SALMON — ON ROCKS. 
225 
These bases are classed, according to the rchtive 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, ffe, 
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 ; 
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 
imder that head. 
XXIV. Chemical relatiom 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 B (Al, ¥e, and Mn.) 
isomorphous with each other, and seven monoxide bases, R (Ca, Mg, K, 
Na, 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 
ije called a peroxide base. 
t Salts ai-e often named from their bases. Thus salts of monoxides or protoxides 
are called monoxide and protoxide salts ; and salts of sesqui- or per-oxides, sesqui- 
oxide or peroxide salts. Some have also been named haloid salts, and consist of 
(1st) certain alcalies and earths united with the soluble acids (carbonic, sulphuric, 
and boracic) ; and of (■2d) chlorine and fluorine with their metals (that is, the 
metals of the alcalies and earths). Of the 1st are — Calcite, magwsUe, dolomite, 
anlujdrit'C, gypsum, haryte and horacite ; of the 2d — J\'ock-salt etud fluor. This 
grouping is useful in considering the rocks made up of these minerals,' which form 
a natural family. 
