=Chemical Aerating Agents.=--In this country a very common method of aerating bread is practiced, based upon the use of certain chemical reagents which when mixed in the dough set free carbon dioxid. These reagents are known as baking or yeast powders and are especially prized by reason of the fact that it is possible with their aid to prepare in a few moments a light spongy loaf or roll which would require from 10 to 24 hours to make by the ordinary fermenting with yeast. The principal objection to the use of baking powder lies in the fact that the residues arising from the chemical reaction are necessarily left in the loaf. While these residues may not have any specific or poisonous properties they increase the quantity of mineral matter in the bread, and this mineral matter is in the inorganic state and as such does not take any part in the process of nutrition. It can only be regarded as a waste product, burdening, to that extent, the excretory organs of the body. =Constituents of Baking Powder.=--The essential constituents of baking powder are a carbonate of some kind and an acid reagent capable of decomposing this carbonate and setting the carbon dioxid free. The common carbonate of a baking powder is bicarbonate of soda. The classification of baking powders rests upon the acid elements which they contain. They may be classified as follows: (1) Cream of tartar baking powder, in which the acid constituent is cream of tartar which is known chemically as acid potassium tartrate. Other forms of tartaric acid may be used in baking powders of this class but they are not common. (2) Phosphate powders, in which the acid constituent is phosphoric acid usually in the form of the acid phosphate of lime. (3) Alum powders in which the acid constituent is alum or some form of aluminium sulfate, usually the basic sulfate of alumina. The acid and basic constituents of these powders may be kept in separate containers and mixed together at the time of making the dough. A more common form is to use them in such a way that until they mix with the dough they do not exert any notable effect upon each other. For instance, perfectly dry bicarbonate of soda and perfectly dry acid potassium tartrate may be mixed together and kept for quite a while without any notable decomposition of the bicarbonate taking place.