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 HYGIENE HYGROMETKY 131 for their immediate and complete removal. Besides the necessary provisions for drainage, the house and apartments should also be fully and completely ventilated. Effluvia and or- ganic vapors of various kinds necessarily be- come developed in every occupied dwelling, from the daily culinary operations and the or- ganic matters of the food and their remains. These effluvia are harmless when fresh ; but they are subject to early decomposition, and at once become noxious if allowed to accumu- late and stagnate. Every house, accordingly, should be swept throughout each day by a cur- rent of fresh air, sufficient to renovate its at- mosphere and remove all vestiges of impurity. A free opening of the windows on opposite sides, early in the morning, is the best way of accomplishing this. In addition, each inhabited apartment should be constantly ventilated in such a manner as to remove the carbonic acid and other products of respiration, by open fires or other effectual means. Proper clothing, adapted to the season and the degree of indi- vidual exposure, is also an important element of hygiene. There are few causes of disease more prolific than undue exposure to cold and dampness, and particularly to sudden changes of weather or draughts of cold air upon un- protected parts. The clothing should be so regulated, as a general thing, that the ordina- ry vicissitudes of the weather shall not be felt by the individual in such a way as to make a permanent impression upon the system. A sufficient suit of woollen underclothing is the best protection in this respect. It is important to remember, however, that for a person in health exposure to cold and dampness is sel- dom injurious so long as the body is in a state of muscular activity. It is remaining in a cold apartment in an inactive condition, or keeping on the wet or damp clothing after muscular ex- ertion has ceased, that gives rise to dangerous consequences. The quality and quantity of the food, and the regularity with which it is taken, are of the next importance in a hygienic point of view. The food, as a rule, should be simple in character, but nutritious, and each article of the best possible quality and proper- ly cooked. An imperfect or careless mode of cooking may often injure materially the nutri- tious and digestible qualities of an article of food, originally of the best kind. Individual peculiarities are to be consulted in regard to the kind of food used by each person ; certain articles being sometimes more or less indiges- tible for one person, which are quite harmless for another. The natural and healthy appe- tite is the best general criterion in regard to the quantity of food to be used, provided it be simple and nutritious in character. It is of great importance, finally, that the food be taken with regularity at the accustomed time, that it be properly masticated, and that its digestion be not interfered with by hurry, anxiety, or any unusual mental or physical dis- turbance at and immediately after the time of meals. A regular and sufficient bodily exer- cise should be taken every day to keep all the organs in a healthy state of activity. The ex- ercise should be neither deficient nor excessive in amount; for bodily exertion which is so violent or so prolonged as to produce a sense of exhaustion and fatigue, instead of being beneficial to the system, is positively injurious to it. Neither can a deficiency of muscular exertion during one period be compensated by an excessive amount taken at another. It is the necessary and appropriate quantity of ex- ercise, taken regularly day by day, which pre- serves the vigor of the system, and keeps it in a condition to resist the attacks of disease. The periods of exertion, furthermore, should alternate daily with periods of repose ; and especially the natural amount of sleep should always be taken with regularity, and in apart- ments which are not too confined and the ven- tilation of which is properly provided for. It is during sleep that the main process of the nutrition and restoration of the nervous and muscular systems takes place ; and if an indi- vidual deprive himself of sleep, wholly or even partially, for one or two nights in succession, he will invariably experience its damaging ef- fects in the consequent temporary failure of the vital powers. An imprudence or neglect, like either of those mentioned above, may he counteracted in a strong and healthy person by subsequent care, so that he may recover from its immediate and. more perceptible ef- fects ; but it is a principle which lies at the basis of hygiene, that causes of disease, however slight, by constant repetition day after day, or even at longer intervals, will certainly at last undermine the health, and produce a perma- nent and often irremediable injury. The easi- est as well as the surest way of avoiding such a result is a constant and regular attention to all the necessary hygienic conditions. (See ALIMENT, BATH, DIETETICS, and GYMNASTICS.) H1GROMETRY (Gr. i-yp6f, moist, and ftirpov, measure), the method of determining the amount of moisture in bodies, more especially in atmospheric air. A hygrometer is an in- strument used for this purpose ; and a hygro- scope is any substance that absorbs moisture from the air, and is in consequence changed in form or weight. Various salts absorb moisture and deliquesce, and are consequently called hygroscopic. These serve as hygrometers in chemical analysis; thus chloride of calcium placed in a glass tube absorbs the moisture from the air passed through the tube, and its increase of weight determines the quantity. The property is exhibited in hemp and cotton ropes, and in small fibres, as those of whale- bone, and in hairs. Paper by absorption of moisture expands to such a degree that it is an imperfect material for preserving accurate plans. Its variation in length in extremely dry and in moist air sometimes exceeds 1 in 40. If a substance could be found which ab- sorbed moisture in proportion to the quantity