o the powerful affinity of
mercury for fluorine, it is a most dangerous experiment to transfer a
tube containing fluorine gas, filled according to either the first or
second method, to the mercury trough; the tube is always shattered if
the mercury comes in contact with the gas, and generally with a loud
detonation. Fluorine may, however, be preserved for some time in tubes
over mercury, provided a few drops of the non-reacting liquid are kept
above the mercury meniscus.

For studying the action of fluorine on gases, a special piece of
apparatus, shown in Fig. 3, has been constructed. It is composed of a
tube of platinum, fifteen centimeters long, closed by two plates of
clear, transparent, and colorless fluorspar, and carrying three
lateral narrower tubes also of platinum. Two of these tubes face each
other in the center of the apparatus, and serve one for the conveyance
of the fluorine and the other of the gas to be experimented upon. The
third, which is of somewhat greater diameter than the other two,
serves as exit tube for the product or products of the reaction, and
may be placed in connection with a trough containing either water or
mercury.

The apparatus is first filled with the gas to be experimented upon,
then the fluorine is allowed to enter, and an observation of what
occurs may be made through the fluorspar windows. One most important
precaution to take in collecting the gaseous products over mercury is
not to permit the platinum delivery tube to dip more than two or at
most three millimeters under the mercury, as otherwise the levels of
the liquid in the two limbs of the electrolysis U-tube become so
different, owing to the pressure, that the fluorine from one side
mixes with the hydrogen evolved upon the other, and there is a violent
explosion.

[Illustration: FIG. 3.]

ACTION OF FLUORINE UPON THE NON-METALLIC ELEMENTS.

_Hydrogen._--As just described, hydrogen combines with fluorine, even
at -23 deg. and in the dark, with explosive force. This is the only case
in which two elementary gases unite directly without the intervention
of extraneous energy. If the end of the tube delivering fluorine is
placed in an atmosphere of hydrogen, a very hot blue flame, bordered
with red, at once appears at the mouth of the tube, and vapor of
hydrofluoric acid is produced.

_Oxygen._--Fluorine has not been found capable of uniting with oxygen
up to a temperature of 500 deg.. On ozone, however, it appears to e