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II Y M 



not absorbable by water ; it is devoid of 

 taste, and is destructive of life when re- 

 spired for any time. It is the lightest 

 body known, 100 cubic inches weighing 

 only 2*25 grains, or being nearly thirteen 

 times lighter than atmospheric air. It is 

 combustible, and, when pure, burns with a 

 yellowish-white flame. Hydrogen enters 

 into the composition of all waters, and is 

 evolved in a compound state from vol- 

 canos, from certain fissures in the earth, 

 and in districts where coal is found. 

 Two volumes of hydrogen unite with one 

 of oxygen in the production of water. 

 As far as the superficies of our planet is 

 concerned, water so predominates, that, at 

 first sight, hydrogen might be considered 

 as constituting a substance of more rela- 

 tive abundance than it really does. The 

 quantity of hydrogen locked up in coal is 

 considerable. According to Dr. Thom- 

 son, cannel coal contains 21 per cent, of 

 it, although in the Newcastle caking coal 

 the proportion is but a trifle more than 

 4 per cent. Hydrogen may be considered 

 as the most important substance of its 

 class next to oxygen, which enters into 

 the composition of the earth's crust. 



HYDRO'GENATED. Combined with hydro- 

 gen. 



HYDRO'GRAPHER. (from uwp and ypa0w, 

 Gr. hydro graphe, Fr. idrografo,It.) One 

 who draws maps of the sea-coast, rocks, 

 islands, shoals, &c. 



HYDROGRA'PHIC. ) Relating to a descrip- 



HYDROGRA'PHICAL. $ tion of the sea- 

 coast, rocks, islands, &c. &c. 



HYDRO'GRAPHY. (from vSup and yp0w, 

 Gr. idrografia, It. hydrographie, Fr.) 

 Description of the watery part of the 

 globe ; the art of measuring and de- 

 scribing the sea, rivers, canals, lakes, &c. 

 With regard to the sea, it gives an ac- 

 count of its tides, counter-tides, sound- 

 ings, bays, gulfs, creeks, &c. as also of 

 the rocks, shelves, sands, shallows, pro- 

 montories, harbours ; the distance and 

 bearing of one point from another ; with 

 every thing that is remarkable either at 

 sea or on the coast. 



HYDRO'METER. (from vSwp and juerpoj', 

 Gr. hydrometre, Fr. idrometro, It.) An 

 instrument for measuring the extent or 

 depth, gravity, density, and velocity of 

 liquids. 



HY'DROPHANE. (from u^wp and ^euvw, 

 Gr. ) A variety of opal which is opaque 

 and white when dry, but by immersion in 

 water becomes transparent. 



HY'DROPHYTE. (from vSwp and <j>vrbv, 

 Gr.) A plant which lives and grows in 

 water. Mr. Lyell observes, " the num- 

 ber of hydrophytes is very considerable, 

 and their stations more varied than could 

 have been anticipated j for while some 



plants are covered and uncovered daily 

 by 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 least to our 

 organs, many of these vegetables are 

 highly coloured." Principles of Geology. 



HYDROSTA'TIC. ^ (from i)wp and trra- 



HYDROSTA"'TICK. \- T-IKOC, Gr.) Relat- 



HYDROSTA'TICAL. j ing to that branch of 

 science termed hydrostatics. 



HY'DROSTATICS. (from inTwp and (TTciTiicbg, 

 Gr. hydrostatique, Fr. i'drostatica, It.) 

 The science which treats of the nature, 

 gravity, pressure, and equilibrium of 

 fluids, and of the weighing of solids in 

 them. 



HY'DRURET. A compound of hydrogen 

 with a metal. 



HYGRO'METER. (from vypoe and fji'tTpov, 

 Gr. hygrometre, Fr. igrometro, It.) An 

 instrument for measuring the degree of 

 moisture of the atmosphere. There are 

 various kinds of hygrometers ; for what- 

 ever either swells by moisture, or shrinks 

 by dryness, is capable of being formed 

 into an hygrometer. 



HYGROME'TRICAL. Relating to hygro- 

 metry. 



HYGRO'METRY. The art of measuring the 

 degree of moisture in the atmosphere. 



HYL^EOSAU'RUS. A fossil lizard discovered 

 in the wealden formation of Tilgate 

 forest by Dr. Mantell, and fully described 

 by him in his Geology of the South-East 

 of England. The Hylseosaurus, or Lizard 

 of the Weald, was discovered in 1832. 

 Its probable length was about twenty-five 

 feet. It is characterised by a series of 

 long, flat, and pointed bones, which ap- 

 pear to have formed a large dermal 

 fringe, resembling the bones on the back 

 of the modern iguana. In this reptile the 

 osteology of the lizard seems blended with 

 that of the crocodile. Dr. Mantell ob- 

 serves, " the most extraordinary parts are 

 many enormous, angular spinous bones, 

 which lie in the direction of the vertebral 

 column, and evidently extended originally, 

 like a serrated fringe, along the back of 

 the animal/' 



HYMENO'PTERA. (from v/rn'OTrrepoc., Gr. 

 alas membranaceas habens : membrane- 

 winged.) Cuvier makes Hymenoptera 

 the ninth order of the class Insecta. 

 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 rnaxillee, yet they do not 

 generally use them to masticate their 

 food, but for purposes usually connected 

 with their sequence of instincts, as the 



