HAL 



HAR 



HAIDI'NGERITE. An ore of anti- 

 mony, consisting of sulphuret of anti- 

 mony and protosulphuret of iron. 



HAIL. Drops of rain, more or less 

 suddenly frozen by exposure to a tempe- 

 rature below 32°. They assume various 

 figures, being sometimes round, at other 

 times pyramidal, cuneated, angular, thin 

 and flat, and sometimes stellated with 

 six radii like the small crystals of snow. 

 Hail occurs only in summer or in warm 

 climates, and when the sun is above the 

 horizon. 



HAIRS OF PLANTS. Minute fila- 

 mentous processes found on the cuticle 

 and in certain cavities of plants, consist- 

 ing of elongated cellular tissue, and con- 

 stituting, in the cotton plant, the pecu- 

 liar substance which envelopes the seeds, 

 and is manufactured into linen. The 

 variable qualities of hairs have given 

 rise to various designations, as pilosity, 

 villosity, pubescence, velvet, bristles, 

 stings, glandular hairs, hooks, barbs, &c. 

 Hairy surfaces are also named, with 

 reference to the quality and form of the 

 hairs, silky, arachnoid, manicate, bearded, 

 rough, and stellate or starry. 



HA'LCYON DAYS {halcyon, the king- 

 fisher). Originally, the seven days which 

 precede and follow the winter solstice, 

 the period of incubation of the halcyon, 

 generally remarkable for calm weather. 

 Hence the term denotes generally days of 

 calmness. 



HALCYO'NID^. Alcedince. The 

 King-fishers ; a family of the Insessores, 

 or Perching birds, remarkable for the 

 great length of their bill, and the extreme 

 shortness of their feet. They feed upon 

 small fishes and insects. See Fissi- 

 rostres. 



UALIO'TWM (aXf, the sea, oZs, the 

 ear). Ear-shells ; a family of the phyto- 

 phagous Gasteropods, named from the 

 genus haliotis, and known by their flat, 

 ear-shaped shells, having only the rudi- 

 ment of a spire, and without any pillar ; 

 hence they may be even called turbi- 

 nated or spiral limpets. 



HALISPO'NGIA. The generic type 

 of a group of sponges, characterized by 

 the presence of siliceous spicules, and 

 thus distinguished from the calcispongia, 

 which contains calcareous spiculae, and 

 from spongia, which is of a horny tubular 

 structure. 



HALLEY'S COMET. A comet named 

 from Edmund Halley, who in 1682 pre- 

 dicted its return in 1759 : the event co- 

 inciding with the prediction, this comet 

 159 



was first proved to belong to the solar 

 system, and to perform its revolution in 

 75 or 76 years. 



HA'LO (a\ft)f , an area). A meteor in 

 the form of a luminous ring, of various 

 colours, appearing round the bodies of 

 the sun, moon, or stars. See Parhelion. 



HA'LOGENE (a\f, salt, -^ewdw, to 

 produce). A term employed by Berze- 

 lius to denote bodies which form salts 

 with metals, as chlorine, bromine, iodine, 

 fluorine, and cyanogen. The salts thus 

 produced are called haloids. 



HALOID SALTS (aXc, the sea, sea- 

 salt, €ido9, likeness). Salt-like com- 

 pounds, consisting of a metal on the one 

 hand, and of chlorine, iodine, and the 

 radicals of the hydracids in general, 

 excepting sulphur, on the other. Be- 

 sides the simple haloid salts, Berzelius 

 distinguishes the three following com- 

 binations : — 



1. Hydro-haloid Salts, or combinations 

 of a simple haloid salt and the hydracid 

 of its radical. 



2. Oxy-haloid Salts, or combinations of 

 a metallic oxide with a haloid salt of the 

 same metal. 



3. Double Haloid Salts, consisting, 

 1. of two simple haloid salts, which con- 

 tain difierent metals, but the same non- 

 metallic ingredient ; 2. of two haloid 

 salts consisting of the same metal, but in 

 which the other element is different ; 

 and, 3. of two simple haloid salts, of 

 which both elements are entirely dif- 

 ferent. 



HA'MITE {ant), hamus, a reaping- 

 hook). A genus of fossil cephalopods, 

 with chambered shells bent in the form 

 of a hook or siphon, with parallel but 

 unequal limbs and sinuous septa, occur- 

 ring in the gait, greensand, and other 

 cretaceous beds. 



HARDNESS OF MINERALS. The 

 comparative hardness of minerals is 

 tested by reference to a scale of sub- 

 stances formed by Mohs. The hardness 

 of a mineral which neither scratches nor 

 is scratched by any particular substance 

 in this scale, is expressed by the number 

 preflxed to that substance. They are 

 1, talc; 2, rock salt; 3, calcareous spar; 

 4, fluor spar; 5, apatite; 6, adularia; 

 7, rock crystal; 8, topaz; 9, corundum; 

 10, diamond. The hardness of minerals 

 is also tested by the application of a file. 



Hardness of Rocks. This character 

 does not require so precise a determina- 

 tion as in the case of simple minerals. 

 The extremes are the hardness of quartz 



