M7 



SILTCA. 



SILK 



511 



it act rmcirntly. The steel point i!oe not come into imme- 

 diate contact with the white P*P er > but with a piece of blacked p&per 

 placed over it, the preamire of the point transferring a nifficient quan- 

 .lir i-fl.mr to form a distinct line. This part of the operation 

 racmblm that of a manifold writer ; and, as in that instrument, several 

 copie* may be produced simultaneously, by using a number of pieces 

 of white and blacked paper, laid alternately upon the swinging board. 

 The siie of the reduced outline drawn on the pa)>er may be regulated 

 by Taryinf the relative proportions of ae and eb; this and several other 

 adjustment* being effected by apparatus which it is unnecessary here 

 to detail. By means of a cord, ttt, held in the hand of the operator, 

 the swinging board d may be drawn back from the steel point when it 

 is required to move the rod without making a murk npon the paper. 

 Greater accuracy may be attained by substituting for the tracer a thin 

 wire, tightly stretched in a bow, and adjusted so as to coincide per- 

 fectly with the axis of the rod. In some canon a kind of knife is 

 substituted for the steel point at 6, and the profile is thus cut out of a 

 piece of thin black paper placed on the swinging board. Some profilist* 

 display considerable talent in cutting silhouettes by hand, with a pair 

 of aciaaors, out of pieces of black paper, without the assistance of an 

 outline. 



SILICA. [Sn.icow.1 



SILICIC ACID. [SILICON.] 



SILICIUM. [SILICOK.] 



BILIOOPLUOBIC ACID. [SILICON.] 



SILICON (Si). Silifium. With the exception of oxygen, silicon is 

 the most abundant constituent of our earth's crust. Its name is 

 derived from tilfjr, Latin for " flint," of which the essential component 

 is silicon. The element silicon has great affinities for other elements, 

 and hence does not exist, in nature, in the free state. Its common, 

 and apparently sole, native form of combination is an oxide known as 

 nttx or nliea. Rock crystal and white quart!! are nearly pure varieties 

 of silica ; while calcedony, agate, opal, flint, and sand are chiefly silica. 

 The natural compounds of silica with oxides are almost innumerable. 

 The individuality of siliceous minerals was recognised at a very early 

 period, but their constitution was not demonstrated till the commence- 

 ment of the present century. 



Silicon itself was first isolated only so lately as 1823. It may be 

 prepared by reducing an alkaline silico-fluoride with potassium, in a 

 glass or iron tube, at a high temperature. It may also be obtained by 

 piTting chloride of silicon vapour over potassium heated in a bulbed 

 glass tube. 



Silicon, like CARBON and Bonos, exists in three distinct modifica- 

 tions. As above prepared, it U a dull-brown powder ; insoluble in, 

 and heavier than, water; in a non-conductor of electricity, is soluble 

 in hydrofluoric acid, and in a warm solution of potash ; burns readily 

 and brilliantly when heated in the air ; and is moreover amorphous 

 (Sty). If the amorphous silicon be strongly heated it becomes semi- 

 crystalline or graphitoidal (Si/3) ; is denser and darker in colour ; docs 

 nut burn in oxygen : ia not soluble in hydrofluoric acid ; and is a con- 

 ductor of electricity. Crystalline silicon (Sia) occurs in the form of 

 six-sided prisms or pyramids, whenever the process for its prepara- 

 tion has included the application of the most intense heat of a wind 

 furnace. 



The equivalent of silicon has, in turn, been represented as 7, 14, 21, 

 and 23 ; but since the experiments of Marignac, who proved that the 

 fluo-stannates and fluo-silicates are isomorphous, it has been usual to 

 consider the number 14 as the true equivalent of silicon. 

 oxygen form two compounds : 



1. Ilydnted protoxide of silicon 



2. Silicic uld . 



2110, 3SiO 

 S10, 



ma, easily reducible to powder by grinding. Perfectly pure 

 ilicic acid may be obtained by igniting hydrated silica. 



Ift/ilratet of silicic acid (iHO, SiO,). Finely powdered silica is 

 soluble in strong alkaliue solutions if heated with them under pressure ; 

 rom the resulting solutions hydrated silica is precipitated in 

 gelatinous state on neutralising by hydrochloric acid. The alkaline 

 silicate may also be formed by fusing powdered flint, &c., with the 

 carbonates of potash or soda; if excess of the latter be used, the 

 silicate is soluble in water, and the solution furnishes hydrated sili,-;,- 

 acid on the addition of hydrochloric acid. Perfectly pure hydrated 

 silica is fonned in a very beautiful manner on passing gaseous tin. >i i.l.- 

 of silicon into water, each bubble of gas forming a little balloon of 

 silica ; hydrofliiosilicic acid (IIF.SiF,) is also fonned at the same tiui.-, 



1. Hydrated protoxide of silicon (2HO, SSiO). When dry hydro- 

 chloric acid gas is passed over crystallised silicon, heated to incipient 

 redness, bydrochlorate of protocliloride of silicon (2HC1, 3SiCl) is pro- 

 duced, and may be condensed to the liquid state in a U-tubo cooled by 

 ice ; hydrogen passes off carrying vapour of protochlorido with it ; the 

 gas being allowed to bubble through water the chloride is decom- 

 posed, and a voluminous white precipitate of the oxide under con- 

 sideration U formed : 



ana, ssia + mo = mo, ssio + snci. 



