if Y D 



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HYD 



plied at will by division. Their natural 

 increase is by shoots. 



HYDRAN'OEA, a genus of arborescent 

 and shrubby plants. DecandriaUiyi/niu. 

 Name from i,ouo, water, and eiyyo? , a 

 vessel. Temperate climates. Some of the 

 species are cultivated in our gardens, as 

 the Chinese guelder-rose, &c. 



HY'DRANT, from t>S%aiw, to irrigate. A 

 pipe with the necessary valves, &c., by 

 which water is raised and discharged 

 from the main conduit of an aqueduct. 

 HYDRARGY'IUA, i/'buQ, and atyvqc;, sil- 

 ver. An eruptive disorder occasioned by 

 the use of mercury. 



H YDRAR'G YHUM,, v^^cifyv^os , from vdug, 

 water, and ajji/joj, silver. Mercury or 

 quicksilver. 



HY'DKATE, from iSwg, water. A com- 

 pound not crystallised, in which water is 

 chemically combined with some other 

 substance in atomic proportion, as in 

 slaked lime, which in chemical language 

 is a hydrate of lime, or lime hydrated, that 

 is, combined with water. The water so- 

 lidified in the process of crystalline bodies, 

 is termed water of crystallization. 



HYDRAU'LIC, from a>j, water, and 

 *uAef , a pipe. Relating to the convey- 

 ance of water through pipes. 



HYDRAU'LICON, the water organ. An 

 ancient musical instrument, acted upon 

 by water. Its construction is now un- 

 certain. 



HYDRAU'LICS, the science which relates 

 to the motion of non-elastic fluids, as 

 water, and the construction of all kinds 

 of instruments and machines by which 

 the force of such fluids is applied to prac- 

 tical purposes. See HYDRODYNAMICS. 



HT'DRIODATE, a compound of the hydri- 

 odic acid with a salifiable base. 



HYDRIOD'IC ACID, an acid formed by 

 the combination of hydrogen with iodine 

 in equal volumes. It occurs in the gas- 

 eous state, but combines readily with 

 water, like the hydrochloric acid. 



HYDROBROM'IC ACID, an acid composed 

 of equal volumes of hydrogen and bromine. 



HYDROCAR'BON, a combination of hy- 

 drogen with carbon; e.g., etherine. 



HY'DRO-CAR'BURETS, compounds of car- 

 bon and hydrogen, all of which are highly 

 combustible. 



HYDROCEPH'ALUS, Lat. from u'oiu^ , water, 

 and i<pJij, the head. Dropsy of the 

 brain or head ; a disease of which there 

 are two sorts, the acute and chronic. The 

 first is particularly fatal among children 

 from two to seven years of age ; in its 

 first stage it is simply inflammation of the 

 brain. The second is often a congenital 

 disease, and is connected with cachexy 

 end debility : it is always dangerous. 



HYDROCHLO'RATE, a salt formed by the 



hydrochloric acid with a base. Accord- 

 ins to some chemists, hydrochlorates exist 

 only when water is present; when the 

 water is withdrawn the salt passes to e 

 chloride. Thus, crystallised culinary salt 

 is a chloride of sodium, but in solution it 

 is a hydrochlorate of soda. 



HYDROCHLO'RIC ACID, an acid formed by 

 equal volumes of hydrogen and chlorine : 

 muriatic acid, or spirit of salt, or marine 

 acid. "NVheii pure it occurs in the gaseous 

 state, but combines readily with water; 

 its solution is extensively employed in 

 the arts. 



HYDRocm.o'RiDE,a compound of hydro- 

 gen, chlorine, and carbon, in atomic pro- 

 portions. 



HYDROCY'ANATE, a salt formed by the 

 union of the hydrocyanic acid with a 

 salifiable base. 



HYDROCYA'NIC ACID, an acid consisting 

 of equal volumes of hydrogen and cyano- 

 gen. It is called also prussic acid, be- 

 cause it was first obtained from Prussian 

 blue. It is a transparent colourless 

 liquid at ordinary temperatures, but is 

 very volatile. Diluted with 8 times its 

 weight of water, it forms medicinal prus- 

 sic acid. It is a deadly poison. One drop 

 of the pure acid introduced into the 

 fauces of the strongest dog produces death 

 after one or two convulsive respirations. 



HYDKODYNA'MICS, from ufiuo, water, and 

 $i>vau.i;. force. The branch of natural 

 philosophy which treats of the mechani- 

 cal effects of non-elastic fluids, whether 

 at rest or in motion. It comprehends both 

 hydrostatics and hydraulics, (q. v.). 



HY'DRO-TER'RO-CY'ANIC ACID, an acid 

 obtained in solution from the ferrocya- 

 nite of potash by the action of sulphuric 

 acid on a solution of that salt. Its colour 

 is lemon-yellow ; and by the action of 

 strong light or a gentle heat it is decom- 

 posed, and hydrocyanic acid and white 

 prussiate of iron are formed. 



HY'DRO-PLC'ATES, salts formed by the 

 hydrofluoric acid with bases, called^uaies 

 by some, and fluorides by other che- 

 mists. 



HYDROFLTJO'RIC ACID. When fluor-spar 

 (fluoride of calcium) is distilled with twice 

 its weight of sulphuric acid, a highly vo- 

 latile and corrosive liquid, which is hy- 

 drofluoric acid, is obtained. Its constitu- 

 tion is as yet imperfectly understood, as 

 its basis fluorine (q. v.), has not yet been 

 obtained in an insulated form. Analogy. 

 however, leads to the inference that it is 

 a compound of equal volumes of hydrogen 

 and fluorine. It acts powerfully on glass, 

 and must therefore be prepared and kept 

 in silver or lead vessels, on which it does 

 not act. Diluted with about six times 

 its weight of water, it is employed tot 

 etching on glass. 



