86 THE REACTIONS OCCURRING IN SOILS. IV. 



found is about 1:4). The reactions really take place in both 

 directions at the same time, and equilibrium is attained when 

 the velocities of the two opposing reactions are equal. The 

 reaction may therefore be better represented thus : 



K 2 CO 3 +BaSO 4 ^ K 2 SO 4 + BaCO H . 



As has been shown by Guldberg and Waage in 1867, the 

 intensity with which two substances tend to react depends 

 upon 



(1) The product of their active masses, i.e., the number of 



equivalents of each of the substances present in the 

 unit volume. 



(2) Another factor, depending upon the nature of the two sub- 



stances, the temperature, and other physical conditions 

 of the reaction. 



Thus, if m and m 1 be the number of gramme equivalents of 

 the two substances A and B present in the unit volume of the 

 solution, the speed of the reaction would be measured by 

 kxmx m' where fc is a constant. It is obvious that other things 

 being uniform the rapidity of a reaction will depend upon the 

 frequency of collisions between substances which are to react. 

 If the number of equivalents m of one substance be doubled 

 the number of collisions per unit time between the molecules 

 of A with those of B will be doubled. So, too, if m 1 be also 

 doubled, the number of collisions per unit time will be again 

 doubled, so that the speed of the reaction will now be repre- 

 sented by fcx2wx2?7i', i.e., four thn^s as great as before. 



If one of the substances is insoluble in water (or so little 

 soluble that there is always some of it present in the solid 

 state) its active mass is constant and the speed of the reaction 

 then varies only as the active mass of the soluble substance 

 varies. 



Now when two substances react they, as a rule, form two 

 other substances, which may react upon each other, re-forming 

 the original two. This occurs in all so-called reversible re- 

 actions. In such cases equilibrium is attained when the 

 speeds of the reaction in the two directions are equal. Con- 

 sider the case to which allusion has already been made and let 

 a number of equivalents of K. 2 C0 3 per unit volume, b the 

 active mass of BaSO 4 (constant because insoluble), c the 



