of liquid methyl chloride, maintained in tranquil ebullition at -23°.
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°. As hydrofluoric acid boils at 19.5°
(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 the
double fluoride HF.NaF. Sodium fluoride also possesses the advantage
of not attracting moisture. After traversing the worm condenser,
therefore, the fluorine is caused to pass through two platinum tubes
filled with fragments of fused sodium fluoride, from which it issues
in an almost perfect state of purity. The junctions between the
various parts of the apparatus are effected by means of screw joints,
between the nuts and flanges of which collars of lead are compressed.
During the electrolysis these leaden collars become, where exposed to
the gaseous fluorine, rapidly converted into lead fluoride, which
being greater in bulk causes the joints to become hermetically sealed.
In order to effect the electrolysis, twenty-six to twenty-eight Bunsen
elements are employed, arranged in series. An ampere meter and a
commutator are introduced between the battery and the electrolysis
apparatus; the former affording an excellent indication of the
progress of the electrolysis.