=Chemistry= is the science that treats of the _composition_ of matter. All matter is made up, as we have seen, of elements. 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 CO₂. The formula for hydrogen oxid, or water, is H₂O; that for starch is C₆H₁₀O₅. 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 1, “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. If you perform the experiment requiring the lime, be sure that you start with a fresh piece of quick or stone lime, which can be had of any lime or cement dealer. During the performance of the following simple experiments, the pupil should describe what he sees at each step. The questions inserted in the text are offered merely as suggestions in the development of the desired ideas. The answers are those which it is desired the pupils shall reach or confirm by their own observation. I. _Oxygen_ Light the candle and place it on a piece of blotting paper (_A_). What do you see burning? Is anything burning besides the candle? The answer will probably be “no.” Let us see. Place the lamp chimney over the lighted candle, and partly cover the top by a piece of stiff paper, as in Fig. A. Ask the pupils to observe and describe how the flame goes out; _i.e._ that it is gradually extinguished and does not go out instantly. Why did the flame go out? The probable thought will be, “Because there was no air.” (If there was no air within the chimney, some could have entered at the top.) [Illustration: _A._--THE BEGINNING OF THE CANDLE EXPERIMENT.] [Illustration: _B._--SUPPLYING AIR UNDERNEATH THE CHIMNEY.]