682 POPULAR SCIENCE MONTHLY. 



least since 1865, Kekule's conception of the benzene ring lias been 

 the guiding clew, and it is certain that without the theory the 

 practice would have advanced much more slowly. Out of research 

 for its own sake has come an enrichment of the world, -which in 

 any previous age would have been inconceivable. 



The atomic theory, while replacing speculation in one sense, 

 stimulated it in another. The human mind is always striving to 

 get back of the known, to see what lies beyond the limits of visi- 

 bility, and the conception of an element with its atomic weight 

 opened up a field for the exercise of the imagination. What is an 

 element ultimately? was an early question to ask. Are the ele- 

 ments really diverse, or do they manifest but one fundamental kind 

 of matter? To such queries the atomic weights offered a promis- 

 ing line for investigation, and more than one mind began traveling 

 along it. In 1815 Prout put forth the supposition that all atomic 

 weights were even multiples of that assigned to hydrogen, and over 

 this hypothesis a long warfare has raged. To-day it is practically 

 abandoned by chemists, but the controversy which it provoked 

 led to some of the most accurate investigations in the history 

 of science, and so served to give precision to our knowledge. 

 "Without the instigation of Front's hypothesis, which hinted at 

 hydrogen as the ultimate form of matter, we might have been con- 

 tent with inferior determinations of atomic weight, and chemistry, 

 as an exact science, would have suffered. 



In due time, however, it was perceived that the elements could 

 be arranged in groups, the members of each group having similar 

 properties and forming similar compounds. Serial relations, analo- 

 gous to those discovered among organic compounds, became mani- 

 fest, and much thought was expended in seeking to trace out their 

 meaning. The classification of the elements was more and more 

 seen to be important, and regularities' came to light which at first 

 were unsuspected. Still, no general law, no one guiding principle, 

 could be found so long as the old system of weights and formulae 

 was retained in common usage. 



The adoption of Cannizzaro's atomic weights and the establish- 

 ment of the theory of valence made possible a new attack upon 

 the problem of classification. In 18G4 Newlands arranged the 

 elements in the order of their atomic weights, and showed that at 

 regular intervals there was a periodic recurrence of certain char- 

 acteristics. This observation, which foreshadowed the periodic law, 

 was received with indifference and, to some extent, with ridicule, 

 l)ut the path had been found which soon led to a great discovery. 

 In 1869 Mendelejeff published his celebrated memoir, and the 

 periodic law took its place as a distinct addition to science. Almost 



