286 On the Doctrine of Uniform and Constant Saturation. 



chemists evolved the theory of compound radicals and the theory 

 of types. The former consisted of a kind of reduplication of 

 the atomic theory, a compound radical being an atom built up 

 of atoms. 



Compound radicals reduced the expressions 



C 4 H 6 O 2 Et H O 



C 4 H 5 C1 to EtCl_ 

 C 8 H 8 4 EtOAO 3 . 



It will be observed that the language of compound radicals 

 expressed the fact that compounds containing certain subordinate 

 groups may be operated upon and changed in a variety of ways 

 without disturbance of the interior arrangement of the subordi- 

 nate group. 



The type was a structure which might be very complicated, 

 but which expressed a variety of different compounds according 

 to the amount of substitution which it underwent. Both theo- 

 ries were embodiments of the uniformities which had been recog- 

 nized between the structures of different compounds. 



But under the theory of compound radicals, the radicals them- 

 selves were regarded as ultimate facts, no attempt being made 

 to deduce their nature and constitution from definite fundamental 

 properties of the elementary atoms of which they were built up. 

 Tn like manner the type was an independent thing, and no limit 

 was hinted at of the numbers or varieties of either radical or 

 type. Although both radical and type were thus made points 

 of departure, the employment of them, and the habits of thought 

 thereby induced, tended continually to the idea of deducing 

 the form of structure of organic compounds from fundamental 

 properties of the elementary atoms. 



Rather more than ten years ago two important steps were 

 taken in this direction ; and to these two attempts we owe the 

 doctrine of uniform and constant saturation such as we have it 

 to-day. The most celebrated of these was Gerhardt's scheme of 

 including all organic compounds under three great types ; viz. 

 Hydrochloric acid, Water, and Ammonia. It was a sort of 

 fusion of the compound-radical with the type theory, organic 

 bodies being brought under these types by the assumption that 

 they contain suitable compound radicals ; e. g., 



HC1 type. H 2 O type. N H 3 type. 



\&W)\ (C*H*)i (CH«J| 



CI J H J w (CH 3 ) In 



(CH 3 )J 

 (C 6 H 5 n (C 6 H 5 )\ n (C 2 H 3 0n 



H J H f H 



H 



