=Experiment to show that Animals give off Carbon Dioxid.=--Place a cardboard over the mouth of a bottle containing pure air. Take a long straw, the hollow stem of a weed, a glass tube, or a sheet of stiff paper rolled into a tube, and pass the tube into the bottle through a hole in the cardboard. Without drawing in a deep breath, send one long breath into the bottle through the tube, emptying the lungs by the breath as nearly as possible (Fig. 4). Next invert the bottle on the table as in the former experiment, afterward withdrawing the cardboard. Move the bottle to the edge of the table and pass the lighted candle up into it (Fig. 5). Does the flame go out as quickly as in the former experiment? If you breathe through a tube into clear lime water, the water turns milky. The effect of the breath on the candle and on the lime water shows that carbon dioxid is continually leaving our bodies in the breath. [Illustration: FIG. 4.--Breathing into a bottle.[1]] [Illustration: FIG. 5.--Testing the air in the bottle.[1]] [1] From Coleman’s “Physiology for Beginners,” Macmillan Co., N.Y. =Oxidation and Deoxidation.=--The union of oxygen with carbon and other substances, which occurs in fires and in the bodies of animals, is called _oxidation_. The separation of the oxygen from carbon such as occurs in the leaves of plants is called _deoxidation_. _The first process sets energy free, the other process stores it up._ Animals give off carbon dioxid from their lungs or gills, and plants give off oxygen from their leaves. But plants need some energy in growing, so oxidation also occurs in plants, but to a far less extent than in animals. At night, because of the absence of sunlight, no deoxidation is taking place in the plant, but oxidation and growth continue; _so at night the plant actually breathes out some carbon dioxid_. The deepest part of the lungs contains the most carbon dioxid. Why was it necessary to empty the lungs as nearly as possible in the experiment with the candle? Why would first drawing a deep breath interfere with the experiment? Why does closing the draught of a stove, thus shutting off part of the air, lessen the burning? Why does a “firefly” shine brighter at each breath? Why is the pulse and breathing faster in a fever? Very slow in a trance? =The key for understanding any animal= is to find _how it gets food and oxygen_, and how it uses the energy thus obtained to grow, move, avoid its enemies, and get more food. Because it moves, it needs senses to guide it. =The key for understanding a plant= is to find _how it gets food and sunlight_ for its growth. It makes little provision against enemies; its food is in reach, so it needs no senses to guide it. The plant is built on the plan of having the nutritive activities _near the surface_ (_e.g._ absorption by roots; gas exchange in leaves). The animal is built on the plan of having its nutritive activities _on the inside_ (_e.g._ digestion; breathing).