mineral by
a feeble affinity, and water be present, a piece of it may be put in
the tube and heated, when the acid gas will be eliminated. The test
paper will indicate its presence, even before it has time to act upon
the glass. If the temperature be too high, fluosilicic acid is
generated, and will form a silicious incrustation upon the cool
portion of the tube.

If the fluorine is too minute to produce either of the above
reactions, then the following process, recommended by Plattner, should
be followed: the assay should be mixed with metaphosphate of soda,
formed by heating the microcosmic salt to dull redness. The mass must
then be placed in an open glass tube, in such a position that there
will be an access of hot air from the flame. Thus aqueous hydrofluoric
acid is formed, which can be recognized by its smell being more
suffocating than chlorine, and also by the etching produced by the
condensation of vapor in the tube. Moist Brazil paper, applied to the
extremity of the tube, will be instantly colored yellow.

Merlet's method for the detection of this acid is the following:[3]
Pulverize the substance for examination, then triturate it to an
impalpable powder, and mix it with an equal part of bisulphate of
potassa. Heat the mass gradually in a moderately wide test-tube. The
judicious application of heat must be strictly observed, for if the
operator first heats the part of the tube where the assay rests, the
whole may be lost on account of the glass being shattered. The
spirit-flame must be first applied to the fore part of the tube, and
then made to recede slowly until it fuses the assay. After the mixture
has been for some time kept in a molten state, the lamp must be
withdrawn, and the part containing the assay severed with a file. The
fore part of the tube must then be well washed, and afterwards dried
with bibulous paper. Should the fluorine contained in the substance be
appreciable, the glass tube, when held up to the light, will be found
to have lost its transparency, and to be very rough to the touch.

[3] Quoted by Plattner.

Great care should be observed not to allow this very corrosive acid to
come into contact with the skin, as an ulcer will be the consequence
that will be extremely difficult to heal.

When hydrofluoric acid comes in contact with any silicious substance,
hydrofluosilicic acid gas is always formed.

(12.) _Selenium_ (Se).--This element occurs in combination with lead
as