A LESSON IN CHEMISTRY 5 



formed. Some elements, oxygen and carbon for instance, 

 readily combine with a great many others, while some, like 

 nitrogen and argon, are called inert, and only with difficulty 

 can be made to unite with others. In chemical reactions heat 

 is often evolved. A good illustration of this is seen in the 

 heat that results from the union of oxygen and carbon when 

 wood or coal is burned, or when water is added to quicklime 

 in the process of making mortar. 



Distribution of the elements. The different elements are very 

 unequally distributed. Some, like radium, are found in very 

 minute quantities and always in combination with other ele- 

 ments, while others may form vast deposits which are nearly 

 pure. Only about forty of the elements are at all common, 

 while but five of these form 96 per cent of the planet on which 

 we live. These five in the order of their abundance are oxygen, 

 silicon, aluminum, iron, and calcium. Since the soil consists 

 of particles from many kinds of rocks, it contains a consider- 

 able number of chemical elements, but only sixteen that are 

 at all abundant, namely, oxygen, silicon, carbon, sulphur, 

 hydrogen, chlorine, phosphorus, fluorine, aluminum, calcium, 

 magnesium, potassium, sodium, iron, manganese, and barium. 



Elements found in plants. Fifteen of the sixteen common 

 chemical elements in the soil are found in plants. Of these, 

 seven are metals and eight are nonmetals. In the first group 

 are potassium, sodium, magnesium, calcium, aluminum, iron, 

 and manganic ; in the second are oxygen, hydrogen, nitrogen, 

 chlorine, ctwHh? phosphorus, sulphur, and silicon. Several of 

 these are not regarded as essential to plant growth, as is also 

 true of lithium, zinc, copper, boron, and fluorine, which are 

 occasionally found in plants in certain regions. Some of the 

 characteristics of the fifteen elements usually found in plants 

 are given below. 



Potassium (K) is a soft white metal, lighter than water. It 

 quickly oxidizes or unites with oxygen when exposed to the 



