HEM 



HEP 



TTEft), to observe). A telescope invented 

 by Scheiner, for making observations on 

 the sun without injuring the eye. The 

 usual method is to place a disc of co- 

 loured glass before the eye-piece of the 

 instrument. 



HE'LIOSTAT {riXtos, the sun, o-raat?, 

 a placing or setting). An instrument by 

 which the. sun-beam can be steadily di- 

 rected to one spot during the whole of its 

 diurnal period. 



HE'LIOTROPE. A sub-species of 

 rhomboidal quartz, found in rocks belong- 

 ing to the secondary trap formation. It 

 is termed bloodstone, from the blood-red 

 specks which occur on its green surface, 

 owing to disseminated jasper. 



HELIX (eXtf, a spiral). A spiral, or 

 winding line; a coil of wire used in 

 magneto-galvanic experiments. 



HELO'PID^. A family of coleopte- 

 rous insects, of the section heteromera 

 and sub-section stenelytra, named from 

 the genus helops, several species of which 

 abound in England, living in rotten wood 

 and under the bark of trees. 



HE'LVINE. A sub-species of dode- 

 cahedral garnet, found in beds subordi- 

 nate to gneiss, in Saxony. 



HE'MATOSIN (al/ua, blood). This 

 and globulin are two of the most charac- 

 teristic constituents of the blood, both 

 closely related to albumen. 



HEMATO'XYLIN. The colouring 

 matter of logwood, the wood of the 

 Hematoxylon campeachianum : it was 

 named hematin by Chevreul, who first 

 distinguished it. 



HEMERO'BIANS {htxepa, day, /?/o9, 

 life). A family of neuropterous insects, 

 of the section planipennes of Latreille, 

 named from the typical genus hemerobius, 

 and, like the ephemerae, of very short 

 existence. 



HEMI- CnfjiKTvi, half). A Greek term 

 frequently employed in composition to 

 denote half, and synonymous with the 

 Latin semi : — 



1. Hem-elytra {eXvrpov, a sheath). A 

 wing, of which one half is opaque and 

 firm, like an elytrum. 



2. Hemi-gyr us {yvpo?, a, circle). A term 

 applied in carpology. by Desvaux, to the 

 fruit of Protseaceae. It differs little from 

 the follicle of other writers. 



3. Hemi-hedral (Upa, a seat). A term 

 applied to a variety in some forms 

 of crystals (particularly those of which 

 the cube and the rhomboid are the pri- 

 mary), characterized by the existence of 

 only half the number of faces belonging 



162 



to any particular modification, which the 

 law of symmetry requires. These hemi- 

 hedral forms are parallel or oblique, ac- 

 cording to the relative position of the 

 existing and omitted faces. The penta- 

 gonal dodecahedron is an example of the 

 former kind, and the tetrahedron, of the 

 latter. As the tetraliedron contains half 

 the planes by which the octahedron may 

 be derived from the cube, it has been 

 sometimes called the hemi-octahedron. 



4. Hemi-ptera (Trrepov, a wing). An 

 order of insects, in which the anterior or 

 upper pair of wings are coriaceous in 

 their texture through one half of their 

 extent, the posterior portion being thin 

 and membranous, as in the notonecta, or 

 water-boatman. 



5. Hemisphere (o-^aTpa, a sphere). A 

 half -sphere, produced by cutting a sphere 

 through the centre by a right line in any 

 direction; thus the equator divides the 

 sphere of the earth into the northern and 

 southern, the eastern and western, the 

 upper and lower, hemispheres. 



6. Hemi-tone (tovo?, tone). An inter- 

 val in ancient music, the ratio of which is 



Ml 



7. Hemi-trope (xpeTro), to turn). A term 

 applied by Haiiy to twin crystals, from 

 their being generally conceived to result 

 from cutting a crystal in halves, and 

 turning one portion half round on the 

 other. The plane common to the two 

 portions of the crystal is called the twin 

 plane, and is parallel either to one of the 

 primary planes, or to a secondary plane 

 resulting from some simple law of decre- 

 ment. These crystals may frequently be 

 recognized by the existence of notches, 

 called re-entering angles, or by lines on 

 the surface, which point out the position 

 of the twin plane. 



HE'PAR (nTTap, the liver). A terra 

 formerly applied to the combinations of 

 sulphur, from their liver-Uke appearance. 

 Thus we had hepar sulphuris, a sulphuret 

 of potash ; hepar antimonii, an oxy sul- 

 phuret of antimony ; hepar calcis, a crude 

 bisulphuret of calcium, &c. 



HEPA'TIC (i^Trap, liver). A term 

 applied to various substances of a brown 

 or liver-like colour. Hepatic pyrites is a 

 variety of prismatic iron pyrites, which 

 becomes brown on exposure to the air. 

 Hepatic cinnabar is a dark-coloured, 

 steel-grey variety of cinnabar. Hepatic 

 air is another name for sulphuretted hy- 

 drogen gas. 



HEPA'TICiE (^Trap, rnrarof, the liver). 

 The Liver-wort tribe of Acotyledonous 