Hydrated protoxide of silicon sinks in water but floats on ether ; 

 the caustic alkalies and their carbonates dissolve it with evolution oi 

 hydrogen and formation of silicate ; it may be heated to 570 Fahr. 

 without undergoing alteration, but at a higher temperature burns ; and 

 in contact with water undergoes oxidation, hydrogen being evolved, at 

 any temperature above 32* Fahr. 



2. tiUcic acid (8iO,\ Silei, Silica, Jiinoxide of silicon. The physical 

 and chemical properties of the many different forms of silica that 

 occur in nature, as well as a list of the minerals in which silica is a 

 characteristic constituent, have already been treated of in the NATURAL 

 HISTORY DIVISION of this Cyclopaedia, article Sn i. \. 



Silicic acid U largely used in the manufacture of glass, chinaware 

 and porcelain. For these purposes it is obtained in a fine state o 

 division by igniting flints or colourless quartz to redness, and thet 

 plunging in cold water. The compact silica is thiu split up into a 



ind remain-, dissolved in the water : 



3S1F, + 4110 



Fluoride of Water, 

 illicon. 



JUO, BiO, + 2(HF, 



Hydrated 

 silicic Held. 



Hrdrofluoiillclc 

 acid. 



Isomeric condition! of silica. Professor Rose has recently directed 

 attention to the two different states in which silica exists. Crystallised 

 silica including rock-crystal, quartz, calcedony, flint, sandstone, and 

 quarteose sand has a specific gravity, of about 'J-!i, and ia only 

 attacked with difficulty by potash, or hydrofluoric acid. Amorphous 

 silica, on the other hand, including gelatinous silica, the pnlv.-rul.Mit 

 variety that results from the decomposition of certain mineral*, or of 

 fluoride of silicon by water, opals, melted quartz, and silica from the 

 carapaces of infusoria is very readily soluble in alkaline solutions, is 

 rapidlv acted upon by hydrofluoric acid, and has a specific irravitv 

 of 2-2" 



Silicates, as a general rule, are insoluble in water. Alkaline silicates 

 are however, soluble in that liquid, and their solutions have lately 

 been used as a varnish for the stonework of buildings with the vi.-w 

 of preventing decay. When thus exposed to the atmosphere, the 

 silica, being but a weak acid, is replaced by carbonic acid ; tbe alkaline 

 carbonate is removed by rain, and silicic acid filling up the pores of 

 the stone is supposed to prevent ingress of moisture or other disinte- 

 grating matter. Th fact is, however, that the decomposition alluded 

 to, being but a slow one, a large portion of the silicate ia washe< 1 

 by the rain, and this process is consequently of but little value. A 

 modification of the method, known as " Uansomo's patent," cons: 

 supplementing the application of the alkaline silicate by a wash of 

 chloride of calcium ; silicate of lime is hereby at once produced, and 

 performs the office of a permanent varnish or cement, in a far m.ire 

 efficient manner than the mere silica before alluded to. Chj 

 felspar, serpentine, mica, zeolites, hornblende, and many other mi* 

 consist of, or contain, silicates of lime, or magnesia, or alumni 

 mixture of those silicates. They are either sesquitiin-:,t,s like m.-.M- 

 schaum (2MgO, 3SiO., + 2HO), or neutral silicates, like Wollu : 

 (CaO, SiO,); or dibasic silicate*, like olivine (2(Mg, Fe)O SiO ) or 

 Silicates (MO, 2SiO,). [SILICA, in NAT. HIST. Div.J 



Silicon and hydrogen form a hydride of silicon. Berzelitis first showed 

 that the hydrogen which is evolved when crude silicon is washed with 

 water, contains a hydride of silicon. This gas has not yet been pro- 

 cured in a pure state, but, according to Wohler and Buff, it is col. ' 

 and spontaneously inflammable. It occurs in the latter-condition when 

 a solution of a chloride is electrotised by a weak voltaic battery of whi.-li 

 the positive pole is aluminium contaminated with silicon. 



Nitride of silicon is a bluish, fibrous-looking body, formed when 

 silicon is heated in nitrogen gas. 



fiil/ihidc of silicon (SiS 2 ). Silicon burns in sulphur vapour, gene- 

 rating bisulphide. The same body may be formed on passing bisulphide 

 of carbon over a heated mixture of carbon and silica. It is a white, 

 earthy-looking powder. 



Hromide of silicon may be obtained, but iodide does not seem to 

 exist. 



Hilicon and chlorine form two compounds corresponding to the two 

 oxides. 



1. Jlydrochlorate of protocltlorule of silicon (2HCl,3SiCl). The 



method of preparing this body has already been described in - 



nection with protoxide of silicon. It is a colourless liquid, refracting 

 light powerfully; emit* suffocating fumes on exposure to air; is 

 inflammable, burning with a pale green Same ; and is decomposed at a 

 red heat. 



2. Chloride of *ilin,,i (SiCl,), or rather, bichloride of silicon. 

 Finely powdered silica is made into a dough-like mass with Limp 

 black and oil, and heated to redness in a covered crucible ; a poreel.uu 

 tube filled with fragment. 4 of the resulting mass is strongly ignit, 



a current of dry chlorine passed through it; the following decom- 

 position occurs : 



8iO, + Cl, + C, = 2(CO) -f SiCl, 



Silica. Chlorine. Carbon. 



Carbonic Dichloride of 

 oxide. silicon. 



The chloride of silicon vapour must bo condensed by the aid < a 

 fi.-.-^iiiL,' mixture, for it is very volatile. It is a transparent, colourless 

 li.jiiid ; fumes in the air, and has an irritating odour. 



