firecracker, the flame travels down the
wick until it reaches the gunpowder, and then the firecracker
bursts with a bang.

493. If you light a small pile of gunpowder in the open, as
you do when you make a squib by breaking the firecracker in
two, you merely have a blaze.

494. Air-filled tires make bicycles ride much more evenly than
solid tires would.

495. When clay has once been baked into brick, you can never
change it back to clay.

496. A photographic negative turns black all over if it is
exposed to the light before it is "fixed."

497. The outside of a window shade fades.

498. A vacuum electric lamp globe feels hot instantly when
turned on, but if turned off again at once, it immediately
feels cold.

499. Coal gas is made by heating coal very hot in an air-tight
chamber.

500. White straw turns yellow when it is long exposed to the
sunlight.

CHAPTER ELEVEN

SOLUTION AND CHEMICAL ACTION

SECTION 53. _Chemical change helped by solution._

Why does iron have to get wet to rust?

Is it good to drink water with your meals?

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.