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XLIII. On the Doctrine of Uniform and Constant Saturation. 

 By J. Alfred Wanklyn, Esq.* 



WITHIN the last ten years a new feature of great philoso- 

 phical interest has made its appearance in organic che- 

 mistry. It has gradually come to be recognized that the com- 

 pounds belonging to organic chemistry, notwithstanding their 

 numbers and diversity, have certain points of resemblance in 

 their structure. And so invariable is this resemblance, that 

 the chemist can confidently predict that many forms of structure 

 which may be proposed are incapable of being realized. Obvi- 

 ously a knowledge of that which is essential to the existence of 

 a chemical structure must involve the power of specifying hypo- 

 thetical forms of structure which, not possessing these charac- 

 teristics, are impossible structures. And the certainty with 

 which the chemist is able to designate an impossible kind of 

 structure, and the number and variety of the structures which 

 may be marked as impossible, must be excellent signs of the 

 degree in which chemical structure is understood. 



From this point of view the progress of chemistry during the 

 last ten years must be regarded as very great ; for the chemist is 

 now able to designate as impossible, numbers of structures the 

 occurrence of which ten years ago would hardly have surprised 

 him. Thus, for example, the forms 



C 3 H 7 , 



C 5 H 14 , 



C 2 H 2 5 , 



C 4 H 6 N 



are recognized as impossible structures ; and the list might be 

 indefinitely extended. 



In investigating the nature of the principles in virtue of which 

 these and similar forms are pronounced to be impossible, it will 

 at once strike the reader that these examples do not offer electro- 

 chemical difficulties. They are not cases of want of sufficient 

 electro-chemical contrast ; and indeed experiment has lent very 

 little support to the otherwise not unreasonable supposition, that 

 groups of atoms too nearly alike in electro-chemical character, 

 or belonging too much to the one or other extreme of the elec- 

 tro-chemical scale, should constitute impossible forms of combi- 

 nation. Hypochlorous acid (C1 2 0) exists, and yet both oxygen 

 and chlorine occupy the negative extreme of the electro-chemical 

 scale. Perchloric acid (HC1 O 4 ), although containing so little 

 positive hydrogen to counterbalance so very negative a grouping 

 as CI O 4 , is a compound endowed with considerable stability. 

 And at the extreme positive end of the scale much the same 

 * Communicated by the Author. 



