FLUORINE.1

By Prof. HENRI MOISSAN,

Membre de l'Académie des Sciences, Paris.

There has long been known a curious mineral, fluor spar, which occurs in nature in great cubic crystals, sometimes colorless, sometimes tinted green or violet. This mineral is a binary compound of a metal, calcium, united with another element hitherto impossible to isolate, which has been named fluorine.

This fluoride of calcium has very often been compared with the chloride of sodium, the composition of which is perfectly well known to chemists. In fact, there are great and profound analogies between the fluorides and the chlorides; potassium chloride and potassium fluoride both crystallize in the cubic system. In their chief properties the chlorides resemble the fluorides. They usually give parallel reactions; treated with sulphuric acid, both yield hydrogen acids which are soluble in water and which fume strongly in the air.

In addition to calcium fluoride, other compounds containing fluorine are found in nature. We know, for example, a complex compound of calcium phosphate and calcium fluoride which is called apatite. This mineral, which occurs sometimes in very pretty crystals, has also been obtained synthetically in the laboratory; but, which is more important, Henri Sainte-Claire Deville has succeeded in preparing a chlorinated apatite, and this new compound forms crystals identical with those of the apatite containing fluorine. We may therefore say with propriety that in these compounds chlorine can replace fluorine, or act as its substitute. Here is a remarkable analogy, a bond which connects well-studied, well-known chlorine with the elementary substance not yet isolated, fluorine.

Need I cite other examples? They are not lacking. We know the mineral wagnerite, which contains fluorine naturally, and we can prepare the similar chlorinated compound.

These analogies between chlorine and fluorine go still further.

A lecture delivered by Prof. Henri Moissan before the Royal Institution of Great Britain, May 28, 1897. Translated from the French, as printed in Proceedings of the Royal Institution, 1897.

Let us treat common salt, the chloride of sodium, with sulphuric acid. You see that it gives at once an abundant disengagement of gaseous hydrochloric acid.

We will do the same with sodium fluoride. Let us add, in a leaden vessel, sulphuric acid to the alkaline fluoride. We shall see copious fumes produced. In each case, at a temperature of 20° C., we shall have disengaged a gaseous body which fumes strongly in the air, is colorless, has the characteristics of an energetic acid, combines in the dry state with ammonia, is very soluble in water, and dissolves in the latter with a great increase in temperature.

If we give to sodium fluoride, to the binary compound of fluorine and sodium, the formula NaF, that of the acid substance produced by the action of sulphuric acid can only be HF. The two reactions are identical.

The acid gaseous body formed in this reaction is, therefore, a compound of fluorine and hydrogen; a body analogous to hydrochloric acid, and to which the name hydrofluoric acid is given.

But in the natural sciences analogy is not sufficient; the scientific method can only accept that which is rigorously proved. It is therefore necessary to demonstrate that hydrofluoric acid is a hydrogen acid. And this will take us back to the beginning of the century. You know well how great was the influence of Lavoisier upon the upward flight of chemistry, and indeed upon all true science. You know how this great genius, by the continual use of the balance in the study of reactions, gave to the science which we follow a mathematical exactness. Struck by the important part which oxygen plays in combustion, he believed that that element was indispensable to the formation of acids. To Lavoisier every acid was an oxygen compound; hydrochloric acid, therefore, according to his theories, was regarded as containing oxygen; and, by analogy, hydrofluoric acid must contain it also.

To your great investigator, Humphry Davy, belongs the honor of having proved that hydrofluoric acid contains no oxygen. But allow me, before coming to the beautiful researches of Davy, to recall to you the history of the discovery of hydrofluoric acid. We need not consider the investigations of Margraff, which were published in 1768, but we must not forget that it was Scheele who definitely characterized hydrofluoric acid in 1771, without, however, obtaining it in a state of purity. In 1809 Gay Lussac and Thénard took up the study of the compound, and succeeded in producing an acid sufficiently pure and highly concentrated, although far from being anhydrous. The action of hydrofluoric acid upon silica and the silicates was then perfectly elucidated.

Let us now come down to about the year 1813, the time when Davy undertook the study of hydrofluoric acid. A little earlier Ampère, in two letters addressed to Humphry Davy, advanced the opinion that hydrofluoric acid might be regarded as formed by the combination of

hydrogen with an element yet unknown-fluorine-or, in brief, that it was not an oxygenated acid.

Davy, who shared this view, sought at once to prove that hydrofluoric acid contained no oxygen. For this purpose he neutralized the pure acid with ammonia, and strongly heating the salt in an apparatus of platinum, collected in the colder parts of the latter only the sublimed fluohydrate of ammonia containing no trace of water.

Let us repeat the experiment, but with an oxygenated acid; let us take sulphuric acid and neutralize it with ammonia. We thus obtain ammonium sulphate. If now we heat this salt in the same platinum apparatus, it will fuse at about 140° C.; then, at about 180°, it will decompose into ammonia and the bisulphate, and the latter, at a still higher temperature, will be transformed into volatile ammonium bisulphite, nitrogen, and water.

Thus, upon strongly heating ammonium sulphate there has been a formation of water, and in this experiment of Davy, when performed with an oxygenated acid, the quantity of water collected is so great as to be unquestionable. The fluohydrate of ammonia, like the chlorhydrate, gives no water upon decomposition, which leads us therefore to say that hydrofluoric acid contains no oxygen, and that it is analogous to hydrochloric acid. Now we know by experimental demonstration that hydrochloric acid is composed of chlorine and hydrogen. It is therefore logical to think that hydrofluoric acid is formed by the combination of hydrogen with fluorine.

This important experiment, made by skillful hands, did not, however, compel a general belief in the existence of hydracids.

