The acids which, when combined with glycerol, produce fats are of two general types. The first of these are the so-called "fatty acids" having the general formula C_{_n_}H_{2_n_+1}·COOH. These are the "saturated" acids, i.e., they contain only single-bond linkages in the radical which is united to the ·COOH group; hence, they cannot take up hydrogen, oxygen, etc., by direct addition. The second type are the "unsaturated" acids belonging to several different groups, as discussed below, but all having one or more double-linkages between the carbon atoms of the alkyl radical which they contain. Because of these double linkages, they are all able to take on oxygen, hydrogen, or the halogen elements, by direct addition. When exposed to the air, for example, these "unsaturated" acids, or the oils derived from them, take up oxygen, increasing in weight, and becoming solid or hard and stiff. Hence, natural oils which contain considerable proportions of glycerides of these "unsaturated" acids are known as "drying oils" and are largely used in the manufacture of paints, varnishes, linoleums, etc.; while oils which contain little of these glycerides are known as "non-drying," and are used for food, for lubrication, or for other technical purposes in which it is essential that they remain in unchanged fluid condition when exposed to the air. The following are some of the more important of the acids which occur as glycerides in natural fats: Saturated Acids: (_a_) Acetic, or stearic, acid series--general formula, C_{_n_}H_{2_n_+1}·COOH. (1) Formic acid, H·COOH, occurs free in nettles, ants, etc. (2) Acetic acid, CH_{3}·COOH, occurs free in vinegar. (3) Butyric acid, C_{3}H_{7}·COOH, in butter fat. (4) Capric acid, C_{9}H_{19}·COOH, in butter fat and cocoanut oil. (5) Myristic acid, C_{13}H_{27}·COOH, in cocoanut oil and spermaceti. (6) Palmitic acid, C_{15}H_{31}·COOH, in palm oil and many fats. (7) Stearic acid, C_{17}H_{35}·COOH, in most fats and oils. Intervening members of this series, such as caprylic acid, C_{7}H_{15}·COOH, and lauric acid, C_{11}H_{23}·COOH, are also found in smaller quantities in cocoanut and palm nut oils, in butter fat, and in spermaceti; while higher members of the series, as arachidic acid, C_{19}H_{39}·COOH, and lignoceric acid, C_{23}H_{47}·COOH, are found in peanut oil; and cerotic acid, C_{25}H_{51}·COOH, and melissic acid, C_{29}H_{59}·COOH, in beeswax and carnauba wax. Unsaturated Acids: (_b_) Oleic acid series--general formula, C_{_n_}H_{2_n_-1}·COOH. (1) Crotonic acid, C_{3}H_{5}·COOH, occurs in croton oil. (2) Oleic acid, C_{17}H_{33}·COOH, occurs in many fats and oils. (3) Brassic acid, C_{21}H_{41}·COOH, occurs in rape-seed oil.