XVI INTRODUCTION. 



from the air which is everywhere above and around them, 

 from the water which exists in the air as vapour and descends 

 from it producing rain, springs, and rivers, or from the soil 

 in which they are fixed. In the infancy of science, when all 

 our knowledge of these subjects was made up of conjectures, 

 to each of these sources the duty of affording the essential 

 food of plants was in turn attributed. We now, however, 

 know that neither the air, the water, or the soil, is by itself 

 sufficient to maintain the growth of the crops that we culti- 

 vate for food, but that they all unite in contributing to supply 

 the simple elements, the raw materials, from which both the 

 parts of vegetables and the nutritive matters which render 

 them valuable to man are produced. 



It must appear astonishing to those who learn it for the 

 first time, that all the varied forms of rock and water, of 

 plant and animal, presented to us in this world in which we 

 live, so different in their appearance and properties, consist 

 of no more than sixty-two different substances; and that of 

 these only about fourteen are extensively employed in nature. 

 These sixty-two substances have as yet resisted all the efforts 

 of the chemist to procure from each of them more than one 

 kind of matter: — thus, from iron, which is one of the number, 

 we can procure nothing but iron. From this circumstance 

 iron, and substances like it, which contain only one kind of 

 matter, have been considered simple or elementary bodies. 

 If, however, we examine a piece of limestone rock, the soil 

 from our fields, or the water which flows in our rivers, we 

 find that they contain several distinct kinds of matter — that 

 they are what are termed compound bodies. The variety of 

 appearance which the elementary bodies present enables us to 

 divide them into classes. Thus some of them, like iron and 

 lead, are solids, while others of them are known as liquids ; 

 and others again, thin and invisible like the gas which 

 illuminates our streets, are termed airs or gases. The elemen- 

 tary bodies are seldom found in a separate state;* gold and 



* The majority of these elementary bodies are seldom met with in 

 nature. The following list contains all those wliich it is essential for 

 the agricultural student to remember, or the teacher to illustrate: — 

 Oxygen, Sulphur, Calcium, 



Hydrogen, Phosphorus, Magnesium, 



Nitrogen, Carbon, Aluminum, 



Chlorine, Silicon, Manganese, 



Bromine, Potassium, Iron; 



Iodine, Sodium, 



