SOILS. I7 



are the same whether the action takes place slowly or 

 rapidly. This process of slow oxidation is constantly 

 going on in the bodies of animals. When they breathe, 

 the air enters the lungs, where the oxygen it contains 

 is taken up or absorbed by the blood, and carried 

 throughout the body. Animals cannot live without 

 oxygen. It is also necessary to the growth of plants. 



23. Hydrogen is another abundant element. It 

 forms one ninth by weight of water and enters into 

 the composition of all plants and animals. It can 

 be prepared easily by the action of dilute sulphuric 

 or hydrochloric acid on zinc or iron. If some scraps 

 of either of these metals be placed in a wineglass 

 and a little acid poured over them, effervescence will 

 be produced by the escape of the hydrogen (5). 



24. Hydrogen is the lightest substance known. 

 Like oxygen it is an invisible gas without color, taste, 

 or smell, but unlike oxygen it will burn when brought 

 in contact with flame. The flame of burning hydro- 

 gen is of a pale-blue color and intensely hot (6). 

 The result of the combustion is water (7). Sixteen 

 pounds of oxygen unite with two pounds of hydro- 

 gen to form eighteen pounds of water. 



25. Water, which is formed by the chemical 

 union of the two elements just mentioned, is one of 

 the most abundant and important compounds in 

 nature. It is easily converted into vapor, \vhich, ris- 

 ing from the surface of the earth in an invisible form, 

 is afterward condensed into rain, and dew, and 

 frost. It dissolves solid, liquid, and gaseous sub- 



