VOLUMES OF SILICON AND CARBON COMPOUNDS. 175 



Another close analogy which exists between the carbonates and the 

 silicates is the fact that many salts of both give alkaline reactions, or under 

 the theory of dissociation are hydrolized as explained (pp. 86-87), and 

 that alkalinity increases with the temperature. 



The specific volumes of the silicates and carbonates also have very 

 close relations. In general the specific volumes (the molecular weights 

 divided by the specific gravities) of the silicon compound are slightly the 

 greater. The comparative specific volumes of a number of the correlative 

 silicon and carbon compounds are as follows:" 



Specific volumes of silicon and carbon compounds. 



Si01 4 112 



SiHCl 3 82 



Si( OC 2 H 5 ) 4 201 



CaSiO, 41 



CC1 4 94 



CHC1 3 81 



C(OC 2 H 5 ) 4 186 



CaCO, 37 



The specific volumes of Si0 2 and C0 2 are wholly different, but this is 

 explained by the fact that one is a solid and the other a gas. 



Since the specific volumes of the carbon compounds are less than those 

 of the silicon compounds, if there be a simple substitution of carbon for 

 silicon the volume is decreased; if silicon for carbon, the volume is 

 increased. However, as a matter of fact, the changes in the rocks are 

 never so simple as this. The volume changes in carbonation with desilica- 

 tion, and in silication with decarbonation in the rocks largely depend upon 

 whether the reacting and resultant compounds are gaseous, liquid, or solid, 

 and whether the products remain as solids or are dissolved and transported 

 elsewhere. 



In the zone of katamorphism carbon dioxide replaces silicon dioxide 

 ordinarily with liberation of heat 



The fact of the carbonation of the silicates is well known. So far as I 

 know, the importance of this process was first realized by Bischof. He 

 attributes the general decomposition of the rocks near the surface mainly 

 to the action of carbonic acid, thus producing the carbonates which are 

 found in spring water. He shows by experiment that "the silicates of 

 alkalies, alkaline earths, protoxides of iron and manganese are decomposed 



«Mendeleeff, D., The principles of chemistry, translated by Geo. Kamensky, Longmans, Green 

 & Co., London, 1897, vol. 2, pp. 99-100. 



