The raising of dough by means of baking soda--bicarbonate of soda--is a very simple process. When soda is heated, it gives off carbon dioxide gas; you can easily prove this for yourself by burning a little soda in a test tube, and testing the escaping gas in a test tube of limewater. When flour and water alone are kneaded and baked in loaves, the result is a mass so compact and hard that human teeth are almost powerless to crush and chew it. The problem is to separate the mass of dough or, in other words, to cause it to rise and lighten. This can be done by mixing a little soda in the flour, because the heat of the oven causes the soda to give off bubbles of gas, and these in expanding make the heavy mass slightly porous. Bread is never lightened with soda because the amount of gas thus given off is too small to convert heavy compact bread dough into a spongy mass; but biscuit and cake, being by nature less compact and heavy, are sufficiently lightened by the gas given off from soda. But there is one great objection to the use of soda alone as a leavening agent. After baking soda has lost its carbon dioxide gas, it is no longer baking soda, but is transformed into its relative, washing soda, which has a disagreeable taste and is by no means desirable for the stomach. Man's knowledge of chemicals and their effect on each other has enabled him to overcome this difficulty and, at the same time, to retain the leavening effect of the baking soda. 211. Baking Powders. If some cooking soda is put into lemon juice or vinegar, or any acid, bubbles of gas immediately form and escape from the liquid. After the effervescence has ceased, a taste of the liquid will show you that the lemon juice has lost its acid nature, and has acquired in exchange a salty taste. Baking soda, when treated with an acid, is transformed into carbon dioxide and a salt. The various baking powders on the market to-day consist of baking soda and some acid substance, which acts upon the soda, forces it to give up its gas, and at the same time unites with the residue to form a harmless salt. Cream of tartar contains sufficient acid to act on baking soda, and is a convenient and safe ingredient for baking powder. When soda and cream of tartar are mixed dry, they do not react on each other, neither do they combine rapidly in _cold_ moist dough, but as soon as the heat of the oven penetrates the doughy mass, the cream of tartar combines with the soda and sets free the gas needed to raise the dough. The gas expands with the heat of the oven, raising the dough still more. Meanwhile, the dough itself is influenced by the heat and is stiffened to such an extent that it retains its inflated shape and spongy nature.