John H. Wolfenden
Dartmouth College Honover, New Hampshire 03755
I
I
The Noble Gases and +he Periodic Table Telling it like it was
I n the mind of t,he chemist, the exciting prospects of the future of his science t,end to eclipse his sense of its past. This underst,andablc indifference to the history of chemistry can however impoverish his sense of perspective and even create siguificant misunderstanding. The true story of how discoveries were made, of why hypotheses were discarded, of how science lived in apparent c~ntent~ment with some COIIflicts between theory and cxperimcntal data whereas other conflicts provoked a speedy reassessment of t,he conceptual scheme t,ends to bc replaced by a vngue and over-simplified myt,h derived from what the story might have been if the investigators of t,he period had been infallible rather than human as well being COIIversant with the intellectual framcworlc of chemistry today. Truth is not always stranger than fictiou but it is consistently more interesting. It is inst,ructive-as well as a t times entertaining-to discover that many great scientists have reported erroneous observations as well as making mistaken interpretations of cxperimenhl data. I t is encouraging to learn how volatile wrong guesses and mistaken inferences prove to be-how quickly and tactfully the healing hand of time rubs out recollection of honest error. There is also some relatively sophisticated fun when reading the contemporary literature associated with a specific discovery in trying to guess which missing piece of the Eddingtonian jigsaw puzzle was holding up the rapid development which hindsight makes so obvious. Almost continually one's reading is illuminated by a feeling comparable with the excitement imparted to a dramatic audience by knowledge of relevant matters of which some or all of the act,ors are unaware. The problems posed to science i11 and by the discovery of the inert or noble gases may serve to illustrate the propositions of the preceding paragraph. Here we have a discovery that is almost the archetype of the scientific success story; it exemplified the importance of great accuracy and the last decimal place; the elements discovered were fresh flowers in the triumphal wreath of the Periodic Table. It is even occasionally implied that the origin of the discoverics in very accurate mcasurements of the density of nit.rogen not only showed the value of extreme accuracy but also s u p p ~ r t ~ ethe d belief that, regardless of conceptual motivation, all observations are valuable if they are sufficientlyaccurate. The period iuvolved (1892-1902, roughly) is sufficiently recent to be free from archaic language and unfamiliar scientific organization and yet old enough to precede the days of intensive editorial censorship and condensation that the information explosiou has imposed on scientific journals. Indeed our insight into much that, happened is appreciably enhanced by the fact that Wil-
liam Croolies would on occasion print almost anything in his Chemical News! Discrepency in Nitrogen Densify
Lord Rayleigh, a physicist strangely unlike a n Iolanthe peer despite his splendid mutton-chop whiskers, was carrying out a program of exact measurements of the densities of the principal elementary gases in the late eighties and mrly nineties. This was not the unreflective aimless accumulation of precise numerical data; Rutherford, who was later in his lighter moments to take pleasure in dividing science into physics and stamp-collecting, would have acquitted Itayleigh of philately although he might have been a little alarmed to find Rayleigh also measuring the combining proportions by weight of hydrogen and oxygen, a suspiciously chemical activity. Rayleigh had been impressed by the numerical coincidences that had led much earlier t o the formulation of Prout's Law; this was the trigger for a massive series of careful measurements. This work had been going on for somc years when Rayleigh noticed the discrepancy in his nitrogen densities t,hat is now so well known. Both the discrepancy and its ultimate explanation reached the scientific world in rather unorthodox forms. The fact that nitrogen derived (partially) from ammonia was about one part in a thousand lighter than atmospheric nitrogen was announced in a short letter to Natuve, published on September 29, 1892 (1) and accompanied by a plea for help from "chemical readers." This intriguing problem was in t,hc public domain from then onwards but it clearly failed to intrigue scientist,^ of the period. Rayleigh made further referencc to it in a paper reporting the densities of air, oxygen and nitrogen in hIarcll 1893 (2); akhough describing it as a "suhject not yet ripc for discussion" he leaned toward the view t,hat nitrogen from ammonia contained somc in :I. "dissocintcd statc." His inclinat~ionto think that the "chemical" nit,rogen was abnormal and the "atmospheric" nitrogen normal is confirmed by his choice of the density dat,n for the latter in his fiual conclusio~is. I t is perhaps a little surprising that all earlier studies of t,he deusit,y of nitrogen seem to have been carried out exclusively on nitrogen derived from the atmosphere. The anomalous densit,y of nit,rogen was clearly ripc for discussiou in April, 1894 because a t that time Rayleigh devoted :L paper (3) exclusively to the densit,y of t,hc two kinds of nit,rogcn; t,he density difference mas found to be around n h:ilf of one per cent,. There was no significant discrep:incy here becausc the "chemical"' nit,rorcn used in 1802 had beeu made bv D:LSS~IIP: air ITo use the convenient if slightly illogical terminology that Rayleigh and Ramsay introduced in their fint (and only) joint paper. Volume 46, Number 9, September 1969
