he
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.

[Illustration: FIG. 1.--FLUORINE APPARATUS.]

As the U-tube contains far more hydrofluoric acid than can be used in
one day, each lateral delivery tube is fitted with a metallic screw
stopper, so that the experiments may be discontinued at any time, and
the apparatus closed. The whole electrolysis vessel is then placed
under a glass bell jar containing dry air, and kept in a refrigerator
until again required for use. In this way it may be preserved full of
acid for several weeks, ready at any time for the preparation of the
gas. Considerable care requires to be exercised not to admit the vapor
of methyl chloride into the U-tube, as otherwise violent detonations
are liable to occur. When the liquid methyl chloride is being
introduced into the cylinder, the whole apparatus becomes surrounded
with an atmosphere of its vapor, and as the platinum U-tube is at the
same instant suddenly cooled the vapor is liable to enter by the
abducting tubes. Consequently, as soon as the current is allowed to
pass and fluorine is liberated within the U-tube, an explosion occurs.
Fluorine instantly decomposes methyl chloride, with production of
flame and formation of fluorides of hydrogen and carbon, liberation of
chlorine, and occasionally deposition of carbon. In order to avoid
this unpleasant occurrence, when the methyl chloride is being
introduced the ends of the lateral delivery tubes are attached to long
lengths of caoutchoue tubing, supplied at their ends with calcium