256 SCIENCE PROGRESS 



and by the similar figure and its mirror image — 



bV t^ 



where only two such chains are present, A and B being other 

 atoms, groups or molecules. 



In the latter case two structurally different isomers are 

 possible, in which the groups A and B are in cis- and trans- 

 positions respectively. 



'" A 



a 



*J B 



Cis Tra ™ 



Optical activity is only possible in the cis-modification, since 

 in the trans form the structure of the complex is identical with 

 that of its mirror image. 



Compounds of these types are well known, containing as 

 the associating atom cobalt, chromium, rhodium or iron, and, 

 as the connecting chains, ethylene diamine 



(NH 9 -CH 2 -CH a -NH a ) 

 or the oxalic acid residue 



— o.co.co.o— 



the former, ethylene diamine, being able to replace two 

 ammonia molecules in compounds of the type [(NH 3 ) 6 Me]X s 

 or [(NH 3 )JVIeX a ]X, where X is some negative atom or group 

 such as a halogen atom or N0 2 , and the latter, the oxalic 

 residue, being able to replace two negative groups in com- 

 pounds of the type [X 6 Me]M' 3 , where M' is a monovalent 

 metallic atom such as that of potassium. 



Werner, to whom the development of the subject is almost 

 wholly due, has resolved many of these compounds, employing 

 for the purpose Pasteur's second method — that is, replacement 

 of the ionisable X or M' by some active acid or base, the separa- 



