THE ELEMENTS OF PLANT FOOD n 



Two Kinds of Changes. When any change takes 

 place of such a nature as to change the molecule, that 

 is, to break up the molecule into atoms or to make the 

 atoms form new combinations, this change is called 

 a chemical change. Any change that does not alter 

 the composition of the molecule is called a physical 

 change. 



Sugar is composed of molecules made by the combi- 

 nation of carbon, oxygen, and hydrogen. Sugar may 

 be pulverized, it may be ground to finest powder, it 

 may be dissolved in water, but it still remains sugar. 

 The molecules are sugar molecules. Such changes 

 are physical changes. If, however, we hold the sugar 

 over a flame till it burns, the molecules of sugar are 

 destroyed, but other combinations of the elements 

 which were in the sugar pass off into the air. There 

 is nothing left of it as sugar. 



Chemical Affinity. - - The tendency of the atoms of 

 certain elements to combine, under favorable conditions, 

 with the atoms of certain other elements is called chem- 

 ical affinity. The atoms of certain elements when 

 brought into intimate relations with the atoms of cer- 

 tain other elements have a strong affinity for them. 

 It is this attraction that holds the atoms together in a 

 molecule of water and prevents them from separating 

 into two elements, hydrogen and oxygen. It is by 

 this attraction that under the influence of sunlight the 

 atoms in certain compounds find stronger affinities in 

 atoms of other compounds than in their present com- 

 pound, and new compounds are formed, resulting in 

 growth of plant life and in decay. 



It takes but little imagination to see these warring 

 affinities asserting themselves in the molecules of every 



