Note Footnote 10 — CHAPTER ELEVEN

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.