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HYGROMETER

906

HYPNOTISM

has thus been warded off from the American coasts by the active supervision of the health-authorities at the ports.

Within the sphere of the home health science has of late vears made satisfactory progress. The principle of safe and sanitary drainage, whereby a house can be trapped off efficiently from the public sewers and the inroads of sewer-gas (giving rise to typhoid fever and other ailments) prevented, is beginning to be everywhere practiced. In other details, also, the health of our homes Is receiving the care it deserves. Questions of ventilation and of lighting are being studied anew, and the warming of houses is no longer left to chance. Personal health, v^hich ranges in its extent from questions of foods and drinks to those of cleanliness and clothes, is not neglected amid the general improvement in hygienic education; so that the outlook in health questions is, on the whole, of the most hopeful kind. Happily, the people at large are beginning at length to perceive and to act on the great truth that only by their personal education in hygiene, and by their knowledge and observance of health laws, can they secure the length of days which of old it was declared Wisdom bore in her right hand.

Hygrom'eter (kygros means moisture, metron, a measure), an instrument used to measure the amount of moisture in the air at any particular time. The hygrometric condition of the air is indicated by the ratio of the amount of moisture in the air at the particular time to the amount that it is capable of holding at that temperature.

The earliest instruments did not afford a means of determining this scientifically. The hair hygrometer or, rather, hygroscope simply indicated roughly whether the air was comparatively wet or dry. The hair expanded on a damp day and contracted on a dry day.

The wet and dry bulb hygrometer consists of two thermometers exactly alike, fastened side by side on a stand, the bulb of one of which is covered with muslin. The muslin is connected with some sort of wick arrangement leading to a jar of water. The idea is to have water conducted up the wick to the muslin surrounding the bulb, the thin character of the muslin giving the water a good chance to evaporate as rapidly as possible. The evaporation, of course, causes a fall of temperature in the bulb underneath. The drier the day, the faster the evaporation and the greater the fall in temperature of the muslin-covered thermometer. The other thermometer records the actual temperature of the air, and by comparing the two and using a set of prepared tables the hygrometric condition can be determined.

There are several other forms of hygrometer, the most scientific of which, perhaps, is that of Regnault. This instrument is similar in principle to the wet and dry bulb

hygrometer, but it is much more precise in construction and makes use of the very rapid evaporation of ether, instead of the slower evaporation of water, to reduce the temperature of one of "';he bulbs. By this means one of the thermometers can easily be cooled down to the dew-pc:.nt, that is, the temperature at which the moisture of the air begins to condense as dew. Having the dew-point and also the temperature of the air as given by the other thermometer, the ratio of the amount of moisture in the air to the amount that it is capable of holding can be determined.

Another form of hygrometer is that in which a definite volume of air is passed over a substance, as calcium chloride or sulphuric acid, that will absorb all of its moisture. This substance is carefully weighed, both before and after the known quantity of air is passed over it, and the increase in weight will be the weight of water in the quantity of air passed over.

Hyksos (hik'soz) (meaning shepherd kings), the name given to the kings of Egypt of the 15th, 16th and iyth dynasties. Their capital was Tanis, called Zoan in the Bible. The Hyksos were not the Hebrews, but probably were wandering tribes of Arabia and Syria, mostly Canaanites. They became Egyptians in manners and customs, even worshiping the Egyptian gods. They ruled for about 500 years, from 2200 B, C. to 1700 B. C. or, according to other accounts, from 2000 to 1500. See Sayce's Ancient Empires and Races of the Old Testament.

Hymet'tus, a ridge of mountains, four miles east of Athens, which reaches a height of nearly 3,000 feet, is famous since ancient times for its honey. The honey appears to be flavored by the wild thyme upon which the bees pasture. Hymettus marble from the mountain quarries has also had a reputation for beauty even from the days of the ancient greatness of Attica.

Hypa^tia a Greek philosopher, daughter of Theon, an astronomer of Alexandria. She was renowned for her knowledge of mathematics and the Platonic philosophy, of which she was a teacher at Alexandria. She was also celebrated both for beauty and modesty. She met a tragic fate. Being accused by the clergy of undue influence over Orestes, prefect of Alexandria, against Cyril, then archbishop there, she was attacked by a mob led by monks, dragged into a church, stripped of her clothing and killed. This occurred about 415 A. D. Hypatia has been made the subject of a romance by Charles Kingsley.

Hy'pha (in plants). The individual thread of the mycelium of a fungus. See FUNGI.

Hyp'notism. This term covers the phenomena associated with an abnormal mental condition resembling somnambulism. The special characteristic of the hypnotized person is liability to suggestion from some one else, ordinarily the one by whose influence