402

EVAPORATION AT DIFFERENT PRESSURES.

[192. at one end and the flue passing horizontally underneath to the other extremity. At Salzburg, in the Tyrol, the same object is effected by pumping the weak brine into reservoirs, whence it is allowed to trickle down through stacks of brushwood, by which means the surface exposed to evaporation in the air is almost indefinitely increased. In the southern parts of Europe the sea-water is admitted into extensive shallow pans excavated on the seacoast, where by exposure to the sun's rays it becomes concentrated, and the salt crystallizes out.

Another circumstance which influences the rate of evaporation is the pressure upon the surface of the liquid. Upon this subject a series of experiments was made by Daniell (Quart. Journ. Sci. 1834, xvii. 46). Under a receiver connected with the airpump, he placed a circular dish of water, 27 inches in diameter, and supported it above a dish containing concentrated sulphuric acid, the object of using the acid being to absorb the aqueous vapour as fast as it was generated: the results of these experiments are given in the following table :

The time in each experiment was 30 minutes, the temperature 45° (7°2 C.). It is obvious that the rapidity of evaporation under these circumstances was inversely as the pressure, which was read off upon the gauge.

The resistance offered by the pressure of a gas or vapour upon the surface of a liquid is purely mechanical; and it follows as a consequence of the law of the diffusion of gases, that the quantity of vapour which rises from a volatile body in a confined space, is the same whether that space be filled with air or not.* The time that is occupied before the space shall have received its full

* Regnault finds that this is not absolutely true,-the pressure of aqueous vapour in air being slightly less than in vacuo, but the difference does not amount to more than 2 per cent. at its maximum. The same thing was found to hold good with the vapour of ether, the pressure of which, whether in air, in hydrogen, or in carbonic anhydride, was always lower than it was at the same temperature in vacuo.

192.]

TENSION OF VAPOUR OF MIXED LIQUIDS.

403

complement of any given vapour varies inversely with the pressure; and with different vapours under similar pressures, the time varies with the diffusiveness of the vapour. The vapour, as it rises, adds its own pressure to that of the air present. When a liquid evaporates into an empty space, the pressure due to the temperature and, consequently, the maximum density of the vapour, is acquired at once; but when it evaporates into a gas, that degree of density is not acquired until after the lapse of a variable interval of time. The circumstance which in both cases finally limits the evaporation of the liquid, is the pressure of its own vapour upon its surface. It is therefore clear that the larger the proportion of moisture that is contained in the air at any given time, the smaller will be the quantity of aqueous vapour that rises from an exposed surface in a given time; and that in proportion as the space is more nearly charged with vapour, the more slowly is each succeeding portion of vapour produced. Evaporation, in short, is more rapid in a dry than in a moist atmosphere. For the same reason, evaporation proceeds more rapidly during a breeze than when the air is still: for the air which rests on the surface of a liquid soon becomes charged to the maximum with vapour, and then all further evaporation would cease were it not for circulating movements, which, even in the stillest air, are occasioned by the change of density due to the accession of moisture; the currents produced by a breeze assist these movements, and the vapour rises into portions of air which are being continually changed, so that the pressure of the aqueous vapour on the surface of the liquid is rapidly removed,

In the case of mixed liquids, Gay-Lussac inferred from his experiments that the pressure of the mixed vapour was equal to the sum of the pressures of the two vapours taken separately. This, however, is true only for liquids which, like carbonic disulphide and water, or like benzol and water, do not sensibly dissolve each other; in other cases, as the experiments of Regnault and of Magnus have shown, the pressure may scarcely exceed that of the more volatile liquid;-for example, in the case of a mixture of ether and water, the pressure is scarcely higher than that due to ether only. If the two liquids be soluble in each other in all proportions, as water and alcohol, the pressure of the mixed vapour is generally greater than that of the less volatile, but less than that of the more volatile liquid.

Wüllner (Pogg. Annal. 1860, cx. 564) has shown that the pressure of aqueous vapour emitted from a saline solution, when

404

IMPORTANCE OF SPONTANEOUS EVAPORATION.

[192.

compared with that of pure water, is diminished by an amount proportional to the quantity of anhydrous salt dissolved, when the salt crystallizes in the anhydrous form, or when it furnishes efflorescent crystals. In cases where the salt is deliquescent, or has a powerful attraction for water, the reduction of pressure is proportional to the percentage of the crystallized salt. For example, sulphates of sodium, nickel and copper, nitrate of calcium, and ordinary or hydric disodic phosphate (Na,HPO) produce a diminution of pressure proportional to the percentage of the anhydrous salt, whilst for caustic potash and soda, and for calcic chloride, it is proportional to the hydrates KHO, 2 H2O; 2 NaHO, 3 H2O; CaCl2, 3 H,O. The amount by which the pressure is reduced for equal quantities of the compounds compared, varies greatly with their nature. Sulphate of nickel, for instance, dissolved in the proportion of 10 per cent. (NiSO) reduces the pressure of the vapour at 100° C. by 13.mm.2, whilst a similar proportion of potassic hydrate (KHO) effects a reduction of 35.6.

