122 MOLECULAR STRUCTURE 



lie in a plane, and the configuration of a compound, 

 C(ab)C(cd), is expressible by the formula 



aCb aCb 



II or || 



cCd dCc 



which shows at the same time that there are two possibili- 

 ties, and that whether the two atoms or groups ab 

 combined with carbon are the same as or 

 different from cd. It is well known that 

 in this case also isomerism is found, and 

 we may discuss this more fully by con- 

 sidering fumaric and maleic acid. These 

 two acids are expressible by the formulae 



COOHCH COOHCH 



II and || 



COOHCH HCCOOH 



and correspond in their behaviour to what 

 might be expected from the difference between these con- 

 figurations. 



In the first place, we are dealing with an isomerism of 

 quite a different kind to that due to a single asymmetric 

 carbon atom. The formulae are symmetrical, and so there 

 is no optical activity and enantiomorphism of crystalline 

 form to be expected ; but, on the other hand, we must not 

 expect the exact agreement in properties specific gravity, 

 melting point, &c. which is produced by the agreement 

 in internal structure when isomerism is due to an 

 asymmetric carbon atom. If we follow out the differences 

 further, we find them in very good accord with the 

 conception of their internal structure arrived at. In the 

 first place, one of the acids, maleic, gives an anhydride 

 very easily : HC CO 



II >0 

 HC-C 



which is in accordance with the proximity of the two 

 carboxyl groups in the formula. In the second place, this 



