a, and the other, some acid such as cream
of tartar. When these substances are dissolved in water and mixed,
effervescence occurs, carbon dioxide escapes, and a solution of
Rochelle salt remains.

212_a_. Source of Soda. An enormous quantity of sodium carbonate, or
soda, as it is usually called, is needed in the manufacture of glass,
soap, bleaching powders, and other commercial products. Formerly, the
supply of soda was very limited because man was dependent upon natural
deposits and upon ashes of sea plants for it. Common salt, sodium
chloride, is abundant, and in 1775 a prize was offered to any one who
would find a way to obtain soda from salt. As a result of this, soda
was soon manufactured from common salt. In the most recent methods of
manufacture, salt, water, ammonia, and carbon dioxide are made to
react. Baking soda is formed from the reaction. The baking soda is
then heated and decomposed into washing soda or the soda of commerce.

CHAPTER XXI

FERMENTATION

213. While baking powder is universally used for biscuits and cake,
it is seldom, if ever, used for bread, because it does not furnish
sufficient gas to lighten the tough heavy mass of bread dough. Then,
too, most people prefer the taste of yeast-raised bread. There is a
reason for this widespread preference, but to understand it, we must
go somewhat far afield, and must study not only the bread of to-day,
but the bread of antiquity, and the wines as well.

If grapes are crushed, they yield a liquid which tastes like the
grapes; but if the liquid is allowed to stand in a warm place, it
loses its original character, and begins to ferment, becoming, in the
course of a few weeks, a strongly intoxicating drink. This is true not
only of grape juice but also of the juice of all other sweet fruits;
apple juice ferments to cider, currant juice to currant wine, etc.
This phenomenon of fermentation is known to practically all races of
men, and there is scarcely a savage tribe without some kind of
fermented drink; in the tropics the fermented juice of the palm tree
serves for wine; in the desert regions, the fermented juice of the
century plant; and in still other regions, the root of the ginger
plant is pressed into service.

The fermentation which occurs in bread making is similar to that which
is responsible for the transformation of plant juices into
intoxicating drinks. The former process is not so old, however, since
the use of alcoholic beverag