x.] THE "SERIES" EVIDENCE. 95 



I think it is quite fair to remark at this stage of our inquiry, that 

 if all the vibratory atoms which produce the spectra of the chemical 

 elements had all been brought to a similar condition of greatest sim 

 plicity, in other words, if we were really dealing with the chemical 

 atom as defined, in each case, the amazing irregularities which we have 

 found could hardly be expected. 



Some Details. 



I will next go a little further into detail in the case of some ele- 

 ments for the sake of instituting comparisons, and seeing whither the 

 results lead us. 



The most remarkable case which I have to refer to is that of 

 hydrogen. We do not know the meaning of it yet, but it has to be 

 taken into account in any consideration of these questions. Until a 

 little time ago only one series was known in the spectrum of this gas, 

 and reasoning on this basis, it was thought that the atom of hydrogen 

 was far more simple than that of any other chemical element, and also 

 that a chemical atom was only competent to produce one series. A 

 short time ago, however, Professor Pickering, in his magnificent work 

 on the stars, to which I have already had the opportunity of referring, 

 pp. 58 et seq., discovered a second series of lines. Not long after, Pro- 

 fessor Rydberg suggested that one of the most important lines seen in 

 a large group of stars really represented a line of the principal series of 

 hydrogen. That conclusion has been generally accepted, although the 

 evidence is considered doubtful by some ; so that we now assume that 

 hydrogen has three series like helium and astqrium, and we seem there- 

 fore to be on solid ground in one direction, at all events, in regard to 

 some gases. That is, we may assume either that a simple atom may 

 by vibrating produce three series, or that hydrogen itself is of at least 

 threefold complexity. We have- another series of metals of low atomic 

 weight, which therefore chemically are supposed to represent a con- 

 siderable simplicity ; we find that in the case of lithium and sodium we 

 also deal with three series, a principal series and two subordinate series. 

 The same remark applies to potassium. It has recently been found that 

 sulphur and selenium also give us three series.' We have a principal 

 series and the first and second subordinates, the suggestion of anything 

 beyond these three is confined to one or two lines in each case. 



But if we pass from the gas hydrogen to the gas oxygen, what do 

 we find 1 



In oxygen we have six series, that is twice as many as we know of in 

 hydrogen, helium, asterium, lithium, sodium, sulphur, and so on. So 

 far as that goes, we are in the same condition that we were some 



