186 NON-METALS AND THEIR COMBINATIONS. 



admitted into the interior, as in a Bunsen burner, is difficult to explain. 

 That it is due to other causes than the presence of the oxygen is shown 

 bv the fact that nitrogen or carbon dioxide will also destroy luminosity. 

 The introduction of cold gases into a flame lowers the temperature of 

 the inner cone, where the dissociation of ethylene takes place. It 

 seems probable that this lowering of temperature and dilution of the 

 gases diminish the decomposition of ethylene to such an extent that 

 not enough carbon is separated to give luminosity. 



Silicon or Silicium, Si = 28.3, is found in nature very abundantly as 

 silicon dioxide, or silica, SiO 2 (rock-crystal, quartz, agate, sand), and in the 

 form of silicates, which are silicic acid in which the hydrogen has been replaced 

 by metals. Most of our common rocks, such as granite, porphyry, basalt, 

 feldspar, mica, etc., are such silicates or a mixture of them. Small quantities 

 of silica are found in spring- waters, as well as in vegetable and animal matters. 



Silicon resembles carbon both in its physical and chemical properties. 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, Si0 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 CO 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 : 



Si0 3 -f 4HF = SiF 4 -f 2H 2 O. 



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 Si0 3 + 2H 2 SiF 6 . 



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

 normal silicic acid, H 4 SiO 4 , and the ordinary silicic acid, H 2 SiO 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 potas- 

 sium the silicates of these metals (known as soluble 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 mixt- 

 ure to complete dryness over a water-bath, and re-dissolving the chlorides of 

 the metals in water acidulated with hydrochloric acid ; silica remains undis- 

 solved 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. 



Silicon carbide, SiO. (Carborundum, Carbon silicide). This compound 



