454 SCIENCE PROGRESS. 



In the second arrangement the oscillations of the bromine 

 atoms are evidently less subject to mutual hindrance. 



As in intra-molecular so in inter-molecular transforma- 

 tions the same phenomenon is observed. As one example 

 among many it may be noted that brom-isobutyric ether 

 re-acts with silver in two ways, thus — 



CH 3 CH 3 



C 2 H 5 C0 2 . C— Br + Ag, + Br— C— C0 2 C 3 H 5 = Ag 2 Br 2 , and 



CH 3 CH 3 



CH 3 CH 3 



either I. C 2 H 5 C0 2 . C— C . C0 2 C 2 H 5 , 



ch/ch 3 



j 3 



CH 3 CH 3 



or II. C 2 H 5 C0 2 . C— CH — C. C0 2 C 2 H 5 



CH 3 H 



According to Bischoff the dynamic hypothesis signalises 

 II. as the favoured configuration, and demands that it be 

 formed in larger quantity ; but an answer to the question 

 why the unfavoured configuration should be formed at 

 all is still lacking. We may hope, however, with Bischoff, 

 that in time stereochemistry will enable us to foretell 

 the course of "abnormal" reactions just as structural 

 chemistry enables us to foretell the course of ordinary re- 

 actions, and we shall then be able to improve the models 

 now in use which are deficient as regards the representa- 

 tion of the distances of the atoms from one another. 



In connection with the subject of atomic motion there is 

 to be mentioned in the first place the simple and wide view 

 of Le Bel, which supposes connected atoms or their repul- 

 sive zones to glide freely over the surface of one another in 

 every direction, no directive force acting from one to the 

 other. This is not inconsistent with the limitations which 

 the atoms of a molecule may impose on one another in the 