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through amtuonin solution and then ovcr red hot copper; as a result t,he greater part of the nitrogeu obtained was still derived from the at,mosphere. This is not the placc to go into t,he care wit,h which Rayleigh had visualized and eliminated the less as well as the more obvious sources of error; these included coutamination by linown impurities as well as possible dissociat,ion or association of the nitrogen molecules. The April papcr was certainly what excited the serious interest, of Rnmsny, although he had already had both verbal and written exchanges wit,h Rayleigh on the puzzling discrepancy. A N e w Constituent of the Atmosphere
Heforc April was ovcr and after seeking t,he permission of Rayleigh, Rnmsay began t,o investigate atmospheric nitrogen; they kept in touch with each other's work and, while Itayleigh was removing nitrogen from the air by sparking with oxygen over alliali, Ramsay was removing the nitrogen by hot magnesium. Thc details of the beginning of thc coll:~borat~ion,the extent to which Rnmsny int,ruded on Rnyleigh's problem, if a t all, and the credit for drawing Rayleigh's attention to Cwendish's work on sparking wit,h oxygen have all been the subject of much subsequent. debate. Indecd a corrcspondcnt, in the Cliemical News hiding behind the pscudonym "Suum cuique" (roughly trnnsl:ltnblc in the modern vcr~~ncul:~r of youth as "To each man his own thing") almost made it, a hobby later of t,rying t,o set Ramsay and Rayleigh a t each other's throats (4). I t is pleasant to lrnow that none of these efforts succeeded; the discovery of argon mas a~lnouncedby Rayleigh and Ramsay jointly in August, 1894. The world hcnrd of argon in a slightly unusual way. Both Itayleigh and Rnmsay had isolated very small quantit,ies of argon rclntively frec from nitrogen by thc beginning of August,, just hefore t,hc am~ualmeet,ing of the British Associ:~tionfor t,hc Adva~lccment.of Science, which wm (to hc in Oxford that year. A special joint meeting of the Physics and Chemist,ry sections was announced for August 13th. The only official statement (and all that the world had in writing about argon from its discoverers for the next five months) was the following (5) Lord Rsyleigh, Sec. RS. and Professor W. Ramsay F.R.S. gave a preliminary X C C O U N ~of a new Gaseous Constituent of t,hc Atmosphere.
The London Times in its quite generous coverage of the annual meeting reported the' methods of isolation of the new const,ituent,, its proportion in the atmosphere (about 1%) and its approximate density (about 20, t,aking that of hydrogen as unity.) Rayleigh and Ramsay made no claim that the "constituent" was a n element. I n the five months that elapscd before a full-dress report on the new constituent was made to the Royal Society a t the end of January, 1895, discussion of its nat,ure in print was inevitably somewhat limited. I n reporting the Oxford meeting in his Chemical News Croobes, who had beeu given the opportunity of photographing the spectrum of thc new substance, reported that it was (6) a very definite and charaeteristie one, and the lines direr in position from those of nitrogen. The appearance mare resembles a metallic spectrum, and no flutings similar to lhose of nitrogen are to be seen.
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This preliminary spectroscopic evidence supported the view that the new gas was n new element even though its discoverers had made no such claim. Nevert,heless Dewar wrote to the London Times (7) three days after the Oxford report and argued that the new gas was an allotrope of nitrogen, Na, formed when nit,rogcn enters into combinat,ion wit,h either oxygen or magnesium; he was not apparently moved t,o make this suggestion by any conccrn ovcr fit,ting a new element into thc Periodic Table but, it would seem, because he was sure that in his work with liquid air he would have noticed any unsuspected one-per-cent constituent. H e anticipated any argument that., like ozone, triatomic nit,rogen might be expected to be very reactive by ohserving that phosphorus, closely allied to nitrogen, passed readily into an allotropic form, red phosphorus, and that t,his "condensed form" was much less active chemically. Dewar believed that the new gas was "ma~lufactured by the respect,ive cxperimenters," a hypothesis t,hat could be readily t&ed by putting "chemical" nitrogen t,hrough t,he Rayleigh (sparking with oxygen) and the Rnmsay (heating wit,h magnesium) techniques. There are vague and anouymous references t o such L: test by an unlinown I?ellow of the Royal Society, who produced the new gas by the action of magnesium on "chemical nitrogen." A well-documented but rather indirect test was reported by Dewar himself to the Chemical Society on December Gth, 1894 (8). Dewar, after descrihing a procedure for observing the condensing point of a gas and the volatilizing rate of the resulting liquid, reported that these characteristics are modified by passing eilhe~atmospheric nitrogen ov nitrogen made from nitric oxide over hot magnesium. The chairman a t the meeting was the President of the Chemical Society, Henry Armstrong, who has been described as "a gallant but perverse champion of lost causes." Something like a bearded John Bull or a more fullblooded Bernard Shaw, Armstrong loved controversy and it must have been a great disappointment to him that neither Rayleigh nor Ramsay attended the meeting. Speaking from the chair after Dewar's paper he remarlced that (8) i n the absence of Lord Ilayleigh and Professor Ramsay, they weie
left ill the position of having to play "Hamlet? with only the ghost present,. . . h e ventured to say that L o ~ dRayleigh and Professor Ramsay now could not hope to keep so remarkable a discovery to t,hemselves mnch longer. After haviug been told so mnch, chemists could not be expected to remain quiet under the imputation that they had been eyeless during a whole century, and they wodd mldoubtedly enquire into the matter. . .
