38o 



NA TURE 



[August 19. 1897 



There should, therefore, be an undiscovered element between 

 helium and argon, with an atomic weight i6 units higher than 

 than of helium, and 20 units lower than that of argon, namely 

 20. And if this unknown element, like helium and argon, 

 should prove to consist of monatomic molecules, then its density 

 should be half its atomic weight, 10. And pushing the analogy 

 still further, it is to be expected that this element should be as 

 indifferent to union with other elements as the two allied 

 elements. 



My assistant, Mr. Morris Travers, has indefatigably aided me 

 in a search for this unknown gas. There is a proverb about 

 looking for a needle in a haystack ; modern science, with the 

 aid of suitable magnetic appliances, would, if the reward were 

 .sufficient, make short work of that proverbial needle. But here 

 is a supposed unknown gas, endowed no doubt with negative 

 properties, and the whole world to find it in. Still, the attempt 

 had to be made. 



We first directed our attention to the sources of helium — 

 minerals. Almost every mineral which we could obtain was 

 heated in a vacuum, and the gas which was evolved examined. 

 The results are interesting. Most minerals give off gas when 

 heated, and the gas contains, as a rule, a considerable amount 

 of hydrogen, mixed with carbonic acid, questionable traces of 

 nitrogen, and carbonic oxide. Many of the minerals, in 

 addition, gave helium, which proved to be widely distributed, 

 though only in minute proportion. One mineral— malacone — 

 gave appreciable quantities of argon ; and it is noteworthy that 

 argon was not found except in it (and, curiously, in much larger 

 amount than helium), and in a specimen of meteoric iron. 

 Other specimens of meteoric iron were examined, but were 

 found to contain mainly hydrogen, with no trace of either argon 

 or helium. It is probable that the sources of meteorites might 

 be traced in this manner, and that each could be relegated to 

 its particular swarm. 



Among the minerals examined was one to which our attention 

 had been directed by Prof. Lockyer, named eliasite, from which 

 he said that he had extracted a gas in which he had observed 

 spectrum lines foreign to helium. He was kind enough to 

 furnish us with a specimen of this mineral, which is exceedingly 

 rare, but the sample which we tested contained nothing but un- 

 doubted helium. 



During a trip to Iceland in 1895, I collected some gas from 

 the boiling springs there ; it consisted, for the most part, of air, 

 but contained somewhat more argon than is usually dissolved 

 when air is shaken with water. In the spring of 1896 Mr. 

 Travers and I made a trip to the Pyrenees to collect gas from 

 the mineral springs of Cauterets, to which our attention had 

 been directed by Dr. Bouchard, who pointed out that these 

 gases are rich in helium. We examined a number of samples 

 from the various springs, and confirmed Dr. Bouchard's results, 

 but there was no sign of any unknown lines in the spectrum of 

 these gases. Our quest was in vain. 



We must now turn to another aspect of the subject. Shortly 

 after the discovery of helium, its spectrum was very carefully 

 examined by Profs. Runge and Paschen, the renowned spectro- 

 scopists. The spectrum was photographed, special attention 

 being paid to the invisible portions, termed the "ultra-violet" 

 and " infra-red." The lines thus registered were found to have 

 a harmonic relation to each other. They admitted of division 

 into two sets, each complete in itself. Now, a similar process 

 had been applied to the spectrum of lithium and to that of 

 sodium, and the spectra of these elements gave only one series 

 each. Hence, Profs. Runge and Paschen concluded that the 

 gas, to which the provisional name of helium had been given, 

 was, in reality, a mixture of two gases, closely resembling each 

 other in properties. As we know no other elements with atomic 

 weights between those of hydrogen and lithium, there is no 

 chemical evidence either for or against this supposition. Prof. 

 Runge supposed that he had obtained evidence of the separation 

 of these imagined elements from each other by means of diffu- 

 sion ; but Mr. Travers and I pointed out that the same altera- 

 tion of spectrum, which was apparently produced by diffusion, 

 could also be caused by altering the pressure of the gas in the 

 vacuum tube ; and shortly after Prof. Runge acknowledged his 

 mistake. 



These considerations, however, made it desirable to subject 

 helium to systematic diffusion, in the same way as argon had 

 been tried. The experiments were carried out in the summer 

 of 1896 by Dr. Collie and myself. The result was encouraging. 

