178 INORGANIC EVOLUTION.- [CHAP. 



To do this we have to see the basis of the atomic weight of oxygen 

 16, and consider the series question in relation to oxygen. This 

 necessitates a reference back to Chapter X, in which I pointed out that 

 the simplest case presented in series phenomena is that placed before 

 us by sodium and other elements which run through all their known 

 spectral changes at a low temperature. Dealing with the line 

 spectrum stage we have three "series/' one principal and two sub- 

 ordinate (first and second). The former contains the orange line D, 

 constantly seen at all temperatures, the first subordinate the red line, 

 the second subordinate the green line, representatives of two series of 

 lines which are best seen both in the flame and arc. 



The two subordinate series of sodium, like those of all other 

 elements so far examined, have the peculiarity that they end at nearly 

 the same wave-length, while the end of the principal series occurs at 

 a different, sometimes widely different, wave-length. This is a touch- 

 stone of the highest importance, as we shall see ; it points to a 

 solidarity of the two subordinate series, and to a difference between 

 them and the principal series. 



Although the original idea was that all three series were produced 

 by the vibrations of the same molecule, observations of the sodium 

 phenomena alone are simply and sufficiently explained by supposing 

 that we have three different masses vibrating, and that two of them, 

 producing the subordinate series, can be broken up by heat, while that 

 producing the principal series cannot. The series represented by the 

 red and green lines seen best at the lower temperatures have been seen 

 alone, and it is a matter of common experience that the yellow line 

 representing the principal series is generally seen alone ; it is not 

 abolished at high temperature as the others are. Because the mass, the 

 vibrations of which give us the yellow lines, is produced by the break- 

 ing up of more complex forms at a low stage of heat, and it cannot 

 be destroyed by the means at our command, it is the common representa- 

 tive of the element sodium. Because the masses, the vibrations of 

 which produce the two subordinate series represented by the red and 

 green lines, are easily destroyed by heat, they are more rarely seen ; 

 scarcely ever at high temperatures when the quantity is small, since, 

 as I pointed out years ago, " the more there is to dissociate, the more 

 time is required to run through the series, and the better the first 

 stages are seen." 



This view is greatly strengthened by considering another substance 



which, if we accept Pickering's and Kydberg's results, has like sodium, 



three series, one principal and two subordinates in quite orthodox fashion. 



I refer to hydrogen. The facts concerning which are given on p. 95. 



Till a short time ago we only knew of one " series" of hydrogen, 



