BEHAVIOUR OF IODONIUM BASES 143 



sulphur, ammonium and sulphonium bases, should appear 

 especially suited to form iodonium bases ; 



4. The new compound shows the greatest similarity to 

 derivatives of certain heavy metals, especially thallium. 



But without the appearance of new valencies certain 

 atomic groupings show quite striking and unexpected 

 chemical properties. We need only think of the discovery 

 by Demole of the easy oxidation of unsymmetrical dibrom- 

 ethylene, H 2 CCBr 2 , and its analogues, which are capable 

 of taking up oxygen at atmospheric temperature, with 

 formation, in the case named, of bromacetyl bromide. 

 H 2 CBrCOBr. Or, finally, of the vista opened up by Victor 

 Meyer's discovery of thiophene, C 4 H 4 S, and -the fact that 

 the replacement of the group C 2 H 2 in benzene by sulphur- 

 changes its properties so little that the preparation and 

 properties of thiophene derivatives in part agree almost 

 word for word with those of benzene. 



Starting at the beginning of this work with the simple 

 phenomena of chemical equilibrium, which are associated 

 with the fundamental laws of physics, we have arrived, 

 here, at the close of it, at those mysterious relations among 

 which only the wonderful instinct of the chemist can find 

 the way. 



