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This table shows that the elastic force of aqueous vapour increases far more rapidly than the temperature. Thus at 50° the tension is only 91.9 millimeters; while at 100° degrees, that is to say, double the temperature, the tension is eight times as great.

237. Latent heat of vapour.-In speaking of ebullition we have seen that, from the moment a liquid begins to boil, its temperature ceases to rise whatever be the intensity of the source of heat. It follows that a considerable quantity of heat becomes absorbed in ebullition, the only effect of which is to transform the body from the liquid to the gaseous condition. And conversely, when a saturated vapour passes into the state of liquid, it gives out an amount of heat.

These phenomena were first observed by Black, and he described them by saying that, during vaporisation, a quantity of sensible heat became latent, and that the latent heat again became free during condensation. The quantity of heat which a liquid must absorb in passing from the liquid to the gaseous state, and which it gives out in passing from the state of vapour to that of liquid, is spoken of as the latent heat of evaporation.

The analogy of these phenomena to those of fusion will be at once seen. The modes of determining them need not be described; but the following results which have been obtained for the latent heats of evaporation of a few liquids may be here given:

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The meaning of these numbers is, in the case of water, for instance, that it requires as much heat to convert a pound of water from the state of liquid at the boiling point to that of vapour at the same temperature, as would raise a pound of water through 540 degrees, or 540 pounds of water through one degree; or that the conversion of one pound of vapour of alcohol at 78° into liquid alcohol of the same temperature would heat 208 pounds of water through one degree.

238. Cold due to evaporation.—Whatever be the temperature at which a vapour is produced, an absorption of heat always takes place. If, therefore, a liquid evaporates, and does not receive from without a quantity of heat equal to that which is expended in producing the vapour, its temperature sinks, and the cooling is greater in proportion as the evaporation is more rapid.

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Water frozen in a Vacuum.

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This may become a source of very great cooling. Thus if a few drops of ether be placed in the hand, and this be agitated to accelerate the evaporation, great cold is experienced. With liquids which are less volatile than ether, like alcohol and water, the same phenomenon is produced, but the cooling is less marked.

On coming out of a bath, and more especially in the open air and with some wind, a very sharp cold is experienced, due to the vapour formed on the surface of the body. Moist linen is cold and injurious, because it withdraws from the body the heat necessary for evaporation.

The cooling effect produced by a wind or draught does not necessarily arise from the wind being cooler, for it may, as shown by the thermometer, be actually warmer; but arises from the rapid evaporation it causes from the surface of the skin. We have the feeling of oppression, even at moderate temperatures, when we are in an atmosphere saturated by moisture in which no evaporation takes place.

The cooling produced by the use of fans is due to the increased evaporation they produce. The freshness occasioned by watering the streets is also an effect of evaporation.

The cold produced by evaporation is used in hot climates to cool water by means of alcarrazas. These are porous earthen vessels, through which water percolates, so that

on the outside there is a continual evaporation, which is accelerated when the vessels are placed in a current of air. For the same reason wine is cooled by wrapping the bottles in wet cloths and placing them in a draught.

239. Water and mercury frozen in a vacuum. From the great quantity of heat which disappears when a liquid is converted into vapour it will be seen that by accelerating the evaporation we have a means of producing intense cold. We have seen that liquids vaporise more rapidly the lower the pressure. Hence, if a vessel containing water be placed in Fig. 187. a space from which the air is exhausted, it should cool very rapidly. Leslie succeeded in freezing water by means of rapid evaporation. Under the receiver of the air-pump is placed a vessel containing

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strong sulphuric acid, a substance which has a great affinity for water, and above it a thin, shallow, porous capsule (fig. 187) containing a small quantity of water. By exhausting the receiver the water begins to boil, and since the vapours are absorbed by the sulphuric acid as fast as they are formed, a rapid evaporisation is produced, which quickly effects the freezing of the water.

By using liquids more volatile than water, more particularly liquid sulphurous acid, which boils at -10°, a degree of cold is obtained sufficiently intense to freeze mercury. The experiment

may be made by covering the bulb of a thermometer with cotton wool, and after having moistened it with liquid sulphurous acid, placing it under the receiver of the air-pump. When a vacuum is produced the mercury is quickly frozen.

Thilorier, by directing a jet of liquid carbonic acid on the bulb of an alcohol thermometer, obtained a cold of 100° without freezing the alcohol. With a mixture of solid carbonic acid, liquid protoxide of nitrogen and ether, M. Despretz obtained a sufficient degree of cold to reduce alcohol to the viscous state.

By means of the evaporation of bisulphide of carbon the formation of ice may be illustrated without the aid of an air-pump. A little water is dropped on a small piece of wood, and a capsule of thin copper foil, containing bisulphide of carbon, is placed on the water. The evaporation of the bisulphide is accelerated by means of a pair of bellows, and after a few minutes the water freezes round the capsule, so that the latter adheres to the wood.

