It is not only important to choose the ingredients carefully; it is also necessary to calculate the respective quantities of each, otherwise there will be an excess of acid or alkali for the stomach to take care of.  A standard powder contains twice as much cream of tartar as of bicarbonate of soda, and the thrifty housewife who wishes to economize, can make for herself, at small cost, as good a baking powder as any on the market, by mixing tartar and soda in the above proportions and adding a little corn starch to keep the mixture dry.

The self-raising flour, so widely advertised by grocers, is flour in which these ingredients or their equivalent have been mixed by the manufacturer.

212.  Soda Mints.  Bicarbonate of soda is practically the sole ingredient of the soda mints popularly sold for indigestion.  These correct a tendency to sour stomach because they counteract the surplus acid in the stomach, and form with it a safe neutral substance.

Seidlitz powder is a simple remedy consisting of two powders, one containing bicarbonate of soda, 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.