224 MODERN SCIENCE READER 



idea of radicles. No objection could be raised against the 

 latter idea, once it were freed from all unnecessary assoc ; a- 

 tions, especially from the belief that radicles were unalter- 

 able substances capable of independent existence. To say 

 that benzoyl chloride C 7 H 5 OC1 ; benzoic acid, C 7 H 6 2 ; and 

 benzamide, C 7 H 7 ON, contain in common the benzoyl radi- 

 cle i.e. the group of atoms, C 7 H 5 was only to express 

 what was evident from their formulae. On the other hand, 

 the recognition of radicles must obviously lead to the dis- 

 covery of the relationship of compounds, and thus, together 

 with the phenomena of substitution, guide in grouping 

 compounds in accordance with the idea of types. In 1849 

 Wurtz and Hofmann discovered a series of compounds that 

 bore an unmistakable resemblance to ordinary ammonia, 

 and could be considered as ammonia in which one or more 

 hydrogen atoms were replaced by radicles. They proposed 

 to group them together as belonging to the "ammonia 

 type." In 1850 Williamson showed that alcohols, ethers, 

 and acids could be referred to the "water type." Ordinary 

 alcohol, for instance, whose formula is C 2 H 6 0, could be con- 

 sidered as water, H 2 0, in which one hydrogen atom has been 

 replaced by the ethyl radicle, C 2 H 5 . Ordinary ether, 

 C 4 H 10 0, could be considered as water, H 2 0, in which two 

 hydrogen atoms have been replaced by two ethyl radicles, 

 ether being thus (C 2 H 5 ) 2 0. Acetic acid C 2 H 4 2 could be 

 considered as water, H 2 0, in which one hydrogen atom has 

 been replaced by the radicle C 2 H 3 0. Now, ether, (C 2 H 5 ) 2 

 was obtained from alcohol C 2 H 5 HO, by the use of dehydrat- 

 ing agents. Williamson therefore held, by analogy, that it 

 ought to be possible to transform acetic acid, C 2 H 3 O.HO 

 into a compound (C 2 H 3 0) 2 0. When in 1852, Frankland 

 actually succeeded in effecting this transformation by the 

 use of dehydrating agents, the usefulness of the type 

 theory was demonstrated. For nothing is more striking 

 proof of the value of a theory than its capacity for reveal- 

 ing unknown facts. To the types ammonia and water 

 Gerhardt added the types hydrogen and h3 T drochloric acid, 



