are done substantially in the way that
you have done the nickel plating, only gold salt or silver salt is
used instead of nickel salt.

Just as electricity helps chemical changes in plating, it helps
changes in a storage battery. But in the storage battery the new
compounds formed by "charging" the battery change back again and
generate electricity when the poles of the battery are connected with
each other by a good conductor.

_APPLICATION 75._ Explain how spoons can be silver plated; how
water can be changed into hydrogen and oxygen.

INFERENCE EXERCISE

Explain the following:

471. Clothes dry best in the sun and wind.

472. Proofs of photographs that have not been thoroughly
"fixed" fade if left out of their envelope.

473. Blowing a match puts it out, yet a good draft is
necessary for a hot fire.

474. A cup does not naturally fall apart, yet after it is
broken it falls apart even if you fit the pieces together
again.

475. Crayon leaves marks on a blackboard.

476. A baked potato tastes very different from a raw one.

477. An air-filled automobile tire is harder at noon than in
the early morning.

478. When a live trolley wire breaks and falls to the street,
it becomes so hot that it burns.

479. Glass jars of fruit should be kept in a fairly dark
place.

480. You wash dishes in _hot_ water.

SECTION 51. _Chemical change releases energy._

Why is fire hot?

What makes glowworms glow?

Why does cold quicklime boil when you pour cold water on it?

If no energy were released by chemical change, we should run down like
clocks, and could never be wound up again. We could breathe, but to do
so would do us no more good than it would if oxygen could not combine
with things. Oxidation would go on in our bodies, but it would neither
keep us warm nor help us to move. A few spasmodic jerks of our hearts,
a few gasps with our lungs, and they would stop, as the muscles would
have no energy to keep them going.

The sunlight _might_ continue to warm the earth, as we are not sure
that the sun gets any of its heat from chemical change. But fires,
while they would burn for an instant, would be absolutely cold; no
energy would be given out by the fuel combining with oxygen. But the
fires could not burn long, because there would be nothing to keep the
gases and fuel hot enough to make them combine with the oxyge