It is unnecessary to believe that the absence of the discoverers was accidental or that they ever regretted it. Their silence in the interval between August 13th, 1894 and January 31st, 1895 when a full joint report was made to the Royal Society, had two very substantial purposes. I n the first place they wanted to learn a great deal more about the "new constituent" as well as to provide experimental refutation of actual or potential criticisms; in the second place they were going to submit a paper to the Smithsonian Institution in competition for a prize of $10,000 from the Hodgkins Fund "for a treatise embodying some new and important discovery in regard to the nature and properties of atmospheric air." Prior publication (other than the
oral statement a t Oxford) would have disqualified them! There were two hundred and eighteen entries for the prize but Rayleigh and Ramsay won it. The Sphericity of a Group of Atoms
On January 31st, 1895, a t a special meeting of the Royal Society in an auditorium of greater capacity than usual a joint paper (9),presented by Ramsay because Rayleigh had made the oral presentation a t Oxford, was read and accompanied by supplementary papers by Crookes on the spectrum (10) and by Olszewski of Cracow on the liquefaction and solidification of the new gas (11). I n this publication, which occupies 72 pages of the Philosophical Tvansactions, the new constituent is christened "argon," its density when separated from the atmosphere either by sparking with oxygen over alkali or by passage over hot magncsium is found to he in thc 19.7-19.9 range (on the basis of hydrogen as unity). It is shown that "chemical" nitrogen subjected to either treatment leaves a much smaller residue than atmospheric nitrogen and one that is reasonably attributable to consequences of the solubility of argon in water. . A large number of unsuccessful attempts to get argon to enter into chemical reaction are listed and, most importantly, the ratio of its specific hests is reported as nearly 1.66. The authors conclude from thc specific heat data that the new "constituent" is an element or a mixture of elements, unless it consists of diatomic or polyatomic molecules whose "atoms acquire no relative motion, even of rotation, a conclusion improbable in itself and one postulating the sphericity of such complex groups of atoms." The possibility that argon is a largely dissociated diatomic gas is rendered very improbable by a n addeudum to the priucipal report that Ramsay submitted in Alarch; in this he showed that. the pressure-volume-temperature relations of thc new gas between 8S°C and 2.50°C give no indication of changing molecular weight. On the q~est~ion of whet,hcr argon is or is not. a mixture they decline to commit themselves; they point out that the red and blue spectra of argon (depending on the conditions of the electric dischsrgc), reported by Crookes in a paper read on the same evening, might suggest a mixture hut that Olszewski's measurements of critical data, boiling point and freezing point, as well as maintenance of constant pressure during liquefaction, suggest a pure substancc. Rayleigh and Ramsay conclude that "the balancc of the evidence seems to point to simplicity" hut note that any element with an atomic weight near 39.9 (coming between potassium and calcium) finds no place in the Periodic Table. They suggest almost wistfully that a mixture of 93.3% of an element between chlorine and potassium of atomic weight 37 with 6.7y0 of an element between bromine and rubidium of atomic weight 82 would take care of the density and the Periodic Table but doubt whether 6.7y0 of a heavier element could have escaped notice. Before passing on to the reception of the paper one or two features of it deserve comment. First, although the work had been done in two different laboratories the writers deliberately offered no clues to the allocation of responsibility. Second, the failure to "manufacture" (the use of Dewar's terminology is amusing) argon from "chemical" nitrogen by either oxygen or magnesium methods was a telling argvment against Dewar's sug-
gestion and others that were to come later. Third, Rayleigh and Ramsay were clearly very well aware of the problem raised by the position of the new element or elements in the Periodic Table; reading between the lines one senses that only the dismaying implications of a single element of atomic weight 39.9 kept them from concluding that argon was almost certainly n,ot a mixture. How was the detailed account of the new element received by the audience? The Periodic Table was a little over 26 years old, had stood the test of time and lent a new excitement to the discovery of any new element. The discussion on the evening of the paper ha9 been fairly fully reported (12) but it is to he remembered that the speakers had not seen the paper beforehand and that any oral presentat,ion was necessarily condensed. Armstrong was congratulatory in a rather grudging way and misinterpreted the caution of the authors for indecisiveness; he thought it entirely probable that two atoms could be "so firmly locked in each others embrace . . . that they are perfectly content to roll on together without. taking up any of t,he energy that is put int,o the molecule." Riicker, president of t,hc Physical Socicty, clearly thought that, his chemical opposite number had beenungenerous nnd hypothecated that t,he special kind of molecule suggcst,ed hy Armstrong would also have to he spherical to give a specific heat ratio of 1.66; he wound up by saying tactfully but firmly it was to he hoped that argon would not upset the "great" hut "empirical" generalization of RiIendeleeff; however, conflict with the "great mechanical generalizations" of physics with their firm dynamical foundations would be much more serious! Rayleigh quoted l%zgerald (of "contraction" fame) suggesting by letter than the