208 PRESENT PROBLEMS OF INORGANIC CHEMISTRY. 



one of my students luid investigated the direct combination of nitro- 

 gen and hydrogen, and I had read Cavendish's memoir on that occa- 

 sion. I mention this fact to show that for some reason which I for- 

 get, a line of work was not followed up, which would have been 

 attended b}'^ 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 inv^estiga- 

 tion of the classification of carbon compounds forms the field of 

 organic chemistry, while general or j^hysical chemistry deals with 

 the laws of reaction and the influence of various forms of energy in 

 furthering or hindering chemical change. And classification centers 

 at present in the periodic arrangement of the elements, according to 

 the order of their atomic weights. Whatever changes in our views 

 may be concealed in the lap of the future, this great generalization, 

 due to Newlands, Lothar Meyer, and Mendelejev, will always retain 

 a place, perhaps the prominent place, in chemical science. 



Now, it is certain that no attempt to reduce the irregular regularity 

 of the atomic weights to a mathematical expression has succeeded ; 

 and it is, in my opinion, very unlikely that any 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 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 connected with the funda- 

 mental chemical ca])acity, atomic weight, or mass. The periodic 

 arrangement is an attem])t 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, 

 AVhy this incomplete concordance? Having stated the general ques- 

 tion, it may conduce to clearness if some details are given. 



L The variation of molecular surface energy with temperature is 



