CARBON 



33 



The second class is noted by the partial rigidity of the mole- 

 cule, owing to the two single bonds linking some units. (See 

 fig. 6, Class II.) 



i I I 



T 



Fig. 6. 



The third class is noted by the complete rigidity of the 

 molecule. (See fig. 7, Class III.) 



Fig. 7. 



