AGKONOMY 



CHAPTER I 



A LESSON IN CHEMISTRY 



Chemical elements. The earth's crust, the animals and 

 plants upon it, and the multitude of substances with which 

 we are familiar are composed of a small number of simpler 

 forms of matter, known as chemical elements, combined in 

 various ways. A chemical element may be denned as a sub- 

 stance that has not been resolved into simpler substances, and 

 as thus defined there are only about eighty chemical elements 

 in the world. Gold may be taken as an illustration. Though 

 it be divided into particles too small to be visible in the 

 microscope, or heated until it becomes liquid, or subjected to 

 strong currents of electricity, it is still gold and nothing else. 

 A few chemical elements may be found " native," that is, 

 uncombined with others, but usually two or more unite to 

 form chemical compounds. By far the larger number are always 

 found thus combined. Oxygen may be cited as a familiar 

 example of an element that exists both free and combined. 

 As a free gas it forms about one fifth of the air we breathe ; 

 combined with other elements it makes up about half of the 

 water and rock of the earth's crust. Chemical elements are 

 often grouped as metals and nonmetals, the metals being 

 greatly in the majority. Usually the metals may be distin- 

 guished by names ending in urn. The difference between a 

 metal and a nonmetal, however, is not easily defined. A 

 metal is supposed to have the following properties: it must 



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