ALLOTROPISM BASED ON THE THEORY OF DIRECTIVE VALENCE. 327 



(2) 



♦>p -3 p-. 



I T 

 -,p 1= p„ 



This is Case A2, in which the single bonds travel in the 

 same direction, but in a direction opposite to that in which 

 the double bonds travel. We have also 

 (3) 



P<=* P." 



T I 



A modification of Case D. (supra) in which the individual 

 bonds in each pair of double bonds travel in opposite direc- 

 tions, (a and b) and the single bonds travel in the one 

 direction (a). This is identical with figure (1). 



(4) 



I T 



In which the double bonds travel as in (3), and the single 

 bonds travel both in the same direction, (b) opposite to 

 that in which they travel in Figure 3. This is also 

 identical with Figure 1. 



Figures 3 and 4 are identical with figure 1, so that there 

 remain only the two possible combinations, figures 1 and 2 

 which result in symmetrical molecular structures. These 

 may be assumed to represent the yellow and red varieties 

 respectively. All other combinations result in unsym- 



