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CHEMICAL COMPOSITION OF MINERALS. 81 
of aluminium to the oxygen in Al,O;, 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 RO;8i, 
and in the other R,O,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 R and the 
acidic element or silicon, as measured by their combinations 
with oxygen, are in the proportion of 1 to 2, for R stands 
for an element in the protoxide state, while 8i stands for sili- 
con, which is in the dioxide state, its oxide being a dioxide ; 
and hence the minerals so constituted are called Bisilicates. In 
the second of these formulas this ratio is 2 to 2, or 1 to 1, and 
hence these are called Unisilicates. 
Multiplying these formulas by 3, they become R,O,8i;, and 
(2R,) O,,Si;; and the same composition is expressed. In this 
form the substitution of sesquioxide bases for protoxide may 
be indicated: thus, R,R O,, 81, signifies that half of the 2R, is re- 
placed by Al or Fe, or some other element in the sesquioxide 
State. 
There are also some species in which the ratio is | to less 
than 1, and these are called Subsilicates. 
The ratio here referred to (formerly known as the oxygen 
ratio) is called the guantivalent ratio. 
The other ternary compounds require no special remarks in 
this place. 
2. CHEMICAL REACTIONS. 
1. Trials in the wet way. 
1. Zest 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- 
