Hygrology

[Gr. hygros, moist, and logos.] (Med.) The doctrine of the humors or fluids of the body. Hygro'ma, n. ( Med.) A tumor containing serum and other not purulent fluids. Hygrometer, ( hi - grom'e-ter,) n. [Gr. hygros, moist, metron, a measure.] An instrument for ascertaining the amount of aqueous vapor present in the atmosphere or other aëriform fluid under examination. Several varieties of apparatus have been invented for this purpose. Any alterations in the state of the atmosphere, with respect to moisture or dryne , are manifested by different phenomena. The various forms of H. are thus very great; but they can generally be divided into two distinct cla es, - those which depend upon absorption, and those which depend upon condensation. A great number of substances in nature absorb moisture in a greater or le degree, and consequently undergo some change, either in regard to their physical qualities, their size, or their weight. Animal fibre is elongated, on account of being softened or relaxed; while vegetable fibre is shortened, on account of its swelling. Moisture is imbibed with avidity by many mineral substances, which grin weight by that means. Many of the H. which depend upon this alteration of dimension or weight are known by the names of their inventors; as, De Luc's, De Sau ure's, Daniell's, . De Luc employed a thin slip of whalebone, the contractions of which indicated the variations of the moisture. De Sau ure employed a human hair, by means of which he constructed a far more delicate instrument; but, unfortunately, it was exceedingly liable to derangement; and, moreover, was uncertain, unle prepared with extreme care. The H. invented by J. F. Daniell is one of the most perfect. It consists (Fig. 1347) of two thin balls of 14 inch diameter, a and b, connected together by a tube having a bore about one-fourth of an inch. The tube is bent at right angles over the two balls, and the arm, bc, contains a small thermometer, de, whose bulb, which should be of a lengthened form, descends into the ball b. This ball, having been about two thirds filled with ether, is heated over a lamp till the fluid boils, IMG:content-0665.jpg:[merged small][merged small][subsumed][ocr errors][merged small] pelled the air from both balls, the capillary tube is hermetically closed by the flame of a IMG:content-0666.jpg:[ocr errors] lamp. The other Fig. 1347. - DANIELL'S HYGROMETER. ball, a, is now to be covered with a piece of muslin. The stand, gh , is of bra , and the transverse socket, i, is made to hold the gla tube in the manner of a spring, allowing it to turn and be taken out with little difficulty. A small thermometer, kl , is inserted into the pillar of the stand. The manner of using the instrument is this: - After having driven all the ether into the ball b , by the heat of the hand, it is to be placed at an open window or out of doors, with the ball b so situated that the surface of the liquid may be on a level with the eye of the observer. A little ether is then to be dropped on the covered ball. Evaporation immediately takes place, which, producing cold upon the ball a , causes a rapid and continuous condensation of the ethereal vapor in the interior of the instrument. The consequent evaporation from the included ether produces a depre ion of temperature in the ball b , the degree of which is measured by the thermometer, de . This action is almost instantaneous, and the thermometer begins to fall in two seconds after the ether has been dropped. A depre ion of 30° or 40° is easily produced, and the ether is sometimes observed to boil, and the thermometer to be driven below zero of Fahrenheit's scale. The artificial cold thus produced causes a condensation of the atmospheric vapor upon the ball b, which first makes its appearance in a thin ring of dew coincident with the surface of the ether. The degree at which this takes place must be carefully noted. In very damp or windy weather the ether should be very slowly dropped upon the ball, otherwise the descent of the thermometer will be so rapid as to render it extremely difficult to be certain of the degree. In dry weather, on the contrary, the ball requires to be well wetted more than once, to produce the requisite degree of cold. - The H. invented by Mason consists of two thermometers placed side by side, one having a dry bulb and the other a bulb covered with muslin, kept moist by means of a string dipping in water. The wet bulb is chilled by the evaporation of the water from it, since this evaporation renders some of its heat latent. The drier the air, the more rapid the evaporation, and the greater the difference between the readings of the two thermometers. - The Edson's Hygrodeik is an improved form of Mason's H. It differs from all other H. in having a dial and pointer, showing at a glance the temperature, the degree of humidity, the absolute amount of vapor in each cubic foot of air, and the dew-point. Hygrometric, or Hygrometrical, a . Pertaining to hygrometry; made by or according to the hygrometer.- This term is commonly applied to substances which readily become moist and dry with cor. responding changes in the state of the atmosphere, or which readily absorb and retain moisture. Seaweed, several saline substances, porous clays, potash and its carbonate, chloride of calcium, sulphuric acid, are in this sense of the term said to be hygrometric.