CHAPTER VIII.

LIQUEFACTION OF VAPOURS AND GASES.

240. Liquefaction of vapours.-The liquefaction or condensation of vapours is their passage from the aëriform to the liquid state. Condensation may be due to three causes-cooling, compression, or chemical affinity.

When vapours are condensed, their latent heat becomes free, that is, it affects the thermometer. This is readily seen when a current of steam at 100° is passed into a vessel of water at the ordinary temperature. The liquid becomes rapidly heated, and soon reaches 100°. The quantity of heat given up in liquefaction is equal to the quantity absorbed in producing the vapour.

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Liquefaction of Gases.

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Liquefaction by chemical affinity.-The affinity of certain substances for water is so great as to condense the vapours in the atmosphere, even when they are far from their point of saturation. Thus, when highly hygroscopic substances, such as quicklime, potass, sulphuric acid, are exposed in the air, they always absorb aqueous vapour. Certain varieties of common salt exposed to the air absorb and condense so much aqueous vapour as to become liquid. Many other salts have the same property, and are hence called deliquescent salts.

Liquefaction by pressure.-Let us suppose a vessel containing aqueous vapour, a cylinder for instance, and in this cylinder a piston which can be depressed at will, like that represented in fig. 4 page 9. As the vapour is not at first in a state of saturation when the piston is depressed, it behaves like a true gas, the pressure increasing its elastic force and density without liquefying it. But the more the piston is depressed the smaller does the volume of the vapour become, and a point is ultimately reached at which the vapour present is just sufficient to saturate the space. From this point the slightest increase of pressure causes a portion of vapour to pass into the liquid state, and the liquefaction continues as long as the excess of pressure lasts; so that if the piston descends to the bottom of the cylinder all the vapour is condensed. In this experiment it is to be observed, that when once saturation is attained, provided there is no air in the cylinder, the resistance to the depression of the piston does not increase in proportion as it descends, which arises from the condensation of the vapour, and confirms what was previously said as to the maximum tension of vapour in a state of saturation.

Liquefaction by cooling.-Cooling, as well as pressure, only causes vapours to liquefy when they are in a state of saturation. But when once a given space is saturated, the slightest lowering of temperature takes from the vapours the heat which gives them their fluidity, the attraction between the molecules preponderates, they agglomerate, forming extremely small droplets, which float in the air and are deposited on the surrounding bodies.

Vapours are ordinarily condensed by cooling. Thus, the vapours exhaled from the nose and mouth of animals first saturate the colder air in which they are disengaged, and then condense with a cloud-like appearance. It is owing to the same phenomenon that the vapours become visible which are disengaged from boiling water, those which rise from chimneys, the fogs formed above

rivers, and so forth. All these vapours are more apparent in winter than in summer, for then the air is colder, and the condensation more complete.

In cold weather, the windows in heated rooms are seen to become covered with dew on the inside. The air of these rooms is in general far from being saturated with vapour, but the layers of air in immediate contact with the windows become colder; and as the quantity of vapour necessary to saturate a given space is less, the colder this space, a moment is reached at which the air in contact with the windows is saturated, and then the vapours they contain are quickly deposited. In a time of thaw, when the air is hotter on the outside than on the inside, the deposit is formed on the outside. To the same cause is due the deposit of moisture formed on walls, which is expressed by saying that they sweat; an unsuitable expression, for the moisture does not come from the walls but from the atmosphere. The walls are colder than the air, and they lower the temperature of the layers in contact with them, and condense the vapours. A similar effect is produced when in summer a bottle of wine is brought from the cellar, or when a glass is filled with cold water; a deposit of dew is formed on the surface of these vessels. The same phenomenon does not occur in winter, for then the temperature of the atmosphere being the same as that of the bottle, or even lower, the layers of air in immediate contact with it are not cooled.

241. Heat disengaged during condensation.-It has been seen that any liquid in vaporising absorbs a quantity of heat. This heat is not destroyed, for in the converse change it reappears in the sensible state; that is to say, capable of acting on our organs and on the thermometer. For instance, we know that a pound of water absorbs in vaporising 540 units of heat (237); that is to say, a quantity of heat necessary to raise 540 pounds of water from o° to 1°: conversely, a pound of water at 100°, which is liquefied and gives a pound of water at 100°, causes 540 units to pass from the latent to the sensible state, an amount of heat which is utilised in heating by steam.

242. Application to heating by steam.-The quantity of heat which becomes free when aqueous vapour is condensed is utilised in the arts for heating private houses, hot-houses, and public buildings. Steam is produced in boilers like those used in steam engines, and passes from thence into metallic tubes concealed behind the wainscot, or into columns which serve at the same time as ornaments

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