10 ENVIRONMENT OF PLANTS AND ANIMALS 



will be more than that of the original nail. Do you see why? 

 Rust is iron oxide and is formed by the union of iron and oxygen. 

 The slow oxidation of many chemical compounds is constantly 

 taking place in nature and is a part of the process of decay and of 

 brealdng down of complex materials into simpler forms. 



The Composition of the. Soil. — The covering of the earth is 

 composed of a mantle of soil and rock. The rock, by the work 

 of wind, frost, heat, water, plants, and animals, gradually breaks 

 down into small bits to form the soil mantle. This is inorganic 

 soil, which is formed usually of several of the elements found in 

 rocks, such as cal'cium, so'dium, magne'sium, siVicon, yotas'sium, 

 and iron, all combined with oxygen. 



A visit to the woods or to a well-kept garden shows us that there 

 is another kind of soil than the inorganic soil just mentioned. This 



is the rich, dark soil containing 

 hu'mus. Humus is made up 

 largely of dead organic matter, 

 the decayed remains of plants and 

 animals. If we could test the 

 chemical elements to be found in 

 humus, we should find nitrogen, 

 hydrogen, oxygen, and also car- 

 bon, an important element found 

 in all organic matter. 



Carbon. — Carbon is found in 

 many conditions in nature. It 

 is found in the bodies of plants 

 and animals, and in coal (fossil 

 plants), and it exists in a nearly pure state in the diamond. The 

 presence of carbon can usually be detected by partly burning or 

 charring a substance ; if carbon is present, some of it remains as 

 a black substance without taste or odor. Carbon may be col- 

 lected by allowing a candle flame to burn in contact with the under 

 side of a sheet of glass. The black deposit is almost pure carbon. 

 The Result of the Oxidation of Carbon. — We have seen that a 

 candle contains carbon. We can also easily show that part of 

 this carbon unites with the oxygen of the air when a candle burns. 

 If a candle is burned in a closed jar it soon goes out. If we now 



Percentages of chemical elements in the 

 human body. 



