PRESENT PROBLEMS 259 



"look into this." I remembered the circumstance which led to the 

 annotation. About ten years before, one of my students had investi- 

 gated the direct combination of nitrogen and hydrogen, and I had 

 read Cavendish's memoir on that occasion. I -mention this fact to 

 show that, for some reason which I forget, a line of work was not 

 followed up, which would have been attended by most interesting 

 results; one does not always follow the clue which yields results 

 of the greatest interest. I regard it therefore as an impossible task 

 to indicate the lines on which research should be carried out. All 

 that I can do is to call attention to certain problems awaiting solution ; 

 but their relative importance must necessarily be a matter of personal 

 bias, and others might with perhaps greater right suggest wholly 

 different problems. 



The fundamental task of inorganic chemistry is still connected 

 with the classification of elements and compounds. The investigation 

 of the classification of carbon compounds forms the field of organic 

 chemistry; while general or physical chemistry deals with the laws 

 of reaction, and the influence of various forms of energy in furthering 

 or hindering chemical change. And classification centres at present 

 in the periodic arrangement of the elements, according to the order 

 of their atomic weights. Whatever changes in our views may be con- 

 cealed in the lap of the future, this great generalization, due to New- 

 lands, Lothar Meyer, and Mendeleeff, will always retain a place, 

 perhaps the prominent place, in chemical science. 



Now it is certain that no attempt to reduce the irregular regu- 

 larity of the atomic weights to a mathematical expression has suc- 

 ceeded; and it is, in my opinion, very unlikely that any such expres- 

 sion, 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 fac- 

 tors of kinetic and of gravitational 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; entropy 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 con- 

 nected 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 incomplete concordance? Having 



