ntil recent years. Methods similar to those which succeed in the
preparation of the other elements of the family cannot be used; for as
soon as the fluorine is liberated it combines with the materials of
which the apparatus is made or with the hydrogen of the water which is
always present. The preparation of fluorine was finally accomplished by
the French chemist Moissan by the electrolysis of hydrofluoric acid.
Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and
placed in a U-shaped tube made of platinum (or copper), which was
furnished with electrodes and delivery tubes, as shown in Fig. 52. This
liquid is not an electrolyte, but becomes such when potassium fluoride
is dissolved in it. When this solution was electrolyzed hydrogen was set
free at the cathode and fluorine at the anode.

[Illustration: Fig. 52]

~Properties.~ Fluorine is a gas of slightly yellowish color, and can be
condensed to a liquid boiling at -187 deg. under atmospheric pressure. It
solidifies at -223 deg.. It is extremely active chemically, being the most
active of all the elements at ordinary temperatures.

It combines with all the common elements save oxygen, very often with
incandescence and the liberation of much heat. It has a strong affinity
for hydrogen and is able to withdraw it from its compounds with other
elements. Because of its great activity it is extremely poisonous.
Fluorine does not form any oxides, neither does it form any oxygen
acids, in which respects it differs from the other members of the
family.

~Hydrofluoric acid~ (HF). Hydrofluoric acid is readily obtained from
fluorspar by the action of concentrated sulphuric acid. The equation is

CaF_{2} + H_{2}SO_{4} = CaSO_{4} + 2HF.

In its physical properties it resembles the binary acids of the other
elements of this family, being, however, more easily condensed to a
liquid. The anhydrous acid boils at 19 deg. and can therefore be prepared at
ordinary pressures. It is soluble in all proportions in water, and a
concentrated solution--about 50%--is prepared for the market. Its fumes
are exceedingly irritating to the respiratory organs, and several
chemists have lost their lives by accidentally breathing them.

[Illustration: HENRI MOISSAN (French) (1853-1907)

Famous for his work with the electric furnace at high temperatures;
prepared artificial diamonds, together with many new binary compounds
such as carbides, silicides, borides, and nitrides; isolate