92 A TREATISE ON METAMORPHISM. 



one another under certain definite conditions which are peculiar to each sub- 

 stance. Therefore not every time such a meeting occurs are compounds 

 formed. The ratio between meeting and union in the case of any two com- 

 pounds is a constant, which can be compared with the constant of any 

 other two compounds, each pair of which has its constant. This is merely 

 another statement of the old law that different substances have different 

 affinities for one another, and it is well known that the chemical affinities 

 are developed only when the molecules are in immediate contact with one 

 another. 



The ions which are present in ground waters in any given case 

 largely depend upon the character of the adjacent rocks. In a lime- 

 stone region, for instance, the water may quickly take into solution all 

 the calcium and magnesium it can hold, considering the acids present. 

 Under such circumstances the acid ions will be mainly balanced by the cal- 

 cium and magnesium. The other substances, such as sodium and potas- 

 sium, perhaps in more readily soluble forms than the calcium and magnesium, 

 will be largely kept from going into solution, or if in solution will be partly 

 thrown down, because these substances are obliged to compete for the acid 

 radicals with the vastly greater number of calcium and magnesium molecules. 

 Is it not possible that the agricultural advantage of having calcium and mag- 

 nesium abundantly in the soil is largely, or at least partly, due to the fact 

 that the presence of these soluble substances in abundance prevents the 

 solution and washing out of the elements potassium and sodium which the 

 plants need? 



Ostwald divides the bases into strong, moderately strong, and weak." 

 The alkalies and alkaline earths, with the exception of magnesium, are 

 strong bases; magnesium is a moderately strong base; iron and aluminum 

 are weak bases — of the two aluminum is the weaker. It follows that, 

 other things being equal, in underground solutions the alkalies and alkaline 

 earths, with the exception of magnesium, largely take possession of the acids. 

 To a less extent this is true of magnesium, and to a still smaller degree of 

 iron and aluminum. Thus we have the partial explanation of the relative 

 solubilities of the bases in the belt of weathering. In this belt the alkalies 

 are dissolved to the greatest extent; next in order comes calcium, then 

 magnesium, and finally iron and aluminum. (See p. 518.) 



a Ostwald, W., The scientific foundations of analytical chemistry, translated by George McGowan, 

 Macmillan & Co., London, 1895, pp. 55-56. 



