CHAPTER XXVIII.

HYGROMETRY.

285. Humidity.-The condition of the air as regards moisture involves two distinct elements: (1) the amount of vapour present in the air, and (2) the ratio of this to the amount which would saturate the air at the actual temperature. It is upon the second of these elements that our sensations of dryness and moisture chiefly depend, and it is this element which meteorologists have agreed to denote by the term humidity; or, as it is sometimes called, relative humidity. It is usually expressed as a percentage; for example, if the weight of vapour present is seven-tenths of that required for saturation, the humidity is said to be 70.

The words humid and moist, as applied to air in ordinary language, nearly correspond to this technical use of the word humidity; and air is usually said to be dry when its humidity is considerably below the average. In treatises on physics, "dry air" usually denotes air whose humidity is zero.

The air in a room heated by a hot stove contains as much vapour weight for weight as the open air outside; but it is drier, because its capacity for vapour is greater. In like manner the air is drier at noon than at midnight, though the amount of vapour present is about the same; and it is for the most part drier in summer than in winter, though the amount of vapour present is much greater.

the same

Bearing in mind that a cubic foot of air is able to take up amount of vapour as a cubic foot of empty space, we may define the humidity of the air as the weight of aqueous vapour in a given volume of air, expressed as a percentage of the weight of vapour at saturation which would occupy the same volume at the actual temperature.

Also, since aqueous vapour nearly fulfils Boyle's law, the humidity

of the air may be obtained by comparing the tension of the vapour present in the air with the maximum tension for the actual tempera

ture.

286. Dew-point.-When air containing aqueous vapour is gradually cooled at constant pressure, its density increases, and the rate of increase is sensibly the same for the vapour as for the dry air with which it is mixed (inasmuch as vapours not in contact with their liquids nearly fulfil Gay-Lussac's law), until a point is reached at which the density of the vapour becomes equal to the maximum density corresponding to the temperature. This temperature is called the dew-point of the given mass, and any further reduction of temperature will be accompanied by the condensation of a portion of the vapour, which will take the form of dew, rain, snow, or hoar-frost, according to circumstances. If the cooling is produced by the low temperature of the sides of the containing vessel, the deposit will be dew or hoar-frost, according as the temperature of the sides is above or below the freezing-point. If the cooling takes place in the interior of the mass of air, the deposit will be rain or snow, according as the temperature of deposition is above or below the freezingpoint.

In the operation of cooling down to the dew-point, the density of the vapour, as we have seen, increases. Let t denote the initial temperature, and T the dew-point, and let d and D be the densities of the vapour at these temperatures. Then we have, by Gay-Lussac's law,

But the tension of the vapour is not sensibly changed by the operation, since the whole pressure is by hypothesis preserved constant, and the changes of temperature and volume affect the dry and the vaporous constituent nearly alike.

If the reduction of temperature from t to T took place at constant volume (in a closed receiver, for example), we should then have

p and P denoting the vapour-tensions at the temperatures t and T; and the density would remain constant, since no vapour enters or escapes. In this case the vapour would not begin to be condensed till a somewhat lower temperature had been attained.

287. Hygroscopes.-Anything which serves to give rough indica

tions of the state of the air as regards moisture may be called a hygroscope (vypos, moist). Many substances, especially those which are composed of organic tissue, have the property of absorbing the moisture of the surrounding air, until they attain a condition of equilibrium, such that their affinity for the moisture absorbed is exactly equal to the force with which the latter tends to evaporate. Hence it follows that, according to the dampness or dryness of the air, such a substance will absorb or give up vapour, either of which processes is always attended with a variation in the dimensions of the body. The nature of this variation depends upon the peculiar structure of the substance; thus, for instance, bodies formed of filaments exhibit a greater increase in the direction of their breadth than of their length. Membranous bodies, on the other hand, such as paper or parchment, formed by an interlacing of fibres in all directions, expand or contract almost as if they were homogeneous. Bodies composed of twisted fibres, as ropes and strings, swell under the action of moisture, grow shorter, and are more tightly twisted. The opposite is the case with catgut, which is often employed in popular hygroscopes.

