icic acid (H_{2}SiF_{6}),
thus:

2HF + SiF_{4} = H_{2}SiF_{6}.

~Silicides.~ As the name indicates, silicides are binary compounds
consisting of silicon and some other element. They are very stable at
high temperatures, and are usually made by heating the appropriate
substances in an electric furnace. The most important one is
_carborundum_, which is a silicide of carbon of the formula CSi. It is
made by heating coke and sand, which is a form of silicon dioxide, in an
electric furnace, the process being extensively carried on at Niagara
Falls. The following equation represents the reaction

SiO_{2} + 3C = CSi + 2CO.

The substance so prepared consists of beautiful purplish-black crystals,
which are very hard. Carborundum is used as an abrasive, that is, as a
material for grinding and polishing very hard substances. Ferrosilicon
is a silicide of iron alloyed with an excess of iron, which finds
extensive use in the manufacture of certain kinds of steel.

~Manufacture of carborundum.~ The mixture of materials is heated in a
large resistance furnace for about thirty-six hours. After the reaction
is completed there is left a core of graphite G. Surrounding this core
is a layer of crystallized carborundum C, about 16 in. thick. Outside
this is a shell of amorphous carborundum A. The remaining materials
M are unchanged and are used for a new charge.

[Illustration: Fig. 73]

~Silicon dioxide~ (_silica_) (SiO_{2}). This substance is found in a great
variety of forms in nature, both in the amorphous and in the crystalline
condition. In the form of quartz it is found in beautifully formed
six-sided prisms, sometimes of great size. When pure it is perfectly
transparent and colorless. Some colored varieties are given special
names, as amethyst (violet), rose quartz (pale pink), smoky or milky
quartz (colored and opaque). Other varieties of silicon dioxide, some of
which also contain water, are chalcedony, onyx, jasper, opal, agate, and
flint. Sand and sandstone are largely silicon dioxide.

~Properties.~ As obtained by chemical processes silicon dioxide is an
amorphous white powder. In the crystallized state it is very hard and
has a density of 2.6. It is insoluble in water and in most chemical
reagents, and requires the hottest oxyhydrogen flame for fusion. Acids,
excepting hydrofluoric acid, have little action on it, and it requires
the most energetic reducing agents to deprive it of oxygen. It is the
anhydride