(28 Sep 1852 - 20 Feb 1907)
By Prof. Henri Moissan
Membre de l’Académie des Sciences, Paris.
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.
[p.259] 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.
[p.260] 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 Thenard 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 Ampere, in two letters addressed to Humphry Davy, advanced the opinion that hydrofluoric acid might be regarded as formed by the combination of [p.261] 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 existence 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 [p.262] 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 uninhabitable. Davy even became quite ill from breathing the vapor of [p.263] hydrofluoric 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 Thenard 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.
Davy varied in many ways the conditions of his experiments. He 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 tine 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. Furthermore, we shall see, in the researches of Fremy, that the action of chlorine upon fluorides tends rather to form addition products—fluochlorides— than 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, aud one of them states that after [p.264] the close of their investigation he spent three years at Genoa and returned still suffering. As for Louyet, carried away by his researches, he took insufficient precautions to avoid the irritating action of the acid vapors, and paid with his life for his devotion to science.
These researches of Louyet led Freiny, about the year 1850, to take up agaiu the question of the isolation of fluorine. Fremy first studied, systematically, the metallic fluorides. He proved the existence of numerous fluohydrates of fluorides, and ascertained their properties and composition. Next he caused many gaseous substances to react upon different fluorides, the action of chlorine and of oxygen being studied with care. Finally, all his attention was drawn to the electrolysis of metallic fluorides.
Most of these experiments were performed in vessels of platinum, at temperatures which were sometimes very high. When, after the general examination of the fluorides, Fremy studied the action of chlorine upon the fluorides of lead, antimony, mercury, and silver, he showed clearly that it was almost impossible then to obtain these compounds in a condition of absolute dryness. Hence we can understand why, in his electrolytic researches, this chemist devoted his attention mainly to calcium fluoride.
Having seen that many fluorides retained water most tenaciously, he fell back upon fluor spar, which often occurs in nature very pure and absolutely dry. This fluoride of calcium, liquefied at a high temperature, he sought to electrolyze in a platinum vessel.
Under these conditions the metal calcium is carried to the negative pole, while around the platinum rod which formed the negative electrode, and which was rapidly corroded, there was visible a boiling, indicating the escape of a new gas.
Undoubtedly, in these experiments, fluorine was set free; but consider that the electrolysis was effected at the temperature of a bright red heat. How difficult experimentation must become under such conditions. How is it possible to collect the gas or to ascertain its properties? This gaseous body displaces iodine from the iodides, but after a few experiments the alkaline metal, set at liberty, pierces the platinum walls of the apparatus, the latter becomes useless, and all must be begun anew.
Far from being discouraged by his failures Fremy, on the contrary, brought to his work an inconceivable perseverance. He varied his experiments, modified his apparatus; the difficulties only encouraged him to continue his labors.
Two important facts at once stood out by themselves. One entered immediately into the domain of science; the other seems to have attracted much less attention.
The first was the preparation of pure, anhydrous hydrofluoric acid. Until the researches of Fremy, the acid absolutely deprived of water was unknown. Having prepared and analyzed the fluohydrate of [p.265] potassium fluoride, Fremy made use of it at once as a source of the pure, dry acid.
He thus obtained a compound which was gaseous at ordinary temperatures, and which condensed in a freezing mixture to a colorless liquid having a great affinity for water. Here, then, is a reaction of great importance—the preparation of hydrofluoric acid in a state of purity.
Allow me to remark incidentally that when Humphry Davy electrolyzed concentrated hydrofluoric acid the badly conducting liquid which he obtained at the end of his experiment was the acid very nearly anhydrous.
The second fact, which, as I have said, was almost unnoticed, and which has been of great interest to me, especially at the end of my researches, was that fluorine has the greatest tendency to unite with nearly all compounds to form addition products.
