tmeal, cornmeal, onions.

[Illustration: FIG. 188. The white powder that is forming is a silver
salt.]

THE LIMEWATER TEST FOR CARBON DIOXID. In crowded and badly ventilated
rooms carbon dioxid in unusual amounts is in the air. It can be
detected by the limewater test.

EXPERIMENT 116. Pour an inch or two of limewater into a
glass. Does it turn milky? Pump ordinary air through it with
a bicycle pump. Now blow air from your lungs through a glass
tube into some fresh limewater until it turns milky. By this
test you can always tell if carbon dioxid (CO_2) is present.

[Illustration: FIG. 189. The limewater test shows that there is carbon
dioxid in the air.]

Carbon dioxid turns limewater milky as it combines with the lime in
the limewater to make tiny particles (a precipitate) of limestone. If
you pour seltzer water or soda pop into limewater, you get the same
milkiness, for the bubbles of carbon dioxid in the charged water act
as the carbon dioxid in your breath did. If you pumped enough air
through the limewater you would produce some milkiness in it, for
there is always some carbon dioxid in the air.

The purpose of these experiments is only to give you a general notion
of how a chemist analyzes things,--by putting an unknown substance
through a series of tests he can tell just what that substance
contains; and by accurately weighing and measuring everything he puts
in and everything he gets out, he can determine how much of each thing
is present in the compound or mixture. To learn to do this accurately
takes years of training. But the men who go through this training and
analyze substances for us are among the most useful members of the
human race.

INFERENCE EXERCISE

Explain the following:

551. A little soda used in canning an acid fruit will save
sugar.

552. The fats you eat are mostly digested in the small
intestine, where there is a large excess of alkali.

553. The dissolved food in the liquid part of the blood gets
out of the blood vessels and in among the cells of the body,
and it is finally taken into the cells through their walls.

554. Ammonia takes the color out of delicate fabrics.

555. Dishes in which cheese has been cooked can be cleaned
quickly by boiling vinegar in them.

556. Prepared pancake flour contains baking powder. It keeps
indefinitely when dry, but if the box gets wet, it spoils.

557. When