8 THE PRINCIPLES OF AGRICULTURE. 



The whole number of elementary substances at present 

 known is from sixty-five to seventy. Some of them, how- 

 ever, are very rare. 



Only fourteen elements are generally found in soil, 

 plants, and animals. Knowledge of these is of impor- 

 tance in the study of agriculture. 



They are as follows : 



Oxygen . . . O Chlorine . . . Cl 



Nitrogen . . . N Potassium ...... K 



Hydrogen . . H Sodium .... Na 



Carbon . . . C Calcium . . . Ca 



Silicon .... Si Magnesium . . Mg 



Sulphur . . . S Aluminium . . Al 



Phosphorus . . P Iron Fe 



These names are used so often that, for convenience, 

 they are abbreviated, each being represented by one or 

 two letters. The abbreviations are called symbols. The 

 symbols for potassium, sodium, and iron are taken from 

 the ancient and foreign names of those substances. 



Atoms. All matter is composed of minute particles 

 called atoms. These are so small that they have never 

 been seen. We cannot even imagine the size of them. 

 Millions of atoms might rest upon the point of a pin. 



The belief that they exist is embodied in what is called 

 the atomic theory. This theory is believed to be true, 

 because all known facts are consistent with it. All the 

 facts in nature with which we are acquainted, and the 

 results of all experiments that have ever been tried, are 

 just what they would be if the theory were true. The 

 fact that atoms cannot be seen is no reason for doubting 

 their existence. We are limited in our power to see 

 and understand. As we are unable to comprehend the 

 distance to the sun, and the immensity of space, so, on 



