HYDRA. 366 HYDRAULIC PRESS. in the autumn, true reproductive orpans may 1)6 ohservoil. holh male njiil female orj.'ans being usually situated on tlie same animal. Propaga- tion by gemination is, however, the most common mode of increase. Minute tulx-rcles appear on the body of the parent animal, which, a.s they increase in size, ^n'adually become perforated at their free extremity, and acquire tentacles. The pedicel by which they originate by degrees be- comes thinner, and finally gives way, leaing the young hydra ]K>rfectly independent. One of the most remarkable points in the historj' of this animal is its |)Ower of being multiplied by me- chanical division. If a hydra be cut into two or even more pieces, every one will in time assume the form and functions of the original animal. Several s|x'cics of hydra have been described, which ditter in size, color, and other respects. Two species are common in America, in still water in nearly all parts of the country-. One of these {Bydra viridis) is bright green in color, while the other (Hydra fiisca) is gniyishbrown. The green hydra is notable because its color is due to the presence of chlorophyll, the coloring matter characteristic of plants. Both may eas- ily he reared in aquariums. HY'DRAGOGTTES ( from T,at. hydragogus, Gk. i'(!pa)'w}(ir, water-carrier, from viup, hydOr, water + ayuyd^, agOyos, carrying, from a}Fn agein, to lead). Active purgatives which give rise to large, watery stools. The most important are eolocynth, elaterium, gamboge, jalap, croton oil. and scammony. These are all powerful drugs, and their administration requires skilled advice. HYDRANGEA, hfdran'j6-ft (Xeo-Lat., from Gk. i'(5up, hydOr, water + ayyelov, angrion. pail). A genus of abnit 2.'5 species of ornamental decidu- ous shrubs of the natural order Saxifragaceie, na- tives of North and South America, China, and Japan. The flowers are white, pink, or bluish, BTDBANOEA. and small, but are borne in large, showy eorj-mbs or panicles, the exterior flowers of which are sterile. The pink varieties, when grown on some soils, produce bluish flowers. The hardier cultivated hydrangeas are Hydrangea panictitata. used as specimens or in borders. Hy- drangea arborescens. and Hydrangea radiata. Hydrangea Hortensia, or Otaska, is a rather dwarf variety, which is extensively grown in greenhouses, esju'cially for the Kaster trade. Hy- drangea.s require a rather lieavy, rich soil, well manured and well watered. They are propagated for the most part by cuttings and division, but sometimes by seed. HYDRAS'TIS (Xeo-Lat., from Gk. iilw/>, hydar, water + ('/mi', dniii, to act), or Wabke- iilA. A genus of plants of the natural order Ka- niuieulacea-, allied to Anemone, the members of which have flowers destitute of petals, and suc- culent or baccate fruit. colIecte<l into a head. The only known species, Hydrastis Canadensis, a jwrennial herbaceous plant, with tuberous roots, and head of fruit resembling a raspberry, is com- mon in waterj' places in Canada, and among the AUeghanies as far south as the Carolinas. Its root is used for dyeing yellow, and also in medi- cine as a tonic. Yellowroot, orangeroot, and gold- enseal are its American names. HY'DRATE (from Gk. Mup, hyddr, water). A name applied to substances whose molecules contain either one or more entire molecules of water: or, STOonymously with the term hydrox- ide, to metallic compounds whose molecules eon- tain one or more hydro.xyl groups (OH). Thus, for example, the question has been discussed, whether substances exist in aqueous solution in the form of 'hydrates,' or not ; that is to say, whether the dissolved molecules arc associated into definite groups with those of the solvent. In the same .sense a substance containing water of erj'stallization is sometimes spoken of as a hydrate, and its erj-stals as 'hydrated crystals.' particularly when they are to be distinguished from the 'anhydrous crystals' of the same sub- stance. On the other hand, chemists often use such terms as potassium hydrate, or calcium hydrate, in the same sense as they use the terms potassium hydroxide and calcium hydroxide. The hydrates, or hydroxides, of the metals are truo chemical compounds. The assumption that sub- stances are chemically combined with their water of crystallization, while corroborated by many facts, has not yet been brought into harmony with the doctrine of valency (q.v.). and cannot therefore be ".'ranteil unconditionally. HYDRATJXIC CEMENT. See Cement. HYDRAULIC ELEVATORS. See Eleva- tors. HYDRAULIC ENGINE. See Htdraijlic PBE.ssrHE Knoi.ne. HYDRAULIC FORGING. Sec Foboe, Foho- 1X0. HYDRAULIC JACK. See -Tack. HYDRAULIC PRESS (from Lat. hydratili- cus,Gk. vdpav/.iKic, hydraulikos, pertaining to the water-organ, from i6f>av?.t^, hydraulis, water- organ, from i'Sup, hydor, water + avi, aulas, pipe). An apparatus for obtaining a heavy pres- sure for compacting fibrous substances, raising hea'y weights, etc.. by means of water under pressure. The first design of a working hy- draulic press is credited to .Joseph Rramah. and for this reason such machines are sometimes called IJramah presses. The operation of the hydraulic press is based upon the principle of hydrostatics that a pressure exerted on any part of the surface of a liquid is transmitted un- diminished to all parts of the mass and in all directions. Thus, if we have a cvlinder filled
