in Ferric Salts and in Chromic Acid. 329 



nature of the metal, and is probably greater the more easily 

 oxidizable, i. e. the more electropositive the metal. When 

 this fraction is near unity the surface of the solid will prac- 

 tically consist of readily oxidizable hydrogen atoms which 

 undergo immediate oxidation on suitable contact with a mole- 

 cule of the oxidizer, so that the latter will be used up at the 

 surface of the solid as fast as supplied by diffusion. This 

 explains Case I. When the fraction is below unity the surface 

 is only partly covered and some accumulation of the oxidizer 

 at the surface results, the molecules which do not encounter 

 active hydrogen on contact with the solid reacting slower, or 

 not at all. This gives Case II, or, in the limit, Case III. The 

 oxidation of the hydrogen of an adsorbed acid molecule is fol- 

 lowed by the prompt escape into the liquid of the molecule of 

 metallic sulphate or chloride so formed, since owing to the 

 low concentration of that kind of molecules in the liquid they 

 would be less strongly adsorbed than the acid molecules. 



The fact that in Case II the observed reaction velocities for 

 different metals follow the same order as the electromotive 

 series can be ascribed either to the dependence of the amount 

 of adsorption upon the electromotive behavior of the metal 

 (see above), or to the influence of the secondary reaction, that 

 is, the one which results when a molecule of the oxidizer finds 

 its way into direct contact with the metal. This reaction may 

 reasonably be assumed to be more rapid the more electroposi- 

 tive the metal. 



The view that the velocity of the chemical reaction increases 

 with the acidity, which we found it expedient to adopt in the 

 general discussion of our results, would evidently be a necessary 

 consequence of this adsorption hypothesis. Finally, since this 

 hypothesis involves nothing to limit or alter the role of diffu- 

 sion, the explanation, from the present point of view, of those 

 phenomena which we have ascribed to the influence of diffusion 

 calls for no change. 



In short, it makes little difference here, in employing the 

 diffusion theory, whether we consider the case from the stand- 

 point of adsorption or not. In general, however, the combina- 

 tion of the two points of view should lead us farther than 

 either one alone. 



Status of the Diffusion Theory. Normal and Abnormal Cases. 



An important result of this investigation is the proof of the 

 existence of cases in which we are almost inevitably led to the 

 conclusion that the reaction at the boundary surface is one of 

 limited velocity. Opinions may differ as to whether these 



