OO THE NEW AGRICULTURE. 



and down underground it conies out again in springs, always 

 more or less mingled with other materials, which it gets from 

 the rocks through which it travels. They are not visible to the 

 eye, for they are held in what is called chemical solution. "When 

 you put a few grains of salt or sugar upon a plate, and pour 

 water over them, they are dissolved in the water and disappear. 

 They enter into union with the water. You can not see them, 

 but you can still recognize their presence by the taste which 

 they give to the water which holds them in solution. So water, 

 sinking from the soil downward, dissolves a little of the sub- 

 stance *of the subterranean rocks, and carries this dissolved ma- 

 terial up to the surface of the ground. One of the important in- 

 gredients in the air is carbonic acid gas, and this substance 

 is both abstracted from and supplied to the air by plants and 

 animals. In descending through the atmosphere rain absorbs 

 a little air. As ingredients of the air, a little carbonic acid gas 

 particles of dust and soot, noxious vapors, minute organisms, 

 and other substances floating in the air, are caught up by the 

 descending rain, which in this way washes the air, and tends 

 to keep it much more w r holesome than it would otherwise be. 



" But rain not merely picks up impurities from the air, it gets a 

 large addition when it reaches the soil. 



" Armed with the carbonic acid which it gets from the air, and 

 with the larger quantity which it abstracts from the soil, rainwater 

 is prepared to attack rocks, and to eat into them in a way which 

 pure water could not do. 



"Water containing carbonic acid has a remarkable effect on 

 many rocks, even on some of the very hardest. It dissolves 

 more or less of their substance, and removes it. "When it falls, 

 for instance, on chalk or limestone, it almost entirely dissolves 

 and carries away the rock in solution, though still remaining 



