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is always limited. For instance, in the above experiment, the depression of mercury in each tube, B, C, D, is not stopped for want of liquid which might form fresh vapours, for care is taken always to add so much that a slight excess remains unvaporised. Thus, in the tube D, enough ether is left; yet we might wait weeks and years, and if the temperature did not increase, we should always see a portion of liquid in the tube, and the level of the mercury remain stationary. This shows that no new vapours can be formed in the tube, and at the same time that the elastic force of the vapour which is there cannot increase, which is expressed by saying that it has attained its maximum tension.

When a given space has acquired all the vapour which it can contain, it is said to be saturated. For instance, if in a bottle full of dry air a little water be placed, and the vessel be hermetically closed, part of the water will evaporate slowly, until the elastic force of the vapour formed holds in equilibrium the expansive force of that which still tends to form; the formation of vapour then ceases, and the space is saturated.

230. The quantity of vapour which saturates a given space is the same whether this is vacuous, or contains air.-For the same temperature the quantity of vapour necessary to saturate a given space is the same, whether the space is quite vacuous, or contains air or any other gas. In the above bottle, whether it be full of air, or has been exhausted, the total quantity which evaporates is exactly the same; the difference being that, in the first case, the evaporation only takes place slowly, while in the second case it is instantaneous. Yet, for the same space, whether it be vacuous or full of air, the quantity of vapour formed which corresponds to the state of saturation, varies with the temperature. The higher the temperature the greater is the quantity of vapour contained in a given space, the denser it is therefore; on the other hand, the lower the temperature, the less is the quantity required to saturate a given space.

The quantity of vapour present in air is very variable; but, spite of the abundant vaporisation produced on the surface of seas, lakes, and rivers, the air in the lower regions of the atmosphere is never saturated, even when it rains. This arises from the fact, that aqueous vapour being less dense than air, in proportion as it is formed, rises into the higher regions of the atmosphere, where, condensed by cooling, it falls as rain.

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Evaporation.

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231. Evaporation.—Causes which accelerate it. We have hitherto designated, under the general term of vaporisation, all production of vapour under whatever circumstances it takes place, whether slow or rapid; in air or in a vacuum; while the term evaporation is especially assigned to the slow formation of a vapour on the surface of a volatile liquid when it is exposed in the open air. It is in consequence of evaporation that the level gradually diminishes in a vessel full of water, and ultimately dries up if it is not fed by a spring. Owing to the same cause the earth moistened by rain dries up and ultimately hardens; that moist linen exposed in the air soon dries up. Several causes influence the rapidity of the evaporation of a liquid: the temperature; the quantity of the same vapour in the surrounding atmosphere; the renewal of this atmosphere; the extent of the surface of evaporation.

Influence of temperature. Heat being the agent of all evaporation, the higher the temperature the more abundant is the formation of vapour. This property is utilised in the arts to hasten and complete the drying of a large number of products which are exposed in stoves; that is to say, in chambers, the temperature of which is kept at 30, 40, 50, and even 60 degrees, and the air of which is continually renewed to allow the vapour formed to escape.

Influence of pressure. We have already seen that the pressure of the atmosphere is an obstacle to the disengagement of vapour, and it will thus be understood that when this pressure is diminished they ought to be formed more abundantly. This, in point of fact, is what takes place whenever liquids are removed from the pressure of the atmosphere. In sugar refineries, in order to concentrate the syrup (that is, to reduce the volume by removing part of the water they contain), they are placed in large spherical vessels; and then, by the aid of large air-pumps of special construction, and worked by steam engines, the air in the boilers is rarefied, which considerably accelerates the evaporation of water, and quickly brings the syrups to the wished-for degree of concentration.

Influence of the renewal of air. In order to understand the influence of the third cause, it is to be observed that no evaporation could take place in a space already saturated with vapour of the same liquid, and that it would reach its maximum in air completely freed from this vapour. It therefore follows that, between

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these two extremes, the rapidity of evaporation varies according as the surrounding atmosphere is already more or less charged with the same vapour.

The effect of the renewal of this atmosphere is similarly explained; for if the air or gas, which surrounds the liquid, is not renewed, it soon becomes saturated, and evaporation ceases.

Thus it is that the wind, removing the layers of air which are in contact with the earth, soon dries up the roads and streets. Hence, too, it is that linen hung out to dry, does so far more rapidly on a windy than on a dry day.

The greater the extent of surface which a liquid presents to the air, the more numerous are the points from which vapour is disengaged. Hence the evaporation of a liquid should be effected in

Fig. 181.

vapour in the mass of a liquid itself.

vessels which are wide and shallow. This is what is done in the process of extracting salt from sea water in salt gardens. The sea water is admitted into broad and shallow pits excavated in the ground. Under the influence of the solar

heat the water evaporates slowly, and when its concentration has reached the point at which the liquid is saturated, the salt then begins to form on the surface and is raked off.

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When a liquid, water for example, is heated at the lower part of a vessel, the first bubbles are due to the disengagement of air

-234]

Laws of Ebullition.

225

which had previously been absorbed. Small bubbles of vapour then begin to rise from the heated parts of the sides, but as they pass through the upper layers, the temperature of which is lower, they condense before reaching the surface. The formation and successive condensation of these first bubbles occasion the singing noticed in liquids before they begin to boil. Lastly, large bubbles rise and burst on the surface, and this constitutes the phenomenon of ebullition (fig. 181).

233. Laws of ebullition. The laws of ebullition have been determined experimentally, and are as follows:

I. The temperature of ebullition, or the boiling point, increases with the pressure.

II. For a given pressure ebullition commences at a certain temperature, which varies in different liquids, but which, for equal pressures, is always the same in the same liquid.

III. Whatever be the intensity of the source of heat, as soon as ebullition commences, the temperature of the liquid remains stationary.

Thus, the boiling point of water under the ordinary atmospheric pressure being 100°, it could not be heated beyond that point, whatever the intensity of the source of heat; hence all the heat which passes from the source into the liquid is absorbed by the vapour disengaged. But, as this vapour is itself at 100°, we must conclude that this heat is not absorbed to raise the temperature of the vapour, but simply to produce it; that is, to change the substance from the liquid into the gaseous state, a phenomenon analogous to that which fusion presents (221). This disappearance of heat during ebullition will be subsequently investigated under the title of latent heat of vaporisation.

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234. Causes which influence the boiling point.-The boiling point of a liquid is affected by the substances in solution, by the

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degree of pressure to which it is subjected, and by the nature of the vessels in which the boiling takes place.

The ebullition of a liquid is the more retarded, the greater the quantity of any substance it may contain in solution, provided that the substance be not volatile, or, at all events, be less volatile than the liquid itself. Water which boils at 100° when pure, boils at 109° when it is saturated with common salt; that is, when it has taken up as much of this salt as it can dissolve. Fatty matters

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combined with water also raise its boiling point; hence it is that fat soup burns more severely than water.

Pressure. The degree of pressure to which a liquid is subjected has a most important influence on its boiling point. The greater the pressure the greater must be the tension, in order that the vapour may be disengaged, and therefore the higher the temperature. On the contrary, the less the pressure, the lower the temperature at which ebullition takes place. If the pressure of the atmosphere

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