MIXTURES 219 



upon plant growths, we see that nearly all of our food 

 comes from the soil in the first place. 



Man's foods may be divided into five general classes. 

 First of all is water, which is needed in all parts of the 

 body, and of which man uses a large amount. Next in 

 quantity are the carbohydrates ( 219) composed of C, 

 H, and O ; these supply energy and heat to the body. 

 Proteids ( 220) contain C, H, O, and N ; they serve 

 to build up muscle and other parts. Small quantities 

 of fats serve to give energy to the system. Last of 

 all are the salts, of which many occur in small quan- 

 tities in other foods. The elements P, Cl, S, Ca, Na, 

 K, Fe, etc., are taken on in slight amounts as salts. 



259. Fuels. The substances commonly used as 

 fuels have already come to our attention ; among them 

 we recall wood, coal, illuminating gas, kerosene, gaso- 

 line, naphtha, benzine, and alcohol. Other things less 

 commonly used as fuels are paper, rags, straw, and peat 

 (partly decomposed vegetable matter). In all these sub- 

 stances note that the elements hydrogen and carbon are 

 present ; both of these burn in air (i.e. combine with O). 



QUESTIONS 



1. What four substances does air usually contain ? State the 

 uses of each of these. In what proportion does the N and the O 

 occur ? 



2. Of what is the soil generally composed ? By what different 

 means may it have been formed? Name any common uses of 

 the soil. 



3. How are earthenware vessels made ? Of what is porcelain 

 made? 



