December 19, 1901] 



NATURE 



16: 



the gas from the Bath springs, which Lord P.ayleigh found to 

 contain argon mixed with about S per cent, of its volume of 

 helium ; and helium has also been found in mineral springs at 

 Wildbad, and at Cauterets, in the Pyrenees. It would appear, 

 then, that helium is not such a very rare constituent of our 

 globe ; and, indeed, it is probable that it is continually escaping 

 from the earth in small quantities in certain regions. 



In 1S97, as president of the Chemical Section of the British 

 Association, I chose the title " An Undiscovered Gas '" for the 

 address to the Section. The arguments in favour of the 

 e.xistenceof such a gas were briefly these : The difterences be- 

 tween the atomic weights of consecutive elements in the 

 columns of the periodic table are appro.\imately i6 to 20 ; thus 

 l6'5 is the difference between the atomic weights of fluorine and 

 chlorine ; 16 between those of o.-iygen and sulphur, and so on. 

 Again, stepping one pace down the scale, we have I9'5 as the 

 difference between chlorine and manganese ; 20'3 between sul- 

 phur and chromium ; 19S between silicon and titanium, &c. 

 The total difference between manganese and fluorine is 36 ; 

 between chromium and o.xygen, 36'3 ; between vanadium and 

 nitrogen, 37'4; and between titanium and carbon, 36'i. This 

 is approximately the difference between the atomic weights of 

 helium and argon, 36. I quote now from that address : " There 

 should, therefore, be an undiscovered element between helium 

 and argon, with an atomic weight 16 units higher than that 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." 



Those who care to read the story of the search for this undis- 

 covered element may find it in the address. Minerals from all 

 parts of the globe, mineral waters from Britain, France and Ice- 

 land, meteorites from interstellar space ; all these were investi- 

 gated without result. Helium from various minerals was 

 separated by long and tedious processes of diffusion into a 

 possibly lighter portion, diffusing more rapidly, and a possibly 

 heavier portion diffusing more slowly, but with no positive 

 result. The systematic diffusion of argon, however, gave a 

 faint indication of where to seek for the missing element, for the 

 density of the more rapidly diffusing portion was I9'93, while 

 that of the portion which diffused more slowly was 20'0i. 



The invention by Dr. Ilampson of an apparatus by means of 

 which it is possible to obtain liquid air at small expense and 

 with little trouble placed a new instrument in our hands ; and 

 Dr. Travers and I prepared 15 litres of argon from the atmo- 

 sphere, with the purpose of distilling it fractionally, after lique- 

 faction ; for we knew, from the researches of Prof. Olszewski of 

 Cracow, who has done so much to determine the'properties of 

 liquefied gases, that argon could be liquefied easily by com- 

 pressing it into a vessel cooled by help of liquid air. And, 

 moreover, we were in hope that by fractionating the air itself 

 gases of even higher atomic weight than argon might possibly 

 be obtained. Both expectations were realised ; on distilling 

 liquid argon the first portions of gas to boil off were found to be 

 lighter than argon, and on allowing liquid air to boil slowly 

 away heavier gases came off at the last. It was easy to recog- 

 nise these gases by help of the spectroscope, for the light gas, to 

 which we gave the name neon, or "the new one," when electric- 

 ally excited emits a brilliant flame-coloured light ; and one of the 

 heavy gases, which we called krypton, or "the hidden one," is 

 characterised by two brilliant lines, one in the yellow and one 

 in (he green part of the spectrum. The third gas, named 

 xenon, or " the stranger," gives out a greenish-blue light and is 

 remarkable for a very complex spectrum, in which blue lines are 

 conspicuous. 



Although neon was first obtained by the fractional distillation 

 of argon, it was afterwards found convenient to prepare it direct 

 from air. The torpedo-compressor, which is used for com- 

 pressing the air before it enters Dr. Hampson's liquefier, was 

 made to take in the air which had escaped liquefaction in the 

 liquefier ; the denser portions were thus liquefied, and the 

 lighter portions were liquefied by compressing them into a 

 vessel cooled by the denser fractions, boiling under reduced 

 pressure, and consequently at a specially low temperature. This 

 liquefied portion was again fractionated, and yielded neon ; and 

 it was not long before we discovered that helium was also pre- 

 sent in the mixture. The presence of helium in atmospheric 



air had previously been noted by Prof. Kayser of Bonn, and by 

 Prof. Friedlander of Berlin, on submitting the spectrum of 

 argon to a searching examination. 



