hydra was one of the twelve “labours” of Heracles, which he accomplished with the assistance of Iolaus. Finding that as soon as one head was cut off two grew up in its place, they burnt out the roots with firebrands, and at last severed the immortal head from the body, and buried it under a mighty block of rock. The arrows dipped by Heracles in the poisonous blood or gall of the monster ever afterwards inflicted fatal wounds. The generally accepted interpretation of the legend is that “the hydra denotes the damp, swampy ground of Lerna with its numerous springs (κεφαλαί, heads); its poison the miasmic vapours rising from the stagnant water; its death at the hands of Heracles the introduction of the culture and consequent purification of the soil” (Preller). A euhemeristic explanation is given by Palaephatus (39). An ancient king named Lernus occupied a small citadel named Hydra, which was defended by 50 bowmen. Heracles besieged the citadel and hurled firebrands at the garrison. As often as one of the defenders fell, two others at once stepped into his place. The citadel was finally taken with the assistance of the army of Iolaus and the garrison slain.
See Hesiod, Theog., 313; Euripides, Hercules furens, 419; Pausanias ii. 37; Apollodorus ii. 5, 2; Diod. Sic. iv. 11; Roscher’s Lexikon der Mythologie. In the article Greek Art, fig. 20 represents the slaying of the Lernaean hydra by Heracles.
HYDRA, in astronomy, a constellation of the southern
hemisphere, mentioned by Eudoxus (4th century B.C.) and
Aratus (3rd century B.C.), and catalogued by Ptolemy (27 stars),
Tycho Brahe (19) and Hevelius (31). Interesting objects are:
the nebula H. IV. 27 Hydrae, a planetary nebula, gaseous and
whose light is about equal to an 8th magnitude star; ε Hydrae,
a beautiful triple star, composed of two yellow stars of the 4th
and 6th magnitudes, and a blue star of the 7th magnitude;
R. Hydrae, a long period (425 days) variable, the range in
magnitude being from 4 to 9.7; and U. Hydrae, an irregularly
variable, the range in magnitude being 4.5 to 6.
HYDRACRYLIC ACID (ethylene lactic acid), CH2OH·CH2·CO2H.
an organic oxyacid prepared by acting with silver oxide and
water on β-iodopropionic acid, or from ethylene by the addition
of hypochlorous acid, the addition product being then treated
with potassium cyanide and hydrolysed by an acid. It may
also be prepared by oxidizing the trimethylene glycol obtained
by the action of hydrobromic acid on allylbromide. It is a
syrupy liquid, which on distillation is resolved into water and
the unsaturated acrylic acid, CH2:CH·CO2H. Chromic and
nitric acids oxidize it to oxalic acid and carbon dioxide.
Hydracrylic aldehyde, CH2OH·CH2·CHO, was obtained in 1904
by J. U. Nef (Ann. 335, p. 219) as a colourless oil by heating
acrolein with water. Dilute alkalis convert it into crotonaldehyde,
CH3·CH:CH·CHO.
HYDRANGEA, a popular flower, the plant to which the name
is most commonly applied being Hydrangea Hortensia, a low
deciduous shrub, producing rather large oval strongly-veined
leaves in opposite pairs along the stem. It is terminated by
a massive globular corymbose head of flowers, which remain a
long period in an ornamental condition. The normal colour
of the flowers, the majority of which have neither stamens nor
pistil, is pink; but by the influence of sundry agents in the soil,
such as alum or iron, they become changed to blue. There are
numerous varieties, one of the most noteworthy being “Thomas
Hogg” with pure white flowers. The part of the inflorescence
which appears to be the flower is an exaggerated expansion of
the sepals, the other parts being generally abortive. The perfect
flowers are small, rarely produced in the species above referred
to, but well illustrated by others, in which they occupy the inner
parts of the corymb, the larger showy neuter flowers being
produced at the circumference.
