imeters of hydrofluoric acid, together with twenty
grms. of the dissolved double fluoride, are submitted to electrolysis
in this new apparatus, and upward of four liters of pure fluorine is
delivered by it per hour.

This improved form of the apparatus is shown in the accompanying
figure (Fig. 1), which is reproduced from the memoir of M. Moissan. It
consists essentially of two parts--the electrolysis apparatus and the
purifying vessels. The electrolysis apparatus, a sectional view of
which is given in Fig. 2, is similar in form to that described in the
paper of 1887, but much larger.

The U-tube of platinum has a capacity of 160 c.c. It is fitted with
two lateral delivery tubes of platinum, as in the earlier form, and
with stoppers of fluorspar, F, inserted in cylinders of platinum, _p_,
carrying screw threads, which engage with similar threads upon the
interior surfaces of the limbs of the U-tube. A key of brass, E,
serves to screw or unscrew the stoppers, and between the flange of
each stopper and the top of each branch of the U-tube a ring of lead
is compressed, by which means hermetic closing is effected. These
fluorspar stoppers, which are covered with a coating of gum lac during
the electrolysis, carry the electrode rods, _t_, which are thus
perfectly insulated. M. Moissan now employs electrodes of pure
platinum instead of irido-platinum, and the interior end of each is
thickened into a club shape in order the longer to withstand
corrosion. The apparatus is immersed during the electrolysis in a bath
of liquid methyl chloride, maintained in tranquil ebullition at -23 deg..
In order to preserve the methyl chloride as long as possible, the
cylinder containing it is placed in an outer glass cylinder containing
fragments of calcium chloride; by this means it is surrounded with a
layer of dry air, a bad conductor of heat.

The purifying vessels are three in number. The first consists of a
platinum spiral worm-tube of about 40 c.c. capacity, immersed also in
a bath of liquid methyl chloride, maintained at as low a temperature
as possible, about -50 deg.. As hydrofluoric acid boils at 19.5 deg.
(Moissan), almost the whole of the vapor of this substance which is
carried away in the stream of issuing fluorine is condensed and
retained at the bottom of the worm. To remove the last traces of
hydrofluoric acid, advantage is taken of the fact that fused sodium
fluoride combines with the free acid with great energy to form t