Place the chimney over the lighted candle. Lower the loop into the chimney and cover the top of the chimney with the paper. Withdraw the wire two minutes after the candle goes out. Note the cloudy appearance of the film of water on the wire. The cloudiness was caused by the carbon dioxid formed while the candle was burning. Omitting the candle, hang the freshly wetted wire in the empty chimney. Let the film of limewater remain within the chimney for the same length of time as when the candle was used. It does not become cloudy now. The cloudiness in clear limewater is a test or indication that carbon dioxid is present. What caused the white scum on the limewater which stood overnight? How does the CO₂ get into the air? It is formed whenever wood, coal, oil, or gas is burned. The amount of CO₂ in ordinary air is very small, being only three parts in ten thousand. If the limewater in the loop be left long enough in the air, it will become cloudy. The reason it clouds so quickly when the candle is being burned is that a large amount of CO₂ is formed. Besides being made by real flames, CO₂ is formed every time we breathe out air. Renew the film of water in the loop and breathe against it gently for two or three minutes. The presence of CO₂ in the breath may be shown better by pouring off some of the clear limewater into a clean glass and blowing into it through a straw. Why does water put out a fire? The answer is, not alone because it wets and shuts off the supply of free oxygen, but because it cools the carbon, which must be hot in order to unite with the oxygen, and prevents the oxygen of the air from getting as near the carbon as before. PLANT BIOLOGY CHAPTER I NO TWO PLANTS OR PARTS ARE ALIKE [Illustration: FIG. 1.--NO TWO BRANCHES ARE ALIKE. (Hemlock.)] If one compares _any two plants_ of the same kind ever so closely, it will be found that they _differ from each other_. The difference is apparent in size, form, color, mode of branching, number of leaves, number of flowers, vigor, season of maturity, and the like; or, in other words, all plants and animals _vary from an assumed or standard type_. If one compares _any two branches or twigs_ on a tree, it will be found that they differ in size, age, form, vigor, and in other ways (Fig. 1). If one compares _any two leaves_, it will be found that they are unlike in size, shape, color, veining, hairiness, markings, cut of the margins, or other small features. In some cases (as in Fig. 2) the differences are so great as to be readily seen in a small black-and-white drawing. [Illustration: FIG. 2.--NO TWO LEAVES ARE ALIKE.] If the pupil extends his observation to animals, he will still find the same truth; for probably _no two living objects are exact duplicates_. If any person finds two objects that he thinks to be exactly alike, let him set to work to discover the differences, remembering that _nothing in nature is so small or apparently trivial as to be overlooked_.