CHEMICAL RELATIONS OF SILICON AND CARBON. 173 



Another set of reactions, of the most fundamental importance and 

 widespread character, which occur in an opposite sense in the two zones of 

 metamorphism are the mutual replacements of carbon dioxide and silicon 

 dioxide. It has already been noted that near the surface, or in the zone 

 of katamorphism, carbonic replaces silicic acid. Deep below the surface, 

 or in the zone of anamorphism, silicic replaces carbonic acid. Under the 

 conditions near the surface, where the pressure is small and the tempera- 

 ture is low, carbonic is the stronger acid; and under the conditions deep 

 below the surface, where the pressure is great and the temperature is 

 high, silicic is the stronger acid. The importance of the mutual replace- 

 ment of these compounds under different conditions makes it advisable to 

 summarize the chemical analogies of silicon and carbon. Silicon is the 

 characteristic element of inorganic compounds; carbon is the characteristic 

 element of organic compounds. How closely analogous are these two 

 elements is shown by the following comparative table: 



Chemical relations of silicon and carbon. 



SILICON. CARBON. 



Si0 2 silica, anhydride, solid C0 2 carbon dioxide, gas. 



SiH 4 silicon hydride, gas CH 4 methane, gas. 



SiCl , silicon chloride, liquid CC1 4 carbon tetrachloride, liquid. 



Boils at 57° Boils at 76°. 



SiHCL, silicon chloroform, liquid CHC1 3 chloroform, liquid. 



Boils at 34° Boils at 60°. 



Si (C 2 H 5 ) 4 silicon ethyl, liquid C(C,H 5 ) 4 tetraethylmethane, liquid. 



" BoilsatloO Boilsatl20°. 



Si(OC,H 5 ) 4 ethyl orthosilicate, liquid C(OC 2 H 5 ) 4 ethyl orthocarbonate. liquid. 



Boils at 160° Boils at 158°. (.See Mendeleeff, Vol. II, 



Chap. XVIII, pp. 99-100.) 



H 4 Si0 4 orthosilicic acid ». H 4 C0 4 orthocarbonic acid. 



OH OH 



qi /OH c /OH 



S"((oH C \OH 



X OH OH 



Si0 4 (C 2 H 5 ) 4 ethyl orthosilicate. 



(MgFe) 2 Si0 4 olivine Exists only in certain artificial organic compounds., 



CaAl 2 (Si0 4 ) 2 anorthite ^Natural orthosilicates. as ethyl orthocarbonate, C0 4 (C 2 H 5 ) 4 . 



R'^R"^ (Si0 4 ) 3 garnet, etc. J 



H,Si0 3 metasilicic acid H,C0 3 carbonic acid. 



.OH ,OH 



0=S< 0=C( 



X OH OH 



Exists in salts and in solution. Forms normal 

 (neutral) and acid salts ("bicarbonates"). 



