MASS ACTION. 87 



number of equivalents of K. 2 SO 4 , and d = the active mass of 

 BaC0 3 (again constant). Then the condition for equilibrium 

 would be 



k (a-x) (b-x) = k 1 (c + x) (d+x) 



but since b and d are constant they are not altered by the 

 subtraction or addition of x, which represents the number of 

 equivalents of K 2 CO 8 (or BaSO 4 ), which undergoes the change. 

 Hence the equation may be written 



k (a-x) (b) = k' (c+x) (d) 



k _ (c+x) d 

 k' ~~ (a x) b 



or if single equivalents were taken of K 2 CO 3 and BaS0 4 



k (l-x) (b) = k' x d 

 k x d 



k< - (l-x) b 

 and since r is constant, we see that equilibrium is reached 





when _. attains a certain value. 

 1 x 



This result means that equilibrium is attained when the 

 ratio of potassium sulphate to potassium carbonate reaches a 

 certain fixed value. 



The same is true (with different values for the ratio) with all 

 other reactions of similar type where there are two soluble 

 and two little soluble or insoluble substances concerned. It 

 applies, for example, to a change which is often made use of in 

 agricultural practice and affords an explanation of what may 

 appear puzzling and contradictory to the student. Calcium 

 sulphate is used as a means of lessening the loss of ammonium 

 compounds from manure heaps ; this it is said to do by under- 

 going double decomposition with the volatile ammonium car- 

 bonate yielding non-volatile ammonium sulphate and calcium 

 carbonate. 



On the other hand, when ammonium sulphate is applied as 

 a manure to a soil, calcium sulphate is found in the drainage 

 water and ammonium carbonate is apparently held back in the 

 soil until nitrification occurs. 



