Chapter XIII: 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.

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.
\____________________________________/
|
176

= =2HFl= + =CaSO₄=
Two molecules of One molecule of
Fluohydric acid Calcic sulphate
40 136
parts by weight. parts by weight.
\___________________________________/
|
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.

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.

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.