In brief, fluorine easily forms ternary and quaternary compounds. Let chlorine act upon a fluoride, instead of isolating fluorine we shall prepare a fluochloride. Employ oxygen, and we shall make an oxyfluoride. This property explains to us the failures of Louyet, of the brothers Knox, and of other experimenters. Even when dealing with dry fluorides in an atmosphere of chlorine, bromine, or iodine we shall obtain ternary compounds instead of free fluorine. This fact was clearly established by Fremy. His memoir covers so great a number of experiments that it seems to have discouraged chemists, to have stopped further attempts. Since 1856, the date of publication of Fremy's memoir, researches upon hydrofluoric acid and the isolation of fluorine have been few. The question seems to have been in a state of arrested development. Nevertheless, in 1809, Gore took up methodically the study of hydrofluoric acid. He started with the anhydrous acid prepared by Fremy's method. He determined its boiling point, the tension of its vapor at different temperatures; indeed, all of its principal properties. His memoir is one of remarkable exactitude. Among the numerous investigations of Gore we will consider for the moment only the following, to which I beg your attention:
In a special apparatus this chemist electrolyzed anhydrous hydrofluoric acid containing a little fluoride of platinum in such manner that the gases produced could be collected at each electrode. At the negative pole he saw hydrogen disengaged abundantly, while the rod which terminated the positive pole was rapidly corroded. This phenomenon was identical with that observed by Faraday during the electrolysis of calcium fluoride. Gore next verified the observation of Faraday, that hydrofluoric acid containing water allows the current to pass, but that the absolutely pure anhydrous acid is a nonconductor. In one of his experiments Gore tried to electrolyze a hydrofluoric acid, which, because of an impurity, was a good conductor; and, seeking to avoid the wasting of the electrode, replaced the latter by a stick of carbon.
[p.266] This carbon was prepared with great care, by heating in a current of hydrogen a dense wood, which gave him a sonorous rod, a good conductor of electricity. The apparatus was put together; the experiment begun. All at once a violent explosion occurred, and fragments of the carbon were thrown to the remotest parts of the laboratory. Gore repeated the experiment several times. The result was always the same. To-day we are able to give an explanation of the phenomenon.
The carbon, which was thus prepared by the distillation of a very hard wood, was filled with hydrogen. You all know how easily gases condense in carbon; the beautiful experiments of Melsens have established this most clearly. When we electrolyze, with a negative pole of such carbon, a conducting hydrofluoric acid, fluorine is set free, which, as we shall see later, unites with hydrogen, producing a violent detonation. In this experiment of Gore a little fluorine was set free, and it was to the combination of that with the hydrogen occluded in the carbon that the explosion was due.
Now I come to the new experiments, to which I call your attention.
I began these researches with a preconceived opinion. If we suppose for a moment that chlorine had not yet been isolated, although we knew how to prepare the chlorides of phosphorus and other similar compounds, it is clear that we should increase our chances of success in attempting to isolate that element by working with the compounds which it could form with the metalloids.
It seemed to me that we could obtain chlorine rather by seeking to decompose the pentachloride of phosphorus or hydrochloric acid than by attempting the electrolysis of chloride of calcium or of an alkaline chloride.
Should not the same considerations hold good for fluorine?
Again, since fluorine, according to the earlier investigations, and especially those of Davy, is a body endowed with very energetic affinities, we should, in order to collect the element, work at the lowest possible temperatures.
Such are the general conceptions which led me to take up systematically a study of the compounds formed by fluorine with the metalloids.
My attention was first given to the fluoride of silicon, and I was struck at once by the great stability of that compound. With the exception of the alkaline metals, which, at a dull red heat, decompose it easily, few substances act upon silicon fluoride. It is easy to account for this property if we remember that its formation is attended by a very great evolution of heat. M. Berthelot showed long since that the stability of compounds is greatest when the most heat is generated during their formation.
I supposed then, rightly or wrongly, before having isolated fluorine, that if we ever succeeded in preparing the element, its combination with crystallized silicon should be attended by incandescence. And every time during my long researches that I hoped to have set fluorine [p.267] free I did not fail to try that reaction, and we shall see later that it succeeded perfectly.
After these first experiments upon silicon fluoride, I took up the investigation of the compounds of fluorine with phosphorus.
Thorpe discovered the compound PF5, the pentafluoride of phosphorus. I prepared the trifluoride PF33, and I gave all my attention to the reactions which might lead to its decomposition. I made the experiment of which Humphry Davy had dreamed, of burning phosphorus trifluoride in oxygen, and I found that there was no formation of phosphoric acid with liberation of fluorine, as the English scholar had expected, but that the trifluoride and the oxygen united to form a new gas—phosphorus oxyfluoride.
Here is a new example of the ease with which fluorine yields products of addition.
I next tried, but without avail, the action of the induction spark upon phosphorus trifluoride. The pentafluoride of phosphorus discovered by Thorpe has, however, been decomposed by very strong sparks into the trifluoride and fluorine.
