34 POPULAR SCIENCE MONTHLY. 



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 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 con- 

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

 lands, Lothar Meyer and Mendelejev, will always retain a place, per- 

 haps 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 con- 

 veyed 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 rela- 

 tion to each other; and while the attempt is in so far a success, inas- 

 much as it is evident that some law is indicated, the divergences are 

 such as to show that finality has not been attained. The central prob- 

 lem in inorganic chemistry is to answer the question — why this incom- 

 plete concordance? Having stated the general question, it may con- 

 duce to clearness if some details are given. 



1. The variation of molecular surface energy with temperature is 

 such that the surface-energy, for equal numbers of molecules distrib- 

 uted over a surface, is equal for equal intervals of temperature below 

 the temperature at which surface-energy is zero — that is, the critical 

 point. This gives a means of determining the molecular weights of 

 liquids, and we assume that the molecular weight of a compound is 

 accurately the sum of the atomic weights of the constituent elements. 