There are upwards of thirty species, found chiefly in Japan, in the mountains of India, and in North America, and many of them are familiar in gardens. H. Hortensia (a species long known in cultivation in China and Japan) is the most useful for decoration, as the head of flowers lasts long in a fresh state, and by the aid of forcing can be had for a considerable period for the ornamentation of the greenhouse and conservatory. Their natural flowering season is towards the end of the summer, but they may be had earlier by means of forcing. H. japonica is another fine conservatory plant, with foliage and habit much resembling the last named, but this has flat corymbs of flowers, the central ones small and perfect, and the outer ones only enlarged and neuter. This also produces pink or blue flowers under the influence of different soils.
The Japanese species of hydrangea are sufficiently hardy to grow in any tolerably favourable situation, but except in the most sheltered localities they seldom blossom to any degree of perfection in the open air, the head of blossom depending on the uninjured development of a well-ripened terminal bud, and this growth being frequently affected by late spring frosts. They are much more useful for pot-culture indoors, and should be reared from cuttings of shoots having the terminal bud plump and prominent, put in during summer, these developing a single head of flowers the succeeding summer. Somewhat larger plants may be had by nipping out the terminal bud and inducing three or four shoots to start in its place, and these, being steadily developed and well ripened, should each yield its inflorescence in the following summer, that is, when two years old. Large plants grown in tubs and vases are fine subjects for large conservatories, and useful for decorating terrace walks and similar places during summer, being housed in winter, and started under glass in spring.
Hydrangea paniculata var. grandiflora is a very handsome plant; the branched inflorescence under favourable circumstances is a yard or more in length, and consists of large spreading masses of crowded white neuter flowers which completely conceal the few inconspicuous fertile ones. The plant attains a height of 8 to 10 ft. and when in flower late in summer and in autumn is a very attractive object in the shrubbery.
The Indian and American species, especially the latter, are quite hardy, and some of them are extremely effective.
HYDRASTINE, C21H21NO6, an alkaloid found with berberine
in the root of golden seal, Hydrastis canadensis, a plant indigenous
to North America. It was discovered by Durand in 1851, and
its chemistry formed the subject of numerous communications
by E. Schmidt and M. Freund (see Ann., 1892, 271, p. 311)
who, aided by P. Fritsch (Ann., 1895, 286, p. 1), established
its constitution. It is related to narcotine, which is methoxy
hydrastine. The root of golden seal is used in medicine under
the name hydrastis rhizome, as a stomachic and nervine
stimulant.
HYDRATE, in chemistry, a compound containing the elements
of water in combination; more specifically, a compound containing
the monovalent hydroxyl or OH group. The first and more
general definition includes substances containing water of
crystallization; such salts are said to be hydrated, and when
deprived of their water to be dehydrated or anhydrous. Compounds
embraced by the second definition are more usually
termed hydroxides, since at one time they were regarded as combinations
of an oxide with water, for example, calcium oxide or
lime when slaked with water yielded calcium hydroxide, written
formerly as CaO·H20. The general formulae of hydroxides
are: MiOH, Mii(OH)2, Miii(OH)3, Miv(OH)4, &c., corresponding
to the oxides M2iO, MiiO, M2iiiO3, MivO2, &c., the Roman index
denoting the valency of the element. There is an important
difference between non-metallic and metallic hydroxides;
the former are invariably acids (oxyacids), the latter are more
usually basic, although acidic metallic oxides yield acidic
hydroxides. Elements exhibiting strong basigenic or oxygenic
characters yield the most stable hydroxides; in other words,
stable hydroxides are associated with elements belonging to the
extreme groups of the periodic system, and unstable hydroxides
with the central members. The most stable basic hydroxides
are those of the alkali metals, viz. lithium, sodium, potassium,
rubidium and caesium, and of the alkaline earth metals, viz.
calcium, barium and strontium; the most stable acidic hydroxides
are those of the elements placed in groups VB, VIB and VIIB
of the periodic table.
