_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
allowed to enter, the water is instantly decomposed, and on looking
through the fluorspar ends a thick dark cloud is seen over the spot
where each drop of water had previously been. This cloud soon
diminishes in intensity, and is eventually replaced by a beautiful
blue gas--ozone in a state of considerable density. If the product is
chased out by a stream of nitrogen as soon as the dense cloud is
formed, a very strong odor is perceived, different from that of either
fluorine or ozone, but which soon gives place to the unmistakable odor
of ozone. It appears as if there is at first produced an unstable
oxide of fluorine, which rapidly decomposes into fluorine and ozone.
_Nitrogen_ and _chlorine_ appear not to react with fluorine.
_Sulphur._--In contact with fluorine gas, sulphur rapidly melts and
inflames. A gaseous fluoride of sulphur is formed, which possesses a
most penetrating odor, somewhat resembling that of chloride of
sulphur. The gas is incombustible, even in oxygen. When warmed in a
glass vessel, the latter becomes etched, owing to the formation of
silicon tetrafluoride, SiF_{4}. Selenium and tellurium behave
similarly, but form crystalline solid fluorides.
_Bromine_ vapor combines with fluorine in the cold with production of
a very bright but low temperature dame. If the fluorine is evolved in
the midst of pure dry liquid bromine, the combination is immediate,
and occurs without flame.
_Iodine._--When fluorine is passed over a fragment of iodine contained
in the horizontal tube, combination occurs, with production of a pale
flame. A very heavy liquid, colorless when free from dissolved iodine,
and fuming strongly in the air, condenses in the cooled receiver. This
liquid fluoride of iodine attacks glass with great energy and
decomposes water when dropped into that liquid with a noise like that
produced by red-hot iron. Its properties agree with those of the
fluoride of iodine prepared by Gore by the action of iodine on silver
fluoride.
_Phosphorus._--Immediately phosphorus, either the ordinary yellow
variety or red phosphorus, comes in contact with fluorine, a most
lively action occurs, accompanied by vivid incandescence. If the
fluorine is in excess, a fuming gas is evolved, which gives up its
excess of fluorine on collecting over mercury, and is soluble in
water. This gas is phosphorus pentafluoride, PF_{5}, prepared some
years ago by Prof. Thorpe. If, on the contrary, the phosphorus is in
excess, a gaseous mixture of this pentafluoride with a new fluoride,
the trifluoride, PF_{3}, a gas insoluble in water, but which may be
absorbed by caustic potash, is obtained. The trifluoride, in turn,
combines with more fluorine to form the pentafluoride, the reaction
being accompanied by the appearance of a flame of comparatively low
temperature.
_Arsenic_ combines with fluorine at the ordinary temperature with
incandescence. If the current of fluorine is fairly rapid, a colorless
fuming liquid condenses in the receiver, which is mainly arsenic
trifluoride, AsF_{3}, but which appears also to contain a new
fluoride, the pentafluoride, AsF_{5}, inasmuch as the solution in
water yields the reactions of both arsenious and arsenic acids.
_Carbon._--Chlorine does not unite with carbon even at the high
temperature of the electric arc, but fluorine reacts even at the
ordinary temperature with finely divided carbon. Purified lampblack
inflames instantly with great brilliancy, as do also the lighter
varieties of wood charcoal. A curious phenomenon is noticed with wood
charcoal; it appears at first to absorb and condense the fluorine,
then quite suddenly it bursts into flame with bright scintillations.
The denser varieties of charcoal require warming to 50° or 60° before
they inflame, but it once the combustion is started at any point it
rapidly propagates itself throughout the entire piece. Graphite must
be heated to just below dull redness in order to effect combination;
while the diamond has not yet been attacked by fluorine, even at the
temperature of the Bunsen flame. A mixture of gaseous fluorides of
carbon are produced whenever carbon of any variety is acted upon by
fluorine, the predominating constituent being the tetrafluoride,
CF_{4}.
_Boron._--The amorphous variety of boron inflames instantly in
fluorine, with projection of brilliant sparks and liberation of dense
fumes of boron trifluoride, BF_{3}. The adamantine modification
behaves similarly if powdered. When the experiment is performed in the
fluorspar tube, the gaseous fluoride may be collected over mercury.
The gas fumes strongly in the air, and is instantly decomposed by
water.
_Silicon._--The reaction between fluorine and silicon is one of the
most beautiful of all these extraordinary manifestations of chemical
activity. The cold crystals become immediately white-hot, and the
silicon burns with a very hot flame, scattering showers of star-like,
white-hot particles in all directions. If the action is stopped before
all the silicon is consumed, the residue is found to be fused. As
crystalline silicon only melts at a temperature superior to 1,200°,
the heat evolved must be very great. If the reaction is performed in
the fluorspar tube, the resulting gaseous silicon tetrafluoride,
SiF_{4}, may be collected over mercury.
Amorphous silicon likewise burns with great energy in fluorine.