302 EARTHS PROPER. 



with alumina of potash. It has been often ana- 

 lyzed ; but the different results obtained do not 

 well agree with each other. The reason is, that 

 felspar almost always occurs in granite, which is 

 a compound roek ; and the other constituents, 

 doubtless, mix themselves with it in greater or 

 smaller proportions. According to the analysis 

 of felspar by Rose, which is one of the latest 

 and most careful that have been made, the con- 

 stituents of felspar are as follows : 



Silica . . . 66-75 



Alumina . . . 17-50 



Potash . . . 12 -00 



Lime . . . 1-05 



Oxide of iron . . 0-75 



98-25 



Let us suppose that all the bases are combined 

 with silica, and in the state of trisilicates, except 

 potash, which is a quadrosilicate ; and let us 

 calculate its constitution according to that sup- 

 position. 



(1.) 2-25 : 6 : : 1 7'5 : 46-666 = silica united to alumina. 



(2.) 6:8:: 12 : 16 = silica united to potash. 



(3.) 3-5 : 6 : : 1-25 : 2-143 = silica united to lime. 



(4.) 4-5 : 6 : : 0-75 : 1 = silica united to oxide of iron. 



Now, 46-666 + 16 + 2-143 + 1 = 65-809, which 

 is less than the silica in the mineral by 0-941. 

 The silica united to the potash, lime, and iron, 



