e amethyst color of manganese and the
yellow of iron together produce white light.

_Nature of glass._ Glass is not a definite chemical compound and its
composition varies between wide limits. Fused glass is really a solution
of various silicates, such as those of calcium and lead, in fused sodium
or potassium silicate. A certain amount of silicon dioxide is also
present. This solution is then allowed to solidify under such conditions
of cooling that the dissolved substances do not separate from the
solvent. The compounds which are used to color the glass are sometimes
converted into silicates, which then dissolve in the glass, giving it a
uniform color. In other cases, as in the milky glasses which resemble
porcelain in appearance, the color or opaqueness is due to the finely
divided color material evenly distributed throughout the glass, but not
dissolved in it. Milky glass is made by mixing calcium fluoride, tin
oxide, or some other insoluble substance in the melted glass. Copper or
gold in metallic form scattered through glass gives it shades of red.

TITANIUM

Titanium is a very widely distributed element in nature, being
found in almost all soils, in many rocks, and even in plant and
animal tissues. It is not very abundant in any one locality,
and it possesses little commercial value save in connection
with the iron industry. Its most common ore is rutile
(TiO_{2}), which resembles silica in many respects.

In both physical and chemical properties titanium resembles
silicon, though it is somewhat more metallic in character. This
resemblance is most marked in the acids of titanium. It not
only forms metatitanic and orthotitanic acids but a great
variety of polytitanic acids as well.

BORON

~Occurrence.~ Boron is never found free in nature. It occurs as boric acid
(H_{3}BO_{3}), and in salts of polyboric acids, which usually have very
complicated formulas.

~Preparation and properties.~ Boron can be prepared from its oxide by
reduction with magnesium, exactly as in the case of silicon. It
resembles silicon very strikingly in its properties. It occurs in
several allotropic forms, is very hard when crystallized, and is rather
inactive toward reagents. It forms a hydride, BH_{3}, and combines
directly with the elements of the chlorine family. Boron fluoride
(BF_{3}) is very similar to silicon fluoride in its mode of formation
and chemical properties.