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Quartz resinite hydrophane of 

 Haiiy. A variety of opal which is 

 opaque and white when dry, but 

 by immersion in water becomes 

 transparent. Hydrophanes, or 

 changeable opals, are highly prized 

 by collectors. In order, however, 

 to preserve their beauty, care must 

 be taken never to immerse them in 

 any but pure water, and to remove 

 them from the fluid as soon as they 

 have acquired their transparency; 

 otherwise, their pores will soon 

 become filled with earthy particles, 

 and their hydrophanous properties 

 destroyed. "Hydrophane has a 

 porous structure, and when im- 

 mersed in water bubbles of air 

 escape from its pores, while the 

 water enters, and its weight is in- 

 creased. When the pores are thus 

 filled with water, a less portion of 

 the light is reflected during its 

 passage through the mineral, than 

 if the same pores were filled with 

 air; consequently more light is 

 transmitted, and the transparency 

 increased." Cleaveland. 



HYDBO'PHANOTJS. Those substances 

 are termed hydrophanous which, 

 when dry, are opaque, but become 

 transparent on being wetted. This 

 property especially occurs in some 

 varieties of common and semi-opal. 



HY'DROPHYTE. (from vdiap and 0vrov, 

 Gr.) A plant which lives and 

 grows in water. Sir C. Lyell 

 observes, "the number of hydro- 

 phytes is very considerable, and 

 their stations more varied than 

 could have been anticipated; for 

 while some plants are covered and 

 uncovered daily with the tide, 

 others live in abysses of the ocean, 

 at the extraordinary depth of one 

 thousand feet : and although in 

 such situations there must reign 

 darkness more profound than night, 

 at leasA to our organs, many of 

 these vegetables are highly co- 

 loured." Principles of Geology, 



HYDBOST'ATICS. (from vwp and 



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Gr. hydrostatique, Fr. 

 idrostatica, It.) The science which 

 treats of the nature, gravity, pres- 

 sure, and equilibrium of fluids, and 

 of the weighing of solids in them. 



HY'DBIJBET. A compound of hydrogen 

 with a metal. 



HYDBOZ'OA. The second class of the 

 sub-kingdom Coelenterata, compri- 

 sing two orders, namely, Lucenna- 

 roida and Hydroida. 



HYGRO'METEB. (from v^pos and /lerpov, 

 Gr. hygrometre, Fr. igrometro, It.) 

 An instrument for measuring the 

 degree of moisture of the atmos- 

 phere. There are various kinds of 

 hygrometers; for whatever either 

 swells by moisture, or shrinks by 

 dryness, is capable of being formed 

 into an hygrometer. 



HYL^EosAu'Etrs. (from vK^, wood, 

 weald, or forest, and aavpos, a 

 lizard, Gr., the Forest Lizard.) A 

 fossil lizard discovered in the weal- 

 den formation of Tilgate forest by 

 Dr. Mantell, in 1832. Its pro- 

 bable length was about twenty-five 

 feet. It is characterised by a series 

 of long, flat, and pointed bones, 

 which appear to have formed a 

 large dermal fringe, resembling the 

 bones on the back of the modern 

 iguana. In this reptile the osteo- 

 logy of the lizard seems blended 

 with that of the crocodile. 



HYMENO'PTEBA. (from v/aevdTnepos, 

 Gr. alas membranaceas habens : 

 membrane-winged.) The first or- 

 der of the class Insecta, comprising 

 the Ichneumon, Saw-fly, &c. It 

 is divided into six families. They 

 have four membranous wings, and 

 the tail of the female is usually 

 armed with a sting. Though the 

 insects of this order are included in 

 the mandibulate section, for their 

 mouth is furnished with mandibles 

 and maxillae, yet they do not ge- 

 nerally use them to masticate their 

 food, but for purposes usually con- 

 nected with their sequence of in- 

 stincts, as the bees in building their 