inertness of argon would be cousistcnt with two very firmly bound atoms with "hardly any internal motion" and t,hat such a diat,omic molecule might give the observed ratio of specific heats; it was Rayleigh's own view that i t was hard to imagine a pair of spherical atoms, however firmly bound, that would not pick up a considerable energy of rotation. The discussion was wound up by the President of the Royal Society, Lord Kelvin, speaking "not as from the chair" t o say that in his judgement no spherical atom would be completely smooth and that a specific heat ratio of 1.66 could only he given by a collection of "ideal Boscovich mathematical points endowed with inertia, and with the other property of acting on neighhouring points with a force depending on distance." The large audience must have found their way home through the streets of London in an excited but somewhat mystified frame of mind. Unwelcome Intrusion upon the Periodic Table
The scientific world was now confronted with the reasonably well-established existence of a gaseous suhstance, just possibly a mixture, without any chemical reactivity; unless accepted physical theory2 about specific heats was defective, the substance was a new element in a monatomic state with an atomic weight. just under 40. Although this unwelcome intrusion a It was in 1857 that Clausius had calculated the value of the ratio of the two specific heats for a gas in which the energy of t,he molecules is wholly translational.
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upon the Periodic Table may not have worried the physicists much, it certainly was extremely disturbing to t,he chemists. Just about the first suggestion, other than that of Dewar already referred to, came from Johnst,on Stoney (IS), father of the word "electron;" he thought argon might be a compound of hydrogen with one of six hypothetical elements between hydrogen and lithium (named infrn-beryllium, infm-boron, ete.) which had escaped from the atmosphere long ago; as paraffins are relatively unreactive the hydro-infracarbons might bc very unreactive indeed. He thought the accepted specific heat theory might be suitable for rigid spheres but that the encounters of long t,hread-like molecules might have quite different characteristics. The suggestion was not taken up by ot,hers and the most interest,ing part of his communication is a two-line footnote; hc had uscd the word l'elect,ron" and-doubtless with justificatio~l-thought it necessary to explain the unfamiliar t,erm, which he did as follows: "Electron, the fixed charge of electricity, t,hc same in all cases, which is associated with each chemical bond." The association of t,hc eleet,ronwith valence two years before J. d. Thomson "discovered" it is a little startling. Gladstonc, one of thc pioneers on the relation of rcfraet,ive index to mo1ccul:ir structure, wrote to Natuw ( I d ) wit,hin tcn clays of the Rayhigh-Ilumsay presentat,ion pointing out how admirably an atomic weight near 20 would fit in with the Periodic Tahlc in fivc different ways nnd list,ing five rcasons why L: new element of at,omie weight near 40 would be cxtremely a d w i t r d t,o reconcile wit,h the Tahlc. Hc was of the opinion that trustworthy conelusions could not be reached until some knowledge of the compounds of argon was available as well as more details about the experimental measurement of the ratio of specific heats. One of his arguments based on the spacing of successive atomic weights may serve to remind us how utterly empirical the I'criodic Table was in 1895; his enthusiasm for the advantages of a new element with an atomic wcight about 20 can be thought of as apremature welcome to lleorl three years before i t was isolated. Although the debate over "this kind of chemical monster brought unexpected and unwelcome, like the cuckoo, into the previously happy family of the elements" was spirited, long, and confused, two simplifying factors should be recorded. The data on argon provided by the original paper of Rayleigh and Ramsay were essentially accurate and, apart from the refractive index of the gas, omitted no information about argon that was relevant to the problem of its position in the Periodic Table. Second, helium was discovered in cleveite by Ramsay less than two months after the argon paper so that the problem almost immediately became that of finding places for two new elements in the Periodic Table; more than three years were to elapse between the discovery of helium and the period (JuneSeptember, 1898) during which Ramsay isolated krypton, neon, and xenon in that order. Third, it is broadly true to say that all remotely plausible hypotheses for nesting the cuckoo had been made within six months of the Rayleigh-Ramsay report to the Royal Society. Escape Hatches, Sophistries, and Erroneous Data
Before describing the hypotheses (or escape-hatches) that were put forward it may be well to outline some of the factors, both obvious and unexpected, that eontrib572
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uted to the atmosphere of the discussion. Spectroscopy was a very fashionable study but in the absence of quantum theory utterly empirical. The gap between physics and chemistry was surprisingly large; neithcr the bridge-building effects of physical chemistry nor the inescapable cooperation imposed on physicists and chemists by the exploration of radioactivit,y had begun to take effect. The Periodic Table a t that time and indeed for many years afterwards was the type of "grand design" that is irresistibly attractive to speculative minds with every degree of competence and fantasy. Let it be finally added that in 1895 only one gas was known with a specific heat ratio of about 5/3, namely mercury vapor, and that the two existing examples of atomic weight inversion (tellurium-iodine and cobaltnickel) were still shadowed with uncertaint,~. What hypotheses