In beginning the bread lessons, it will be found that there are no new principles to teach. It will, however, be necessary to explain the new means of producing gas which is used in this particular mixture, namely, yeast. From their lessons on the "Preservation of Food" and "Canning", the pupils are already acquainted with one class of microscopic plants. The little plants, in that case, were a source of great inconvenience to the housekeeper. Yeast may be introduced as another family of one-celled plants, but one which is most useful. Under good conditions these tiny plants will produce a large amount of carbon dioxide gas, provided they are given sufficient time. If, however, the gas be required quickly, soda and acid must be used. For this reason, plain flour mixtures, in which the carbon dioxide is quickly made, are called quick breads, to distinguish them from breads in which yeast is used. Examples of these are baking-powder biscuits, gems, corn-bread, etc. The use of yeast is the simplest and cheapest way of obtaining carbonic acid gas, and mixtures so made remain moist longer than those in which baking-powder is used. Throughout the introductory lesson, this fact must be kept prominently before the class, that yeast is a plant and, as such, requires plant conditions. The necessary conditions will be known from the lesson on "Bacteria", so that they have only to be reviewed. The pupils may be told that although they cannot see the plants, they can very plainly see the bubbles of gas which the plants give off when the latter are made active under favourable conditions. LESSON I OUTLINE OF LESSONS ON YEAST 1. Description of yeast: Yeast is a one-celled plant which can be seen only with a microscope. Under good conditions it becomes very active and multiplies rapidly by a process called _budding_. It is used by the housekeeper for the carbonic acid gas it gives off. [Illustration: Yeast plants magnified] 2. Conditions necessary for the activity of yeast: (1) Oxygen (2) Water (3) Food.--This must be sugar, or starch which it will change into sugar. Potato starch is more easily used by yeast than flour starch. It uses also some nitrogenous food and mineral matter. (4) Heat.--The yeast plant thrives in a heat of about the same temperature as our bodies. A little extra heat will only make it grow faster; but excessive heat will kill it. Freezing will not kill the plant, though cold makes yeast inactive. 3. Sources of yeast: Yeast was first found as _wild yeast_ in the air, but now it may be obtained at grocery stores, in three forms: (1) Liquid yeast.--The plants are put into a starchy liquid. This will keep only a few days, as the starch sours.