1:^ The rKixciPLEs of A(;kicl ltlki;. 



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 arraugement of atoms 

 in its molecules. The molecules of cane sugar, and 

 those of gum arable, for instance, contain exactly the 

 same number of cori'cs])onding atoms (CjoHo^Oij). There 

 are other instances in wliich 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 sul)stanccs. 



Chemical Equations. — The action of chemical force in 

 breaking up molecules of different l<inds when they are 

 brought together, and forming new mulecides, is reju'e- 

 sented by an equation. 



In the case of water and lime the erpiation is : 



CaO + H,0 = CaOoH^. 



(Lime.) (Water.) (Slakeil Lime.) 



For zinc and hydrochloric acid it would be: 

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



(Acid.) (Zine.) (Ziue Chloriilo.) (Ihdrogen.) 



In the latter case it requires two molecules of the acid 

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

 chloride and lil)erating two atoms of hydrogen. 



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

 of sultstances with which we must become acquainted 

 in order to understand the chemical principles of agri- 

 culture. 



They arc called acids, bases, and salts. 



The acids arc? a class of substances wliich genei-ally 

 hav(^ a sour taste. Annegar contains acetic acid. 



tSome of the more counnon acids are: sul}ihuric acid 



