189.]

EVAPORATION.

397

and inverting them in a bath of the same metal. One of these tubes (1) may be kept as a standard of reference: if into one of the others (2) a few drops of water be allowed to ascend, an immediate depression of the column of mercury

is observed, due to the elasticity of the aqueous vapeur furnished by the evaporation of the water. If into a third tube (3) alcohol be introduced, a greater depression will be perceptible; carbonic disulphide in a fourth tube (4) will produce a still greater depression, and if ether be admitted to a fifth (5), the height of the mercurial column will be still less. Now let a second wider tube, closed below by a cork, be placed round the exterior of any one of these tubes, so as to inclose nearly its whole length, as in fig. 147: let the outer case thus formed be filled with water, the temperature of which is gradually raised, so as to communicate the heat uniformly to the tube within. A progressive depression of the mercurial column is thus produced; and by measuring the amount of this depression, it is found that the pressure of the vapour emitted from each liquid increases as the temperature rises, until at the boiling-point of the liquid the pressure becomes equal to that of the air.

If the temperature increase according to the terms of an arithmetic ratio, the pressure rises according to the terms of a geometric progression, the ratio of which differs for each liquid.

The following table comprises some of the results of Regnault's

398

DALTON'S LAW OF PRESSURE OF VAPOurs.

[189.

experiments upon the pressure of the vapours of various liquids at equal temperatures. The pressure of the vapour is measured by the height of a column of mercury in millimetres which each vapour will support at the temperatures quoted. That of water is taken from Regnault's Relations des Expériences, &c. (Mem. de l'Institut, 1847, xxi. 624), and those of the other liquids the temperatures of which were determined by a mercurial thermometer from the Comptes Rendus, 1860, l. 1063

(190) Dalton's Law of Pressure of Vapours.-It was assumed by Dalton that the pressure of all vapours was equal, if compared at temperatures which represented differences of an equal number of degrees above or below the boiling-points of their respective liquids, the pressure of the vapour increasing according to the terms of a geometric progression uniform for all liquids, as the temperature rose in terms of an arithmetic progression. This law is not strictly in accordance with the results of experiment. However, for short distances above and below the boiling-point, it is very nearly true, excepting in the case of mercury, and may be employed for the purpose of correcting the observations of the boiling-points of liquids made at atmospheric pressures which are but little above or below the standard pressure of 760mm.

190.]

PRESSURE OF VAPOURS.

399

The following table exhibits the pressure of the vapours of five different liquids at corresponding distances above and below their boiling-points.

Pressure of Vapours at equal distances from the Boiling-Points

The ether used in these experiments could not have been perfectly pure, as its boiling-point is too high. The boiling-point of mercury was estimated by a mercurial thermometer without correction for the increasing rate of expansion at high tempe

ratures.

The increase of pressure produced by heat in those vapours which are in contact with the liquids by which they are furnished, indicates also a corresponding increase in their density: the one may, in fact, be calculated from the other. When the temperature is reduced, the pressure falls, and a portion of the vapour is condensed. There is, indeed, for every vapour a maximum density for each temperature, which, when the liquid is in contact with the vapour, is speedily attained, but which cannot be surpassed, no matter how much the pressure to which the vapour is subjected may vary; an increase of pressure immediately condenses a part of the liquid that had evaporated, and a diminution of pressure is attended with immediate volatilization of a fresh portion of the liquid: consequently a cubic centimetre of vapour of any particular liquid at any given temperature, is always of the same pressure, and possesses the same density.

400

LIMIT OF EVAPORATION.

[190.

If a small quantity of ether be thrown up into the vacuum of the barometer tube, represented in fig. 148, the length of the column of mercury, a b,

FIG. 148.

above the level of that in the bath, will continue to be nearly the same whether the tube be raised or lowered in the outer vessel. If it be raised, fresh ether will evaporate; if depressed, part of the vapour will be condensed.

(191) Limit of Evaporation.-From what has just been stated it might be supposed that all liquids, even at the lowest temperature, were constantly emitting vapour. That mercury does so at common atmospheric temperatures may be shown by a very simple experiment. Place at the bottom of a bottle a few drops of mercury, and suspend in the neck a bit of gold leaf; in a few weeks the lower portions of the gold will become white from the condensation of the vapour of mercury upon it. In the tube of a well-made barometer the same thing is shown by the formation of a dew of metallic globules in the space above the column of metal. Faraday has, however, proved that there is a temperature below which this volatilization ceases, a temperature which varies for different substances; for mercury the limit is about 4°C.: for sulphuric acid the limit is much higher, since the acid undergoes no sensible evaporation at ordinary atmospheric temperatures. The cohesion of the liquid here appears to overcome the feeble tendency to evaporation.

It is not necessary for the evaporation of a body that it should be in the liquid form. Solid camphor is constantly emitting vapour, which condenses in a crystalline form on the sides and upper part of the vessel which contains it. Ice, if introduced into the vacuum of a barometer, immediately causes a depression of the mercurial column amounting at o° C. to upwards of 0*457mm., and even at o° F. the pressure of the vapour of ice is found to amount to o'1' It is owing to this evaporation that patches of snow and tufts of ice are observed gradually to disappear even during the continuance of a severe frost.

mm.

Regnault found in his experiments that no appreciable change in the curve which represents the pressure of a vapour is produced by the passage of a body from the solid to the liquid state;

192.]

CIRCUMSTANCES WHICH INFLUENCE EVAPORATION.

401

that is to say, that there is no abrupt diminution in the amount of vapour emitted from a body when it becomes solid.

It has been shown that if the temperature of one of the tubes, shown in fig. 145, which contains a volatile liquid, be uniformly raised throughout its entire length, the pressure of the vapour increases rapidly till the liquid reaches its boiling-point. The application of heat to one portion only of the tube, however, is attended with a very different result: the liquid may even be heated to ebullition, and it will distil and be condensed, but unless the whole of that portion of the tube which is filled with vapour he heated to the same degree, no corresponding increase of pressure will be observed: the pressure can never exceed that of the vapour which would be emitted if the liquid were at the same temperature as that of the coolest portion of the tube above the liquid; because the excess of vapour is at once condensed as soon as it reaches this colder part of the space. The ether, for example, in the barometer-tube 5, fig. 147, may be made to boil by the heat of the hand, but the height of the column of mercury undergoes little change; the ether vapour being condensed in the colder portions of the space as rapidly as it is produced.*

(192) Circumstances which Influence Evaporation.-In the process of evaporation, the vapour is supplied only from the superficial layer of the liquid. It is therefore evident that the extent of surface exposed must greatly influence the amount and rapidity of evaporation independently of the temperature. Now if the evaporating surface be in any way protected, as by allowing a small quantity of oil to become diffused over it, evaporation is entirely suspended. Advantage is sometimes taken of this fact in the laboratory in cases where it is necessary to maintain a gentle heat for many hours: the vessel to be heated is supported in a larger one containing water, upon the top of which a little oil has been poured; under these circumstances the danger of the waterbath becoming dry is obviated, and the temperature required is kept up by a smaller expenditure of fuel, because the escape of latent heat by evaporation is prevented. When, on the contrary, a rapid evaporation is necessary, a large extent of surface must be exposed. In the salt works of Cheshire, for instance, the brine is evaporated in shallow pans, 4 or 5 feet (12 or 15 metre) wide and 40 or 50 feet (12 or 15 metres) in length, the fire being lighted

In the Appendix will be found a Table giving the pressure of aqueous vapour for each degree C. between 0° and 100°.