12 THE PRINCIPLES OF AGRICULTURE. 



unite with the zinc, and the atoms of hydrogen unite with 

 one another, forming molecules of free hydrogen. 



Chemical force sometimes changes the nature of a 

 substance by simply changing the arrangement of atoms 

 in its molecules. The molecules of cane sugar, and 

 those of gum arabic, for instance, contain exactly the 

 same number of corresponding atoms (C 12 H 2 20 n ). There 

 are other instances in which the same is true. The only 

 explanation of this is that the atoms must occupy different 

 positions with respect to one another in the molecules of 

 the different substances. 



Chemical Equations. The action of chemical force in 

 breaking up molecules of different kinds when they are 

 brought together, and forming new molecules, is repre- 

 sented by an equation. 



In the case of water and lime the equation is : 



CaO + H 2 = Ca0 2 H 2 . 



(Lime.) (Water.) (Slaked Lime.) 



For zinc and hydrochloric acid it would be : 

 2.HC1 + Zn = ZnCl a + H 2 . 



(Acid.) (Zinc.) (Zinc Chloride.) (Hydrogen.) 



In the latter case it requires two molecules of the acid 

 to combine with one of zinc, forming a molecule of zinc 

 chloride and liberating two atoms of hydrogen. 



Acids, Bases, and Salts. There are three general classes 

 of substances with which we must become acquainted 

 in order to understand the chemical principles of agri- 

 culture. 



They are called acids, bases, and salts. 



The acids are a class of substances which generally 

 have a sour taste. Vinegar contains acetic acid. 



Some of the more common acids arc : sulphuric acid 



