138 CHEMICAL PROPERTIES AND COMPOSITION 



a plausible explanation *. Thus, with regard to velocity of 

 reaction, the accelerating effect of oxygen, when acting on 

 a carbon linkage, must first be noted, and its possible cause 

 in a commencement of formation of negative ions. A 

 second result of great importance is that in reactions where 

 carbon linkages are not specially concerned, presence of 

 hydrogen has an accelerating effect. That this cannot be 

 due to a tendency to form positive ions is shown by the 

 stability of the saturated hydrocarbons, so that it has been 

 rightly suggested, from several sides, that another cause 

 must be looked for, such as the small volume or small mass 

 of hydrogen. However the facts now to be discussed may 

 be connected with the preceding ones, these considerations 

 of spatial influences start from an entirely different method 

 of research, which must now be described. 



Already in Part II, p. 122, we have mentioned, as pre- 

 cursor of stereochemistry, explanations, such as that de- 

 veloped mainly by Wislicenus, of the anhydride formation 



of maleic acid, 



HCCOOH 



II 

 HCCOOH 



This is easily seen to result from the closeness of the 

 carboxyl groups, in contradistinction to fumaric acid, 



HCCOOH 



. II 

 COOHCH 



which does not yield an anhydride. 



With this is connected the fact that if in maleic acid the 

 hydrogen is replaced by larger groups, so by the greater 

 space occupied the carboxyls are driven closer together, 

 formation of anhydride is so facilitated that, e.g. pyro- 



cinchonic acid, 



H 3 CCCOOH 



II 

 H 3 CCCOOH 



1 The investigations of Michael ( Journ. f. Prakf. Chem. (2) 37. 473) only 

 reached me after correcting the proof. 



