RICHARDS. — SOLID SOLUTIONS. 381 



Turning to the results of Kiister and ThiePs work, we find other 

 arguments indicating the same thing. On pages 439-440 of their third 

 paper they point out that the addition of acid diminishes very much the 

 occlusion of the iron complex. This is wholly consistent with the present 

 thesis : for (according to the Mass Law) the addition of acid must 

 diminish the concentration of the basic complex in the solution, and 

 hence (according to the distribution law) the amount which finds its way 

 into the solid. It is true that this argument is somewhat diminished in 

 force by two circumstances : first, because Kiister and Thiel have not 

 here considered the fact that hydrochloric acid increases the weight of 

 the precipitate by introducing into it more baric chloride ; and second, 

 because the addition of hydrochloric acid undoubtedly causes a chloride- 

 complex which is not occluded to nearly so great an extent as the sulphate- 

 complex by the baric sulphate. In order really to study the effect of 

 the hydrogen ion one should not rely simply upon the total weight of 

 the mixed precipitate, but should actually determine the amount of iron 

 present in the precipitate and compare it with the amount present in a 

 precipitate obtained from a solution containing the same amount of iron 

 and an amount of sodic chloride equivalent to the hydrochloric acid 

 used in the first case. In spite of these objections, however, the effect 

 observed by Kiister and Thiel is so great that one must ascribe a 

 portion of it to the action of the hydrogen ion, and hence, according to 

 Whitney and Recoura. infer the substance concerned to be a basic com- 

 plex. Kiister and Thiel offered no explanation for the action of the 

 hydrogen ion. 



Again. Kiister and Thiel point out (pp. 437-438) that the hydrolysis 

 caused by dilution prevents the addition of much water from causing as 

 considerable a decrease in the absorbed material as would otherwise have 

 been expected. This is equivalent to saying that the complex which is 

 concerned is a basic one : for the hydrolysis undoubtedly involves the 

 ionization of hydrogen. 



Yet another, although less cogent, argument in favor of the basic 

 nature of the complex is the fact that Jannasch and I found the impure 

 baric sulphate to be capable of retaining about three per cent of water at 

 250°. While it is not impossible that this should be at least in part 

 simply held in minute cells or as crystal-water, one is inclined to 

 ascribe part of it to hydroxyl existing in the precipitate. It may 

 well be, however, that the extra water is simply dissolved in the 

 precipitate, as other uudissociated substances are dissolved by baric 

 sulphate. This tendency to dissolve water may be the reason why 



