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 deg. or 60 deg. 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.