were available to those who were not prepared to accept a deliberate inversion of atomic weights? The answer depended on whether the specific heat ratio was accepted as evidence of monatomicity or not. If monatomieity was accepted, argon could not be a compound and, if the molecular weight was accepted as about 39.9, it could find no place in the Periodic Table unless i t was a mixture of two or more monat,omic elements. A subtler possibility, consistent with both density and specific heat data (but not with the P-V-T measurements of Ramsay) was that argon might be a largely (95Yo was suggested) dissociated diatomic gas; the specific heat ratio would be quite close to that of a monatomic gas but the small percentage of the diatomic species would lead to an overestimate of the atomic weight of the new element, which might, when corrected, fall neatly between chlorine and potassium. If the evidence for monatomicity could be ignored, argon might be the triatomic nitrogen analog of ozone or the diatomic molecule of a new element belonging to a new "triad" in Group VIII. If it were not an element but a compound, the theoretical possibilities were infinite, if highly improbable when considered individually; the Periodic Table was then in no danger, although a compound that would not react, would not decompose, and could not be synthesized would be something of a nove1t.y. Without attempting any exhaustive chronological record it may be of interest to examine the role played by t,he various hypotheses, the sophistries sometimes employed in their support and, occasionally, the erroneous pieces of experimental information that eomplicated and delayed arrival a t the final solution. Let it first be said that Rayleigh and Ramsay were themselves exceptionally cautious and open-minded; they found themselves unable to ignore the evidence for monatomicity and without committing themselves to any single interpretation were often able to supply experimental data relevant (and sometimes damaging) to hypotheses suggested by others. For a while they were far from sure that helium and argon were not mixtures3 and seem rather to have drifted gradually (if one can drift alertly) into the presently accepted "Group 0 plus atomic weight inversion" interpretation. The attitude of Rfendeleeff is 'of especial interest. He was not prepared to accept inversions of atomic weight. Less than two months after the RayleighRamsay paper, and indeed before terrestrial helium
was discovered, Mendeleeff, who had obviously devoted much thought to the matter, reported to the Russian Chemical Society his tentative conclusion that argon was triatomic nitrogen (15). He took the ratio of specific heats more seriously than some of his fellow chemists but ingeniously argued that an examination of the ratio for a variety of diatomic gases suggested an inverse correlation between the magnitude of the specific heat ratio and the chemical reactivity of a given gas; the extreme inertness of argon might be responsible for a specific heat ratio higher than would otherwise he expected for a molecule containing two or three atoms.' I n reviewing the other possibilities (and the tone of his report is far from dogmatic) his second preference was the hexatomic molecule of a new element of atomic weight around 6.5 and his third preference was the diatomic molecule of a new elcment of atomic weight about 20, "giving a new eighth group to an even series." He thought his third possibility unlikely but "much more probable" than the A = 40 supposition. Indeed he wrote that "if we admit the molecule of argon contains but one atom, there is no room for i t in the periodic system." An elaboration of his 1895 discussion of the nature of argon was published as Appendix 111 to the second English edition (1897) of the "Principles of Chemistry" (18) and it has a more thoughtful and more temperate atmosphere than the opinions of some others with much less of a personal stake in the Periodic System. One of the most passionate believers that argon was triatomic nitrogen was Bohuslav Brauner of Prague; Brauner was familiar with England-indeed by a happy accident he was in Ramsay's laboratory on the Saturday afternoon when the gas from cleveite was identified as helium. Brauner not only believed that argon was N3 but was also inclined to think that helium was Ha or H, (19). His respect for the "laws of vsleucy" was strictly limited; in one of his papers he suggested the following possible structural formulas for helium, argon, and an as yet undiscovered form of 03,other than ozone
An interesting line of chemical experimentation to test the triatomic nitrogen hypothesis was carried out by A. Peratoner and G. Oddo a t Palermo in 1895 (20); they looked for argon in the decomposition products of a The suspicion that mixtures of substances of different molecular weight but closely similar properties might be involved haunted the whole debate. As will be seen later, Ramsay at one time thought he had partidly separated two kinds of helium with identical spectra but different atomic weights; at another he believed he had discovered an element of almost the same atomic weight as argon but strikingly different spectrum. Written fifteen years before isotopy had surfaced, some of these discussions produce uncanny echoes in the mind. This suggestion is perhaps less fanciful than a speculation of Ramsay's in the final paragraphs of the first edition of his "The Gases of the Atmosphere" (18). He suggested that the weights of reactive atoms might conceivably be reduced by their reactivity and defended this notion by reminding readers that phlogiston, though mistaken, had sewed a useful purpose for a period. The book review in Nature (17) dryly suggested that "the compa"son was unfair to the phlogistonists" and this escrtpe-hatch from the atomic weight inversion problem did not appear in later editions.
