the upper part of a testtube full of water is heated by the lamp, the water will boil at the top while it remains quite cold, and may even contain ice, held down by a wire, at the bottom. Testtubes are cylindrical glass vessels with rounded bottoms. To prevent their breaking on the application

[graphic]

of heat, the bottom must be thin, and blown of a uniform shape. A simple wooden rack, as in the annexed figure, serves as a convenient stand for them.

Convection in Gases.-Currents of Air.-A great many phenomena of daily occurrence may be explained by the difference in lightness between warm and cold air. When a fire is lighted in a stove for the heating of an apartment, the air immediately in contact with the stove is first heated, becomes lighter, and ascends; colder air rushes in to supply its place, and this likewise becomes heated and ascends; consequently a constant circulation of air is kept up. By a similar circulation, the whole atmosphere of the earth is kept in continual motion. At the equator the strongly heated air ascends and moves in the upper regions of the atmosphere towards the poles, while in the lower regions the current of cold air flows from the arctic zone towards the equator, in order here to restore again the equilibrium, disturbed every moment by the ascent of the warm air. These regular currents of air, the direction of which is somewhat diverted by the revolution of the earth on its axis, are called tradewinds.

In every heated apartment, a difference between the heat of the air near the ceiling and that near the floor is very perceptible. If a door or window in such a room be opened, a current of air is produced, the direction of which may easily be perceived by holding a lighted candle in the opening; the

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