Beer and Wine Making.--Brewers' yeasts are cultivated with the greatest care; for the different flavors of beer seem to depend largely upon the condition of the yeast plants. Beer is made in the following manner. Sprouted barley, called malt, in which the starch of the grain has been changed to grape sugar by digestion, is killed by drying in a hot kiln. The malt is dissolved in water, and hops are added to give the mixture a bitter taste. Now comes the addition of the yeast plants, which multiply rapidly under the favorable conditions of food and heat. Fermentation results on a large scale from the breaking down of the grape sugar, the alcohol remaining in the fluid, and the carbon dioxide passing off into the air. At the right time the beer is stored either in bottles or casks, but fermentation slowly continues, forming carbon dioxide in the bottles. This gives the sparkle to beer when it is poured from the bottle. In wine making the wild yeasts growing on the skin of the grapes set up a slow fermentation. It takes several weeks before the wine is ready to bottle. In sparkling wines a second fermentation in the bottles gives rise to carbon dioxide in such quantity as to cause a decided frothing when the bottle is opened. Commercial Yeast.--Cultivated yeasts are now supplied in the home as compressed or dried yeast cakes. In both cases the yeast plants are mixed with starch and other substances and pressed into a cake. But the compressed yeast cake must be used fresh, as the yeast plants begin to die rapidly after two or three days. The dried yeast cake, while it contains a much smaller number of yeast plants, is nevertheless probably more reliable if the yeast cannot be obtained fresh. [Illustration: _a_ _b_ _c_] The cut illustrates an experiment that shows how yeast plants depend upon food in order to grow. In each of three fermentation tubes were placed an equal amount of a compressed yeast cake. Then tube _a_ was filled with distilled water, tube _b_ with a solution of glucose and water, and tube _c_ with a nutrient solution containing nitrogenous matter as well as glucose. The quantity of gas (CO{2}) in each tube is an index of the amount of growth of the yeast cells. In which tube did the greatest growth take place? Bread Making.--Most of us are familiar with the process of bread making. The materials used are flour, milk or water or both, salt, a little sugar to hasten the process of fermentation, or "_rising_," as it is called, some butter or lard, and yeast.