468 SCIENCE PROGRESS 



In the first case the two groups R lie one above the other, while 

 in the second figure R lies over R^ ; and since free rotation of 

 the carbon atoms is assumed to be precluded by the nature of the 

 double bond, no change in configuration can take place under 

 ordinary conditions. It should be noted that all four groups 

 RRR^ R^ lie in one plane, so that no isomerism due to enantio- 

 morphous arrangements of atoms is to be expected, nor have 

 these isomers any action upon the plane of polarisation. To 

 distinguish these isomers from those containing asymmetric 

 carbon atoms it is usual to term them geometrical isomers, 

 as distinct from optical isomers. 



The best known case of geometrical isomerism is that of 

 maleic and fumaric acids, which may be represented thus, if 

 the tetrahedra be omitted : 



H-C-COOH COOH-C-H 



II 

 H-C-COOH H-C-CpOH 



Maleic Acid Fumaric Acid 



When maleic acid is heated above its melting-point it is converted 

 into fumaric acid. The former body is therefore termed the 

 labile form, while fumaric acid is known as the stable isomer. As 

 in the case of optical isomers, we may give here a table showing 

 the principal ph3'sical properties of maleic and fumaric acids : 



Maleic Acid Fumaric Acid 



Melting-point 130° Sublimes unchanged 



Affinity Constant . . . . ri? o'o93 



Specific Gravity . . . . i"590 i'625 



Solubility in water . . . .1:2 1:150 



An extension of the hypothesis of geometrical isomerism to 

 nitrogen compounds was made in 1890 by Hantzsch and Werner.^ 

 They assumed that when a carbon atom is doubly linked to a 

 nitrogen atom, the third valency of the nitrogen atom is directed 

 at a certain angle to the other two. 



Now, in the compound 



g'>C = N-X, 



it is obvious that X may be supposed to lie near R or R^; so 

 that in this case also we have the possibility of two geometrical 

 isomers : 



R-C-R' R-C-R' 



II II 



N-X X-N 



' Hantzsch and Werner, Ber., 23, n (1890). 



