ENVIRONMENT OF LIVING THINGS 21 



is found that the water separates into two gases, one of which 

 occupies twice, as much space as the other in the tubes. If we 

 test the gas present in smaller quantity, we find it to be oxygen. 

 The other gas, colorless, tasteless, and odorless like the oxygen, 

 differs from it by igniting with a slight explosion if a burning match 

 or splinter is introduced in it. As it burns, drops of water are 

 formed, showing that it is passing back to its original condition, 

 that is, it is uniting with oxygen to form water. This gas is hydro- 

 gen. Hydrogen has a great chemical affinity or hking for other 

 elements, hence it is usually found in nature combined with other 

 elements, as with oxygen to form water. 



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

 probably very different in former ages from what it now is. Its 

 molten plastic mass after cooling formed rock. This rock, by the 

 work of the wind, frost, heat, and water, and plants, has in part 

 been broken into small bits. This is inorganic soil, such as sea 

 sand and gravel. Such soil is formed usually of several elements 

 found in rocks, such as calcium, sodium, magnesium, silicon, 

 potassium, and iron 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 humus. Humus is made up in part 

 of dead organic matter, the decayed remains of plants and ani- 

 mals. In such soil we should find relatively more water than in 

 inorganic soil. If we could test the chemical elements to be found 

 in humus, we should find nitrogen, hydrogen, oxygen, and also 

 carbon, an important element found in all organic matter. 



Carbon. — Carbon is found in many conditions in nature. It 

 makes up a large part of the bodies of plants and animals, and of 

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

 presence of carbon can usually be detected by partially burning 

 the substance, carbon showing as a black substance without taste 

 or odor. Carbon may be collected by allowing a candle flame to 

 burn in contact with the underside of a sheet of glass. The 

 black deposit is almost pure carbon. 



Oxidation of Carbon and its Result. — If we burn a candle in 

 a closed jar containing air, the flame soon begins to flicker, and then 

 goes out. If the cover of the jar is carefully removed, and a 



