When an acid and any alkaline carbonate are dissolved together, a chemical action takes place, a gas is given off (carbon dioxide) and another substance is formed that is neutral, being neither acid nor alkaline, and known as a “salt.” In selecting the two substances we must bear in mind this neutral substance that remains in the batter and insure its harmlessness. The _lactic acid_ of sour milk is probably the earliest used, being a domestic product. The lactic acid is neutralized by bicarbonate of sodium, the latter being also called “baking soda.” The resulting salt is harmless. _Acid molasses_ with soda is another old-fashioned method. Here the acid is developed by the fermentation of the molasses. _Cream of tartar_ (acid potassium tartrate), obtained from crystals deposited in wine vats, came into use later, neutralized by bicarbonate of soda, two parts of cream of tartar to one of soda. _Baking powder._——The first baking powders were made of cream of tartar and bicarbonate of soda, mixed with a starch, to prevent the slight chemical action which would cause the powder to lose strength; and these two substances are now used in the best baking powders. The resulting salt is the Rochelle salt of medicine. An _acid phosphate_ is sometimes used with soda, and this gives a harmless neutral substance. Cheaper acids have sometimes been used, especially _alum_. It is best not to use an alum powder. Select a standard kind, avoiding those that offer prizes for a certain number of boxes purchased. Even if these latter do not contain alum, there is probably an excess of starch or flour. The advantage of baking powder is in the accuracy of the proportions of the two substances by weight. Even though the measuring of the cream of tartar and soda separately is accurate, the proportions may not be correct. There is no great advantage in homemade baking powder. It costs almost as much as the manufactured, and is not as perfect a product. =The proportions of the main ingredients.=——Attempts are made to define the degrees of stiffness of batters and doughs, but these distinctions are not very accurate. A “pour batter” is liquid enough to pour, and a “dough batter” soft enough to drop from a spoon; a “soft dough” is next in grade, and “dough” is the stiffest of all.