42 CHEMISTRY, GEOLOGY 



magnesia oxide of iron, oxide of manganese, 

 sulphuric acid, phosphoric acid, and chlorine. 

 These substances are found, in different propor- 

 tions, in the ashes or inorganic part of all plants, 

 and the plants derive them solely from the soil, 

 as none of them are found in the air. 



Plants take up these inorganic substances 

 through their roots ; but before they can do so 

 they must be dissolved, or made into a solution, 

 by the rain water, or the waters of springs. So 

 long as they remain solid, or undissolved, they 

 are valueless to the plant, which can derive no 

 food from them. (See Appendix, 22.) 



All fertile or productive soils must contain 

 these inorganic substances ; because without them 

 plants must languish, and the crops be poor and 

 unproductive. 



All plants do not require the same quantity 

 of all these substances ; some take up a large 

 quantity of one or more of them, and only a very 

 small portion of the others. (See Appendix, 23.) 



Questions. Where are the last-named substances found, 

 besides in the soil ? Do plants take them up ? How do they 

 take them up ? In what form do they take them up ? Must 

 fertile soils possess these inorganic substances ? Why ? Do 

 all plants require the same proportion of these substances ? 



