98 NON-METALS AND THEIR COMBINATIONS. 



Silicon resembles carbon both in its physical and chemical proper- 

 ties. Like carbon, it is known in the amorphous state, and forms 

 two kinds of crystals, which resemble graphite and diamond. Like 

 carbon, silicon is quadrivalent, forming silicon dioxide, SiO 2 , silicic 

 acid, H 2 SiO 3 , silicon hydride, SiH 4 , silicon chloride, SiCl 4 , which 

 compounds are analogous to the corresponding carbon compounds, 

 C0 2 , H 2 C0 3 , CH 4 , and CC1 4 . 



The compounds formed by the union of silicon with hydrogen, chlorine, and 

 fluorine are gases. The latter compound, silicon fluoride, SiF 4 , is obtained by 

 the action of hydrofluoric acid on silica or silicates, thus : 



SiO 2 + 4HF = SiF, + 2H 2 0. 



This reaction is used in the analysis of silicates, which are decomposed and 

 rendered soluble by the action of hydrofluoric acid. 



Silicon fluoride is decomposed by water into silicic acid and hydrofluosilicic 

 acid, H 2 SiF 6 , thus : 



3SiF 4 + 3H 2 O = H 2 SiO 3 + 2H 2 SiF 6 . 



Several varieties of silicic acid are known, of which may be mentioned the 

 normal silicic acid, H 4 Si0 4 , and the ordinary silicic acid, H 2 Si0 3 , from the latter 

 of which, by heating, water may be expelled, when silicon dioxide, SiO 2 , is left. 



Tests for silicic acid and silicates. 



(Soluble glass or flint may be used ) 



1. Silicic acid and most silicates are insoluble in water and acids. 

 By fusing silicates with about 5 parts of a mixture of the carbonates 

 of sodium and potassium, the silicates of these metals (known as solu- 

 ble glass) are formed. By dissolving this salt in water and acidifying 

 the solution with hydrochloric acid a portion of the silica separates 

 as the gelatinous hydroxide. Complete separation of the silica is 

 accomplished by evaporating the mixture to complete dryness over 

 a water-bath, and re-dissolving the chlorides of the metals in water 

 acidulated with hydrochloric acid; silica remains undissolved as a 

 white, amorphous powder. 



2. Silica or silicates when added to a bead of microcosmic salt (see 

 index) form on heating before the blowpipe the so-called silica- 

 skeleton. 



Boron, B /r/ = 1O.9, is found in but few localities, either as boric 

 (boracic) acid or sodium borate (borax). Formerly the total supply 

 of boron was derived from Italy ; large quantities of borax are now 

 obtained from Nevada. 



Boric acid, Acidum boricum, H 3 BO 3 =61.9 (Boracic acid), is a 



