252 



NA TURE 



[January i6, 1902 



SOME SCIENTIFIC CENTRES. 

 III.— The Laboratory of Henri Moissan. 



THE isolation of the element fluorine in i8S6, the 

 result of a long series of investigations carried out 

 with an enthusiasm and manipulative skill and natural 

 resourcefulness amounting to genius, has ensured Henri 

 Moissan a high place in the history of modern chemistry. 

 His appointment in 1900 to fill the chair at the Sor- 

 bonne, rendered famous by its association with the names 

 of the illustrious Dumas and Sainte-Claire Deville, sets a 

 seal on a career of splendid activity. 



Born in Paris in the year 1852, he commenced the 

 systematic study of chemistry in the laboratory of the late 

 M. Paul Ueherain at the Museum d'Histoire naturelle. 

 His first researches were in the direction of vegetable 

 physiology, and in 1874 he made his dSut in a paper on 

 the respiration of plants. 



In the meantime, from 1872 until 1876, he was attending 

 the course of Henri Sainte-Claire Deville. Under the 

 influence of this teacher his interest was aroused in "la 

 chimie minerale," and he carried out several investi- 

 gations on the nature of the various oxides of iron, 

 amalgams and the salts of chromium. 



Obtaining his degree of " Docteur 6s Sciences ",in 1880, 

 he received an appointment on the staff of the Ecole de 

 Pharmacie as "chef des travaux pratiques de I"' annee." 

 In spite of the laborious routine duties of his position, he 

 found time to commence the examination of some of the 

 compounds of fluorine " in the uncertain hope of at last 

 being able to isolate the element." The next three years 

 find him engaged on the problem which had baffled in- 

 vestigators from the time of Davy onwards. With patient 

 and indefatigable zeal he continued, bearing up under 

 repeated failure, and accumulating that valuable experi- 

 ence which enabled him to triumph over difficulties, and 

 in 1886 to announce to the world that his efiforts had 

 been crowned with success. Fluorine was at length a 

 chemical fact. 



Before going on to describe the experiments by which 

 Moissan led up to his final result, it will be interesting to 

 glance at the attempts of previous workers and view them 

 in the light of later discoveries ; we shall then be in a 

 better position to appreciate the value and the scope of 

 his research. 



Margaff and Scheele commenced the investigation of 

 hydrofluoric acid in the latter half of the eighteenth 

 century. Davy devoted considerable time to the isolation 

 of the hypothetical element, and suffered a severe illness 

 from breathing the fumes of the acid : he advised chemists 

 to take special precautions against its action on the skin 

 and lungs. Gay Lussac and Thcnard fared in the same 

 way. Knox, after repeating the work of Ainie, a French 

 chemist, who attempted to replace fluorine by chlorine in 

 silver fluoride, had to recruit his health in Italy for three 

 years. Louyet, the next worker on the subject, paid the 

 price of his enthusiasm with his life. 



Fremy, in 1850, was the first to obtain anhydrous 

 hydrofluoric acid, but the failure of his laborious re- 

 searches seemed only to discourage further etiforts in that 

 direction ; and, with the exception of some work by Gore 

 in 1869, nothing more was attempted. Chemists accepted 

 the inevitable, and were content to wait. Moissan, in 

 approaching the subject in 1880, may be supposed to have 

 recognised the dangers as well as the difficulties of his 

 task. 



The various experiments which culminated in the 

 isolation of the element fall under four headings. 



First of all Moissan made use of the fact that the high 

 temperature of the electric spark is often capable of 

 splitting up binary compounds into their constituent 

 elements. He tried its effect on certain gaseous fluorides. 

 Those of silicon and carbon were unaffected by it. The 

 trifluoride of phosphorus was decomposed, probably 



NO. 1681, VOL. 65] 



into phosphorus and fluorine, but the latter element 

 immediately combined with the excess of undecomposed 

 trifluoride, and yielded the pentafluoride ; thus 



5PF3 = 3PF5 -I- 2P. 



He then tried the pentafluoride, but found it far more 

 stable than the corresponding chloride. It only split up 

 under the influence of a very strong spark. Since the 

 experiment had to be made in a glass vessel over 

 mercury, it resulted only in the formation of the fluorides 

 of mercury and silicon. 



The trifluoride of arsenic was equally unsatisfactory. 

 It was vaporised and sparked. There was reason to 

 believe that decomposition occurred, but from the con- 

 ditions of the experiment it was impossible to isolate the 

 element. 



Moissan now adopted a different plan of attack. 

 Fremy had noticed that platinum fluoride, produced 

 accidentally in one of his experiments, splits up under 

 the influence of heat into platinum and fluorine. It was 

 only logical to assume that, if the fluoride could be formed 

 at a dull red heat, a sudden rise in temperature would 

 result in the liberation of the element. The difficulty, 

 however, was to obtain the platinum fluoride. On heat- 

 ing platinum in a current of the trifluoride of phosphorus, 

 combination occurred, but not in the way desired, the 

 chief product being a fluorophosphide of platinum. 



Similar experiments with the pentafluoride of phos- 

 phorus indicated the hopelessness of expecting the 

 desired result from work carried on at so high a 

 temperature. 



Moissan now had recourse to electrolysis. In the 

 first instance he used trifluoride of arsenic contained in 

 a platinum vessel, and found that if the material were 

 quite pure and dry (B.P. 63") it was decomposed into 

 its elements. The fluorine, however, combined at once 

 with the undecomposed trifluoride to form the penta- 

 compound, while the arsenic sank to the bottom of the 

 vessel, or remained suspended in the form of fine par- 

 ticles in the liquid. 



It was found impossible to prepare the pentafluoride 

 of arsenic by any chemical process, so that this substance, 

 which would probably have solved the difi'iculty, had to 

 be abandoned. The extremely poisonous nature of the 

 arsenic compounds also rendered it desirable, if not 

 indeed necessary, to find some more convenient electro- 

 lyte. 



Moissan now returned to the early experiments of 

 Davy, and took up the investigation of the effect of the 

 electric current upon hydrofluoric acid. As both products 

 of the electrolvsis might be expected to be gaseous, a 

 platinum U-tube had to be employed, so that the gases 

 could be separated at the moment of their liberation. 

 Further, to ensure the complete liquefaction of the acid, 

 which boils at -I- 19'' 5 C, the apparatus was immersed 

 in a bath of methyl chloride (B.P. -23° C). 



His first experiments confirmed the results which 

 Davy had obtained, and which later on Faraday and 

 (iore had explained, viz. that the anhydrous acid is a 

 non-conductor. Moissan showed, in addition, that if the 

 acid contains a small quantity of water, this latter is 

 decomposed, until only the anhydrous acid remains in 

 the U-tube, when the current ceases to pass. It was 

 necessary, therefore, to add to the acid some substance 

 which would enable it to conduct the electricity : such a 

 substance is the acid potassium fluoride having the 

 formula KF . HF. This can be obtained quite pure, and 

 is very soluble in the anhydrous acid. 



By this means it was found possible to decompose the 

 hydrogen fluoride. The fluorine, however, immediately 

 attacked the corks which had been covered with paraffin. 

 F'luorspar stoppers were therefore substituted, and the 

 experiment repeated. Hydrogen was evolved at the 

 negative pole. In the other limb there collected an 



