FLUORINE.
Of Fluorine it is necessary to made the remarkable statement that it
has never been known to be produced isolated, that is in a separate or
uncombined form. Many experiments have been performed for the purpose
of reaching this result, and though none of these have resulted in
the production of the element sought, they lead us to believe that
the element is a gas. For, if it were a solid or a liquid at ordinary
temperatures, it may be safely supposed that some processes that have
been devised would be capable of producing at least a small quantity
of the elementary substance, and that from this the observer would be
enabled to recognize and discover at least some of the properties of
the fluorine itself.
Properties of Fluorine.
There are a number of compounds known whose various properties, powers
of chemical interchange, and special molecular weights, clearly point
out the existence in them all of a certain peculiar element analogous
in many respects to chlorine, bromine, and iodine. To this element the
name fluorine has been given. Although, as before intimated, it has not
been known to have been obtained _liberated_, its properties in these
combined forms have been carefully studied and well made out. Thus,
like chlorine and its family associates it combines with hydrogen to
form an acid, fluohydric acid (HFl), properly comparable with the acids
formed by the three elements last discussed:
Chlorohydric acid, HCl.
Bromohydric acid, HBr.
Iodohydric acid, HI.
It also combines with the metals to form fluorides. The best example of
these fluorides is that compound in which fluorine most commonly occurs
in nature: that is fluor-spar, the mineral substance whose chemical
name is calcic fluoride, and whose composition is expressed by the
formula CaFl₂.
The property above all others that is characteristic of fluorine is,
however, its striking affinity for silicon. With this element it
readily combines under almost any circumstances. More wonderful still,
the compound produced with it is a gas. Now in general the compounds of
silicon are solids. These solids are many of them familiarly known in
those materials which constitute the principal portions of the stable
earth on which we tread, of the rock beneath it and of the enduring
mountain masses that here and there pierce through the soil and raise
their crests above the general level. The majority of these earthy and
rocky substances are silicates. It is apparent then that the compounds
of silicon are types of solidity and stability. They cannot be melted
except in the most powerful heating appliances, and the chemist can
hardly imagine conditions such as would change them into vapor. So
then it seems strange and almost contradictory that fluorine should
have the power of attacking compounds that seem to be the embodiments
of permanency itself;—yet it readily does so. Thus if fluohydric acid
comes in contact with silicon, whether that substance is in combination
as sand or as hard rocky minerals, the fluorine atoms pluck out the
silicon and then they fly away together in the form of gas or vapor.
Again, fluohydric acid may be spoken of as the unique agent that
readily attacks glass and dissolves, and even destroys, this ordinarily
unchangeable substance.
Finally, there may be added what can be said of no other element,
namely: that fluorine is never known to form any compound with oxygen.
Discovery of Fluohydric Acid.
It is not easy to refer the first knowledge of fluorine to any
particular discoverer. Perhaps however renewed mention of the ingenious
Scheele is not out of place here; for it seems to have been he who
for the first time, and as early as 1771, recognized fluohydric acid
as a special acid. He called it fluoric acid, but he did not obtain
a correct idea of its composition. Scheele prepared the acid from a
well-known mineral, fluor-spar, and by the addition of sulphuric acid.
This operation cannot be performed to advantage in a glass or porcelain
vessel for they contain silicon, and as has been suggested already,
silicious matters are freely attacked by the acid produced. The
decomposition therefore is commonly conducted in a retort of lead, or
in one of platinum, and the acid produced is collected in a receiver,
also constructed of one of these metals.
[Illustration: FIG. 30.—Platinum retort and receiver shown with its
several parts separated.]
The chemical change is represented by the following equation:
=CaFl₂= + =H₂SO₄=
One molecule of One molecule of
Calcic fluoride, Sulphuric acid,
78 98
parts by weight. parts by weight.
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|
176
= =2HFl= + =CaSO₄=
Two molecules of One molecule of
Fluohydric acid Calcic sulphate
40 136
parts by weight. parts by weight.
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|
176
Ordinarily the product is a liquid, and consists of water holding in
solution the fluohydric acid (HFl). It is possible however to prepare
the acid free from water, and still in a liquid form. But in this
condition it is one of the most dangerous, poisonous, and corrosive
substances known. It produces painful burns if it falls upon the flesh,
and fatal results have been known to follow injuries received from it.
Thus in 1869, Professor Nicklès, an eminent French chemist, died from
injuries sustained by the accidental inhalation of fluohydric acid
vapor, while studying the properties of the substance.
Etching Glass by Fluohydric Acid.
The effect of fluohydric acid upon glass may be shown in attractive
form, and without much difficulty or danger, by the help of a small
dish of lead and a plate of glass to cover it. These being provided,
the experiment may be conducted somewhat as follows: Melt a little
beeswax upon the glass so that the wax may form a thin film upon one
side of it. Then allow the wax to cool and harden. Next, by use of
any convenient pointed instrument, draw some sketch or design deep
in the wax—in fact, to the surface of the glass. Next place some
powdered fluor-spar in the leaden dish, and add to it some concentrated
sulphuric acid. Now cover the dish, with the glass already prepared,
in such a way that the sketch or design is turned downward so as to
receive the fumes of fluohydric acid as they rise from the mixture in
the dish. It is easily understood from what has been said already that
the fluohydric acid will attack the glass, carrying away some of its
silicon in the form of gas or vapor. As a result of this action, minute
channels are formed in the glass. When the experiment is thought to be
sufficiently advanced, the wax may be removed from the plate by melting
it off or otherwise; thereupon it will be discovered that the glass has
actually become etched or engraved by the fluohydric acid gas.
[Illustration: FIG. 31.—Platinum retort and receiver shown as arranged
for production of fluohydric acid (HFl).]
In 1788 Puymaurin presented to the French Academy of Sciences such
a glass plate, upon which there was a beautiful fluoric etching
representing Chemistry and Genius weeping at the tomb of Scheele, who
had contributed so much to the history of fluohydric acid. “This work,”
says Haüy, “was of interest to the Academy on account of the fitness of
the subject as well as the elegance of its execution.”
Practical Application of Fluohydric Acid.
Fluohydric acid, formerly a mere chemical curiosity, has now become
a familiar article upon the shelves of the druggists. It is sold in
gutta-percha bottles with rubber stoppers. It is often used by jewelers
to correct errors in the application of silicious enamels upon their
work. Thus if the enamel has been incorrectly placed, it may be removed
by fluohydric acid and afterward a new portion may be introduced in
the proper position. Again, it is largely used in the decoration of
artistic glass objects, such as globes for gas chandeliers, and the
multitude of articles of table glass ware. In engraving such objects,
they are first covered with a suitable varnish that will resist the
fluohydric acid, then the design is drawn through the varnish with a
sharp needle; afterward the article is exposed to the gas and etched in
a manner similar to that already described.
[Illustration: FIG. 32.—Leaden tray and glass plate. The tray is
intended to receive the materials for production of fluohydric acid;
the plate is represented as covered with a varnish, through which a
sketch has been drawn, preparatory to etching.]
READING REFERENCES.
Fluohydric Acid.
=Gore=, G.—Jour. of Chem. Soc. of London. xxii, 368.
Fluorides.
=Fremy=, E.—Annales de Chimie et de Physique. 3 Sér. xlvii, 5.