Evaporation in a confined space, in which the atmosphere is kept constantly in a state of dryness, is often resorted to in the laboratory. Crystallizations on a small scale are frequently effected in this way: the liquid evaporates, and is absorbed by a surface of sulphuric acid, as in the experiment of Leslie (185). The evaporation may be rendered quicker or slower according to the extent to which the exhaustion of the receiver is carried. Many compounds which would be injured by exposure to air, or to a moderate rise of temperature, may be dried effectually in this

manner.

As a necessary consequence of the evaporation which is continually going on over the entire surface of the earth, the atmosphere is at all times charged with moisture, the percentage of which is perpetually varying, but it is almost always below that which experiment gives as the maximum density for aqueous vapour due to the observed temperature. It is owing to the cir cumstance that the air is rarely fully charged with vapour, that wet bodies become dry, and that the surface of the soil, although saturated with moisture, yet in a few hours or days becomes parched and dusty. By the process of evaporation from the surface of the land as well as of the ocean, a natural distillation is thus continually effected, by which a perpetual circulation of water is maintained; the waters conveyed by the rivers into the sea return imperceptibly into the atmosphere. The vapour thus raised either assumes an invisible form, or it floats about in masses of cloud; these are at length arrested, particularly by mountains

193.]

DEW-POINT.

405

and elevated ridges of land, and becoming condensed, descend as showers, and supply stores of water, which flow down the sides of the hills, and collect in the ravines, or else are absorbed into the porous strata. The waters thus absorbed sink into the soil until they meet with a bed of clay or some other stratum impervious to moisture; by this they are arrested, and flow along its surface till they burst out as springs in the valleys. These springs in their turn furnish constant supplies to the rivers, and the rivers, after irrigating the countries through which they flow, again empty themselves into the ocean. The frequency of rain, and various other meteorological phenomena of the highest interest and importance,-in fact, many of the great peculiarities of climate, are mainly influenced by the variations in the percentage of moisture which is contained in the atmosphere. The knowledge of the quantity of aqueous vapour which exists at any given time in a certain volume of air, becomes, therefore, a problem which is constantly requiring solution for meteorological purposes. Instruments employed for this purpose are termed hygrometers (from vypòs, moist, and μérpov, a measure). Various methods have been proposed for determining the percentage of moisture in the air; the simplest and the most accurate of these consists in the determination of the dew-point.

(193) Dew-point.—It is evident that a reduction of temperature in a space already charged to the maximum with vapour, must produce a deposit of moisture in the liquid form. Such a result, in fact, accords with daily observation: for example, when a glass of cold water is brought into a warm and moist room, its surface becomes bedewed with moisture. This observation has been ingeniously turned to account for the purpose of determining the quantity of moisture present in the air at any given time. If the cold liquid be poured from one vessel to another, its temperature will be gradually raised; the quantity of dew which is formed on the outside of the vessel into which it is poured will become less and less, until it ceases to be formed at all. By noting with a sensitive thermometer the exact temperature at which this formation of dew ceases, the pressure of the aqueous vapour present in the air at that period can be readily ascertained from tables constructed for that purpose, and the corresponding percentage of moisture calculated. If the temperature of the air at the time be noted, it is easy to determine the additional percentage of moisture which the air at that time is capable of taking up. This comparison is generally made by calling the quantity of invisible vapour which it is possible for air to retain at the particular tem

406

DANIELL'S DEW-POINT HYGROMETER.

[.193.

perature at the time of observation icoo, and calculating from the observed dew point the ratio which the amount actually present bears to that which might exist at that temperature. Suppose, for example, when the air is at 15° C., that the dewpoint be as low as 10°; that is, the temperature at which dew begins to be formed is 10°. On reference to the table, it appears that the pressure of vapour at 15° amounts to 12-mm.699 of mercury, while at 10° it is equal to only 9mm.165. Now the quantity of vapour is directly proportioned to its pressure; therefore, by proportion:

12699 : 9*165 :: 1000 : Ꮳ (=722). 722 represents the degree of atmospheric saturation at the time of observation.

Practically, however, it is desirable also to know the actual rate of evaporation at the time (or the number of grammes of water which evaporate from a given surface, such as a square metre of water freely exposed to the air), since it is this which in great measure determines the drying influence of the atmosphere upon the human body, and upon the substances exposed to its action.

FIG. 149.

k

(194) Daniell's Hygrometer.-The method of observing the dew-point above mentioned, although it affords very exact results, is tedious in practice.

To facili

tate this operation, a beautiful instrument was contrived by Daniell, and termed by him the Dew-point Hygrometer. It consists essentially of a small cryophorus (fig. 149) containing ether instead of water, one limb of which, e, is longer than the other, and terminates in a ball, b, make of black glass, for the purpose of rendering the moment at which the deposition of dew occurs more readily ob servable. In the long limb of the instru ment is placed a sensitive thermometer, d, the bulb of which is partially immersed in the ether. The second bulb, a, is covered with muslin. In constructing the appa ratus, the ether is boiled to expel the air, and the instrument is hermetically sealed whilst the ether is still boiling. When the hygrometer is to be used, all the ether is driven into b, by inverting the instrument, and warming the bulb a with the hand; the instrument is then placed in the clip h, on the top of the stand g. On allowing a few drops of ether to fall on the muslin, the vapour within the ball a is condensed by the reduction of temperature occasioned by the rapid evaporation thus produced on its outer surface: fresh vapour rises from the surface of the ether in the blackened ball, owing to the diminished pres