342 CHEMICAL COMPOSITION OF MINERALS. 



peroxyds or protoxyds, the ratio still holds. Suppose an 

 analysis of Garnet affords the per-centage, Silica 39*6, alu- 

 mina 22-5, lime 32*6, protoxyd of iron 5*3 : we ascertain the 

 oxygen in each constituent in the manner explained, (in the 

 Silica, by multiplying 0-5298 by 39*6, — in the alumina, by 

 multiplying 0*467 by 22*5,— ^in the lime by multiplying 

 0*2857 by 32*6, — -in the protoxyd of iron by multiplying 

 0*2222 by 5*3) ; then on adding the oxygen of the protoxyd 

 of iron to that of the lime, the amount just equals that of 

 the alumina, as the oxygen ratio requires. Moreover the 

 oxygen of all the protoxyds and peroxyds together equals 

 the oxygen of the silica. 



As different protoxyds may thus replace one another, and 

 as different peroxyds likewise admit of mutual replacement, 

 it is common to write R as a general symbol for the prot- 

 oxyds of a compound, and & for the peroxyds. It is also 

 common to write the special symbols of the protoxyds which 

 replace one another, in parentheses, with a comma between 

 them. Thus in the Garnet referred to, in which lime and 

 protoxyd of iron replace one another, the general formula 

 may be either 



R 3 Si+£lSi ; or (Ca, #e) 3 Si+£lSi. 



The proportions of the lime and protoxyd of iron are not 

 here stated, but may be ; if -1 : 2, the formula becomes 

 (iCa-(-tFe)3Si+£lSi. 



Again, the formula (Oa, ]frg)C, signifies that the compound 

 is a carbonate of lime and magnesia, in definite or indefinite 

 proportions ; (^Ca-l-pig)C that the proportion is 1 : 1 ; (f Ca-f 

 |Mg)C that the proportion is 2 : 3, f and f having this ratio, 

 and together equaling a unit. EC is a general expression for 

 a carbonate of any protoxyd. 



4. The formula for Garnet, K 3 Si-f-XlSi, may also be writ- 

 ten with equal precision as follows : 



(£R*-H£l)Si, 

 the ratio 1:1:2 being still retained, and the fact being also 

 presented to the eye that the oxygen of all the oxyds is to 

 that of the silica as 1 : 1. In Gehlenite, another silicate of 

 alumina and lime, the oxygen ratio is 3 : 3 : 4, which gives 

 6 : 4 or 3 : 2 for the ratio of the oxyds and silica (that is, the 

 oxygen of the silica is two-thirds that of all the oxyds), whila 



