XXIV.] FROM A PHYSICAL STANDPOINT. 191 



complex systems. These various first forms bring about the possi- 

 bility of evolution along several parallel lines, as well as of the possi- 

 bility of an infinite number of intererossings. In this connection we 

 must not forget that the constituents of the reversing layer of Bellatrix 

 and of protoplasm are nearly identical, while the particular forms of 

 matter of which they are composed make so little show in the sun. 



A consideration of the central congeries of material units and the 

 ion revolving round it, suggests that the ion may be the more con- 

 stant in its structure, and that it is to a large extent to the varying 

 mass and charge representing the centre of force that spectral changes 

 are due. It may be that the subordinate " series " indicate that very 

 small variations of complexity are possible, as well as greater ones. 



The ions visible in the simple spectra of the hottest stars may be 

 those associated with the smallest centres of force. These are, so far as 

 we know at present, hydrogen, helium, asterium, oxygen and nitrogen 

 among the gases ; carbon and silicium, and calcium, magnesium, and 

 sodium among the metals in the forms we study by their spectra at 

 the highest temperatures we can employ in our laboratories. 



As the stars cool larger aggregates of material units in the centres 

 of force round which these ions revolve become possible, and hence 

 the complexity of the spectrum of uranium and of the sun, repre- 

 senting a cool star, are both explained by the same process, the various 

 stages of which can be reproduced in the reverse direction by various 

 degrees of dissociation. 



