SIL 



SIL 



occupied by one entire revolution of the 

 earth upon its axis. See Hour. 



SIDEREAL YEAR {sidus, a star). 

 The period of time in which the earth 

 makes one complete revolution in its 

 orbit ; that is, from any given star to the 

 same again. See Hour. 



SIDERUM. The name given by Berg- 

 mann to phosphuret of iron. 



SI'ENITE or SY'ENITE. A compound 

 granular aggregated rock, composed of 

 felspar and hornblende, and sometimes 

 quartz and black mica. It is found at 

 Syene in Egypt, and other places. 



SIGHT, FIELD OF. The field of a 

 telescope maybe measured by directing 

 the instrument to some star in or very 

 near to the equator, care being taken 

 that it shall pass over the middle of the 

 field, and then count the number of 

 seconds which elapse during its passage : 

 four seconds of time will make an 

 angle of one minute for the field of 

 vision. 



SIGNS of the ZODIAC. The ecliptic 

 is usually divided by astronomers and 

 by globe-makers into 12 signs of 30 de- 

 grees each, answering to twelve constel- 

 lations, with the following names and 

 signs ; the parts adjacent to the ecliptic 

 being called the zodiac:— 



Aries T from 0° to 30° 



Taurus » — 30° — 60° 



Gemini 11 — 60° — 90° 



Cancer 25 — 90° — 120° 



Leo Q, — 120° — 150° 



Virgo TYK — 150° — 180° 



Libra :Dz— 180° — 210° 



Scorpio in — 210°— 240° 



Sagittarius ... / — 240° — 270° 

 Capricornus ... Vf — 270° — 300° 



Aquarius S» — 300° — 330° 



Pisces ^ — 330°— 360° 



Of these signs, the first six are called 

 northern, lying on the north side of the 

 equator ; and the last six are called 

 southern, being situated to the south of 

 the equator. 



SILENA'CEiE. A sub-order of the 

 family of plants termed Caryophyllaceae, 

 distinguished from the other sub-order, 

 or Al^inacece, by the possession of a 

 tubular calyx and clawed petals. 



SILEX. The Latin term for flint; 

 the name of one of the pure earths, of 

 which flint is wholly composed ; in 

 chemical language, it is an oxide of 

 silicon, forming the basis of chalce- 

 dony, cornelian, jasper, &c. French 

 geologists have applied the term as a 

 generic name for all minerals composed 

 303 



entirely of silex, of which there are 

 many different external forms. 



1. Silica. Siliceous earth; the oxide 

 of silicon, constituting almost the whole 

 of silex, or flii.t. It combines with many 

 of the metallic oxides, and is hence 

 sometimes called silicic acid. 



2. Silicate. A chemical compound of 

 silica and another substance, as silicate 

 of iron. 



3. Siliceous. Of or belonging to the 

 earth of flint. A siliceous rock is one 

 composed mainly of silex. 



4. Silicified. Any substance which is 

 petrified or mineralized by siliceous 

 earth. 



5. Silicon or Silicium. An elementary 

 body very abundant in the mineral king- 

 dom, constituting the basis of silica. It 

 is a dark-brown, inflammable substance, 

 without metallic lustre. 



SILI'CULA. A diminutive of siliqua, 

 and applied, in botany, to a fruit of the 

 same construction, but shorter and 

 broader than the siliqua, as in candy- 

 tuft. 



SILIQUA. A fruit consisting of two 

 carpels cohering together, the placentae 

 of which are parietal, and separate from 

 the valves, presentir'g a kind of frame 

 called Sireplum, and connected by a mem- 

 branous expansion, as in the stock. 



SI'LLIMANITE. A crystallized va- 

 riety of silicate of alumina, found at 

 Saybrook in Connecticut, in a vein of 

 quartz penetrating gneiss. 



SILT. The more comminuted sand, 

 clay, and earth, which are transported by 

 running water. It is often accumulated 

 by currents in banks. Thus, the mouth 

 of a river is silted up when its entrance 

 into the sea is impeded by such accumu- 

 lations of loose materials. 



SILU'RIAN SYSTEM. The name 

 given by Mr. Mtirchison to the upper 

 part of the lower series of secondary 

 rocks, from its being highly developed 

 in that part of Wales which was formerly 

 inhabited by the Silures, viz., the coun- 

 ties of Hereford, Radnor, Brecon, Caer- 

 marthen, and Pembroke. He divides it 

 into two portions — the Lower, comprising 

 the Llandeilo Flags and the Caradoc 

 Limestoiie; and the Upper, consisting 

 of the Wenlock Limestone and the Lud- 

 low Rocks. 



SILURIDiE. A family of malacoptery- 

 gious, or soft-spined fishes, distinguished 

 from all other families of the order by 

 the absence of true scales, having only a 

 naked skin, or large bony plates. 



