CHAP. XXIV.] FROM A PHYSICAL STANDPOINT. 185 



Imagine a series of substances " chemically " different, the intrinsic 

 difference of which, from A the simplest to Z the most complex, really 

 consists merely of their being built up of different numbers of units. 

 When Z is simplified by heat, its complex system of centre of force 

 and ion with their electric charges will undergo changes which we may 

 expect to result in the formation of less complex systems doubtless 

 built on a like pattern, and therefore capable of producing spectra ; 

 hence we are bound to see the spectra of some of the intermediate 

 forms which, when they are stable and go about in company, it may 

 well be that physicists have already recognised. These we may call 

 B or C, or R or S, or X or Y, as representatives of various com- 

 plexities. 



The more complex the form experimented on and the higher the 

 temperature employed m the laboratory, the more spectral lines 

 indicating different chemical " elements " in intermediate stages may 

 we see. 



I say in the laboratory, because in the stars the result will be dif- 

 ferent. There, in consequence of the long continued action of heat 

 and the shielding of the reversing layer from the effects of lower tem- 

 perature, we may only see at the highest temperature the spectra of 

 the forms A and near A. We now know what these are. 



To take another case ; let us assume that the electric charges or 

 arrangement, as well as the number of the units of matter, may vary. 

 Under these conditions, when we dissociate Z, not all, but only some, 

 of possible intermediate forms may be expected to afford spectral 

 evidence. Say, to take an example, those in the vertical columns of 

 Mendeleef 's table ; and I am led to make this suggestion, because 

 Kayser has shown that in " series " the duplicity or triplicity of lines 

 is associated with the position of the elements producing them in these 

 columns. A concrete case would be afforded by contrasting the be- 

 haviour of sodium and caesium, representing relatively simple and 

 complex substances. We might observe the lines of sodium when 

 caesium is dissociated ; we should not expect to see the lines of caesium 

 when sodium is dissociated. 



The two cases taken it is possible may illustrate the difference 

 between related and not related groups of " elements." 



The apparently constant appearance of representative lines of the 

 spectrum of one substance of a group in that of the other members of 

 the same group may be thus explained, although it has generally been 

 attributed to the presence of impurities, as in the case of all common 

 long lines seen in spectra ; and this -in spite of the protest that if the 

 purest specimens known (I have worked on beads of Stas' silver which 

 had never been touched) were so impure, some of the decimals used to 