The views of Lavosier concerning the part played by oxygen in the formation of acids, ideas which had been opposed at first, were then so generally admitted that many persons refused to accept the exist ence of hydrogenated acids at all. It was only after the memorable researches of Guy Lussac upon cyanogen and hydrocyanic acid that it was proved beyond discussion that energetic acids could exist which contained no trace of oxygen.

Furthermore, when we compare the acid compounds formed by chlorine, for example, or sulphur with hydrogen we have two types of combination which are entirely different.

Let us take one volume of chlorine and one volume of hydrogen. By the action of light or of a spark from an induction coil they unite to form two volumes of hydrochloric-acid gas, a compound having all the properties of a very energetic acid.

If we combine two volumes of hydrogen with one volume of sulphur vapor we shall obtain two volumes of sulphuretted hydrogen gas, which has, it is true, an acid reaction, but incomparably weaker than that of hydrochloric acid.

It is very evident that by virtue of its powerful reactions, by the disengagement of heat which it produces upon contact with water or

with bases, that hydrofluoric acid should be compared with hydrochloric acid and not with the sulphur compound. It resembles absolutely the acid formed from one volume of chlorine and one volume of hydrogen united without condensation.

Let me now recall to you a much more recent experiment of Gore. This chemist heated fluoride of silver in an atmosphere of hydrogen. Under these conditions he saw the volume of gas double itself; it was apparent, then, that hydrofluoric acid was formed by the union of one volume of hydrogen with one volume of the element not yet isolated, fluorine. Furthermore, it was evidently that same element which had left the silver fluoride to unite with hydrogen and to generate the hydrofluoric acid of which we have spoken.

Thus, without preparing fluorine, without being able to separate it from the substances with which it is united, chemistry has been able to study and to analyze a great number of its compounds. The body was not isolated, and yet its place was marked in our classifications. This well demonstrates the usefulness of a scientific theory, a theory which is regarded as true during a certain time, which correlates facts and leads the mind to new hypotheses, the first causes of experimentation; which, little by little, destroy the theory itself, in order to replace it by another more in harmony with the progress of science.

Thus certain properties of fluorine were foreseen even before its isolation became possible.

Let us now see what attempts were made not only with hydrofluoric acid, but also with the fluorides to isolate fluorine.

I have already spoken of Davy's experiments, in which, most notably, he proved that hydrofluoric acid contained no oxygen. In addition to these experiments Davy made many others which I will briefly recall. We can in a general way divide the researches upon fluorine into two great classes:

1. Experiments made by the electrolytic method, either upon the acid or upon fluorides.

2. Experiments in the dry way. From the beginning of these researches it was foreseen that fluorine, when isolated, would decompose water; consequently all the attempts made by the wet way since the first work of Davy had no chance of success.

Humphrey Davy made many electrical experiments, and these were carried out in apparatus of platinum or of fused (cast) chloride of silver with the powerful voltaic pile of the Royal Society.

He found that hydrofluoric acid was decomposed, despite the fact that it contained water, and then that the electric current seemed to pass with much more difficulty. He tried also throwing the electric sparks into the acid liquid, and was able in some attempts to obtain by this method a small quantity of gas. But the acid, although cooled, was rapidly dissipated in vapor and the laboratory soon became uninhab itable. Davy even became quite ill from breathing the vapor of hydro

fluoric acid, and he advised all chemists to take the greatest precautions to avoid its action upon the skin and the bronchial tubes. Gay Lussac and Thénard also suffered much from the same acid vapors.

The other experiments of Davy (I can not cite them all) were chiefly directed to the reaction of chlorine upon fluorides. They presented very great difficulties, for at that time the fluohydrates of the fluorides. were unknown, nor was it known how to prepare the majority of the anhydrous fluorides.

These researches of Davy are, as should be expected, of the highest importance, and one remarkable property of fluorine was put in evidence. In those experiments which yielded a small quantity of this radicle of the fluorides the vessels of gold or platinum in which the reaction took place were profoundly attacked. In this case fluorides of gold or of platinum were formed.

He

Davy varied in many ways the conditions of his experiments. repeated the reaction of chlorine upon a metallic fluoride in vessels of sulphur, of carbon, of gold, of platinum, etc.; and he never attained to a satisfactory result. He was thus led to think that fluorine undoubtedly possessed a chemical activity much greater than that of known substances.

In closing his memoir, Humphry Davy suggests that these experiments might succeed if they were performed in vessels of fluor spar. We shall see that this idea has been taken up by different investigators. To read the work of Davy will interest you, captivate you to the highest degree. I can best compare this fine memoir with those pictures of the masters to which time only adds new charms. One never tires of admiring them, and discovers in them without end new details and new beauties.

It was by operating in apparatus made of calcium fluoride that the brothers Knox sought to decompose silver fluoride with chlorine. The chief objection to their experiments is based on the fact that the fluoride of silver employed was not dry. In fact, it is extremely difficult to completely dehydrate the fluorides of silver and mercury. Further more, we shall see, in the researches of Fremy, that the action of chlorine upon fluorides tends rather to form addition products-fluochloridesthan to set the fluorine free.

In 1848, Louyet, also working with apparatus of fluor spar, studied an analogous reaction. He acted with chlorine upon the fluoride of mercury. The objections raised against the researches of the brothers Knox also apply to the labors of Louyet. Fremy has shown that fluoride of mercury prepared by Louyet's process contains a notable amount of water. Furthermore, the results obtained were quite variable. The gas collected was a mixture of air, chlorine, and hydrofluoric acid, whose properties varied during the course of preparation.

The brothers Knox complained much of the action of hydrofluoric acid upon the respiratory passages, and one of them states that after