azoimide and some of its derivatives with com~letelv negative results. There can be little doubt that the evidence most damaging to these triatomic formulas, although it does not appear to have exerted great influence a t the time, was the refractive index of the new gases measured by Rayleigh by interferometry in January 1896 (21). The refractive index of helium was less than that of hydrogen and that of argon was less than that of nitrogen. Enough was already known about the refraction of elements and compounds to suggest the extreme improbability of allotropes of higher molecular weight having lower refractivity. Undoubtedly the gap between physics and chemistry prevailing a t the period contributed to the relatively minor impact of these optical data on the controversy. And yet it was perhaps a little unreasonable to expect an element that was totally unreactive to supply chemical evidence as to its own nature. No more unreasonable, however, than the opinion solemnly recorded by one participant in the debate (22) and supported by others (2.9) to the effect that elements that had no chemical reactions could not expect a place in the Periodic Table. If this had been the view of an English scientist, one might have been forgiven for muttering something about "the old school tie." The opinion is not unrelated in spirit to the following statement in the 1926 Kendall revision of Alexander Smith's "Inorganic Chemistry" (24) Since the atomic weight of s. substance is a quantity showing the proportion in which it enters into combination, it will be seen that argon, since it has not yet been found t,o combine with anything, has, to speak strictly, no atomic weight.
The degree of chemical inertness of argon (and later of helium) was inevitably the subject of much investigation. In their original paper Rayleigh and Ramsay reported some fifteen diversified attempts to secure chemical reaction; in June, 1895 Moissan reported the absence of any reaction between argon and fluorine (25). An even more famous French chemist, Berthelot, arrived a t a different conclusion; he had previously recorded reactions between nitrogen and various hydrocarbons including benzene under the influence of the silent electric discharge. Ramsay sent him samples of argon to discover whether it would react under similar conditions. Berthelot reported that under the protracted action of the discharge argon disappeared and a "yellow, resinous, odorous matter" was formed (20). The most plausible explanation for this is curious indeed. I n a letter to Ramsey thanking him for the argon Berthelot added that an assistant had hurt himself removing the iron wire used to secure the rubber tubing a t the ends of the sample vessel. He recommended copper wire for the purpose. This puzzled both Ramsay and his assistant, Morris Travers, as they felt sure they had used copper wire. They gave the matter little attention-probably because the letter was received after they sent off the last of a series of argon samples. Forty or Inore years later Travers working with Ramsay's papers suddenly realized the probable explanation of the iron wire and also some at least of Berthelot's results (27). The sample had probably been opened by the French customs officials so that Berthelot's experiment may well have been carried out with air rather than argon. I t is preVolume 46, Number 9, September 1969
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sumably safe to assume that the douane failed to identify an undutiable nonreactive gas, that had been discovered only six months previously. The possibility of argon and helium being mixtures rather than pure substances was thought quite likely for awhile by Rayleigh, Ramsay, and many .others. Not only might a mixture solve the atomic weight problem but the spectra were for a time thought to point in this direction. Ramsay and Collie (28) examined the effect of diffusion through pipe-clay on the density of samples of argon and of helium. With argon the maximum density difference produced was insignificant but with helium their first experiments showed density values of 1.874 and 2.133 as well as substantial differences in refractive index, which Rayleigh measured for them. They were puzzled by the fact that the spectra of all the helium diffusion fractions were virtually identical and wondered whether a gas might be a mixture of molecules of different weight but identical in other properties. It was not till 1898 that Ramsay and Travers after more elaborate studies of the diffusion of helium became satisfied that the density differences produced in the diffusion of helium through pipe-clay were due to contamination of their helium with argon. The original work of Ramsay and Collie was first reported to the French Academy of Sciences a t its meeting on July 27, 1896, and included some reasoning that should cheer any freshman whose aspirations to infallibility are coming to prove unrealizable, especially when he also learns that the French Academy of Sciences seems to have accepted the reasoning without blinking an eye. The following is a literal translation of their line of thought as printed in Conzptes Rendus Let us now consider what. happens when we submit a mixture of the two gases to diffusion. Let us take, e.g., a. mixture of hydrogen with an excess of oxygen. After a sufficient number of operat,ions we obtain pure oxygen on the one hand, snd on the other a mixture of 1 part of hydrogen with 4 parts of oxygen. It will not he possible to separate this mixture into its constituents on account of the eqoal difiusiou of oxygen and hydrogen when thus mixed. The idenLity of the spectra of helium prevent us from deciding which is the pure gas and which is the mixture. Calculation establishes that if we suppose the heavier gas is a mixture, the density of the lighter, supposed pure, ought to be 1.58. Helium, in effect,,if it consists of a mixture of two gases, is formed either of two gases of the densities 2.366 and 1.874 or of two gases of the densities 2.133 and 1.580.
