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FORMATION OF VAPOURS. MEASUREMENT OF THEIR ELASTIC
226. Vapours.-- We have already seen (108) that vapours are the aëriform Auids into which substances, such as ether, alcohol, water, and mercury, are changed by the absorption of heat.
In respect to the property of disengaging vapours, liquids are divided into two classes, volatile liquids, and fixed liquids. The first are those which have a tendency to pass into the state of vapour, at the ordinary or even at lower temperatures ; such, for instance, are water, ether, chloroform, alcohol, which rapidly disappear when exposed to the air in open vessels. To this class belongs a numerous family of liquids met with in nature, such as essence of turpentine, oil of lemons, of lavender, of thyme, of roses, etc.
Fixed liquids, on the contrary, are those which emit no vapour at any temperature ; such, for instance, are the fat oils, as olive, rape, etc. When strongly heated these oils are decomposed, and give rise to gaseous products; but they do not emit vapours of the same nature as their own. There are some known as drying oils, which become thicker in the air ; but this is in consequence of their having absorbed oxygen, and not in consequence of evaporation.
Some substances give vapours even in the solid state. Ice gives an instance of this, as is seen in dry cold winters, where the snow and ice quite disappear from the ground, without there having been any fusion. Camphor and odoriferous bodies, in general, present the same phenomenon.
227. Elastic force of vapours.- Vapours formed on the surface of a liquid are disengaged in virtue of their elasticity; but this force is generally far lower than the pressure of the atmosphere, and hence liquids exposed to the air only evaporate slowly.
The following experiment renders evident the elastic force of vapours. A bent glass tube has the shorter limb closed (fig. 179); this branch and part of the longer are filled with mercury. A drop of ether is then passed into the closed leg, which in virtue
of its lower density rises to the top of the tube at B. The tube thus arranged is immersed in a water bath at a temperature of
about 45o. The mercury then sinks slowly in the short branch, and the space AB is filled with a gas which has all the appearance of air. This gas or aeriform fluid is nothing but the vapour of ether, whose elastic force CA counterbalances not only the pressure of the column of mercury, but also the atmospheric pressure exerted at C.
If the water in the vessel be cooled, or if the tube be withdrawn, the mercury gradually rises in the short leg, and the drop of liquid which seemed almost to have disappeared is re-formed. If, on the contrary, the water in which the tube is immersed be still more heated, the drop diminishes and the mercury descends further in the short leg; thus showing that fresh vapours are formed, and that the elastic force increases. This increase of tension with the tempe
rature continues as long as any Fig. 179.
liquid remains to be vaporised. The crackling of wood in fires is due to the increased tension of the vapours and gases formed in the pores of the wood during combustion. In roasting chesnuts it is usual to slit the outer skin ; the object of this is to allow the vapour formed to escape, for otherwise it would acquire such a tension as to burst the chesnut and scatter the particles far and wide.
228. Formation of vapours in a vacuum.-In the previous experiment the liquid changed very slowly into the vaporous condition; the same is the case when a liquid is freely exposed to the air. In both cases the atmosphere is an obstacle to the vaporisation. In a vacuum there is no resistance, and the formation of vapours is instantaneous, as is seen in the following experiment. Four barometer tubes, filled with mercury, are immersed side by
-229] Formation of Vapours in a Vacuum. 221 side in the same trough (fig. 180). One of them, A, serves as a barometer, that is, only contains dry mercury, and a few drops of water, alcohol, and ether are respectively introduced into the tubes, B, C, D. When the liquids reach the vacuum a depression of the mercury is at once produced. But this depression cannot be produced by the weight of the liquid, for it is but an infinitely small fraction of the weight of the displaced mercury. Hence, in the case of each liquid, some vapour must have been formed whose elastic force has depressed the mercurial column, and as the depression is greater in the tube D than in the tube C, and greater in this than the tube B, it is concluded that, for the same temperature, the elastic force of ether is greater than that of alcohol vapour, and that this in turn has a greater elastic force than that of water. If the depression
Fig. 180. be measured by means of a graduated scale, it will be found that at a temperature of 20° the elastic force of ether is twenty-five times as great as that of water, and that of alcohol almost four times as great. From these experiments we obtain the two following laws for the formation of vapours :
I. In a vacuum all volatile liquids are instantaneously converted into vapour.
II. At the same temperature the vapours of different liquids have different elastic forces.
229. Limit to the formation and to the tension of vapours. Saturated space. The quantity of vapour which can be formed in a given space, whether at the ordinary or at higher temperatures,
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.
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