Chemical substances used instead of yeast in bread-making, their action being to give off carbonic acid gas, which puffs up the dough, or causes the effect called "rising." They are usually composed of tartaric acid and bicarbonate of soda. A chemical action takes place in these in contact with the wet dough, and the gas is given off. Other substances are used, alum being common, though it is thought to be injurious; also acid phosphate of calcium. The use of baking-powders has grown enormously in recent years, and yeast is little used.