This making of bread light is usually accomplished by the introduction of air into the dough, or by carbonic acid gas generated within the mass, either before or during the baking, by a fermentative or chemical process. When air is the agency used, the gluten, by its glue-like properties, catches and retains the air for a short period; and if heat is applied before the air, which is lighter than the dough, rises and escapes, it will expand, and in expanding distend the elastic glutinous mass, causing it to puff up or rise. If the heat is sufficient to harden the gluten quickly, so that the air cells throughout the whole mass become firmly fixed before the air escapes, the result will be a light, porous bread. If the heat is not sufficient, the air does not properly expand; or if before a sufficient crust is formed to retain the air and form a framework of support for the dough, the heat is lessened or withdrawn, the air will escape, or contract to its former volume, allowing the distended glutinous cell walls to collapse; in either case the bread will be heavy. If carbonic acid gas, generated within the dough by means of fermentation or by the use of chemical substances, be the means used to lighten the mass, the gluten by virtue of its tenacity holds the bubbles of gas as they are generated, and prevents the large and small ones from uniting, or from rising to the surface, as they seek to do, being lighter than the dough. Being thus caught where they are generated, and the proper conditions supplied to expand them, they swell or raise the dough, which is then termed a loaf. (This word "loaf" is from the Anglo-Saxon _hlifian_, to raise or lift up.) The structure is rendered permanent by the application of heat in baking. BREAD MADE LIGHT BY FERMENTATION. For general use, the most convenient form of bread is usually considered to be that made from wheat flour, raised or made light by some method of fermentation, although in point of nutritive value and healthfulness, it does not equal light, unfermented, or aërated bread made without the aid of chemicals.