SALMON — ON ROCKS. 



225 



These bases are classed, according to the relative amount of oxygen 

 they contain, into monoxide bases (Ca, Mg, Ba, K, Na, Li, Fe, and Mn) 

 symbolized by the general formula R ; and sesqui-oxide bases (Al, ^e, 

 and Mn) symbolized by the general forbula E.* 



The terms acid and base are only relative. One substance may be 

 electro-positive with regard to a second, but electro-negative with 

 regard to a third : in the first case it is a base, in the second an acid. 

 In rock-minerals Al is usually a base, with respect to the acid Si, but 

 in Spinelle (Al Mg) it acts as an acid, with regard to the base Mg j 

 and in many of the compound silicates, the variety of which in com- 

 position cannot be accounted for by the doctrine of isomorphism, Al 

 is supposed similarly to act as an acid. 



If one equivalent of a base always combined with one equivalent of 

 an acid to form salts, the chemical relations of these latter would be 

 simple enough. But this is not so : the acids and bases combine in 

 very variable proportions, particularly in the silicates. The salt re- 

 sulting from the union of one equivalent of base and one of acid is 

 termed a neutral salt ; those in which the base preponderates, basic 

 salts ; and those in which the acid, acid^ salts. t 



As the only salts connected with rock-minerals which present any 

 complication are silica-salts, I shall now proceed with the subject 

 under that head. 



XXIV. Chemical relations of Silicates. — I have already mentioned 

 in XIX. the bases which unite with Silica to compose rock-forming 

 Silicates. They are three sesqui-oxide bases E, (Al, ¥e, and Mn.)^ 

 isomorphous with each other, and seven monoxide bases, R (Ca, Mg, K, 

 . JS[a, Li; Fe, and Mn), isomorphous with each other. 



* When the monoxide happens to be a protoxide, the base may also be called a 

 protoxide base ; and similarly when the sesqui-oxide is a peroxide, that base may 

 be called a peroxide base. 



t Salts are often named from their bases. Thus salts of monoxides oy protoxides 

 are called monoxide and protoxide salts ; and salts of sesqui- or per-oxides, sesqui- 

 oxide or peroxide salts. Some have also been named lialoid salts, and consist of 

 (1st) certain alcahes and earths united with the soluble acids (carbonic, sulphuric, 

 and boracie) ; and of (2d) chlorine and fmorine with their metals (that is, the 

 metals of the alcalies and earths). Of the 1st are — Colcite, wagnesite, dolomite, 

 ankydritt, gypsum, haryte and boracite ; of the 2d — Rock-salt smd fluor. This 

 grouping is useful in considering the rocks made up of these minerals, v/hich form 

 a natural family. 



