PRELIMINARY EXPERIMENTS xix 



force, electricity, sound, friction, density, weight, and the 

 like. 



Chemistry is the science that treats of the composition of 

 matter. All matter is made up, as we have seen, of ele- 

 ments. Very few elements exist in nature in a free or 

 uncombined form. The nitrogen and oxygen of the air 

 are the leading uncombined elements. 



In order to express the chemical combinations clearly, 

 symbols are used to represent each element, and these 

 symbols are then combined to represent the proportions 

 of each in the compound. If C stands for carbon and O 

 for oxygen, the carbon dioxid might be represented by the 

 formula COO. In order to avoid the repetition of any 

 letter, however, a number is used to denote how many 

 times the element is taken : thus the formula always used 

 for carbon dioxid is CO 2 . The formula for hydrogen 

 oxid, or water, is H 2 O ; that for starch is C 6 H 10 O 5 . N 

 stands for nitrogen ; P, for phosphorus ; K, potassium ; 

 Fe, iron ; S, sulfur. 



Biology is the science that treats of life ; that is, of all 

 knowledge of plants and animals of all kinds. (See page 

 i, "Animal Biology.") 



How A CANDLE BURNS 



Some of the foregoing suggestions may be readily explained 

 and illustrated by simple experiments with a burning candle. 

 The following directions for such experiments are by G. W. 

 Cavanaugh. 



The materials needed for this exercise are : a piece of candle 

 about two inches long, a lamp chimney (one with a plain top is 

 best), a piece of white crockery or window glass, a piece of fine 

 wire about six inches long, a bit of quicklime about half the 

 size of an egg, and some matches. . All of these, with the possible 

 exception of the quicklime, can be obtained in any household. 



