12 BIOLOGY FOR BEGINNERS 



absolutely necessary as food and drink. But its chief importance 

 in biology is that, by uniting with the substance of both plant 

 and animal, it sets free the energy which keeps them alive. Without 

 oxygen, no life can exist. 



NITROGEN 



Where it is Found. Nitrogen (N) is another important element. 

 It makes up four-fifths of the air. It is found combined with several 

 minerals in the soil and exists in the living tissue of all organic 

 things. 



Properties. Nitrogen resembles oxygen in being colorless, odor- 

 less, and tasteless, and in that it will not burn. It is less soluble 

 in water and lighter in weight. It is the exact opposite of oxygen 

 in its behavior, for it will not cause combustion, nor will it combine 

 readily with other elements. Its compounds decompose easily. 



Uses. It is found in the active living substance of all plants 

 and animals and is essential to their life. Its various compounds 

 are our most necessary foods. 



All fertilizers which we use for plants, as well as meat, milk, 

 eggs, and many other animal foods contain very important com- 

 pounds of nitrogen. 



If the air were pure oxygen, fires could not be controlled and 

 things would oxidize too rapidly. Thus, another important use of 

 nitrogen is to restrain the activity of oxygen and make the at- 

 mosphere suitable for life. 



HYDROGEN 



Where it is Found. Hydrogen (H) occurs combined in water, 

 plant and animal tissue, wood, coal, gas, and all acids. 



Properties. It resembles both nitrogen and oxygen in being 

 colorless, odorless, and tasteless. It does not dissolve much in 

 water and it will not cause things to burn, but unlike either nitrogen 

 or oxygen it burns readily and even explodes when mixed with air 

 and brought into contact with fire. It is the lightest substance 

 known and, because of this fact, is used to fill balloons. 



