38 MANUFACTURE OF FOODS AND FUELS 



Water and ^Iineral Salts. — Water, which also 

 occurs as vapor in the earth's atmosphere, is taken by 

 plants from the soil in which it is always present. It 

 dissolves the soluble soil ingredients and covers the sur- 

 face of the soil particles with films of different thickness, 

 varying with the wetness of the soil. The composition 

 and significance of these films will be discussed again 

 (Chap. VI). The soil ingredients are mineral salts, the 

 nitrates, sulphates, and phosphates of calcium, potassium, 

 and magnesium. 



The Composition of Food Materials, — The food 

 materials are all comparatively simple compounds, stable 

 and saturated; they do not readily change chemically. 

 They are composed of ten chemical elements only, out 

 of the many (about 100) known, namely, carbon (C), 

 hydrogen (H), oxygen (0), nitrogen (N), calcium (Ca), 

 potassium (K), magnesium (Mg), sulphur (S), phos- 

 phorus (P), and iron (Fe). The compounds in which 

 they occur as food materials are all soluble in water, most 

 of them freely so. Hence they are of practically universal 

 occurrence. 



The Absorption of Food Materials, to be discussed 

 in Chapter VI, takes place through roots from soil and 

 through leaves from air, in solution. Land plants, with 

 roots in the soil and stems in the air, live in two sets of 

 circumstances. Their underground parts, in darkness, 

 in contact with hard, heavy, solid particles, take up 

 soluble soil constituents by their roots and move them to 

 the organs where they will be converted into food. Their 

 leaves spread in light and air and take up from the latter 

 such gases as will enter their cells. 



The Working Over of Food Materials. — Water and 

 salts taken up by the roots from the soil, and carbon 

 dioxid taken by the leaves from the air, are converted 

 from raw materials into finished products, food. The 

 changes taking place in the manufacture of foods from 

 raw materials involve changes in energy. Energy must 



