soon after the jar is opened, and cider
soon turns to vinegar. All fruit juice does this even in cold weather.
But in cold weather it keeps for a longer time.

[Illustration: =Fermentation in a jar of cherries.=]

=59. Preserving fruit.=--If your mother wishes to keep fruit all winter
she boils it and at once puts it into tight jars. This shuts out the air
and then the fruit keeps good all winter. Boiling kills all living
things, and no more can get in through the tight jars. Does a living
thing have anything to do with making the fruit juice turn sour?

=60. Yeast.=--Yeast will make all sweet things ferment. Bakers make
yeast grow in bread sponge. Yeast is alive. It is made of millions of
tiny round cells. New cells sprout out from the side of the old cells
like young lilies on an old lily bulb. Soon each new cell breaks off
and lives all by itself. In a single night enough new cells will form
to fill the whole loaf of bread.

[Illustration: =Yeast plant cells (x500).=]

=61. How yeast makes alcohol.=--Yeast will grow only where sugar is.
When it has grown for some time there is no more sugar, and instead of
a sweet taste there is a sharp or sour taste. The yeast has changed
the sugar to alcohol. All alcohol is made from sugar by yeast.

The seeds of the yeast plant are everywhere in the air. Some are on
the skins of fruit and so are found in the juice when it is squeezed
out. There they begin to grow at once and soon change the sugar to
alcohol. They do this by taking a gas away from the sugar. The gas
rises in little bubbles, and makes a froth upon the top of the juice.
Boiling kills the yeast plant. If the juice is at once put into tight
jars no new yeast plants can get in, and so the juice keeps.

=62. Vinegar.=--Sometimes fruit juice turns sour. The sourness is due
to vinegar. Besides yeast, other little living plants fall into the
juice and turn the sugar to vinegar. But if there is much alcohol in
the juice, the vinegar plants will not grow.

=63. Yeast in bread.=--Growing yeast plants always make alcohol. They
change some of the sugar of bread dough to alcohol and a gas. The gas
bubbles through the bread and makes it light. When bread is baked, the
heat of the oven drives off the alcohol, and so we do not eat any in
bread.

=64. Alcohol.=--Alcohol is a clear liquid and looks like water. It has
a sharp taste and smell. It burns very easily and makes a very hot
flame. Its smoke cannot be seen, and its f