SLA 



SMI 



(TTona, a mouth). A designation of ani- 

 mals with a suctorious mouth like a 

 tube. The term is usually applied to an 

 order of the edentulous Crustacea which 

 are so characterized, as the caligus. 

 Under the term Siphonobranchiata, De 

 Blainville describes the first order of his 

 first sub-class of MoUusca, Paracephalo- 

 phora dioica. 



SIPHU'NCLE. A long tube, partly 

 calcareous and membranous, which passes 

 through all the compartments of the shells 

 of certain cephalopods ; the membranous 

 siphuncle is continued into the animal, 

 and terminates in a cavity contained 

 within its body. 



SIRE'NE. An instrument, invented 

 by Caignard Latour, for determining the 

 number of musical vibrations. The notes 

 are produced in it by breaks in a con- 

 stant stream of air, these breaks occur- 

 ring at equal intervals of time, so that 

 after each break there is a new wave of 

 air. When the number of waves amounts 

 to 32 in a second, the lowest note is 

 emitted, and the number may be in- 

 creased to about 16,000 per second, when 

 the note degenerates into a mere whistle. 



SIR'IUS. The Dog-star; a star of the 

 first magnitude in the southern constel- 

 lation Canis major. 



SIRO'CCO. The name given to the 

 hot wind which occasionally blows in 

 Sicily, and which is supposed to derive 

 its origin from the excessively heated 

 deserts of Africa. See Solano. 



SI'TTINiE {sittia, the nuthatch). Sit- 

 tine birds, or Nuthatches ; a family of the 

 Reptatrices of Macgillivray. In the form 

 of the feet and claws these birds resemble 

 the Certhiadae, while in the structure of 

 the bill they somewhat resemble Wood- 

 peckers, and in their general aspect and 

 habits approach the Parinae. By other 

 writers, the Nuthatches are included in 

 the family Certhiadae, belonging to the 

 Insessores or Perchers. 



SKO'RODITE (<r*c6po5oi/, garlic). Cu- 

 preous arseniate of iron ; a mineral sub- 

 stance which appears to be closely allied 

 to Bournon's martial arseniate of copper. 

 The name is derived from the garlicky 

 odour it emits when heated. 



SKY. The popular name for the blue 

 expanse of atmospheric air. The blue 

 colour of the sky has been found by 

 Brewster to be due to light which has 

 suffered polarization, and which is, there- 

 fore, reflected light, like the white light 

 of clouds. 



SLAG. The glassy compounds pro- 

 305 



duced during the reduction of metallic 

 ores by means of fluxes. 



SLATE. Clay slate. A geological term 

 for the well-known stone with which 

 houses are roofed, and which consists 

 mainly of clay. It agrees with mica- 

 slate in its fissile structure, but differs in 

 having its cleavage more or less trans- 

 verse to the line of its stratification, 



SLATE-CLAY. Another name for 

 shale, or argillaceous matter, indurated, 

 with a slaty structure, and a dull grey 

 streak. 



SLATE-SPAR. Schiefer Spar. The 

 name of a sub-species of limestone. 



SLATE, STONESFIELD. A com- 

 ponent part of the Lower Oolitic Series, 

 consisting of slaty calcareous lime- 

 stone. 



SLATY GNEISS. A variety of gneiss, 

 of which the texture is usually minute, 

 and the scales of mica or crystals of 

 hornblende form small laminae, render- 

 ing the rock easily fissile. 



SLEET. Half-melted snow, consti- 

 tuting an intermediate condition between 

 that of snow and that of rain. 



SLI'C KENS IDE. The name given 

 by the Derbyshire miners to the compact 

 and specular variety of galena, or sul- 

 phuret of lead, from the smoothness of 

 its surface. It occurs lining the walls of 

 very narrow rents. 



SLIDING RULE. An instrument 

 consisting of two parts, one of which 

 slides along the other, for the mechanical 

 performance of addition and subtraction, 

 and also of multiplication and division, 

 by the use of logarithmic scales, instead 

 of scales of equal parts. 



SMALT {schalmz, Germ.). A blue- 

 coloured glass, obtained by heating zaffre, 

 or the impure oxide of cobalt, with sand 

 and potash, reduced to powder. It is the 

 blue-stone used in washing. 



SMARA'GD (o-yuapa75o9)- Anciently, 

 this term was applied to a semi-trans- 

 parent stone like the aqua marina, but is 

 now usually employed to designate the 

 emerald. 



SMELTING. The reduction of me- 

 tallic ores, for" the purpose of extricating 

 the pure metal. With the ore and fuel a 

 third substance, called a. flux, is usually 

 added, the object of which is to form a 

 fusible compound with the earthy matter 

 of the ore. 



SMILA'CEuE. The Smilax tribe of 

 Monocotyledonous plants. Herbaceous 

 climbing plants ; flowers hexapetaloide- 

 ous, hermaphrodite, sometimes dicBci- 



