PASSAGE INTO SOLUTION 277 



dissolved crystalloid out of this film into the body of the fluid. 

 This leads to the equation: 



log -^_ = S A*, (i) 



where a is the concentration of a saturated solution of the crys- 

 talloid, x the concentration of the solution at time t, s the area 

 of the surface of the crystalloid which is exposed to the action 

 of the solvent and k a constant which varies with the rate of 

 stirring and with the rate of diffusion of the crystalloid (11), 

 and, consequently, with the temperature. 



On the other hand, Boguski (4) (5) and Veley (104) (105) 

 have shown that the rate of solution of basic substances such 

 as marble or zinc in acids, a process involving chemical inter- 

 action, is proportional at each instant to the concentration of 

 the unneutralized acid and to the surface of the solid. Upon 

 the supposition that the surface remains appreciably constant 

 in area during the period occupied by the experiment (a readily 

 realizable condition) this leads to the equation : 



log -r^-- = fat, (ii) 



Cl ~~ */ 



which is identical with the above, save that a is, in this case, 

 the initial concentration of the acid. In this case also Nernst 

 believes (60) the velocity of solution is determined by a diffusion- 

 velocity, namely the velocity with which the acid diffuses into 

 the film in contact with the solid. Naturally, were the velocity 

 of the chemical interaction low in comparison with the velocity 

 of diffusion of the acid (or any reagent playing a similar part) 

 the determining velocity would be a chemical and not a diffusion 

 velocity. An examination of this case in the light of the mass 

 law shows, however, that the form of the relation between time 

 and mass dissolved would be the same as that expressed in 

 equation (ii).* 



In the light of what has been said concerning the indefinite 

 solubility of the caseinates in water, it might be anticipated 

 that equation (i) would not apply to the dissolving of solid casein 

 in alkaline water. It is quite imaginable, however, that equa- 

 tion (ii) might apply, a, in that equation, being the number of 



* Provided only one molecule of acid (or other dissolved reagent) took part 

 in the reaction. 



