12 THE ELEMENTS OF PLANT FOOD 



substance. Under some conditions the changes are 

 more active, but affinity never ceases to be present. 



Light, heat, and electricity are influences that aid 

 in the separation of the atoms of one compound and 

 the forming of others. Most compounds are destroyed 

 at very high temperatures and remain unchanged at 

 very low temperatures. Under these potent influ- 

 ences chemical changes are going on about us all the 

 time. 



Chemists have agreed on certain symbols which shall 

 represent an atom of the element. This symbol is 

 usually the first letter or the first and second letters 

 of the name of the element, but to avoid confusion some 

 elements have for symbols the first and second letters 

 of their Latin names ; thus, P represents the atom of 

 phosphorus and K (from Latin kalium) represents an 

 atom of potassium. 



A compound is represented by the symbols of its 

 elements written one after the other. HC1 represents 

 one atom of hydrogen (H) combined with one atom of 

 chlorine (Cl), making one molecule of hydrochloric acid. 

 If more than one atom of an element is present in the 

 molecule of a compound, as in phosphoric acid (H 3 PO 4 ), 

 a small subscript is used. H 3 PO 4 means that the 

 molecule of phosphoric acid is composed of three atoms 

 of hydrogen (H), one atom of phosphorus (P), and four 

 atoms of oxygen (O). 



A coefficient placed before an element or a compound 

 indicates a number of atoms or molecules. Thus, 2 Cl 

 indicates that two atoms of chlorine are meant; 

 5 NaCl indicates that five molecules of sodium chloride, 

 or common salt, are meant, making ten atoms repre- 

 sented. 



