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SCIENCE PROGRESS. 



was long ago foreseen by it. The figures show also how 

 " cis-trans " isomerism occurs in ring-compounds ; when R' 

 changes places with R" we get the other isomer. 1 



Plate III. 



Among tetramethylene derivatives this kind of isomerism 

 is encountered in the truxillic acids 



C 6 H 5 CH - CHCCXH QH 5 CH _ CHCO.H 



and 



QH 5 CH - CHCO.H C0 2 HCH - CHC 6 H 5 

 of which at least four isomers are already known, and for 

 which the theory foresees the existence of many more. 



In every case where the R groups become identical, the 

 isomerism vanishes ; thus in the monosubstituted poly- 

 methylene derivatives it is non-existent. Again, the theory 

 is borne out by the hexamethylene derivatives, hydro- 

 terephthalic and hydrophthalic acids, which may be shown 



in skeleton, thus : — 

 co z h 



C0 2 H 



Hy droterepht Italic acid. 



C0 2 H y C0 2 H 



Hydrophthalic acid. 

 It will be seen that in the last case enantiomorphism exists, 

 together with the " cis-trans " isomerisms, a complication 

 Similarly in Plate III., hydrophthalic acid, if C0 2 H and H be transposed. 



