d tight to keep out other
germs.

=Yeast or the Alcohol Plant.=--Sweet cider and other fruit juices are
sometimes spoiled by a plant named yeast. This plant has the form of a
football and is so small that a million of its kind together would not
make a mass as large as the head of a pin. It floats about in the air
and is present on the skins of fruits.

Yeast is also called the alcohol plant because whenever it grows in a
sweet substance like fruit juice it changes part of it into a biting
substance called alcohol. At the same time it gives off a gas. It is
this gas which forms the bubbling or frothing in beer.

[Illustration: FIG. 19.--Yeast plants used in making bread and beer.
Those on the right are growing new plants. Much enlarged.]

The millions of yeast plants in the yeast cake bought at the store,
when put into the dough for bread, grow and form gas. This pushes the
bits of dough apart and makes it light. The little alcohol formed is
all driven off in the baking.

The alcohol which yeast forms by growing in sweet cider is in a few
weeks changed to vinegar by other germs called the vinegar plants.
Sour cider may make those who use it sick and drunk because it
contains alcohol. Yeast makes wine out of grape juice.

PRACTICAL QUESTIONS

1. Where are germs found?

2. What is the form of microbes?

3. Name some diseases caused by germs.

4. What is mold?

5. Why should food be kept covered when not in use?

6. What causes meat to spoil?

7. How may fruit be kept from spoiling?

8. Where is yeast found?

9. What effect has yeast on fruit juice?

10. Why should you not drink sour cider?

CHAPTER VI

MILK MAY BE A FOOD OR A POISON

=Of what Milk is Made.=--Milk is the most perfect food known. It
contains everything needed to build and strengthen the body. In one
gallon of milk there is about one teacupful of pure fat, nearly the
same amount of sugar, one teacupful of body-building food needed to
make muscle and blood. There is also some lime and other mineral
matter to make the bones of the young grow strong. The remaining seven
pints are water.

=Kinds of Milk.=--When milk is left standing in a jar for several
hours, much of the fat, which is present in the form of tiny balls,
rises to the upper part. This upper layer of milk full of fat is
called _cream_. If this is removed, the rest is called _skim milk_.

Milk after standing in a