Hydra
or IDRA, an island of the Grecian archipelago, on the E. coast of the Morea, between the gulfs of Nauplia and Ægina; arca, 20 square miles. Pop. about 6,500.
HYDRA (hī'drä), in classical mythology, a monster which infested the Lake Lerna in Peloponnesus. It was the off- HYDRATE spring of Ichidna's union with Typhon.
It had 100 heads, and as soon as one was cut off, two grew up if the wound was not stopped by fire. It was one of the labors of Hercules to destroy this monster; this he effected with the assistance of Iolaus, who applied a red-hot iron to the wound as soon as one head was cut off. The conqueror dipped his arrows in the gall of the hydra, and all the wounds which he gave proved incurable.
In astronomy, the hydra or watersnake, one of the 15 ancient southern constellations. It is so long that it has been divided into four parts: (1) Hydra-i. e., Hydra proper; (2) Hydra et Crater; (3) Hydra et Corvus; and (4) Hydræ continuatio. Hydra proper is a little S. of the bright star, Regulus, which is in Leo.
In zoölogy the typical genus of the family Hydride. The animal is locomotive, single, naked, gelatinous, sub-cylindrical, but very contractile and variable in form; the mouth surrounded by a single row of filiform tentacles. Propagation by the formation of gemmæ and ova upon or within the substance of the animal's body. If cut into pieces each will become a new hydra. If turned inside out the exterior surface will digest good and the interior one respire. Hydra was first described by Trembley in 1774. The genus contains the fresh water polypes. Hydra viridis has 6 to 10 tentacles, shorter than the body; it is leaf-green, and is found in ponds and still waters adhering to the roots of duckweed. H. vulgaris has 7 to 12 tentacles, at least as long as the body; it is orange, brown, yellow, or red.
H. attenuata and H. fusca are rare.
HYDRANGEA (hi-drān'jēa), a genus of shrubs or herbs of the order Saxifragaceæ, containing about 33 species, natives of Asia and America. The garden hydrangea (H. hortensis) is a native of China, and was introduced into Great Britain by Banks in 1790. It is a favorite for the beauty and size of its flowers.
HYDRARTHRUS (hi-drar'thrus), a white swelling. The joints most subject to this disease are the knee, ankle, elbow, and wrist. It can be distinguished from rheumatic swelling of the joints by its fixed and wearing pain, which often exists for a long time before any enlargement of the part is perceptible. HYDRATE. In chemistry, in combination with certain metallic oxides, water seems to play the part of an acid, forming a compound that may be con- HYDRAULIC ENGINES sidered as a pseudo-salt. Thus, with oxide of sodium water forms the compound NaOH2O, or hydrate of soda, which is quite a different body to the simple NaO; in fact, such is the attraction existing between the two bodies that they cannot be separated by the strongest heat.