Light a piece of charcoal, or let a splinter of wood burn a few minutes and then blow it out so that a glowing coal will be left on the end of it. Lift the cardboard off the bottle and plunge the glowing stick into it for a couple of seconds. Cover the bottle after taking out the stick, and repeat, using a lighted match or a burning piece of wood instead of the glowing stick. If you dip a piece of iron picture wire in sulfur and light it, and then plunge it into the bottle, you will see iron burn. [Illustration: FIG. 165. Filling a bottle with oxygen.] [Illustration: FIG. 166. The iron really burns in the jar of oxygen.] Both manganese dioxid and potassium chlorate have a great deal of oxygen bound up in them. When they join together, as they do when you heat them, they cannot hold so much oxygen, and it escapes as a gas. In the experiment, the escaping oxygen passed through the tube, filled the bottle, and forced the water out. WHAT BURNING IS. When anything burns, it is simply joining oxygen. When a thing burns in air, it cannot join the oxygen of the air very fast, for every quart of oxygen in the air is diluted with a gallon of a gas called _nitrogen_. Nitrogen will not burn and it will not help anything else to burn. But when you have pure oxygen, as in the bottle, the particles of wood or charcoal or picture wire can join it easily; so there is a very bright blaze. Although free oxygen helps things to burn so brilliantly, a match applied to the solids from which you got it would go out. And while hydrogen burns very easily, you cannot burn water although it is two-thirds hydrogen. Water is H_2O, you remember. WHAT COMPOUNDS ARE. When elements are combined with other elements, the new substances that are formed are called _compounds_. Water (H_2O) is a compound, because it is made of hydrogen and oxygen combined. When elements unite to form compounds, they lose their original qualities. The oxygen in water will not let things burn in it; the hydrogen in water will not burn. Salt (NaCl) is a compound. It is made of the soft metal sodium (Na), which when placed on water sputters and drives hydrogen out of the water, and the poison gas chlorine (Cl), combined with each other. And salt is neither dangerous to put in water like sodium, nor is it a greenish poison gas like chlorine. MIXTURES. But sometimes elements can be mixed without their combining to form compounds, in such a way that they keep most of their original properties. Air is a mixture. It is made of oxygen (O) and nitrogen (N). If they were _combined_, instead of _mixed_, they might form laughing gas,--the gas dentists use in putting people to sleep when they pull teeth. So it is well for us that air is only a _mixture_ of oxygen and nitrogen, and _not_ a compound.