LATER YEARS 177 



such expression, of not insuperable complexity, and having a 

 basis of physical meaning, will ever be found. I have already, 

 in an address to the German Association at Cassel, given an 

 outline of the grand problem which awaits solution. It can be 

 shortly stated then : While the factors of kinetic and of gravita- 

 tional energy, velocity, and momentum on the one hand and force 

 and distance on the other are simply related to each other, the 

 capacity factors of other forms of energy surface in the case 

 of surface energy ; volume in the case of volume energy ; en- 

 tropy for heat ; electric capacity when electric charges are being 

 conveyed by means of ions ; atomic weight when chemical 

 energy is being gained or lost all these are simply connected 

 with the fundamental chemical capacity, atomic weight or mass. 

 The periodic arrangement is an attempt to bring the two sets 

 of capacity factors into a simple relation to each other, and 

 while the attempt is in so far a success, inasmuch as it is evident 

 that some law is indicated, the divergences are such as to show 

 that finality has not been attained. The central problem in 

 inorganic chemistry is to answer the question, Why this incom- 

 plete concordance ? " 



This question still remains incompletely answered 

 notwithstanding the important advances which have 

 resulted from the observations, especially of Soddy, 

 Fajans, Fleck and Russell, on the existence and pro- 

 perties of isotropic elements, that is, of elements which 

 are not identical as to atomic weight though occupying 

 the same place in the periodic scheme and are not 

 separable by chemical means from one another. Two \ 

 varieties of lead, for instance, exist in nature. The 

 skill of the best workers on atomic weights has thus 

 been severely tested. 



There are many other questions raised in this address 



