The little yeast plants multiply quickly when they find something which they like to feed upon, and it happens that they like a mixture of flour and water which is neither very hot nor very cold. Therefore, when we put yeast into dough the little plants feed and multiply and in doing so change the character of the dough. They cause it to ferment, just as grape or apple juice ferments. When the carbohydrate substances in the flour, that is, the starches and sugars, ferment they change, and in the change form alcohol and carbon dioxide. When this performance is at its height, we put the dough into the oven, the yeast plant is killed by the heat and a stop thus put to its activities. Another result of putting the bread into the oven is that the bubbles of gas formed by fermentation expand with the heat. The gas escapes, but not before the walls of the bubbles have been hardened sufficiently by heat to make the bread full of tiny holes--"porous" we call it, and "light." The following receipt is a usual one for a small batch of bread: 2 quarts of flour. 1 tablespoonful of salt. 1 " " sugar. 1 " " lard. 1 half cake of yeast soaked in a cup of milk. 2 cups of milk or water. The sugar and the lard are not necessary to bread making but are frequently used; the lard because it makes bread tender and moist, the sugar to take the place of some of the sugar in the flour which is used up in fermentation. Without the other four ingredients, flour, salt, "wetting" and yeast, we could not have bread. The yeast is either a little compressed cake of useful bacteria, or it is a liquid in which this bacteria has congregated. The flour is a nourishing but unattractive substance which we wish the yeast plant to change into spongy, pleasant-flavoured, digestible food. The salt assists in making the pleasant flavour and also helps to prevent fermentation from going beyond the desired point. Unless the fermentation of bread stops at the right time, changes occur in the dough like those which take place in milk when it sours, and in cider when it turns to vinegar. Milk or water are necessary to give the flour the moist consistency which is agreeable to the growth of yeast plants. It is sometimes necessary to heat the "wetting" a little, for the temperature of the dough to be favourable to the activity of the yeast must be not less than 70° F. nor more than 90° F.