THE SOIL. 13 



Rain, in falling, not only carries down with it these 

 particles of dust around which it clings, but acids, 

 which help to dissolve the rocks. Rock thus dis- 

 solved is caught in the soil to remain as plant food; 

 or is borne away into streams, and thence to the 

 sea. Here some of it is taken up by such animals 

 as oysters and corals. They are thus enabled to 

 secrete the hard outer skeletons of their bodies. 

 Some of it finds its way down through the earth 

 to the permanent water layers, or veins, from 

 whence we get it as the hard water of wells and 

 springs. 



Carbonic acid is brought down by every rain. It 

 is constantly poured into the air from the chimneys 

 of houses and factories, from the lungs of animals, 

 and from decomposing vegetation. It has the 

 power, when mixed with water, of dissolving lime- 

 stone. Many caves in Kentucky and Indiana have 

 been formed by this action. Water bearing this 

 acid in solution has trickled down into the crevices 

 of the limestone deposits of those regions and 

 gradually eaten out cavities so large that people 

 can walk into some of them for miles. 



Rain, then, not only falls upon the earth to make 

 it productive, but it also washes the air clean of its 

 impurities, making it wholesome and pure for us to 

 breathe. The atmosphere of cities has more 

 impurities in it than that of the country. For this 

 reason the free, open country is a more healthful 

 place for dwelling. 



