This process is known as _saponification_, since it is the one which takes place in the manufacture of soaps. The ordinary soaps are made by heating fats with a solution of sodium hydroxide. The reactions involved may be illustrated by the following equation representing the reaction between palmitin and sodium hydroxide: C_{3}H_{5}(C_{16}H_{31}O_{2})_{3} + 3 NaOH = 3 NaC_{16}H_{31}O_{2} + C_{3}H_{5}(OH)_{3}. In accordance with this equation the ethereal salts in the fats are converted into glycerin and the sodium salts of the corresponding acids. The sodium salts are separated and constitute the soaps. These salts are soluble in water. When added to water containing calcium salts the insoluble calcium palmitate and stearate are precipitated. Magnesium salts act in a similar way. It is because of these facts that soap is used up by hard waters. ETHERS When ethyl alcohol is heated to 140 deg. with sulphuric acid the reaction expressed by the following equation takes place: 2C_{2}H_{5}OH = (C_{2}H_{5})_{2}O + H_{2}O. The resulting compound, (C_{2}H_{5})_{2}O, is ordinary ether and is the most important member of the class of compounds called _ethers_. Ordinarily ether is a light, very inflammable liquid boiling at 35 deg.. It is used as a solvent for organic substances and as an anaesthetic in surgical operations. KETONES The most common member of this group is acetone (C_{3}H_{6}O), a colorless liquid obtained when wood is heated in the absence of air. It is used in the preparation of other organic compounds, especially chloroform. ORGANIC BASES This group includes a number of compounds, all of which contain nitrogen as well as carbon. They are characterized by combining directly with acids to form salts, and in this respect they resemble ammonia. They may, indeed, be regarded as derived from ammonia by displacing a part or all of the hydrogen present in ammonia by hydrocarbon radicals. Among the simplest of these compounds may be mentioned methylamine (CH_{3}NH_{2}) and ethylamine (C_{2}H_{5}NH_{2}). These two compounds are gases and are formed in the distillation of wood and bones. Pyridine (C_{5}H_{6}N) and quinoline (C_{9}H_{7}N) are liquids present in small amounts in coal tar, and also in the liquid obtained by the distillation of bones. Most of the compounds now classified under the general name of _alkaloids_ (which see) also belong to this group. CARBOHYDRATES The term "carbohydrate" is applied to a class of compounds which includes the sugars, starch, and allied bodies These compounds contain carbon, hydrogen, and oxygen the last two elements generally being present in the proportion in which they combine to form water. The most important members of this class are the following: