HOE 



[219] 



H Y A 



horns. According to Kirwan, its 

 constituents are, silica 72, alumina 

 22, carbonate of lime 6. "A speci- 

 men yielded to Faraday," says 

 Phillips, "silica 71 '3, alumina 

 15-3, protoxide of iron 9*3, and a 

 trace of lime." Its colours are 

 numerous, and commonly dull. Its 

 infusibility by the blow-pipe dis- 

 tinguishes it from petrosilex and 

 jade. Its translucency serves to 

 render it distinct from jasper. It 

 is generally more dull than flint, 

 and emits sparks more feebly with 

 steel. It is deficient in lustre in 

 comparison with quartz. 



HO'RXSTONE PORPHYRY. The Horn- 

 stein porphir of Werner. A variety 

 of porphyry of a red, brown, pur- 

 ple or blackish colour. Fracture 

 splintery and conchoidal. Emits 

 sparks when struck with steel. Is 

 susceptible of a fine polish. 



HO'RSTED SAND. (So called from Hor- 

 sted in Sussex.) The Horsted sand 

 forms one of the members of the 

 Hastings beds, or central group of 

 the Wealden formation. It con- 

 sists of grey, white, fawn coloured, 

 and ferruginous sands and friable 

 sandstone, with numerous small 

 portions of lignite. It has been 

 found principally in Sussex and 

 Kent The Horsted sand is char- 

 acterized by the dissemination 

 throughout of small portions of 

 lignite, which appear to have orig- 

 inated from the carbonization of 

 plants of the fern tribe. Mantell. 



HO'RTUS SICCTJS. (Lat.) A collection 

 of dried plants. 



HU'MBOLDITE. A rare mineral, thus 

 named after Humboldt. 



HU'MITE. A reddish-brown mineral, 

 found at Somma, thus named after 

 Sir A. Hume. 



HY'ACINTH. (vaKtvOos, Gr. hyacintfatt, 

 Lat. hyacinthe, Fr.) The Hiazinth 

 of Werner; Zircon hyacinthe of 

 Brongniart. A variety of pyra- 

 midal zircon, of a hyacinth-red 



colour; it occurs also, but more 

 rarely, of a yellow, grey, and green 

 colour, and sometimes, though very 

 rarely, it is found white. It is 

 crystallized, and when in distinct 

 crystals its ordinary form is a four- 

 sided rectangular prism, terminated 

 by four rhombic planes. Each 

 plane angle at the summit is 73 

 44'. It is found in beds of streams 

 and rivers along with rubies, sap- 

 phires, &c., but sometimes it occurs 

 in the primary rocks. It consists 

 of, zirconia 70, silica 25, oxide of 

 iron 0-5. It is considered a gem, 

 but is little used as such. The 

 finest specimens are brought from 

 Ceylon, but it is found in Europe in 

 many places. It is cut by the 

 jewellers, like the diamond, into 

 rose and table forms, and possessing, 

 as it does, though in an inferior 

 degree, the play of the diamond it 

 is frequently sold for that more 

 precious gem. 



HY ACI'NTHINE. A mineral of a brown 

 or greenish colour, usually crys- 

 tallized in rectangular eight-sided 

 prisms. Fracture imperfectly con- 

 choidal. Transparent. Causes 

 double refraction. 



HYAL^'A. (So named from its semi- 

 transparent shell.) The Hyalaa 

 has the appearance of a bivalve 

 with soldered valves, the upper one 

 being the largest ; this difference 

 of size of the seeming valves causes 

 an aperture through which the 

 animal sends forth two large yellow 

 and violet wings, or sails, rounded 

 and divided at their summit into 

 three lobes. When its wings, or 

 sails, are unfolded, it moves with 

 great velocity on the surface of the 

 sea. The Pteropods, both from the 

 beautiful colouring of their filmy 

 wings, and from their number and 

 symmetry, are better entitled to 

 the appellation of the butterflies of 

 the ocean, than the escalop shells, 

 which have sometimes been so 

 called. .Zfrv. W. Kirby. 



