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° 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°. On ozone, however, it appears to exert
some action, as will be evident from the following experiment. It was
shown in 1887 that fluorine decomposes water, forming hydrofluoric
acid, and liberating oxygen in the form of ozone. When a few drops of
water are placed in the apparatus shown in Fig. 3, and fluorine