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 deg.,
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.
ACTION OF FLUORINE UPON METALS.
_Sodium_ and _potassium_ combine with fluorine with great vigor at
ordinary temperatures, becoming incandescent, and forming their
respective fluorides, which may be obtained crystallized from water in
cubes. Metallic _calcium_ also burns in fluorine gas, forming the
fused fluoride, and occasionally minute crystals of fluorspar.
_Thallium_ is rapidly converted to fluoride at ordinary temperatures,
the temperature rising until the metal melts and finally becomes red
hot. Powdered _magnesium_ burns with great brilliancy. _Iron_, reduced
by hydrogen, combines in the cold with immediate incandescence, and
formation of an anhydrous, readily soluble, white fluoride.
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