~Chemical changes in bread making.~ The average composition of wheat flour is as follows: Water. 13.8% Protein (nitrogenous matter) 7.9 Fats 1.4 Starch 76.4 Mineral matter 0.5 In making bread the flour is mixed with water and yeast, and the resulting dough set aside in a warm place for a few hours. The yeast first converts a portion of the starch into dextrose or a similar sugar, which then undergoes alcoholic fermentation. The carbon dioxide formed escapes through the dough, making it light and porous. The yeast plant thrives best at about 30 deg.; hence the necessity for having the dough in a warm place. If the temperature rises above 50 deg., the vitality of the yeast is destroyed and fermentation ceases. In baking the bread, the heat expels the alcohol and also expands the bubbles of carbon dioxide caught in the dough, thus increasing its lightness. SOME DERIVATIVES OF BENZENE Attention has been called to the complex nature of coal tar. Among the compounds present are the hydrocarbons, benzene, toluene, naphthalene, and anthracene. These compounds are not only useful in themselves but serve for the preparation of many other important compounds known under the general name of coal-tar products. ~Nitrobenzene~ (_oil of myrbane_) (C_{6}H_{5}NO_{2}). When benzene is treated with nitric acid a reaction takes place which is expressed by the following equation: C_{6}H_{6} + HNO_{3} = C_{6}H_{5}NO_{2} + H_{2}O. The product C_{6}H_{5}NO_{2} is called nitrobenzene. It is a slightly yellowish poisonous liquid, with a characteristic odor. Its main use is in the manufacture of aniline. ~Aniline~ (C_{6}H_{5}NH_{2}). When nitrobenzene is heated with iron and hydrochloric acid the hydrogen evolved by the action of the iron upon the acid reduces the nitrobenzene in accordance with the following equation: C_{6}H_{5}NO_{2} + 6H = C_{6}H_{5}NH_{2} + 2H_{2}O. The resulting compound is known as aniline, a liquid boiling at 182 deg.. When first prepared it is colorless, but darkens on standing. Large quantities of it are used in the manufacture of the _aniline or coal-tar dyes_, which include many important compounds. ~Carbolic acid~ (C_{6}H_{5}OH). This compound, sometimes known as _phenol_, occurs in coal tar, and is also prepared from benzene. It forms colorless crystals which are very soluble in water. It is strongly corrosive and very poisonous.