3. As has been pointed out, the ingredients that are actually required in the making of hot breads are flour, liquid, salt, and leavening, and to give variety to breads of this kind, numerous other materials, including sugar, shortening, eggs, fruit, nuts, etc., are often added. With the exception of leavening agents, none of these ingredients requires special attention at present; however, the instruction that is given in _Bread_ regarding flour should be kept in mind, as should also the fact that all the materials for hot breads should be of the best quality that can be obtained.
As is known by this time, leavening agents are the materials used to leaven, or make light, any kind of flour mixture. These agents are of three classes, namely, _organic, physical_, and _chemical_. The organic agent is the oldest recognized leavening material, it being the one that is used in the making of yeast breads; but as a complete discussion of this class of leavening agents is given in _Bread_ and as it is not employed in the making of hot breads, no consideration need be given to it here. Physical leavening is accomplished by the incorporation of air into a mixture or by the expansion of the water into steam, and chemical leavening agents are the most modern and accurate of all the agents that have been devised for the quick rising of flour mixtures.
4. PHYSICAL LEAVENING consists in aerating, or incorporating gas or air into, a mixture that is to be baked, and it is based on the principle that air or gas expands, or increases in volume, when heated. It is definitely known that when air is incorporated into dough and then heated, the air increases 1/273 of its own volume for each degree that the temperature is increased. For instance, if the temperature of an aerated mixture is 65 degrees Fahrenheit when it is put into the oven, the air or gas will have doubled in volume by the time it has reached 338 degrees Fahrenheit. Thus, the success of aerated bread depends to some extent on the temperature of the mixture when it goes into the oven. The colder it is at that time, the greater is the number of degrees it will have to rise before it is sufficiently baked, and the more opportunity will the gas have to expand.