218 CHEMICAL DISCOVERY AND INVENTION 



Van 't Hoff long ago indicated that when two carbon atoms 

 are united together by a single bond they may be supposed to be 

 free to rotate about an axis which is in the line representing the 



FIG. 60. 



direction of the uniting valencies, and that if two carbons are 

 joined by two or more bonds rotation becomes impossible. This 

 hypothesis was first made use of by Wislicenus in 1886, and has 

 been the subject of a good deal of discussion since. 



It may be assumed that the radicles united to two adjacent 

 atoms of carbon will be likely to influence each other, and, ac- 

 cording as they attract or repel each other, rotation may or may 

 not occur. Thus it may be supposed that in ethylene dichloride, 

 C 2 H 4 C1 2 , chlorine and hydrogen atoms probably attract each 

 other. Hence there can be only one stable form of this 

 compound, viz. : 



H 

 H\ ] /Cl 



other arrangements passing spontaneously by rotation into this. 

 Succinic acid also is known in only one foim, which piobably 

 has the following structure : 



