d fluorine
and studied its properties and its compounds very thoroughly]

~Chemical properties.~ Hydrofluoric acid, like other strong acids, readily
acts on bases and metallic oxides and forms the corresponding fluorides.
It also dissolves certain metals such as silver and copper. It acts very
vigorously upon organic matter, a single drop of the concentrated acid
making a sore on the skin which is very painful and slow in healing. Its
most characteristic property is its action upon silicon dioxide
(SiO_{2}), with which it forms water and the gas silicon tetrafluoride
(SiF_{4}), as shown in the equation

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

Glass consists of certain compounds of silicon, which are likewise acted
on by the acid so that it cannot be kept in glass bottles. It is
preserved in flasks made of wax or gutta-percha.

~Etching.~ Advantage is taken of this reaction in etching designs
upon glass. The glass vessel is painted over with a protective
paint upon which the acid will not act, the parts which it is
desired to make opaque being left unprotected. A mixture of
fluorspar and sulphuric acid is then painted over the vessel
and after a few minutes the vessel is washed clean. Wherever
the hydrofluoric acid comes in contact with the glass it acts
upon it, destroying its luster and making it opaque, so that
the exposed design will be etched upon the clear glass. Frosted
glass globes are often made in this way.

The etching may also be effected by covering the glass with a
thin layer of paraffin, cutting the design through the wax and
then exposing the glass to the fumes of the acid.

~Salts of hydrofluoric acid,--fluorides.~ A number of the fluorides are
known, but only one of them, calcium fluoride (CaF_{2}), is of
importance. This is the well-known mineral fluorspar.

CHLORINE

~Historical.~ While studying the action of hydrochloric acid upon the
mineral pyrolusite, in 1774, Scheele obtained a yellowish, gaseous
substance to which he gave a name in keeping with the phlogiston theory
then current. Later it was supposed to be a compound containing oxygen.
In 1810, however, the English chemist Sir Humphry Davy proved it to be
an element and named it chlorine.

~Occurrence.~ Chlorine does not occur free in nature, but its compounds
are widely distributed. For the most part it occurs in combination with
the metals in the form of chlorides,