Kneading bread is for the purpose of distributing the gas and breaking up the large bubbles into small ones, to give the loaf a fine grain. One will immediately see that kneading before the bread is raised is a more or less useless task. Kneading is a process which should be done gently, by handling the dough with great tenderness; for if it is pressed hard against the molding-board, the bubbles will be worked out through the surface, and the loaf consequently less porous than if all the gas is kept in it. The best temperature for the raising of bread (in other words, for the growing of yeast) during the first part of the process is from 70° to 75° Fahr. It may touch 80° without harm, but 90° is the limit. Above that acetic fermentation is liable to occur, and the bread becomes sour. When the bread is made into loaves, it may be placed in a very warm temperature, to rise quickly if it is intended for immediate baking. Besides killing the yeast, the object sought in baking is to form a sheath of cooked dough all over the outside, for a skeleton or support for the inside mass while it is cooking. Baking also expands the carbonic acid, and volatilizes the alcohol. The latter is lost. A good temperature in which to begin the baking of bread is 400° Fahr. This may gradually decrease to not lower than 250°, and the time, for a good-sized brick loaf, is one hour. If it is a large loaf, increase the time by a quarter or a half hour. "The expansion of water or ice into 1700 times its volume of steam, is sometimes taken advantage of in making snow bread, water gems, etc. It plays a part in the lightening of pastry and crackers. Air at 70° Fahr. expands to about twice its volume at the temperature of a hot oven, so that if air is entangled in a mass of dough it gives a certain lightness when the whole is baked. This is the cause of the sponginess of cakes made with eggs. The viscous albumen catches the air and holds it."[41] There are other means of obtaining carbonic acid to lighten bread, besides by the growing of yeast. The most convenient, perhaps the most valuable, method is by causing cream of tartar and bicarbonate of soda to unite chemically. (The products of the union are carbonic acid and Rochelle salts.) The advantage of using these over everything else yet tried is, that they do not unite when brought in contact except in the presence of water and a certain degree of heat. Rochelle salts, taken in such minute quantities as it occurs in bread made in this way, is not harmful. Cream of tartar bread, if _perfectly_ made, is more nutritious than fermented bread, for none of the constituents of the flour are lost, as when yeast is used.[42]