288. Hygrometers.—Instruments intended for furnishing precise measurements of the state of the air as regards moisture are called hygrometers. They may be divided into four classes:

1. Hygrometers of absorption, which should rather be called hygroscopes.

2. Hygrometers of condensation, or dew-point instruments.

3. Hygrometers of evaporation, or wet and dry bulb thermometers.

4. Chemical hygrometers, for directly measuring the weight of vapour in a given volume of air.

289. De Saussure's Hygrometer.-The best hygrometer of absorption is that of De Saussure, consisting of a hair deprived of grease, which by its contractions moves a needle (Fig. 266). When the hair relaxes, the needle is caused to move in the opposite direction by a weight, which serves to keep the hair always equally tight. The hair contracts as the humidity increases, but not in simple proportion, and Regnault's investigations have shown that, unless the most

Fig. 266.

De Saussure's Hygroscope.

DEW-POINT HYGROMETERS.

367

minute precautions are adopted in the construction and graduation of each individual instrument, this hygrometer will not furnish definite numerical measures.

Fig. 267 represents Monnier's modification of De Saussure's hygrometer, in which the hair, after passing over four pulleys, is attached to a

light spring, which serves instead of a weight, and gives the advantage of portability.

These instruments are never employed for scientific purposes in this country.

290. Dew-point Hygrometers. -These are instruments for the direct observation of the dew-point, by causing moisture to be condensed from the

Fig. 267.-Monnier's Hygroscope.

air upon the surface of a body artificially cooled to a known tempera

ture.

The dew-point, which is itself an important element, gives directly, as we have seen in § 286, the tension of vapour; and if the temperature of the air is at the same time observed, the tension requisite for saturation is known. The ratio of the former to the latter determines the humidity.

The principle of these instruments may be illustrated by a description of their simplest type, the hygrometer of Leroy, a French philosopher of the last century.

291. Leroy's Hygrometer. The instrument consists of a tin vessel containing water, in which a thermometer is immersed. The temperature of the water and containing vessel is gradually lowered by the introduction of ice, and when it has fallen below the dew-point of the adjacent air, a portion of the vapour will be condensed as dew upon the exterior of the vessel. This is at once recognized by the metallic surface losing its brilliancy.

We may observe that the deposition of dew does not begin till the

point of saturation has been passed, and that the indication of the thermometer is consequently somewhat too low. Leroy proposed an empirical correction of half a degree. There are, however, other defects in the instrument; the use of ice does not afford a speedy and regular diminution of temperature; and it is especially objectionable to place an open vessel containing water in the very place where the humidity of the air is to be determined.

292. Danielle Hygrometer.-Daniell's hygrometer is an instrument of much greater precision, and has been very extensively used. It consists of a bent tube with a globe at each end, and is partly filled with ether. The rest of the space is occupied with vapour of ether, the air having been expelled. One of the globes A contains a thermometer t. This globe is generally made of black glass, which presents a brilliant surface. The method of using the instrument is as follows:-The whole of the liquid is first passed into the globe A, and then the other globe B, which is covered with muslin, is moistened externally with ether. The evaporation of this ether from the muslin causes a partial condensation of vapour of ether in the interior of the globe, which produces a fresh evaporation from the surface of the liquid in A, thus lowering the temperature of that part of the instrument. By carefully watching the surface of the globe, the exact moment of the deposition of dew may be ascertained. The temperature is then read on the inclosed thermometer. This temperature is a little lower than the dew-point.

If the instrument be now left to itself, the exact moment of the disappearance of the dew may be observed; this corresponds to an indicated temperature a little above the dew-point, and the usual plan is to take the mean between this temperature and that first observed. The temperature of the surrounding air is given by a thermometer ť attached to the stand.

Daniell's hygrometer, though capable of furnishing accurate indications, has some defects, which have been removed by the improvements effected by Regnault.

293. Regnault's Hygrometer.-Regnault's hygrometer consists (Fig. 270) of a glass tube closed at the bottom by a very thin silver cap D. The opening at the upper end is closed by a cork, through which