Magnesia and its compounds give beads with borax and microcosmic salt similar to those of lime.  By igniting magnesia or its compounds very strongly in the oxidizing flame, moistening with nitrate of cobalt, and re-igniting in the oxidation flame, they present, after a continued blowing, a pale flesh-color, which is more visible when cold.  It is indispensable that the magnesia compounds should be completely white and free of colored substances, or the color referred to cannot be discerned.  In general the reactions of magnesia before the blowpipe are not sufficient, and it will be necessary to confirm its presence or absence by aid of reagents applied in the wet way.

THIRD GROUP.—­THE EARTHS, ALUMINA, GLUCINA, YTTRIA, THORINA, AND ZIRCONIA.

The substances of this group are distinguished from the preceding by their insolubility in water, in their pure or hydrated state—­that they have no alkaline reaction upon litmus paper, nor form salts with carbonic acid.  The earths are not volatile, and, in the pure state, are infusible.  They cannot be reduced to the metallic state before the blowpipe.  The organic salts are destroyed by ignition, while the earths are left in the pure state, mixed with charcoal, from the organic acids.  The most of their neutral salts are insoluble in water; the soluble neutral salts change blue litmus paper to red, and lose their acids when ignited.

(a.) Alumina (Al^{2}O^{3}).—­This earth is one of our most common minerals.  It occurs free in nature in many minerals, as sapphire, etc.; or in combination with sulphuric acid, phosphoric acid, and fluorine, and chiefly silicates.  Pure alumina is a white crystalline powder, or yellowish-white, and amorphous when produced by drying the hydrate, separated chemically from its salts.  Alumina is quite unalterable in the fire; the hydrate, however, losing its water at a low red heat.  The neutral salts of alumina, with most acids, are insoluble in water.  Those soluble in it have an acid reaction upon litmus paper, changing the blue into red.

The sulphates of alumina eliminate water when heated in a glass tube closed at one end.  By ignition, sulphurous acid (SO^{2}) is given off, which can be recognized by its smell, and by its acid reaction upon blue litmus paper, when a small strip of it moistened is brought within the orifice of the tube; an infusible residue is left in the tube.

The greater part of the alumina compounds give off water with heat; the most of them are also infusible, except a few phosphates and silicates.