Many housewives look askance at ready-made baking powders and prefer to bake with soda and sour milk, soda and buttermilk, or soda and cream of tartar. Sour milk and buttermilk are quite as good as cream of tartar, because the lactic acid which they contain combines with the soda and liberates carbon dioxide, and forms a harmless residue in the dough. The desire of manufacturers to produce cheap baking powders led to the use of cheap acids and alkalies, regardless of the character of the resulting salt. Alum and soda were popular for some time; but careful examination proved that the particular salt produced by this combination was not readily absorbed by the stomach, and that its retention there was injurious to health. For this reason, many states have prohibited the use of alum in baking powders. 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