90 INORGANIC EVOLUTION. [CHAP. 



If the lines are more difficult to see, and if the sub-series of 

 lines get stronger towards either the red end or the blue end, then 

 we are more likely to see one line than two, and more likely to see 

 two lines than three. 



With regard to this suggestion made by Rydberg, it is interesting 

 to riote that Professor Kayser is not inclined to hold the same opinion 

 and does not look upon triplets, doublets, or single lines of the series as 

 remnants of flutings, whose other members are too weak to be seen. 

 He points out that we have for the elements of the first vertical column 

 in Mendeleeff's table, doublets ; for the second column, triplets ; for 

 the third, doublets. As the first column contains monavalent elements, 

 the second bivalent ones, the third trivalent, it seems as if the elements 

 with uneven valencies had doublets, those with even valencies triplets. 

 This is confirmed by the triplets of oxygen, sulphur, and selenium, 

 which belong to the sixth column, with even valency. As in every 

 natural group of elements, the first elements show the series strongest, 

 and they get weaker as the atomic weight increases (i.e., in the group 

 of alkalies we cannot see the weaker second series for rubidium 

 and caesium ; in the group copper, silver, gold we can find no series 

 in gold ; in the group of magnesium, calcium, strontium, barium, for 

 strontium the second series is already weak, for barium we cannot 

 find the series). We should expect to find, according to Rydberg's 

 hypothesis, in the spectra of every group first triplets, then doublets, 

 then single lines. But that is not so : so long as we find anything of 

 the series the members are and remain triplets or doublets. 



There is only a very small number of the chemical elements 

 which give us single lines ; in the principal series, so far, we only 

 know of helium and asterium ; in the subordinate series we only know 

 of asterium. The number of doubles is very much greater, but it is 

 not so great in relation to the principal series as it is in the case 

 of the subordinate series ; but although we have nine elements giving 

 us triplets in the subordinate series, we have only three which give 

 them in the principal series. These results are shown in the following 

 table. 



It is well that I should indicate the basis of these statements, and 

 for this purpose I give in Fig. 33 a very small part of the spectra of 

 three different elements, in order that the way in which the work has- 

 been done may be followed. In the lower horizon we are dealing with 

 zinc, and the way in which the triplets have been picked out will be 

 easily gathered. The triplet in each case has its central line nearer to- 

 one side of the triplet than the other. All the triplets in the zinc 

 spectrum are perfectly symmetrical from that point of view. If we 

 take the upper spectrum that of calcium we find also that the 