This mistaken echo of azeotropy was corrected in a short letter to Nature on October Sth, 1896 (29), (and in an almost simultaneous communication to the French Academy) (SO) which frankly admitted that the conclusion that a mixture of four volumes of oxygen and one volume of hydrogen (and analogous mixtures of two fractions of helium) could not be separated by diffusion was "of course, wholly wrong." It is a t first sight curious that a paper submitted to the Royal Society (31) on the matter a t almost exactly the same time on the same experimental findings does not include the howler: a reasonable explanation might be that as the Royal Society did not meet between June and November, Ramsay got a chance to modify his paper with Collie before it was printed in the Proceedinys. The moral is, of course, not that Ramsay or Collie was stupid but that "you can't win them all." 574
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Journal of Chemical Education
Shori-Lived "Metargon"
The story of the discovery of the inert gases ends with the explosive period between June and September 1898 when three more inert gases were discovered by Ramsay and Travers as well as a further gas "metargon," which died young. The search for other noble gases had been going on ever since argon and terrestrial helium were discovered. In an address to the British Association a t Toronto in August, 1897, entitled "An undiscovered gas" (Sb), Ramsay marshalled arguments based largely on atomic weight differences between elements of similar chemical behavior (a type of regularity frequently discussed after the Periodic Table was put forward and also occasionally before that event) for expecting a noble gas of atomic weight near 20. He went on to describe his fruitless searches for the element in minerals, the gases from hot springs and in the heavy fractions from the diffusion of helium. I t is not too fanciful to suggest that "the undiscovered gas" was the focus of much of Ramsay's excitement after helium had been discovered in cleveite. The rosy tinge of the contemporary urban landscape a t night would certainly have startled him and i t might have been hard to persuade him that neon would become the best known noble gas outside the laboratory. The same address to the British Association drew a parallel between the argon-potassium atomic weight anomaly and the similar anomaly involving iodine and tellurium; the implication of the acceptability and reality of an atomic weight inversion was pretty clear. The search for other inert gases could not be pursued effectively without access to substantial quantities of liquid air, both as a working material and as a refrigerant. For this they were indebted to Hampson of Brin's Oxygen Company who "moonlighted" on the company equipment a t week-ends and other slack periods. The amount Ramsay and Travers required must not be exaggerated: krypton, neon, and metargon were detected with the first two deliveries of liquid air, totalling less than two liters! I n the period between May 31st and the end of July Ramsay and Travers got evidence of the existence of krypton, neon, metargon, and xenon, although the fourth of these was not reported until the British Association meeting in September. The "discovery" of metargon and indeed its name is all but forgotten now. If a substantial quantity of liquid argon, in whose most volatile fraction the spectrum of neon was first observed, was allowed to evaporate completely a solid residue was left behind. Its spectrum seemed a good deal more complex than that of argon although its density seemed very close to that of argon. Ramsay and Travers thought that this was a new element "metargon" (SS), perhaps bearing something of the same relation to argon that nickel did to cobalt. Schuster, a British physicist, was quick to point out that many lines in the metargon spectrum corresponded to lines in the so-called Swan bands (S4), observed in the spectrum of carbon monoxide and nowadays known to arise from C2. Ramsay, Travers, and Baly ($5) replied that the metargon lines survived various procedures likely to remove carbon compounds and further reported that they had been able to remove spectroscopically detectible carbon lines from a 50-50 mixture of carbon monoxide and argon. Schuster
stuck to his guns (36) and Ramsay and his associates gradually came to realize that metargon did not exist and that thc spectroscopic evidence arose from carbon contamination from yellow phosphorus used to remove oxygen in the course of preparation of the origiml argon sample. The change of opinion was slow, a t any rate in Ramsay, for he included metargon in a lecture to the German Chemical Society in December, 1898 (37) and formal repudiation of metargon does not seem to have come until a paper read to the Royal Society in November, 1900 (38). Metargon did not, in fact, prove to be a serious distraction even while its existence was still believed in by Ramsay; this was probably hecause of fairly widespread scepticism coupled with a vague idea that metargon might share a slot in the Periodic Table with "regular" argon to which it seemed so closely related. It would be a pity to allow the "metargon" mistake to obscure the resourcefulness of Ramsay and Travers during the two months in which they identified three new elements; they were by now well schooled in the dangers inherent in using the brightness of spectra as a form of semi-quantitative analysis of a gas mixture and also sometimes showed remarkable intuition in surmising the density (when pure) of a gas also containing one or more related elements. Group 0: Argon and Its Companions
