(4) Ricinoleic acid, C_{17}H_{32}OH·COOH, occurs in castor oil. (_c_) Linoleic acid series--general formula, C_{_n_}H_{2_n_-3}·COOH. (1) Linoleic acid, C_{17}H_{31}·COOH, occurs in linseed and other drying oils. (_d_) Linolenic acid series--general formula, C_{_n_}H_{2_n_-5}·COOH. (1) Linolenic acid, C_{17}H_{29}·COOH, occurs in many drying oils. It will be observed that all of these acids contain a multiple of two total carbon atoms. No acid containing an uneven number of carbon atoms has been found in a natural fat. Furthermore, the acids which occur most commonly in natural fats are those which contain eighteen carbon atoms; in fact, more than 80 per cent of the glycerides which compose all animal and vegetable fats are those of the C_{18} acids. This fact, in addition to the one that the sugars and starches all contain multiples of six carbon atoms in their molecules, indicates a very great biological significance of the chain of six carbon atoms. This has been alluded to in connection with the discussion of the biological significance of molecular configuration (see page 57) and will be mentioned again in other connections. THE ALCOHOLS WHICH OCCUR IN NATURAL FATS =Glycerol=, as has been pointed out, is by far the most common alcoholic constituent of natural fats and oils. This substance, which is familiar to everyone under its common name "glycerine," is a colorless, viscid liquid having a sweetish taste. It is a very heavy liquid (specific gravity 1.27) which mixes with water in all proportions and when in concentrated form is very hygroscopic. Glycerine is made from fats and oils by commercial processes which clearly prove that the constitution of fats is as described above. The fat is boiled with a solution of caustic soda and is decomposed, the sodium of the alkali taking the place of the glyceryl (C_{3}H_{5}) group, the latter combining with three (OH) groups from the three molecules of alkali necessary to decompose the fat. A sodium salt of the organic acid, or soap, and glycerol are thus produced, and are separated by saturating the hot solution with common salt, which causes the soap to separate out as a layer on the surface of the liquid, which, on cooling, solidifies into a solid cake, which is then cut and pressed into the familiar bars of commercial soap. From the remaining solution, the glycerine is recovered by evaporation and distillation under reduced pressure. Taking stearin, a common fat, as the example, the reaction which takes place in the above process may be expressed by the following equation: C_{3}H_{5}(C_{17}H_{35}·COO)_{3} + 3NaOH = 3C_{17}H_{35}COONa + Stearin Sodium stearate--a soap C_{3}H_{5}(OH)_{3}. Glycerol