294 



NA TURE 



[July i8, 1901 



ON THE SEPARATION OF THE LEAST 

 VOLATILE GASES OF ATMOSPHERIC 

 AIR, AND THEIR SPECTRA} 



"T^HE separation of these gases from each other was effected 

 by collecting them in a bulb in the solid state and allowing 

 the solid gradually to evaporate at as low a temperature as 

 possible, while the vapour was continually pumped away with a 

 mercurial pump. Between the bulb containing the solidified 

 gases and the pump a sparking tube was interposed, where the 

 spectrum emitted by the vapour under the influence of an 

 electric discharge was from time to time observed. 



The success of the operation of separating the gases which 

 boil at different temperatures depends on keeping the tempera- 

 ture of the solid mass as low as possible, as will be seen from the 

 following consideration : — 



The pressure, p, of a gas G, above the same material in the 

 liquid state, at temperature T, is given approximately by the 

 formula 



log /* = A--, 



where A and B are constants for the same material, 

 other gas G' the formula will be 



For some 



and 



log ^ = A- 



A,+ 



B, - B 



Now for argon, krypton and xenon, respectively, the 

 values of A are 6782, 6'972 and 6'963, and those of B 

 are 339, 496'3 and 669'2 ; and for these and many other 



substances A - A, is always a small quantity, while ', ^ 



is considerable and increases as T diminishes. Hence the ratio 

 of/ to/i increases rapidly as T diminishes, and by evaporating 

 the gases always from the solid state and keeping the solid at as 

 low a temperature as possible, the gas first coming off consists 

 in i)y far the greatest part of that which has the lowest boiling 

 point, and is succeeded, with comparative abruptness, by the 

 gas which has the next higher boiling point. .So abrupt indeed 

 is the succession that the nitrogen is almost completely removed 

 before the argon makes its appearance, and the necessity for 

 removing the nitrogen by sparking with oxygen almost wholly 

 avoided. The change from one gas to another is easily detected 

 by examining the spectrum in the sparking tube, and the 

 reservoirs into which the gases are pumped can be changed 

 when the spectrum changes and the fractions separately 

 stored. 



The general sequence of spectra, omitting those of nitrogen, 

 hydrogen and compounds of carbon, which were never entirely 

 removed by the process of distillation alone, was as follows : 

 The spectrum of argon was first noticed in succession to nitrogen, 

 and then as the distillation proceeded the brightest rays, green 

 and yellow, of krypton appeared, and then the intensity of the 

 argon spectrum waned, and it gave way to that of krypton until, 

 as predicted by Runge, when a Leyden jar was in the circuit, 

 the capillary part of the sparking tube had a magnificent blue 

 colour, while the wide ends were bright pale yellow. Without 

 a jar the tube was nearly white in the capillary part, and yellow 

 about the poles. As the distillation proceeded, the temperature 

 of the vessel containing the solid mixture being allowed to rise 

 slowly, the brightest of the xenon rays began to appear, namely, 

 the green rays about \ 5420, 5292 and 4922, and then the 

 ^krypton rays soon died out and were superseded by the xenon 

 ■rays. At this stage the capillary part of the sparking tube is, 

 with a jar in circuit, a brilliant green, and is still green, though 

 less brilliant, without the jar. The xenon formed the final 

 fraction distilled. 



The authors give a long list of the approximate wave-lengths 

 of rays they have observed to be emitted by xenon and krypton 

 .under the influence of electric discharge. 



The variation of the spectra of both xenon and krypton with 

 variation in the character of the electiic discharge is very striking, 

 and has already been the subject of remark, in the case of 

 krypton, by Runge, who has compared krypton with argon in 

 its sensitiveness to changes in the electric discharge. Runge 

 distinguishes krypton rays which are visible without a jar and 



1 Abridged from a piper by Prof. G. D. Liveing,' F.R.S.,and Prof. J. 

 Dewar, F.R.S., read before the Royal Society on June 20. 



NO. 1655, VOL. 64] 



those which are only visible with a jar discharge. The difference 

 in the intensity of certain rays, according as the discharge is 

 continuous or oscillatory, is no doubt very marked, but, with 

 rare exceptions, the authors have found that the rays which are 

 intensified by the oscillatory discharge can be seen with a con- 

 tinuous discharge when the slit of the spectroscope is wide. 