The discovery of three more noble gases in the spriug and summer of 1898 contributed to the resolution of the Periodic Table problem in several ways. First, the discovery of a gas with a molecular weight of about 20 and t,he same specific heat ratio as argon undermined t,he view that argon was a diatomic molecule unless one was prepared t,o postulate a new "monster" of atomic weight near 10 to compensate for the rationalization of the nature of argon. Second, the triatomic nitrogen hypothesis when extendcd to krypton and xenon suggested Siafor the former and for the latter the triatomic molecule of a new element between calcium and scandium. Third, the inversion of atomic weight for argon became more acccptable to many when it pulled that element into line with three other elements that were obviously closcly related to it and whose atomic weights fell comfortably between halogen and alkali metal without skulduggery. I t is impossible to pinpoint the time at which "Group 0" became generally' accepted, although it was certainly not before the middle of 1898 and certainly not later than 1902. Brauner, who had so early and so spiritedly championed the tri-atomic nitrogen hypothesis of Mendeleeff and of Dewar, was still defending it in 1899. Plus royaliste que le roi he published a paper in thc Berichte early that year (39) in which he admitted his lack of allies but reaffirmed his position. One of the factors that encouraged him to do this was, he wrote, the discovery of metargon. This gas was a compound of carbon according to its spectrum (conceivably, he thought, with the formula C20) and yet its specific heat ratio was 1.66. He concluded irresistibly from these facts (a) that a gas could have a specific heat ratio of 1.66 without being monatomic (b) that not all gases that neither show chemical reactivity nor are dissociable into simpler substances must consist of elements in a monatomic state. Such resourceful
dialectic was worthy of a better cause and of more reliable experimental data. By an unusual coincidence i t was a t the same meeting of the Royal Society on June 9th, 1898, that Ramsey and Travers reported the discovery of krypton (40) and that Crookes showed a model of a three-dimensional Periodic Table in something of a figure-of-eight in which helium and argon fell on the "neutral" line represented by the crossing point of the loops (41); the atomic weight inversion of argon was of course implicit in the model. Before the model was illustrated in Chemical News (48) Crookes was able to include both neon and krypton and to surmise that, if metargon proved to have an atomic weight near 40, i t must share a position with argon. I n November, 1900 Ramsay and Travers read a paper to the Royal Society on "Argon and its Companions" (38)in which after reporting the latest information on the preparation and properties of the gases they suggested that with the possible exception of helium all belonged to a new group in the Periodic Table. Acceptance of this view could not he called universal even in 1902 because in the third edition of the widely used "Lehrhuch der Anorganischen Chemie" by H. Erdmann (43) (who seems, incidentally to have coined the term "Edelgase") helium was placed in the nitrogen group, neon as the first discovered member of a new triad like iron, cobalt, and nickel while argon, krypton, and xenon were somewhat less closely connected to "Group VIII." The best symbol of the final acceptance of the noble gases as a new group in the Periodic Table, which also marks the amearance of the term "Grouu 0." is the third (and la& English edition of Mendelkeffk "Principles of Chemistry" (44) in which he recalled and withdrew his earlier opinion that argon was a nitrogen allotrope and placed all five gases in a separate group preceding the alkali metals. It is true that he gave argon an atomic weight of 38 as he was still patiently awaiting the improved atomic weight determinations that would, he believed, eliminate all inversions of atomic weight. "Argon and its Companions" ($8) may be thought of as a triumphant conclusion to an important chapter in the history of chemistry. It is curious that the mood of its final paragraph is undeniably one of disappointment
. ..
we were not without a strong hope that their discovery would solve the problem [of irregularities in the periodic arrangement of the elements]. But our hope has been fruitless.
One would like to summon William Ramsay from the shades and allow him to read, perhaps by a neon lamp, on page 1 of Sidgwick's "Chemical Elements" (45) . . . (the inert gases) have provided the key to the whole problem of vdency and the interpretation of the Periodic Classification.
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