After mixing the materials thoroughly by a process called "kneading," the bread is put aside in a warm place (about 75 deg. Fahrenheit) to "rise." If we examine the dough at this time, we find it filled with holes, which give the mass a spongy appearance. The yeast plants, owing to favorable conditions, have grown rapidly and filled the cavities with carbon dioxide. Alcohol is present, too, but this is evaporated when the dough is baked. The baking cooks the starch of the bread, drives off the carbon dioxide and alcohol, and kills the yeast plants, besides forming a protective crust on the loaf. Sour Bread.--If yeast cakes are not fresh, sour bread may result from their use. In such yeast cakes there are apt to be present other tiny one-celled plants, known as _bacteria_. Certain of these plants form acids after fermentation takes place. The sour taste of the bread is usually due to this cause. The remedy would be to have fresh yeast, to have good and fresh flour, and to have clean vessels with which to work. Importance of Yeasts.--Yeasts in their relation to man are thus seen to be for the most part useful. They may get into canned substances put up in sugar and cause them to "work," giving them a peculiar flavor. But they can be easily killed by heating to the temperature of boiling. On the other hand, yeast plants are necessary for the existence of all the great industries which depend upon fermentation. And best of all they give us leavened bread, which has become a necessity to most of mankind. BACTERIA IN THEIR RELATION TO MAN What Bacteria do and Where They May be Found.--A walk through a crowded city street on any warm day makes one fully alive to odors which pervade the atmosphere. Some of these unpleasant odors, if traced, are found to come from garbage pails, from piles of decaying fruit or vegetables, or from some butcher shop in which decayed meat is allowed to stand. This characteristic phenomena of decay is one of the numerous ways in which we can detect the presence of bacteria. These tiny plants, "man's invisible friends and foes," are to be found "anywhere, but not everywhere," in nature. They swarm in stale milk, in impure water, in soil, in the living bodies of plants and animals and in their dead bodies as well. Most "catching" diseases we know to be caused directly by them; the processes of decay, souring of milk, acid fermentation, the manufacture of nitrogen for plants are directly or indirectly due to their presence. It will be the purpose of the next paragraphs to find some of the places where bacteria may be found and how we may know of their presence. [Illustration: A steam sterilizer.]