 It was found possible to separate helium into two portions of 



different rates of diffusion, and consequently of different density 

 by this means. The limits of separation, however, were not 

 very great. On the one hand, we obtained gas of a density 

 close on 2*0; and on the other, a sample of density 2*4 or 

 thereabouts. The difficulty was increased by the curious be- 

 haviour, which we have often had occasion to confirm, that 

 helium possesses a rate of diffusion too rapid for its density. 

 Thus, the density of the lightest portion of the diffused gas, 

 calculated from its rate of diffusion, was i '874 ; but this corre- 

 sponds to a real density of about 2'0. After our paper, giving 

 an account of these experiments, had been published, a German 

 investigator, Ilerr A. Hagenbach, repeated our work and con- 

 firmed our results. 



The two samples of gas of different density differ also in other 

 properties. Different transparent substances differ in the rate 

 at which they allow light to pass through them. Thus, light 

 travels through water at a much slower rate than through air, 

 and at a slower rate through air than through hydrogen. Now 

 Lord Rayleigh found that helium offers less opposition to the 

 passage of light than any other substance does, and the heavier 

 of the two portions into which helium had been split offered 

 more opposition than the lighter portion. And the retardation 

 of the light, unlike what has usually been observed, was nearly 

 proportional to the densities of the samples. The spectrum of 

 these two samples did not differ in the minutest }>articular j 

 therefore it did not appear quite out of the question to hazard 

 the speculation that the process of diffusion was instrumental, 

 not, necessarily in separating two kinds of gas from each other, 

 but actually in removing light molecules of the same kind from 

 heavy molecules. This idea is not new. It had been advanced 

 by Prof. Schiitzenberger (whose recent death all chemists have 

 to deplore), and later, by Mr. Crookes, that what we term the 

 atomic weight of an element is a mean ; that when we say the 

 atomic weight of oxygen is 16, we merely state that the average 

 atomic weight is 16 ; and it is not inconceivable that a certain 

 number of molecules have a weight somewhat higher than 32, 

 while a certain number have a lower weight. 



We therefore thought it necessary to test this question by direct 

 experiment with some known gas ; and we chose nitrogen, as 

 a good material with which to test the point. A much larger 

 and more convenient apparatus for diffusing gases was 

 built by Mr. Travers and myself, and a set of systematic 

 diffusions of nitrogen was carried out. After thirty rounds, 

 corresponding to 180 diffusions, the density of the nitrogen was 

 unaltered, and that of the portion which should have diffused 

 most slowly, had there been any difference in rate, was identical 

 with that of the most quickly diffusing portion — i.e. with that 

 of the portion which passed first through the porous plug. 

 This attempt, therefore, was unsuccessful ; but it was worth 

 carrying oUt, for it is now certain that it is not possible to 

 separate a gas of undoubted chemical unity into portions ot 

 different density by diffusion. And these experiments rendered 

 it exceedingly improbable that the difference in density of the 

 two fractions of helium was due to separation of light molecules 

 of helium from heavy molecules. 



The apparatus used for diffusion had a capacity of about two 

 litres. It was filled with helium, and the operation of diffusion 

 was carried through thirty times. There were six reservoirs, 

 each full of gas, and each was separated into two by diffusion. 

 To the heavier portion of one lot, the lighter portion of the 

 next was added, and in this manner all six reservoirs were 

 successfully passed through the diffusion apparatus. This pro- 

 cess was carried out thirty times, each of the six reservoirs 

 having had its gas diffused each time, thus involving 180 

 diffusions. After this process, the density of the more quickly 

 diffusing gas was reduced to 2 '02, while that of the Jess quickly 

 diffusing had increased to 2-27. The light portion on re-diffusion 

 hardly altered in density, while the heavier portion, when 

 divided into three portions by diffusion, showed a considerable 

 difference in density between the first third and the last third. 

 A similar set of operations was carried out with a fresh quantity 

 of helium, in order to accumulate enough gas to obtain a 

 sufficient quantity for a second series of diffusions. The more ' 

 quickly diffusing portions of both gases were mixed and re- 

 diffused. The density of the lightest portion of these gases was 

 1-98 ; and after other 15 diffusions, the den.sity of the Hghtest 

 portion had not decreased. The end had been reached ; it was 

 not possible to obtain a lighter portion by diffusion. The 

 density of the main body of this gas is thereiore i'98 ; and its 

 refractivity, air being taken as unity, is 0-1245. The spectrum 



NO. 1 45 I, VOL. 56] 



