When iron rusts, it is really slowly burning (combining with oxygen). If your house is on fire, you throw water on it to stop the burning. Yet if you throw water on iron it rusts, or burns, better than if you leave it dry. What do you suppose is the reason for this? The answer is not difficult. You know perfectly well that iron does not burn easily; we could not make fire grates and stoves out of iron if it did. But when iron is wet, a little of it dissolves in the water that wets it. There is also a little oxygen dissolved in the water, as we know from the fact that fish can breathe under the water. This _dissolved_ oxygen can easily combine with the _dissolved_ iron; the _solution_ helps the chemical change to take place. The chemical change that results is oxidation,--the iron combining with oxygen,--which is a slow kind of burning; and in iron this is usually called _rusting_.[10] But when we pour water on burning wood, the wood _stops_ burning, for there is not nearly enough oxygen dissolved in water to combine rapidly with burning wood; and the water shuts off the outside air from burning wood and therefore puts the fire out. [Footnote 10: The rusting of iron is not quite as simple as this, as it probably undergoes two or three changes before finally combining with oxygen. But the solution helps all these changes.] Another chemical change, greatly helped by solution, is the combining of the two things that baking powder is made of, and the setting free of the carbon dioxid (CO_2) that is in one of them. Try this experiment: EXPERIMENT 104. Put half a teaspoonful of baking powder in the bottom of a cup and add a little water. What happens? The chemical action which takes place in the baking powder and releases the gas in bubbles--the gas is carbon dioxid (CO_2)--will not take place while the baking powder is dry; but when it is dissolved, the chemical change takes place in the solution. If you ate your food entirely dry, you would have a hard time digesting it; and this would be for the same reason that baking powder will not work without water. Perhaps you can drink too much water with a meal and dilute the digestive juices too much; certainly you should not use water to wash down your food and take the place of the saliva, for the saliva is important in the digestion of starch. But you need also partly to dissolve the food to have it digest well. Crackers and milk are usually more easily digested than are plain crackers, for the milk partly dissolves the crackers, and drinking one or two glasses of water with a meal hastens the digestion of the food. _APPLICATION 78._ Explain why paint preserves wood; why iron will rust more quickly in a wet place than it will either under water or in a dry place; why silver salts must be dissolved in order to plate a spoon by electricity. INFERENCE EXERCISE Explain the following: 501. There is dew on the grass early in the morning.