CHAPTER I 



more experiments are suggested to emphasize the nature of two gases, 

 oxygen and carbonic acid gas. At any rate, the teacher should famil- 

 iarize the pupils with the names of the gases and some every-day facts 

 about them. For instance, that we are uncomfortable in a close room 

 because the oxygen is being exhausted, so we let in fresh air, which 

 means air with fresh supplies of oxygen ; that in all life and growth 

 material is constantly being used up, so that waste substance is being 

 breathed out in the form of carbonic acid gas by all living things ; 

 that when material burns more rapidly, wood, coal, coal-oil, illumi- 

 nating gas, etc., the same gas is given off. The wonderful story of 

 how this product of waste, carbon dioxide, is made over by green 

 plants into food for themselves, and so more or less indirectly becomes 

 food for all the living world, is a story that interests the youngest 

 children. 



Children old enough to use the reader are likely to ask questions 

 that will demand knowledge of some of the following facts: 

 the water in which Algae live always contains carbon dioxide or 

 CO 2 a compound of two parts oxygen and one of carbon which it 

 has absorbed from the air. The formula for water isEUO two parts of 

 hydrogen to one of oxygen. Nowvhile starch and similar substances, 

 all classed as carbohydrates, are known to consist of carbon, hydrogen 

 and oxygen in definite proportions, it is not possible to artificially 

 manufacture them from these elements. It is only in the cell labora- 

 tories of green plants that this combining or synthesis (photosynthesis 

 some botanists call it) takes place, so green plants only can bridge 

 the gulf between the inorganic and the organic world. Just how this 

 is done is not known. The protoplasm of the chloroplasts must be 

 the builder, the sun's rays supply the motive force, and there are 

 theories about the part played by the chlorophyll, but no one claims to 

 have solved fully this mystery of life. 



These primary organic substances are, by chemical changes, con- 

 verted into all the other compounds, such as albumen, cellulose, 

 starch, fats, pigments, etc., of which the bodies of plants and animals 

 are composed. The technical term applied to these changes in plants 

 is metabolism. The impelling force for these chemical changes is 

 obtained by oxidation ; that is, the protoplasm withdraws oxygen 

 from the air and uses it in burning a portion of the carbohydrates, 

 but only a fraction sometimes ^ or -^ of the amount manufactured 

 by the plants. So plants, like animals, respire, that is, use oxygen 

 and give off carbon dioxide. Other inorganic substances than carbon 

 di9xide and water are used in these chemical processes. Nitrogen is 

 necessary and so are several mineral salts, such as sulphur and iron. 



