A HISTORY OF SCIENCE 



with an atom of oxygen, announcing this theory in 

 1818. Ten years later, Liebig and Wohler undertook 

 a joint investigation which resulted in proving that 

 compound radicals are indeed very abundant among 

 organic substances. Thus the theory of Berzelius 

 seemed to be substantiated, and organic chemistry 

 came to be defined as the chemistry of compound 

 radicals. 



But even in the day of its seeming triumph the dual- 

 istic theory was destined to receive a rude shock. This 

 came about through the investigations of Dumas, who 

 proved that in a certain organic substance an atom of 

 hydrogen may be removed and an atom of chlorine 

 substituted in its place without destroying the integ- 

 rity of the original compound much as a child might 

 substitute one block for another in its play-house. 

 Such a substitution would be quite consistent with the 

 dualistic theory, were it not for the very essential fact 

 that hydrogen is a powerfully electro-positive element, 

 while chlorine is as strongly electro-negative. Hence 

 the compound radical which united successively with 

 these two elements must itself be at one time electro- 

 positive, at another electro-negative a seeming in- 

 consistency which threw the entire Berzelian theory 

 into disfavor. 



In its place there was elaborated, chiefly through the 

 efforts of Laurent and Gerhardt, a conception of the 

 molecule as a unitary structure, built up through the 

 aggregation of various atoms, in accordance with " elec- 

 tive affinities " whose nature is not yet understood. A 

 doctrine of "nuclei" and a doctrine of "types" of 

 molecular structure were much exploited, and, like the 



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