BIOLOGY'S BUILDING MATERIALS 19 



Carbon dioxide is like nitrogen in many ways (mention them), 

 but if it be mixed with lime water, it causes the clear liquid to be- 

 come milky, while nitrogen does not. This is the test for carbon 

 dioxide. 



Carbon dioxide is a plant food; plants having the power to 

 take this gas from the air, combine it with water, and make it 

 into their tissues in fact it is from this source that all organic 

 carbon comes. 



Water. When hydrogen combines with oxygen, water (H^O) is 

 formed as we found when studying hydrogen. This compound is 

 so familiar that we do not need to learn any test for its presence. 

 It may be well to realize, however, that water constitutes much 

 over half the weight of all organic matter; that it is absolutely 

 essential to all life; and that it is not only a food, but a means of 

 carrying food to the tissues of all plants and animals. 



Mineral Compounds. The next compounds we shall take up 

 are made of the elements mentioned last in our list: sulphur, 

 phosphorus, iron, potassium, sodium, and calcium. 



Calcium unites with sulphur and oxygen to form calcium sulphate, 

 and with phosphorus and oxygen to form calcium phosphate. 

 Sodium and potassium unite with oxygen and nitrogen to form 

 sodium or potassium nitrates and so on with many other com- 

 pounds. 



Fortunately we do not have to learn to test for these separately. 

 When found in organic tissue, they are usually grouped together 

 and called " mineral matter " or " mineral salts," and the fact 

 that they remain as ash, when organic matter is completely burned, 

 is a sufficient test for these compounds at present. 



Notice that all the elements except carbon and hydrogen may 

 exist, combined as mineral compounds, in the soil where the plants 

 can get them. Hydrogen is obtained from soil water and carbon 

 from the carbon dioxide of the air. 



All the compounds mentioned so far, water, carbon dioxide, 

 and numerous mineral salts, are inorganic substances. 



One of the most important ways in which plants differ from 

 animals is that they can use inorganic substances solely for food 



