36 HOW CROPS GROW. 



modes of decomposition of the body. For example, the 

 real arrangement of the atoms in ammonium carbonate 

 is believed to be expressed by the rational (or structural) 

 formula 



"=^\0-H 



in which the carbon is directly united to oxygen, to 

 which latter one hydrogen and the nitrogen are also 

 linted, the remaining hydrogens being combined to the 

 nitrogen. 



Valence. — The connecting lines or dashes in thg fore- 

 going formula show the valence of the several atoms, i. e. , 

 their "atom-fixing power." The single d&sh from H 

 indicates that hydrogen is univalent or has a valence of 

 one. The two dashes connected with express the 

 bivalence of oxygen or that the atom of this element can 

 combine with two hydrogens or other univalent atoms. 

 The nitrogen is united on one hand with 4 hydrogen 

 atoms, and also, on the other hand, satisfies half the val- 

 ence of oxygen ; it is accordingly quinquivalent, i. e., has 

 five units of valence. Carbon is quadrivalent, being 

 joined to oxygen by four units of valence. 



Equations of Formulas serve to explain the results 

 of chemical reactions and changes. Thus,, the breaking 

 up by heat of potassium chlorate into potassium chloride 

 and oxygen is expressed by the following statement: 



Potassium Chlorate. Potassium Chloride. Oxygen. 



2 KCIO3 = 2 KCl + 3 Oj 



The sign of equality, ^, shows that what is written 

 before it supplies and is resolved into what follows it. 

 The sign -|- indicates and distinguishes separate com- 

 pounds. 



The employment of this kind of short-hand for exhib- 

 iting chemical changes will find frequent illustration as 

 we proceed with our subject. 



Modes of Stating Composition of Chemical 



