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.

502. Cold cream makes your hands and face soft.

503. Glowworms and fireflies can be seen on the darkest
nights.

504. A lake looks gray on a cloudy day and blue on a clear
day.

505. Dried fruit will keep much longer than fresh fruit.

506. If you scratch a varnished surface, you can rub the
scratch out completely by using a cloth wet with alcohol.

507. Soda is usually dissolved in a little water before it is
added to a sour-milk batter.

508. Iron rusts when it gets wet.

509. Peroxide is usually kept in brown bottles.

510. Dry lye may be kept in tin cans, but if the lye is
_moistened_ it will eat the can.

SECTION 54. _Acids._

Why are lemons sour?

How do acids act?

Some acids are v