[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
chloride drying tubes, so as to convey dry air from outside the
atmosphere of methyl chloride vapor. If great care is taken to obtain
the minimum temperature, this difficulty may be even more simply
overcome by employing a mixture of well pounded ice and salt instead
of methyl chloride; but there is the counterbalancing disadvantage to
be considered, that such a cooling bath requires much more frequent