This experiment was made in a glass tube over mercury. You will see that at once fluoride of mercury and fluoride of silicon were formed. There was no hope, under these conditions, of preserving the fluorine, even when it was diluted by an excess of pentafluoride. I then thought of another reaction.
We have known, since the researches of Fremy, that the fluoride of platinum, produced during the electrolysis of alkaline fluorides, decomposes at a high temperature. Having found that the fluorides of phosphorus are easily absorbed by hot platinum sponge, with the final production of platinum phosphide, I thought that this method of preparing fluoride of platinum might lead to the isolation of fluorine. Heating gently at first, the absorption of phosphorus fluoride should give a mixture of phosphorus and platinum fluoride; and, the quantity of the latter being sufficient, a subsequent increase of temperature should disengage fluorine. These experiments and others analogous to them were tried under conditions most favorable to success. They yielded interesting results, which were not, however, sufficiently sharp to settle the question of the isolation of fluorine.
While still pursuing the above-mentioned studies, I prepared the trifluoride of arsenic, which had already been obtained by Dumas in great purity. I determined its physical constants, together with some new properties, and investigated with great care the action of the electric current upon it.
The fluoride of arsenic, liquid at ordinary temperatures (a binary compound, formed by a solid, arsenic, with a gas, fluorine) seemed to be admirably suited to electrolytic experiments.
I was obliged at four different times to interrupt these researches upon arsenious fluoride, the manipulation of which is more dangerous [p.268] than that of anhydrous hydrofluoric acid, and whose toxic properties made it impossible for me to continue the experiments.
I succeeded, however, in effecting the electrolysis of this compound upon employing the current produced by a battery of 90 Bunsen cells.
Under these conditions the current passed continuously; pulverulent arsenic was deposited at the negative pole, and at the positive electrode gaseous bubbles were formed which rose in the liquid, but were almost instantly absorbed. The liberated fluorine was at once taken up by the trifluoride of arsenic, AsF3, which was transformed into the pentafluoride, AsF5. This investigation, carried on for a long time, gave me no fluorine, but it yielded me precious data concerning the electrolysis of the liquid compounds of fluorine, and led me to the decomposition of anhydrous hydrofluoric acid.
In order to effect the electrolysis of hydrofluoric acid I had made the small apparatus which is before you, and which consists of a platinum U tube, carrying on each limb an exit tube placed above the level of the fluid.
The two openings of the U tube were closed by corks previously saturated with paraffin, as was done in all of my experiments upon the electrolysis of arsenious fluoride.
A platinum wire passed through each stopper and was connected with a battery of fifty Bunsen elements.
I prepared at first pure anhydrous hydrofluoric acid, and found, as shown by Faraday and by Gore, that it was a nonconductor.
The experiment was varied in many ways. The result was always the same. With the current given by ninety Bunsen cells decomposition occurred only with the hydrous acid, and it stopped as soon as all the water had been separated into hydrogen and oxygen. It therefore seemed impossible to effect by this process the decomposition of hydrofluoric acid into its elements—hydrogen and fluorine.
At this point I remembered that in the previous study of arsenious fluoride 1 had sought to make that liquid a good conductor by adding to it a little fluoride of manganese or acid fluoride of potassium. This process was applied to the hydrofluoric acid, and then, after three years of investigation, I reached the first important experiment upon the isolation of fluorine.
Hydrofluoric acid containing the acid fluoride of potassium decomposes under the action of the current; and in the apparatus which is before your eyes one could obtain at the negative pole a regular disengagement of hydrogen. What was there at the positive pole? Nothing. A slight increase of pressure—that was all. Only, in dismounting the apparatus it was found that the cork of the positive pole had been burnt, carbonized, to the depth of a centimeter. The paraffined stopper of the negative pole was unaltered. Hence there had been disengaged at the positive pole a substance having an action upon cork quite different from that of hydrofluoric acid.
[p.269] I should add that in order to lessen the vapor tension of the hydrofluoric acid the liquid was cooled by means of methyl chloride, which by rapid evaporation produces a cold of —50° C.
It was necessary to modify the apparatus, and especially the, closing of the U tube. Stoppers of fluorspar smoothly ground did not give me good results. The gum lac or gutta-percha which surrounded them was rapidly attacked by the gas produced at the positive pole. It was necessary, therefore, to resort to a closure by means of platinum screws, and after much groping the experiment was thus arranged.