The purification of this mixture of neon and helium from 

 argon, although a lengthy process, was not attended by any 

 special difficulty. It was accomplished by repeated distillation, 

 the lighter portions being always collected separately from the 

 heavier portions, and again distilled by themselves. But aftei 

 this separation had been accomplished, we found that we were 

 unable by means of liquid air to liquefy the mixture, or indeed 

 any portion of it. We effected a partial .separation by diffusion ; 

 but it is not possible to separate by this method two gases of 

 which the quantity is limited. Another attempt was made by 

 dissolving the gases in liquid oxygen, on the supposition that 

 neon might prove more soluble than helium ; but without satis- 

 factory results. It was evident that a lower temperature than 

 that possible by help of liquid air was necessary. 



Prof. Dewar had by that time succeeded in producing liquid 

 hydrogen in quantity, and had indicated the principle, which is 

 identical with that of Dr. Hampson's air-liquefier, although he 

 has not published any detailed account of his apparatus. Dr. 

 Travers undertook to investigate the subject ; and after four 

 unsuccessful trials he made a liquefier, with the help of Mr. 

 Holding, the laboratory mechanician, by means of which a 

 hundred cubic centimetres of liquid hydrogen could be easily and 

 cheaply produced. There was then no difficulty in effecting the 

 separation of neon from helium ; for, while neon is practically 

 non-volatile, when cooled by liquid hydrogen, remaining in the 

 state of solid or liquid, even that enormously low temperature 

 is not sufficient to convert helium into a liquid. Hence the 

 gaseous helium could be pumped away from the non-gaseous 

 neon, and the latter was obtained in a pure state. 



The residues obtained from the evaporation of about thirty 

 litres of liquid air, after being freed from o.xygen and nitrogen, 

 were liquefied by help of liquid air, and fractionated from each 

 other. The separation offered no special difficulty, but was long 

 and tedious. It soon appeared that when most of the argon had 

 been removed the residue solidified when cooled ; but while it 

 was possible to remove the krypton by pumping, for it goes into 

 gas slowly even at the low temperature of liquid air, very little 

 xenon accompanied it ; for at that temperature xenon is hardly 

 at all volatile. 



Having finally separated the gases, their densities and other 

 properties were carefully determined ; and it was also proved 

 that they are like argon and helium, inasmuch as their 

 molecules consist of single atoms. Neon, as was expected, 

 turned out to be the missing link between helium and argon ; 

 the atomic weight of krypton was found to be 81 '6, and that of 

 xenon 128. The volumes occupied by equal numbers of mole- 

 cules of the liquefied gases were determined ; and also the boil- 

 ing-points and melting-points of argon, krypton and xenon. 

 These figures are shown in the following table : — 



Helium. Neon. Argon. Krypton. Xenon. 



Density of gas ... i"98 g'gS I9'96 4o'78 64*0 



Atomic weigbt 3*96 I9"92 39 '92 8i"56 i28'o 



Density of liquid ... 03 C?) io(?) 1-212 2-155 3"52 



Boiling-points — — — i86-i''C. -t5i-7°C. -1091^0. 



Melting-points — — — rSjg^C. - i69'°C. -i4o*°C. 



Critical temperatures — — — 117'4'C. — 62*5°C. -i- 14'75°C. 



Critical pressures ... — — (Metres.) 40-20 41-24 43'50 



Refractivity of gas... 0-I24 0-235 0-968 1-450 2-368 



In every case there is seen what is termed periodicity ; that is, 

 a gradual alteration with rise of atomic weight, of the densities 

 of the liquids, of the melting-points, of the boiling-points, and 

 of the retardation of light when passed through the gas. 



Let us consider, in conclusion, the position of these elements 

 in the periodic table ; and it will be sufficient to confine our 

 attention to he groups of elements which form the neighbour- 

 ing columns. The atomic weights are given in round numbers. 



Hydrogen. Helium. Lithium. Beryllium. 



I 479 



Fluorine. Neon. Sodium. Magnesium. 



19 20 23 24 



Chlorine. Argon. Potassium. Calcium. 



35 '5 40 3g . 40 _ 



Bromine. Krypton. Rubidium. Strontium. 



80 82 35 87 



Iodine. Xenon. Caesium. Barium. 



127 128 133 137 



It is evident that these new elements fall into their natural 

 places between the strongly electro-negative elements of the 

 fluorine group, and the very electro-positive elements of the 



NO. 1677, VOL. 65] 



