Physical and Chemical Behavior of Solids. 243 



accepted theory of solution, by the relative solubilities of 

 BaC0 3 and BaS0 4 under the particular conditions of experi- 

 ment, or in other words, by the relative concentrations of car- 

 bonate ion and sulphate ion in the solution. Equilibrium 

 conditions are reached with comparative slowness; but in any 

 given length of time — as, for instance, the time required to 

 wash away the soluble material, as in Spring's method of 

 analysis — the amount transformed would depend primarily on 

 the intimacy of contact of the particles, and would tend 

 towards the amount corresponding to equilibrium conditions. 

 This then is sufficient explanation of the results of successive 

 alternate compressions and pulverizations, and of the apparent 

 limiting amount transformed ; as pointed out by Spring him- 

 self, " this result demonstrates the influence exercised by the 

 renewal of the surfaces of contact of reacting substances upon 

 the amount of the reaction product," 



These observations then cannot be used as evidence in 

 support of the contention that compression favors chemical 

 reaction ; they do indicate, however, the possibility of inter- 

 diffusion of the solid substances after compression. Thus the 

 tables show that the apparent amount transformed in the com- 

 pressed blocks gradually increased with time ; this increase we 

 attribute to more intimate mixture of the solid phases — for 

 this would further the apparent extent of the reaction when 

 water was added — and the most probable cause of this is to be 

 sought in a slow process of diffusion* occurring between the 

 grains which have been brought into intimate contact by the 

 previous compression. 



Before passing, one point remains to be noticed ; namely that 

 the compressed cylinders heated for six hours to 160° showed 

 less transformation than those not so heated. Now it would 

 be easy to devise several hypotheses to account for this, but it 

 is quite unnecessary to do so, because this shows only that on 

 heating some factor intervened which influenced the contact 

 between the particles, and because it can afford us no informa- 

 tion whatever about the actual state of the solid system. 



2. The Reaction between Potassium Nitrate and Sodium Acetate. 



Equally unconvincing is the evidence for the reaction between 

 solid potassium nitrate and sodium acetate. f Spring expected 

 an interchange of bases and acids, and left the mixture of the 

 dry fine powders for four months in a desiccator. On remov- 

 ing them from the desiccator a deliquescence was noticeable ; 

 from which he concludes that the interchange had taken place, 

 since the original salts do not easily deliquesce, but the product 

 * Cf. p. 248. f Spring, Zs. phys. Chem., ii, 537, 1888. 



