FERMENTATIVE AGENTS.--Fermentation in vegetable matter is always accompanied by the growth of living organisms. The development of these minute organisms is the exciting cause of fermentation and putrefaction. The germs or spores of some of these fermenting agents are always present in the air. It is well known to housekeepers that if a batter of flour and water and a little salt be kept in a jar of water at a temperature of from 100° to 110°, it will ferment in the course of five or six hours. Scientists assure us that this fermentation is occasioned by the introduction of the spores of certain species of fungi which are continually floating in the atmosphere, and the proper conditions of warmth and moisture being supplied, they at once begin to grow and multiply. This method of securing fermentation is utilized by housewives in making what is termed salt-rising bread. The raising of dough by this process is lengthy and uncertain, and a far more convenient method is to accelerate the fermentation by the addition of some active ferment. The ancient method of accomplishing this was by adding to the dough a leaven, a portion of old dough which had been kept until it had begun to ferment; but since the investigations of modern chemistry have made clear the properties of yeast, that has come to be considered the best agent for setting up the process of alcoholic fermentation in bread. The use of leaven is still practiced to somewhat in some European countries. The bread produced with leaven, although light and spongy in texture, has an unpleasant, sour taste, and is much less wholesome than that produced with fresh yeast. Yeast is a collection of living organisms or plants belonging to the family of fungi, which, like all other plants, require warmth, moisture, and food, in order to promote growth, and when properly supplied with these, they begin to grow and multiply rapidly. Fermentation will not take place at a temperature below 30°, it proceeds slowly at 45°, but from 70° to 90° it goes on rapidly. Fermentation may be arrested by the exhaustion of either the fermenting agent or the food supply, or by exposure to heat at the temperature of boiling water. This latter fact enables the housewife to arrest the process of fermentation, when the loaf has become sufficiently light, by baking it in a hot oven. Heat destroys most of the yeast cells; a few, however, remain in the loaf unchanged, and it is for this reason that yeast bread is considered less wholesome for dyspeptics than light unleavened bread. It is apparent, then, that the more thoroughly fermented bread is baked, the more wholesome it will be, from the more complete destruction of the yeast germs which it contains.