412 SCIENTIFIC THOUGHT. 



certain stable qualities with a multitude of changing 

 varieties was a notion familiar to other branches of natural 

 history. The idea of substituting one element for another 

 gave the death-blow to the theory of Berzelius, which 

 assumed that elements paired with each other, according 

 to some polar contrast. It was found, for instance, that 

 the element chlorine, which stood on one side of the 

 scale the electro-negative could take the place of the 

 opposite electro-positive element hydrogen. 



In the course of time the conception of types was much 

 changed, and became more and more complicated ; it had 

 however the effect of finally destroying the binary view of 

 chemical composition, and restoring in its place the older 

 unitary conception. 



All these attempts to simplify the study of chemical 

 compounds, by reducing them to simple or complex ele- 

 ments, or to pairs of simpler combinations, or by ranging 

 them according to types, were useful in many ways in 

 extending the knowledge of bodies, in indicating new 

 methods of inquiry, and in suggesting instructive experi- 

 ments : l none of them were universally accepted in the 



1 About that time so far as 

 chemistry proper, i.e., the study of 

 compounds and of reactions was 

 concerned there existed two main 

 currents of thought, the most illus- 

 trious and influential representa- 

 tives of which were Kekule (1829- 

 96, first professor at Ghent, then 

 since 1865 at Bonn), and Kolbe 

 (1818-1884, first professor at Mar- 

 burg, then since 1865 at Leipsic). 



Liebig, Wohler, and Bunsen. To 

 them as a third can be added the 

 name of A. W. von Hofmann (1818- 

 1892), who, through his twenty 

 years' residence in London, did much 

 to introduce a knowledge of German 

 chemistry and German teaching 

 methods in England, and who from 

 1865 established the modern Berlin 

 school of chemistry. It would be 

 impossible to enter here into details 



As teachers and centres of aca- ! as to how mainly through the influ- 



demic influence, though in differ- ence of these three men the work 



ent, frequently opposite directions. begun by Liebig and Wohler was 



these two eminent men continued j extended, and how especially also 



the work started in Germany by . the great development of chemical 



