PROGRESS IN NINETEENTH CENTURY 227 



ward order reigned. The full significance of valence could not appear 

 until the old system of chemical equivalents had been set aside. 



Naturally, as the mass of chemical data increased, specialism 

 became necessary. No man could expect to know the whole of chem- 

 istry; a small part of it was all that any one could handle, and the 

 inevitable results followed. A specialist may be broad, but the direct 

 tendency of specialism is to narrow one's field of view, and to con- 

 centrate the attention upon details rather than generalities. The 

 theories which fit immediate conditions then become satisfactory, and 

 the chance that they may be only partial glimpses of greater laws is 

 disregarded. Only the stronger and more philosophical minds can 

 escape these limitations and see things in their larger aspects. 



To the organic specialist, at least in most cases, the doctrine of 

 valence was adequate ; for it explained the combinations with which 

 he had to deal. Relatively few of the chemical elements were seri- 

 ously considered by him, and they offered no insuperable difficulties. 

 Carbon was the typical element, the key to all organic matter; its 

 quadrivalency in terms of the hydrogen unit was assured; its ability 

 to unite with itself in chains or rings was established; with these 

 data constitutional formulae became truly significant, and useful for 

 the correlation of existing knowledge. Even more can be said in 

 their favor, for they had a certain prophetic ability which guided 

 research and foretold discovery. But, after all, carbon was only one 

 among many elements, and nobody was justified in assuming that 

 its modes of combination represented general laws, or that ideas 

 drawn from the study of organic matter alone were applicable else- 

 where. The theory of valence must be tested with regard to all 

 the elements before its full validity could be recognized, and that 

 test implied a renewal of interest in inorganic problems. It was neces- 

 sary to discriminate between special cases and fundamental principles, 

 and so a much larger field than organic chemistry could offer had to 

 be surveyed. Clues had been found in the study of carbon compounds, 

 but where were they to lead? 



So far as actual knowledge went, the chemical elements were dis- 

 tinct entities, and speculation as to their nature had been looked 

 upon generally with disfavor. And yet they had points in common 

 which rendered their classification possible, and it was perfectly 

 evident that they could be arranged in a small number of natural 

 groups. Certain elements were obviously types of others; some were 

 isomorphous, as shown by Mitscherlich, and some exhibited serial 

 relations as in Dobereiner's triads; but no one scheme of classifica- 

 tion covered the entire ground. Analogies were numerous enough, 

 but their meaning was not clear. A process of evolution was at work, 

 however, and in due time it culminated in Mendelejeff's develop- 

 ment of the periodic system. All partial classifications, all the dim 



