176 INORGANIC EVOLUTION. [CHAP. 



that chemists when hypothetically considering the possible dissociation 

 of the chemical elements would favour the view of depolymerisation ; 

 that is, the breaking up of a substance A into finer forms (a) weighed 

 by A/2 (or A/3), rather than a simplification of A into x and y. 



The method of attacking this problem from the chemical point of 

 view in the first instance, must be a somewhat indirect one. 



The Stars and the Periodic Law. 



In Chap. XXI I referred to the important hypothesis put forward 

 by Newlands, Mendeleef and others in relation to the so-called " periodic 

 law," which law indicates that certain chemical characteristics of the 

 elements are related to their atomic weights. 



It will be well now to study this question with a view of discussing 

 it more fully in the light of all the facts known to us, among which 

 the stellar evidence and that afforded by the study of series are, I 

 think, of especial importance ; since it may be said that we are now 

 absolutely justified in holding the view that of the lines which make 

 their appearance in the spectra of chemical substances when exposed 

 to relatively high temperatures, a varying proportion is produced b)/ 

 the constituents of the substance, whether it be a compound like the 

 chloride of magnesium, to take an instance, or of magnesium itself. 



Now the periodic law based upon atomic weights deals with each 

 " element " as it exists at a temperature at which the chemist can 

 handle it ; that is, if it be a question, say of magnesium, the chloride or 

 some other compound of the metal must have been broken up, and 1 

 the chlorine entirely got rid of before the pure magnesium is there to 

 handle, and of this pure magnesium the atomic weight is found, and r 

 having also regard to its chemical characteristics, its position in the 

 periodic system determined. 



But if the magnesium be itself compound, the position thus assigned 

 for the element is cei'tain not to tally with the stellar evidence if the 

 temperature of the star from which information relating to it is obtained 

 is high enough to continue the work of dissociation ; that is, to break 

 up magnesium itself into its constituents as certainly as the chloride of 

 magnesium was broken up in the laboratory in the first instance. 



It is now known that dealing with this very substance magnesium, 

 high electric tension brings us in presence of a spectrum which con- 

 sists of at least two sets of lines, numerous ones seen also at the 

 temperature of the arc, and a very restricted number which make their 

 appearance in the spark. 



If this be the work of dissociation and, as I have shown, the 

 proofs are overwhelming the " atomic weight " of the -particle, mole 



