THE STORY OF NINETEENTH-CENTURY SCIENCE 



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 integrity 

 of the original compound much as a child might sub- 

 stitute 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 hy- 

 drogen 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 inconsistency 

 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 molec- 

 ular structure were much exploited, and, like the doc- 

 trine of compound radicals, became useful as aids to 

 memory and guides for the analyst, indicating some of 

 the plans of molecular construction, though by no means 

 penetrating the mysteries of chemical affinity. They 



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