XXIII. ] FROM A CHEMICAL STANDPOINT. 179 



and on this ground Rydberg assumed it to represent the finest form of 

 matter known, regarding the other substances which give three normal 

 series as more complex. This idea is in harmony with the view ex- 

 pressed above. 



If we accept the recent suggestions, we must regard hydrogen as 

 identical with sodium in its series conditions. But there is this tre- 

 mendous difference. In sodium we easily at low temperatures the 

 Bimsen is sufficient see all three series, while in the case of hydrogen 

 even the Spottiswoode coil can show us nothing more than one of the 

 subordinate series. At the same time, the other subordinate and the 

 principal series are visible in stars which we have many reasons for 

 believing to be hotter than the spark produced by the Spottiswoode 

 coil. 



The argument for the existence of at least three different masses pro- 

 ducing the three different series, derived from the sodium observations, 

 is therefore greatly strengthened by what we now know of hydrogen. 



I shall therefore assume it in what follows, which has reference to 

 more complicated phenomena. 



Oxygen, instead of having three series like metals of low melting 

 point such as sodium, and the gas hydrogen, has six. These six have 

 been divided by Runge and Paschen into two normal sets of three,, 

 each set possessing one principal and two subordinate series. 



There is evidently a new problem before us ; we require to add the 

 series of hydrogen to the series of sodium to get a " series " result 

 similar to that obtained from oxygen. 



Before we go further it will be well to consider the possible order of 

 simplifications. Let us take the simplest case represented by sodium 

 and hydrogen in the first instance. The facts are shown in the follow- 

 ing table : 



High temperature. 

 Sodium. Hydrogen. 



p Celestial 



C Principal I and ter- 

 I Subordinate J restrial 

 Line stage <j [_ vapour. 



f Terres- 



^ Subordinate -< trial 

 Flutings . . . . [ vapour, 



f Solid and 



Continuous 



" \ liquid. 



fPrincipal "1 Celestial! 



frmcipal 1 L'elest 

 Line stage J Subordinate / gas. 



^Subordinate ") 

 Structure spectrum . }. Terrestrial 



Continuous 



J 



Low temperature. 



We may now bring these results to bear upon oxygen. We learned 

 first from Egeroff that this gas at ordinary temperature and pressure 

 is so molecularly constituted that it produces a fluted absorption in 

 the red part of the spectrum. On account of the constancy of the 



N 2 



