(B.) Sesquioxide of Cerium (Ce^{2}O^{3}).—­This oxide, in the pure state, is a red powder.  When heated with hydrochloric acid, it produces chlorine gas, and is dissolved to a salt of the protoxide.  It is not affected by either the flame of oxidation or of reduction; when fused with borax or microcosmic salt, it acts like the protoxide.  It does not fuse with soda upon charcoal.  In the reduction flame it is reduced to the protoxide, which remains of a light grey color, while the soda is absorbed by the charcoal.

(b.) Lanthanium (La.)—­This metal is invariably associated with cerium.  It presents, in its metallic state, a dark grey powder, which by compression acquires the metallic lustre.

The oxide of lanthanium (LaO) is white, and its salts are colorless.  Heated upon charcoal, it does not change either in the oxidation flame or that of reduction.  With borax, in the flame of oxidation or reduction, it gives a clear colorless bead.  This bead, if saturated, and when hot, presents a yellow appearance, but is clouded or enamelled when cold.  With microcosmic salt the same appearance is indicated.  It does not fuse with carbonate of soda, but the soda is absorbed by the charcoal, while the oxide remains of a grey color.

(c.) Didymium (D).—­This metal occurs only in combination with the preceding ones, and it is therefore, like them, a rare one.

Oxide of Didymium (DO).—­This oxide is of a brown color, while its salts present a reddish-violet or amethyst color.  The oxide is infusible in the oxidation flame, and in that of reduction it loses its brown color and changes to grey.  With borax in the oxidation flame, it fuses to a clear dark red or violet bead, which retains its clearness when highly saturated with the oxide, or if heated with a fluctuating flame.

The reactions with microcosmic salt are the same as with borax.

It does not melt with carbonate of soda upon charcoal, but the oxide remains with a grey color, while the soda is absorbed by the charcoal.

(d.) Columbium, (Tantalum—­Ta).—­This rare metal occurs quite sparingly in the minerals tantalite, yttrotantalite, etc., as columbic acid.  In the metallic state, it presents the appearance of a black powder, which, when compressed, exhibits the metallic lustre.  When heated in the air it is oxidized into columbic acid, and is only soluble in hydrofluoric acid, yielding hydrogen.  It is oxidized by fusion with carbonate of soda or potash.

Columbic Acid (Ta^{2}O^{3}) is a white powder, and is infusible.  When heated in the flame of oxidation or reduction, it appears of a light yellow while hot, but becomes colorless when cold.  With borax, in the flames of oxidation and reduction, it fuses to a clear bead, which appears by a certain degree of saturation, of a yellow color so long as it continues hot, but becomes colorless when cold.  If overcharged, or heated with an intermittent flame, it presents an enamel white when cool.