CHEMICAL FORMULAS OF MINERALS. 343 



that of the protoxyds and peroxyds is 1 : 1, The formula 

 may hence be 



ft?5i*+BSi 1 ' ; or (ift»+ifi)Si*. 



In the first of these formulas each of the two members has 

 the same oxygen ratio 3:2; in the second this ratio is also 

 retained, and is more briefly expressed, without the hypothet- 

 ical idea that the silica in the compound is divided off between 

 the protoxyd and peroxyd bases. 



5. To deduce the per-centage atomic relations >from a for- 

 mula, the process above described is reversed. For example : 

 for Feldspar we have 4 of silica, 1 of alumina, 1 of potash. 

 In the preceding table the atomic weight of silica is 566*25, 

 and four times this is 2265. Setting this down and tho 

 atomic weights of alumina and potash below it, and adding, 

 we have 



4 of silica, 2265 



1 of alumina, - - - - 6425 



1 of potash, .... 5889 



Total atomic weight of the feldspar, 3496*4 



Now if this amount (3496*4) of feldspar contains 2265 

 of silica, what will 100 parts contain? Hence, to obtain the 

 per-centage, we divide the atomic weight of each constituent 

 in succession by the sum of the whole, and this gives the per- 

 centage relation for each; viz. silica 64*78, alumina 18*38, 

 potash 16*84. 



The following are the formulas of the more common min* 

 eral species following the order of the book. 



Table of Chemical Formulas of Minerals. 



Sal Ammoniac (100), 



NH4CI 



Niter (101), 



Kff 



Glauber Salt (102), 



NaS-flofl 



Nitratine (Nit. Soda, 1 03), 



Nat 



Natron (103), ' 



NaC+lOH 



Trona (103), 



N"a*C»+4H: 



Common Salt (104), 



NaCl 



Borax (107) 



NaB' + lOS 



Barytes (Heavy Spar, 108), 



BaS 



Witherite(109), 



*Ba<3 



