The candle is made mostly of a combination of hydrogen and carbon. The hydrogen combines with part of the oxygen in the air that is in the bottle to form a little water. The carbon combines with the rest of the oxygen to make carbon dioxid, much of which dissolves in the water below. So there is practically empty space in the bottle where the oxygen was, and the air outside forces the water up into this space. The rest of the bottle is filled with the nitrogen that was in the air and that has remained unchanged. About how much of the air was oxygen is indicated by the space that the water filled after the oxygen was combined with the candle. [Illustration: FIG. 167. The water rises in the bottle after the burning candle uses up the oxygen.] CARBON AND HYDROGEN THE CHIEF ELEMENTS IN FUEL. Carbon and hydrogen make up the larger part of almost every substance that is used for fuel, including gas, gasoline, wood, and soft coal; alcohol, crude oil, kerosene, paper, peat, and the acetylene used in automobile and bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly plain carbon. Since burning is simply the combining of things with oxygen, it is plain that when the carbon of fuel joins oxygen we shall get carbon dioxid (CO_2). When the hydrogen in the fuel joins oxygen, what must we get? When things do not burn up completely, the carbon may be left behind as charcoal. That is what happens when food "burns" on the stove. But if anything burns up entirely, the carbon or charcoal burns too, passing off as the invisible gas, carbon dioxid, just as the hydrogen burns to form steam or water. It is because almost every fuel forms water when it burns, that we find drops of water gathering on the outside of a cold kettle or cold flatiron if either is put directly over a flame. The hydrogen in the fuel combines with the oxygen of the air to form steam. As the steam strikes the cold kettle or iron, it condenses and forms drops of water. NOTHING EVER DESTROYED. One important result of the discovery that burning is only a combining of oxygen with the fuel was that people began to see that nothing is ever destroyed. There is exactly as much carbon in the carbon dioxid that floats off from a fire as there was in the wood that was burned up; and there is exactly as much hydrogen in the water vapor that floats off from the fire as there was in the wood. Chemists have caught all the carbon dioxid and the water vapor and weighed them and added their weight to the weight of the ashes; and they have found them to weigh even more than the original piece of wood, because of the presence of the oxygen that combined with them in the burning.