10 THE PRINCIPLES OF AGRICULTURE. 



unite in regular groups. A group of this kind is called 

 a molecule, which means a little mass. 



Two atoms of hydrogen uniting with one of oxygen 

 form a molecule of water. Twelve atoms of carbon, 

 twenty-two atoms of hydrogen, and eleven atoms of 

 oxygen, produce a molecule of sugar. 



Molecules, though larger than atoms, are yet too small 

 to be seen. 



A molecule is represented by writing together the 

 symbols of the different kinds of atoms of which it is 

 composed, giving the number of atoms of each kind. 

 The expression H 2 represents either a molecule of 

 water or water in general. H 2 S0 4 denotes sulphuric 

 acid, and indicates that in a molecule of the acid there 

 are united two atoms of hydrogen, one of sulphur, and 

 four of oxygen. 



When more than one molecule is to be represented, 

 the number is indicated by a figure prefixed. 2 C0 2 

 represents two molecules of carbonic acid. 



In all molecules of the same substance the atoms are 

 supposed to maintain a uniform order of arrangement. 



They always hold regular positions with respect to each 

 other. The positions depend upon the relative degree 

 of attraction which the atoms have for each other. 



The arrangements are supposed to be somewhat like 



the following : 



H 



Na-Cl. H-O-H. H-N-H. H-0-Ca-O-H. 



(Salt.) (Water.) (Ammonia.) (Slaked Lime.) 



The Nature of Matter, The nature of any substance 

 depends upon the nature of the molecules of which it is 

 composed. It may be entirely different from the sub- 

 stances whose atoms unite to form the molecule. 



