192 DESCRIPTIONS OF MINERALS. 
of muriatic acid depends not only on the amount of oxygen in the ore, 
but also on the presence or absence of baryta and such other earths as 
may combine with this acid. The binoxide of manganese, when pure, 
affords 18 parts by weight of chlorine, to 22 parts of the oxide ; or 234 
cubie inches of gas from 22 grains of theoxide. ‘The best ore should 
give about three-fourths its weight of chlorine, or about 7,000 cubic 
inches to the pound avoirdupois. 
Iron ores containing some manganese are used for making spiegelcisen, 
a hard highly crystallized pig-iron, containing a large amount of car- 
bon and some manganese. A manganesian iron carbonate or siderite is 
thus used, and also the franklinite of New Jersey. 
Manganese is also employed to give a violet color to glass. The 
sulphate and the chloride of manganese are used in calico printing. 
The sulphate gives a chocolate or bronze color. 
' ALUMINUM. 
The aluminum compvuunds among minerals include only 
one oxide—a sesquioxide Al O;—hydrated oxides, fluorides, 
and, among ternaries, sulphates, phosphates, and numerous 
silicates. There are no sulphides or arsenides, and no ¢ar- 
bonate, with a single imperfectly understood exception. — 
The silicates are described in the following section. Many 
aluminum compounds may be distinguished by means of a 
blowpipe experiment, as explained on page 87. 
Corundum. 
Rhombohedral. RA PR or r Ar=86° 4’. Cleavage some- 
times perfect parallel with O, and sometimes par- 
allel to the rhombohedral faces. Usual in six- 
sided prisms, often with uneven surfaces, and 
somctimes so irregular that the form is scarcely 
traceable. Occursalso granular. Colors blue, 
and grayish-blue most common ; also red, yel- 
low, brown, and nearly black ; often bright. 
When polished on the surface O, a star of six 
rays, corresponding with the six-sided form of 
the prism, is sometimes seen within the crystal. 
Transparent to translucent. H.=9, or next below the 
diamond. Exceedingly tough when compact. G.=3:9-4'16. 
Composition. AlO,=Oxygen 46-8, aluminum 5382=100; 
pure alumina. B.B. remains unaltered both alone and with 
soda. The fine powder moistened with cobalt nitrate and 
ignited assumes a blue color. 

