=60. Yeast.=--Yeast will make all sweet things ferment. Bakers make yeast grow in bread sponge. Yeast is alive. It is made of millions of tiny round cells. New cells sprout out from the side of the old cells like young lilies on an old lily bulb. Soon each new cell breaks off and lives all by itself. In a single night enough new cells will form to fill the whole loaf of bread. [Illustration: =Yeast plant cells (×500).=] =61. How yeast makes alcohol.=--Yeast will grow only where sugar is. When it has grown for some time there is no more sugar, and instead of a sweet taste there is a sharp or sour taste. The yeast has changed the sugar to alcohol. All alcohol is made from sugar by yeast. The seeds of the yeast plant are everywhere in the air. Some are on the skins of fruit and so are found in the juice when it is squeezed out. There they begin to grow at once and soon change the sugar to alcohol. They do this by taking a gas away from the sugar. The gas rises in little bubbles, and makes a froth upon the top of the juice. Boiling kills the yeast plant. If the juice is at once put into tight jars no new yeast plants can get in, and so the juice keeps. =62. Vinegar.=--Sometimes fruit juice turns sour. The sourness is due to vinegar. Besides yeast, other little living plants fall into the juice and turn the sugar to vinegar. But if there is much alcohol in the juice, the vinegar plants will not grow. =63. Yeast in bread.=--Growing yeast plants always make alcohol. They change some of the sugar of bread dough to alcohol and a gas. The gas bubbles through the bread and makes it light. When bread is baked, the heat of the oven drives off the alcohol, and so we do not eat any in bread. =64. Alcohol.=--Alcohol is a clear liquid and looks like water. It has a sharp taste and smell. It burns very easily and makes a very hot flame. Its smoke cannot be seen, and its flame will not make anything black, as a match flame will do. =65. Use of alcohol.=--Alcohol will dissolve more things than water will dissolve. It is used to dissolve drugs, varnishes, perfumery, and many other things. It will dissolve even oil and fat. Tailors clean grease spots from clothes with it. It takes water away from flesh and makes it dry, hard, and tough. It will keep anything from rotting. In museums we pour alcohol over pieces of flesh or plants in glass jars. Then they will keep and we can look at them at any time. Thus alcohol is a very useful thing, and we could hardly do without it. =66. Strong drink.=--Some men use alcohol in a wrong way. They swallow it as a drink. But men cannot drink pure alcohol, for it would burn their mouths. They always drink it mixed with some water. Alcohol in water is called _strong drink_.