prepared by
the action of aluminium or magnesium on their oxides and in impure state
by reduction with carbon in an electric furnace. They are very hard and
melt only at the highest temperatures. At ordinary temperatures they are
not attacked by oxygen, but when strongly heated they burn with great
brilliancy. Silicon and boron are not attacked by acids under ordinary
conditions; titanium is easily dissolved by them.

SILICON

~Occurrence.~ Next to oxygen silicon is the most abundant element. It does
not occur free in nature, but its compounds are very abundant and of the
greatest importance. It occurs almost entirely in combination with
oxygen as silicon dioxide (SiO_{2}), often called silica, or with oxygen
and various metals in the form of salts of silicic acids, or silicates.
These compounds form a large fraction of the earth's crust. Most plants
absorb small amounts of silica from the soil, and it is also found in
minute quantities in animal organisms.

~Preparation.~ The element is most easily prepared by reducing pure
powdered quartz with magnesium powder:

SiO_{2} + 2Mg = 2MgO + Si.

~Properties.~ As would be expected from its place in the periodic table,
silicon resembles carbon in many respects. It can be obtained in several
allotropic forms, corresponding to those of carbon. The crystallized
form is very hard, and is inactive toward reagents. The amorphous
variety has, in general, properties more similar to charcoal.

~Compounds of silicon with hydrogen and the halogens.~ Silicon hydride
(SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties
are more like those of phosphine (PH_{3}). It is a very inflammable gas
of disagreeable odor, and, as ordinarily prepared, takes fire
spontaneously on account of the presence of impurities.

Silicon combines with the elements of the chlorine family to form such
compounds as SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most
familiar and interesting. As stated in the discussion of fluorine, it is
formed when hydrofluoric acid acts upon silicon dioxide or a silicate.
With silica the reaction is thus expressed:

SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.

It is a very volatile, invisible, poisonous gas. In contact with water
it is partially decomposed, as shown in the equation

SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}.

The hydrofluoric acid so formed combines with an additional amount of
silicon fluoride, forming the complex fluosil