CHEMICAL COMPOSITION OF MINERALS. 81 



of aluminium to the oxygen in Alj,0 3 , from which the percent- 

 age proportion may be obtained. 



2. Ternary Oxygen Compounds. 



Silicates. — Of these compounds there are two prominent 

 groups. In one of these groups the general formula Is R0 3 Si, 

 and iu the other R 2 4 Si. In both of these formulas R stands 

 for any basic elements in the protoxide state, as Ca, Mg, Fe, 

 etc., either alone or in combination. In the first of these for- 

 mulas the combining values of the basic element E and the 

 acidic element or silicon, as measured by their combinations 

 with oxygen, are in the proportion of 1 to 2, for E, stands 

 for an element in the protoxide state, while Si stands for sili- 

 con, which is in the dioxide state, its oxide being a dioxide • 

 and hence the minerals so constituted are called Bisilicates. Ii 

 the second of these formulas this ratio is 2 to 2, or 1 to 1, and 

 hence these are called UnisUicates. 



Multiplying these formulas by 3, they become R^OjSia, and 

 (2RJ 0,. 2 Si 3 ; and the same composition is expressed. In this 

 form the substitution of sesquioxide bases for protoxide may 

 be indicated : thus, KR G 12 Si 3 signifies that half of the 2R 3 is re- 

 placed by Al or Ee, or some other element in the sesquioxide 

 state. 



There are also some species in which the ratio is 1 to less 

 than 1, and these are called Subsilicates. 



The ratio here referred to (formerly known as the oxygen 

 ratio) is called the quantivalent ratio. 



The other ternary compounds require no special remarks in 

 this place. 



2. Chemical Reactions. 

 1. Trials in the wet way. 



1. Test for Carbonates. — Into a test tube put a little hydro* 

 chloric acid diluted with one half water, and add a small por- 

 tion in powder of the mineral. If a carbonate, there will be a 

 brisk effervescence caused by the escape of carbonic dioxide 

 (carbonic acid), when heat is applied, if not before. With cal- 

 cium carbonate no heat or pulverization is necessary. 



2. Test for Gelatinizing Silica. — Some silicates, when pow 



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