THE PRINCIPLES OF AGPJCULTURE. 



CHAPTER I. 



THE SUBSTANCES OF THE EAETH. 



Simple Substances. — All matter of which the earth, the 

 atmosphere, and all plants and animals are composed 

 consists of a c'omparatively small number of simple snlj- 

 stances or elements. 



We are accustomed to think of the ordinary olijects 

 about us as simple in their nature, and as composed of 

 but one kind of matter. 



This is true of a few substances, like pure gold, silver, 

 and iron ; but the greater number of the objects with 

 which we are familiar are composed of several sim]jle 

 substances mixed or combined. 



Water is composed of two gases. Wood may be di- 

 vided into ten or more different elements. If we burn a 

 pile of wood we see the smoke, we know that there are 

 gases escaping which we cannot see, and we find a small 

 quantity of ashes remaining. Neither the smoke, the 

 gases, nor the ashes appear at all like wood, and yet we 

 know that in some way the wood has been transformed 

 into these substances. 



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