September 26, 1895] 



NATURE 



521 



two series ending at the same place, and the third leaping 

 over the first two in large bounds and ending in the more 

 refrangible part of the spectrum. This third series wc 

 suppose to be analogous to the so-called principal series 

 in the spectra of the alkalis, which show the same 

 features. It is not impossible, one may even say not un- 

 likely, that there are principal series in the spectra of the 

 other elements. But so far they have not been shown to 

 exist. 



Each of our two spectra now shows a close analogy to 

 the spectra of the alkalis. 



We therefore believe the gas in cle\citc to consist of 

 two, and not more than two, constituents. We propose 

 to call only one of the constituents helium, the one to 

 which the bright yellow double line belongs, whose spec- 

 trum altogether is the stronger one, while the other 

 constituent ought to receive a new name. 



We have confirmed this rather hypothetical conclusion 

 by the following experiment. The connection leading 

 from our supply of cleveite gas to the vacuum tube con- 

 tained a side branch parting from it and joining it again. 

 There were stopcocks on either side of the side branch, 

 and a third one in the side branch. In the main tube 

 between the ends of the side branch a plug of asbestos 

 was tightly inserted. To prepare the vacuum tube only 

 the taj) leading to the supply was closed, the whjle space 

 up to this tap being carefully evacuated. Now the side 

 branch was closed, and the tap leading to the supply was 

 opened. Then we obserxed that the light of the electric 

 discharge in the vacuum tube was at first greenish, and 

 after a while grew yellow. By cutting off the current of 

 gas after a sufficiently short time, we succeeded in making 

 a vacuum tube which remained greenish. On examining 

 it in a small spectroscope with which we could overlook 

 the whole spectrum, we found that the intensities of the 

 lines had changed. The yellow line was scarcely as 

 bright as the green line 5016, and the red line 7065 had 

 apparently decreased relatively to 7282 and 6678, although 

 it was still stronger than 7282. The two lines that had 

 decreased in intensity belong to the second set of series, 

 while the others are meinbers of the first set. The other 

 visual lines of the second set could not very well be 

 examined because they are more in the violet part. 



This observation confirms our spectroscopic result. 

 The gas in cleveite may be taken to be a mixture of two 

 gases of different density, of which the lighter one is more 

 rapidly transmitted through the plug of asbestos. There 

 is, however, the objection to be raised, that in the green 

 tube the pressure is less, and that the difference of in- 

 tensities is due to the pressure being different. This must 

 be further inquired into. 



We were not satisfied with the visual observation of 

 the change of intensities in our green tube, but thought 

 it desiraljle to test the conclusion by the bolometric 

 measurement of the two lines that we have discovered 

 in the ultra-red part of the spectrum. If we were right, 

 the ultra-red line of smaller wave-length, which belongs 

 to the second set of series, ought to have decreased in 

 intensity relatively to the other ultra-red line. This we 

 found to be so indeed. In the yellow tubes the intensity 

 of the smaller wave-length was to that of the other on an 

 average as 3 to i, while in the green tubes it was as r8 

 to I. This confirmation we consider the more valuable as 

 it does not de|)end on any estimation which may be biassed 

 by the personal opinion of the observer, but is based on 

 an objective numerical determination. 



Another confirmation may be gathered from the spec- 

 trum of the sun's limb and that of several stars. Let us 

 confine our attention to the six strongest lines in the 

 visible part of the spectrum : 



7066, 6678, 5S76, 5016, 4922, 4472. 



The first, third, and sixth belong to the second set of 

 series ; the second, fourth and fifth to the first set. These 



six lines have all been observed in the spectrum of the 

 sun's limb, as Norman Lockyer and Deslandres have 

 pointed out. Now, according to their appearance in the 

 spectrum of the sun's limb, they may be classed in 

 two groups, one group being always present, the other 

 group being sometimes present. C. \. Young long ago 

 called attention to the difference in the frequency of 

 appearance of the chromospheric lines. He has given them 

 frequency numbers, roughly estimating the percentage of 

 frequency with which the lines were seen during the six 

 weeks of observation at Sherman in the summer of 1872. 

 According to Young, 7066, 5876, 4472 have the frequency 

 number 100, while 6678, 5016, 4922 have the numbers 25, 

 30, 30, showing that one of the two constituents was 

 always present, while the other was only seen about once 

 in every four cases. 



The lines of both constituents have been observed in 

 the spectra of a considerable number of stars ^, S, e, f, 7 

 Orionis, a. Virginis, /3 Persei, 3 Tauri, 7; Ursae majoris, 

 /3 Lyra;. In the spectrum of /3 Lyne, thirteen lines 

 have been identified with certainty. But the most 

 interesting case in point is the spectrum of Nova Auriga?, 

 that wonderful star whose sudden appearance was 

 announced to astronomers in 1892 by an anonymous post- 

 card. In the spectrum of Nova Aurig;e the two lines 

 5016 and 4922 were veiy strong, while 4472 was weak and 

 5876 has only been seen by Dr. Huggins, we believe only on 

 one occasion, and appears to have been very weak. Now 

 5016 and 4922 belong to the lighter constituent, and are 

 together with 6678 the strongest lines in the visible part 

 of the spectrum ; while 5876 and 4472 are the strongest 

 lines of the other constituent in the visible part of the 

 spectrum. In Nova Auriga;, therefore, the lighter con- 

 stituent gave a much brighter spectrum than helium 

 proper. But there may here be raised an objection, 

 which indeed we do not know liow to refute. Why has the 

 line 6678 not been observed ? It is a pity that the red part 

 of the spectrum cannot be more easily photographed. 

 Nova Auriga; has now become very weak, and besides 

 the spectrum is quite altered, so that we shall never know 

 whether the red line 6678 was really absent or has only 

 escaped notice. 



From the fact that the second set of series is on 

 the whole situated more to the refrangible part of the 

 spectrum, one may, independently of the diffusion ex- 

 periment, conclude that the element corresponding to 

 the second set is the heavier of the two. In the spectra 

 of chemically related elements like Li, Na, K, Rb, Cs, 

 or Mg, Ca, Sr', or Zn, Cd, Hg, the series shift to the 

 less refrangible side with increasing atomic weight. 

 But it appears that in the sjiectra of elements following 

 each other in the order of their atomic weights in a row 

 of the periodic system like 



Na, Mg, Al ; 

 K, Ca ; 

 Cu, Zn ; 

 Rb, Sr ; 

 .\g, C(l, In ; 



the scries shift the opposite way, so that the spectrum of 

 the element of greater atomic weight is as a whole 

 situated further to the more refrangible side. Now in 

 our case the density of the gas has been determined by 

 Langlet (published by Cleve) and by Ramsay to be about 

 double the density of hydrogen. Assuming the atomic 

 weights of the two constituents to be between that of 

 lithium and that of hydrogen, they would both belong to 

 the same row- of the periodic system, and therefore the 

 more refrangible set of series would correspond to the 

 greater atomic weight. 



For convenience of reference all the observed lines are 

 given in the following table, the wave-lengths being 

 abridged to tenth-metres. 



NO. 1352, VOL. 52] 