 Runge used a grating, whereas they have, for the sake of more 

 light, used a prism spectroscope throughout, and were therefore 

 able to observe many more rays than he. 



There is one very remarkable change in the xenon spectrum 

 produced by the introduction of a jar into the circuit. Without 

 the jar xenon gives two bright green rays at about A 4917 and 

 A 4924, but on putting a jar into the circuit they are replaced by 

 a single still stronger ray at about K 4922. In no other case 

 have the authors noticed a change so striking as this on merely 

 changing the character of the discharge. It is noteworthy that 

 the ray A 4922 is close to a well-known helium ray, but other 

 helium rays were not seen in the same spectrum. Changes of 

 the spectrum by the introduction of a jar into the circuit are, 

 however, the rule rather than the exception, and there are 

 changes in the spectrum of krypton which seem to depend on 

 other circumstances. Of many tubes filled with krypton in the 

 manner above indicated, some give with no jar the green ray 

 A. 557 'i the yellow ray A 5S71 and the red ray A 7600 very 

 bright, while other rays are very few, and those few barely 

 visible. Puttinga jar into the circuit makes very little difference ; 

 the three rays above mentioned remain much the brightest, 

 nearly, though not quite, so bright as before, and the blue rays, 

 so conspicuous in other tubes, though strengthened by the use 

 of the jar, are still very weak. In other tubes the extreme red 

 ray is invisible, the rays at A 5571 and 5871 absolutely, as well 

 as relatively, much feebler, while the strong blue rays are bright, - 

 even brighter than the green and yellow rays above named. In 

 one tube the blue rays could be seen, though not the others. 

 This looks very much as if two different gases were involved, 

 but the authors have not been able to assure themselves of that. 

 The case seems nearly parallel with that of hydrogen. There 

 are some hydrogen tubes which show the second spectrum of 

 hydrogen very bright, and others which show only the first 

 spectrum ; the second spectrum is enfeebled or extinguished by 

 introducing a jar into the circuit, while the first spectrum is 

 strengthened ; and the conditions which determine the appear- 

 ance of the ultra-violet series of hydrogen rays have not yet 

 been satisfactorily made out. 



It is to be noted that putting the jar out of circuit does not in 

 general immediately reduce the brightness of the rays which are 

 strengthened by the jar discharge. Their intensity fades gradu- 

 ally, and is generally revived, more or less, by reversing the 

 direction of the current, but this revival gets less marked at 

 each reversal until the intensity reaches its minimum. The 

 rays strengthened by the jar discharge also sometimes appear 

 bright, without a jar, on first passing the spark when the elec- 

 trodes are cold, and f.ade when the electrodes get hot, reappear- 

 ing when the tube has cooled again. Moreover, if the discharge 

 be continued without a jar, the resistance in the krypton tubes 

 increases rather rapidly, the tube becomes much less luminous 

 and finally refuses to pass the spark. With an oscillatory 

 discharge the passage of the spark and the brightness of the 

 rays are much more persistent. This seems to point to some 

 action at the electrodes which is more marked in the case of 

 krypton than in that of xenon. 



The xenon spectrum is characterised by a group of four 

 conspicuous orange rays of about equal intensities, a group of 

 very bright green rays of which two are especially conspicuous, 

 and several very bright blue rays. The list of xenon rays 

 published by Erdmann does not present any close agreement 

 with that of the authors except as to the strongest green lines. 

 The number of xenon rays observed is very considerable, and 

 some of them lie very near to rays of the second spectrum o. 

 hydrogen, but inasmuch as these rays are more conspicuous 

 with a jar in circuit than without, which is not the character of 

 the second spectrum of hydrogen, and as, moreover, many of 

 the brightest of the hydrogen rays are absent from the spectrum 

 of the tubes, the authors conclude that these rays are not due to 

 hydrogen. 



Certain rays, tabulated separately, have been as yet observed 

 in only one xenon tube ; they include a very strong ultra-violet ray 

 of unknown origin, and due either to some substance other than 

 xenon or to some condition of the tube which has not been 

 repeated in the other tubes. 