The platinum U tube was closed by screw stoppers. Each stopper was formed by a cylinder of fluorspar, carefully inserted in a hollow cylinder of platinum, whose outer surface carried the screw thread. Each stopper of fluorspar was penetrated by a square rod of platinum. The lower ends of these rods, which served as electrodes, dipped into the liquid. Finally, two branches of platinum, soldered to the two limbs of the U tube below the stoppers, but above the level of the liquid, allowed the gases generated by the action of the current to escape.
In order to obtain pure anhydrous hydrofluoric acid, one begins by preparing the fluohydrate of potassium fluoride, taking all the precautions indicated by Fremy. Having obtained this salt in a state of purity, it is dried over the water bath at a temperature of 100° C.; and afterwards the capsule containing it is placed in vacuo in presence of sulphuric acid and of caustic potash fused in a silver crucible. The acid and potash are replaced every morning during fifteen days, and the vacuum in the bell jar is always maintained to a pressure of about 1 centimetre of mercury.
During this desiccation it is necessary to pulverize the salt from time to time in an iron mortar, in order to expose fresh surfaces. When the fluohydrate no longer contains water, it falls into fine powder, and can then be used for the preparation of hydrofluoric acid. It is to be. noted that well-made fluohydrate of potassium fluoride is much less deliquescent than the normal fluoride.
When the fluohydrate is thoroughly dry, it is quickly transferred to a platinum alembic, which has been dried at a red heat a little while before. It is heated gently for an hour or an hour and a half, in order that decomposition may begin slowly; and the first portions of the hydrofluoric acid formed, which may- contain traces of water remaining in the salt, are rejected. The platinum receiver is then attached to the retort, which is heated more strongly, but still so as to effect the decomposition of the fluohydrate somewhat slowly. The receiver is surrounded by a mixture of ice and salt; and from this point all the hydrofluoric acid is condensed as a clear liquid, boiling at 19.5° C, very hygroscopic, and, as we know, fuming abundantly in presence of the moisture of the air.
During this operation the platinum U tube, dried with the utmost [p.270] care, has been fixed by means of cork in a cylindrical glass vessel and surrounded by methyl chloride. Up to the moment of introducing the hydrofluoric acid the exit tubes have been connected with exsiccators containing fused potash. The hydrofluoric acid is brought into this little apparatus by inserting one of the lateral tubes into the receiver in which it is condensed.
When a determinate volume of liquid hydrofluoric acid has been collected in the platinum apparatus, and cooled by gently boiling methyl chloride to a temperature of—23° C, the current from twenty-five large Bunseu cells, mounted in series, is passed through it. An ampere meter placed in the circuit enables us to take account of the intensity of the current.
In order to make the acid a conductor there is added to it before the experiment a little of the dried and fused fluohydrate of potassium fluoride, about 2 grams to 10 cubic centimeters of the liquid. Under these conditions the decomposition takes place continuously, and we obtain, at the negative pole, a gas which burns with a colorless flame, and which has all the characteristics of hydrogen. At the positive pole there is a colorless gas of a very disagreeable, penetrating odor, resembling that of hypochlorous acid, and irritating to the mucous membrane of the throat and the eyes. The new gas is endowed with very energetic properties—for instance, sulphur inflames upon contact with it.
Phosphorus takes fire in the gas and yields a mixture of oxyfluoride and fluoride. Iodine combines with it, giving a pale flame and losing its color. Powdered arsenic and antimony combine with fluorine incandescently.
Crystallized silicon, even when cold, kindles immediately upon contact with the gas, and burns with much brilliancy, sometimes giving off sparks. The product is silicon fluoride, which can be collected over mercury and clearly identified.
Pure boron ignites also, giving fluoride of boron. Amorphous carbon becomes incandescent upon contact with fluorine. In order to make these different experiments it suffices to put the solid substance in a small glass tube, which is brought close to the extremity of the platinum tube from which the fluorine emerges. We can also repeat the experiments by putting small fragments of the solid bodies to be studied upon the cover of a platinum crucible held near the opening of the exit tube.
The gas decomposes water in the cold, yielding hydrofluoric acid and ozone: it ignites carbon disulphide, and when collected in a platinum crucible containing carbon tetrachloride it produces a continuous liberation of chlorine.
