Dr. W. Ramsay on Picoline and its Derivatives. 27 



Theoretical considerations. — If the formula N 



proposed for pyridine by Alder Wright, be u/ ^pt-t 

 accepted as correct, six isomeric acids of the „ . 



formula C 5 H 3 N (CO . OH) 2 are theoreti- HC CH 

 cally possible. They are as follows : — ^ptt 



Pyridine. 



1. 2. 



H(f ^C-CO . OH HC \lH 



HC 6-00. OH HC C-CO.OH 



CH C-CO . OH 



3. 4. 



N N 



HO . OC-d" *VcO . OH KU \l-CO . OH 



ii i li l 



HC CH HC CH 



\ S \ S 



CH C-CO . OH 



5. 6. 



N N 



B.G \h Hcf X C-CO . OH 



HO.OC-C C-CO.OH HO.OC-C CH 

 \ // \ // 



CH CH 



Even if structural formulas be discarded, it is evident from 

 the analogy between pyridine and chinoline, and benzol and 

 naphthalene, that a number of isomerides are to be expected 

 when two external groups replace two atoms of hydrogen in 

 pyridine. In the case of benzol, three isomerides are known 

 under such conditions ; and it is to be expected that pyridine, 

 a more complicated substance than benzol, owing to its con- 

 taining an atom of nitrogen, should yield more than three 

 isomeric bodies. 



Now, that a-, /5-, and 7-dicarbopyridenic acids are isomeric, 

 and not identical, is rendered highly probable by the following 

 considerations : — the /3-acid is much less soluble in water than 

 the other two modifications ; as a rule its salts are much less 

 soluble : its crystalline form is different. Copper sulphate 

 produces no precipitate with the a-acid, whereas the other two 



