330 Van Name and Hill — Solution of Metals 



cases represent real or only apparent exceptions to Nernst's 

 hypothesis that the attainment of equilibrium at the boundary 

 surface between two phases is practically instantaneous. It is 

 after all largely a matter of definitions. The practical bearing 

 of these cases, however, is obvious. They show that we can 

 not assume, as an over-strict interpretation of Nernst'a view- 

 point might lead us to do, that every case in which different 

 solids react at different rates with the same dissolved substance 

 can be ascribed to some tangible and experimentally demon- 

 strable interfering effect.* If the cases in point are due to sec- 

 ondary interference it must be interference of a sort which 

 only Maxwell's demon could recognize as not being a normal 

 part of the mechanism of a reaction of the given type. In 

 order to avoid such fine and elusive distinctions it is much 

 better to admit the existence of phenomena which may or may 

 not be theoretically equivalent, but are certainly practically 

 equivalent, to a limited reaction velocity at the actual boundary 

 surface. 



What, then, is the status of the diffusion theory? It is evi- 

 dent that by this modification the diffusion theory loses a good 

 deal of its simplicity and beauty, but its usefulness is not 

 seriously impaired. We must now admit, as strictly normal, 

 cases in which the influence of diffusion upon the observed rate 

 of the reaction has any value between 100 per cent and zero. 

 It may be that values close to 100 per cent will prove far more 

 common in practice than lower ones, or, in other words, that 

 ]S T ernst's hypothesis is sustained much more often than it is 

 contradicted, but its validity in a given case cannot always be 

 assumed a priori. 



Finally, in employing the diffusion theory we must be on 

 our guard against cases in which the normal relation between 

 diffusion and reaction velocity is disturbed by secondary inter- 

 fering effects. Since it is desirable to be able to predict these 

 exceptions in advance, a knowledge of the different known 

 types, and of their specific effects upon the results, is important. 

 The present investigation has furnished examples of four. 

 These are 



(a) Insolubility of one of the products of the reaction, with 

 consequent formation of a coating on the solid phase. 



(b) Occurrence of the reaction in two stages of which the 

 second is of limited velocity and is not confined to the actual 

 contact surface. 



* If, however, the solid is somewhat soluble in the pure solvent we are 

 really dealing with a two-stage reaction, and agreement between the rates for 

 different solids would not necessarily be expected, even when the second 

 stage is instantaneous. (Example: Solution of different slightly soluble 

 acids in the same dissolved base.) 



