108 HOW CROPS FEED. 
In nature alumina is found in the form of emery. The 
sapphire and ruby are finely colored crystallized varieties 
of alumina, highly prized as gems. 
Hydrated alumina dissolves in acids, yielding a numer- 
ous class of salts, of which the sulphate and acetate are 
largely employed in dyeing and calico-printing. The sul- 
phate of alumina an1 potash is familiarly known under 
the name of alum, with which all are acquainted. Other 
compounds of alumina will be noticed presently. 
§ 2. 
MINERALOGICAL ELEMENTS OF ROCKS. 
The mineralogic:l elements or minerals * which compose 
rocks are very numerous. * 
But little conception can be gained of the appearance 
of : mineral from a description alone. Actual inspection 
of the different varietivs is necessary to enable one to rec- 
ognize them. The teacher should be provided with a 
collection to illustrate this subject. The true idea of their 
composition and use in forming rocks and soils may be 
gathered quite well, however, from the written page. For. 
minute information concerning them, see Dana’s Manual 
of Mineralogy. We shall notice the most important. 
Quartz.—Chemically speaking, this mineral is anhy- 
drous silica—silicic acid—a compound of silicon and ox- 
ygen, Si O,. It is one of the most abundant substances 
met with on the earth’s surface. It is found in nature in. 
six-sided crystals, and in irregular masses. It is usually 
colorless, or white, irregular in fracture, glassy in luster. 
It is very hard, readily scratching glass. (See H. C. G., 
p- 120.) 
Feldspar (field-spar) is, next to quartz, the most abund- 
* The word mineral, or mineral “species,” here implies a definite chemical 
compound of natural occurrence. " 
