226 THE GEOLOGIST, 



The .(sombination of one equivalent of one of these monoxide bases 

 with one equivalent of the acid (R Si) forms a neutral Silicate. If the 

 Sihca preponderates and the salt is an acid salt, then, when the Si is 

 to the base as 2 : 1, it is called a bisilicate ; when 3:1a trisilicate, 

 and so on. Conversely, when the base preponderates and the salt is 

 basic, then, when the base is to the Si as 2 : 1, it is called a bibasic 

 silicate, when 3:1a tribasic silicate, and so on. This nomenclature 

 is only applicable to the monoxide salts ; but it is also applied to the 

 sesqui-oxide salts in the manner explained below. 



It is found, however, that the chemical relations of silica-salts are 

 not so adequately or simply expressed by a statement of the propor- 

 tionate combination of these equivalents of bases and acids, as by 

 tracing the ratio of the oxygen contained in the base to that contained 

 in the acid. If we take b to be the number of equivalents of oxygen 

 in the base, and a the number in the acid, the ratio between those 

 (b : a) will always express the most important chemical relations of a 

 silica-salt. For instance, in the simple silicate Wollastonite (Ca) Si 

 there is one equivalent of oxygen in the base and two in the acid, hence 

 6 : a : : 1 : 2. In the compound silicate Orthoclase (K Si^ + Al Si^) 

 b:a: 'A: 12. 



This relation can be expressed more compactly by dividing out. the 

 ratio, and taking the quotient, which we call the oxygen-quotient 



(abbreviated O.Q.). In Wollastonite O.Q. would equal - = -i = .5 ; 



a 2i 



b 4 



in orthoclase O.Q. = - = = '333. 



a \Z 



^e following table of the principal orders of silicate-salts will give 

 a general idea of their relations — and particularly of the relation of the 

 monoxide (R) salts to the sesqui-oxide (R) salts. It must be under- 

 stood, however, that it is to a great extent theoretical, — many of the 

 salts having no natural existence. It will be seen by this that a 

 nomenclature devised for, and only strictly applicable to, the R salts is 

 likewise applied to the R salts, which are arranged upon the othei's 

 according to their O.Q. It will likewise be observed, and should 

 always be borne in mind, that minerals of the same 'per centage of Si, 

 may belong to very different orders of combination. 



