160 ELEMENTARY SCIENCE 



use of soap in hard water causes a sediment to be pre- 

 cipitated, or "thrown down." 



In Chapter XII, in studying the origin of soil, you 

 learned that oxygen and water act on rock, producing 

 changes known as oxidation and hydration. Rusting you 

 noted to be a process which includes both oxidation and 

 hydration; that is, the chemical union of oxygen and water 

 with other substances. Here we have another case of 

 reduction, similar to that noted in burning. 



Then, in Chapter XIII, in considering the nitrogen-fixing 

 bacteria, you learned that certain soil bacteria are able to 

 act upon the nitrogen of the air, and to "fix" it in such form 

 that it becomes available for the use of plants. This 

 nitrogen-fixing process results in the production of sub- 

 stances called nitrates, whose molecules are much more 

 complex than the molecules of atmospheric nitrogen; it 

 is a building-up process. 



Similarly, from the bacteria of the soil up to the highest 

 living creatures, all life is based on a constant succession 

 of chemical changes actions and reactions, as they are 

 called. By the process of digestion, the molecules of the 

 food we eat are reduced to simpler substances which 

 enter our bodies, and there through a long series of changes, 

 these substances are built up into more and more complex 

 substances, the most complex of all being protoplasm, 

 which is the living substance itself. 



So we must have food to build up and maintain our 

 bodies. But we must also have air, whose oxygen is carried 

 by our blood to all parts of the body. There it acts chem- 

 ically upon the substances which have been built up from 

 our food. This action (oxidation) is a reducing process, 

 like the action you have noted in the process of burning. 