Fused potassium chloride is attacked in the cold with disengagement of chlorine. In presence of mercury the gas is completely absorbed, forming light-yellow mercurous fluoride. Potassium and [p.271] sodium become incandescent, yielding fluorides. In general, however, the metals are attacked less vigorously than the metalloids. This, we 1 think, is due to a superficial formation of fluoride which hinders further attack. Powdered iron and manganese burn in the gas with a shower of sparks.
Organic bodies are violently attacked. A piece of cork placed near the mouth of the platinum exit tube carbonizes at once and inflames. Alcohol, ether, benzene, turpentine, and petroleum take fire upon contact with fluorine.
Working under good conditions one can obtain from each pole of the apparatus two to four liters of gas per hour.
When the experiment has lasted for several hours and the quantity of hydrofluoric acid remaining at the bottom of the U tube is not sufficient to separate the two gases they recombine in the apparatus with a violent detonation.
We are assured, by direct experiment, that a mixture of ozone saturated with hydrofluoric acid produces none of the reactions just described. The same is true of gaseous hydrofluoric acid. It may be added that the hydrofluoric acid employed, and also the fluohydrate, were absolutely free from chlorine. Finally, it can not be objected that the new gas might be a perfluoride of hydrogen; for, passed over iron heated to redness in a platinum tube it is completely absorbed, without liberation of hydrogen.
In the most recent investigations I have found that it is possible to make these experiments in an apparatus of copper, constructed like the platinum device which is before you.
By the electrolysis of hydrofluoric acid rendered conductive with the acid fluoride of potassium, we have obtained, at the negative pole, hydrogen, and at the positive pole the continuous evolution of a gas having new properties and endowed with very energetic affinities; that gas is fluorine. We have been able to determine its density, its color, and its spectrum, and to study its action upon both elements and compounds.
Now that we know the chief properties of fluorine, now that the element has been isolated, I am convinced that in spite of its energetic reactions, new means for its preparation will be discovered.
We may even suppose that purely chemical methods for the preparation of fluorine may be found, which shall give a better yield than the electrolytic process.
Will fluorine ever have practical applications?
It is very difficult to answer this question. I may, however, say in all sincerity that I gave this subject little thought when I undertook my researches, and I believe that all the chemists whose attempts preceded mine gave it no more consideration.
A scientific research is a search after truth, and it is only after discovery that the question of applicability can be usefully considered.
[p.272] It is evident that, as we see the great industrial transformations which take place to-day before our eyes, we can not well dogmatize on the subject. After the preparation of Bessemer steel, the manufacture of manganese in the blast furnace, and the synthesis of alizarin, the chemist dares not deny the industrial vitality of any reaction of his laboratory.
When we think of the value which certain metals, such as sodium and potassium, had when Davy prepared them by electrolysis; when we recall that by the process of Gay-Lussac and Thenard they cost some thousands of francs a kilogram, and that today, by electrolytic methods, they can be made for not more than 5 francs, we can not say of any chemical reaction that it shall have no industrial uses.
Only—and here I shall close—it is curious to see how many continuous efforts, how many different points of view, are involved in the solution of one of these scientific questions; or rather, I should say, to advance one of them, for in reality no subject is ever closed. It remains always open to our successors; we can only add a link to an infinite chain.
The advancement of science is slow; it is effected only by virtue of hard work and perseverance. And when a result is attained, should we not in recognition connect it with the efforts of those who have preceded us, who have struggled and suffered in advance? Is it not truly a duty to recall the difficulties which they vanquished, the thoughts which guided them; and how men of different nations, ideas, positions, and characters, moved solely by the love of science, have bequeathed to us the unsolved problem? Should not the last comer recall the researches of his predecessors while adding in his turn his contribution of intelligence and of labor? Here is an intellectual collaboration consecrated entirely to the search for truth, and which continues from century to century.
This scientific patrimony which we ever seek to extend is a part of the fortune of humanity; we should preserve it with full recognition of those who gave it the warmth of their hearts and the best of their intelligence.
- Science Quotes by Henri Moissan.
- 28 Sep - short biography, births, deaths and events on date of Moissan's birth.
- Henri Moissan Obituary - by George Fredereck Kunz read at American Electrochemical Society (1907)
- Booklist for Fluorine.