12 PLANT BIOLOGY 



II. DEFINITIONS 



14. A chemical element is a ..substance that has never 

 been separated into two or more different kinds of matter. 1 

 Over seventy of these elements are known at the present time, 

 and of these seventy, twelve are found constantly in the liv- 

 ing substance of plants and animals. The most common of 

 these twelve elements are carbon (symbol, C), hydrogen 

 (H), oxygen (0), nitrogen (N), sulphur (S), phosphorus (P), 

 iron (Fe), and calcium (Ca), which is found in lime. 



[In addition to the elements already studied (C, 0, H, N), 

 the others mentioned should be shown to students; and if 

 time permits, some of these elements may be burned or oxi- 

 dized in oxygen and the characteristics of the oxids thereby 

 formed may be discussed.] 



15. A chemical compound is a substance formed by the 

 union of two or more chemical elements. Two of the im- 

 portant compounds considered in biology are carbon dioxid 

 (formula CO 2 ) , which means that it is composed of one part 

 of carbon and two parts of oxygen, and water (formula H 2 O), 

 which means that it is composed of two parts of hydrogen and 

 one part of oxygen. 



16. A mixture differs from a compound in the fact that 

 the elements or compounds of which the former is composed 

 are not chemically united. In air, for instance, the oxygen 

 and nitrogen are not chemically combined, but are simply 

 put together as one might mix pepper and salt. Again, when 

 sugar is dissolved or mixed with water, the two compounds 

 are mingled so closely that the sugar disappears; it may 

 easily be obtained unchanged in its composition by evaporat- 

 ing the water. 



1 There are, however, exceptions to this statement, but they are 

 too technical for discussion in an elementary text-book. 



