82 OX ID A TION. 



Then comes a period of middle life, in which they are 

 about equal. Finally, in old age, it often happens that 

 the organs cannot make use of enough food for their 

 complete repair, and therefore slowly diminish in size. 

 The muscles and bone of an old man often become 

 " wasted" and feeble. 



3. A Second Use of Foods is to give us strength and 

 keep up our animal heat, A starving man not only be- 

 comes lighter every day, but weaker and colder. This 

 use of foods may be compared to the use of coal in the 

 furnace of a steam-engine. And just as the coal would 

 be useless if it did not burn, and will not burn unless 

 there be a draught of air in the furnace, so foods would 

 neither make us strong nor warm did they not contain 

 things which could very gently burn inside the body; 

 and in order to burn, these things must be supplied with 

 air by our breathing. 



4. Oxidation. The air which we breathe into our 

 bodies is a mixture of two gases; only one of them is 

 useful in keeping a fire alight or in burning foods 

 inside our bodies. It is named oxygen. Generally 

 when anything burns, it unites with oxygen. The 

 thing burned is then said to be oxidized, and the process 

 of uniting with oxygen is named oxidation. When oxi- 

 dation takes place very fast, a great deal of heat is given 

 out along with light, as in a fire or candle. But oxida- 

 tion sometimes goes on slowly; and then the tempera- 



3. What is the second use of foods ? How shown ? To what may 

 the use of foods be compared ? In order that a fire may burn what is 

 necessary besides coal or wood ? What must food contain ? What 

 is necessary that these things may burn ? How is it supplied ? 



4. What is oxygen ? When is anything said to be oxidized ? What 

 is oxidation ? What are the effects of rapid oxidation ? Of slow ? 

 Which kind of oxidations occurs in our bodies ? Why ? 



