The greater number of the fatty acids belong to a few homologous series. The series to which stearic acid belongs may be represented by the general formula, CnH2nO2, and is made up of homologues of acetic acid. The principal members of physiological importance are as follows:

Acids Of The Series CNH2NO2

Butyric acid (C4H8O2) occurs as glyceride to the extent of about 5 to 6 per cent in butter and in very small quantities in a few other fats.

Caproic acid (C6H12O2) is obtained from goat and cow butter and coconut fat.

Caprylic acid (C8H16O2) is obtained from coconut oil, butter, and human fat.

Capric acid (C10H20O2) is obtained from coconut oil, butter, and the fat of the spice bush.

Lauric acid (C12H24O2) occurs abundantly as glyceride in the fat of the seeds of the spice bush, and in smaller proportions in butter, coconut fat, palm oil, and some other vegetable oils.

Myristic acid (C14H28O2) is obtained from nutmeg butter, coconut oil, butter, lard, and many other fats, as well as from spermaceti and wool wax.

Palmitic acid (C16H32O2) occurs abundantly in a great variety of fats, both animal and vegetable, including many fatty oils, and also in several waxes, including spermaceti and beeswax.

Stearic acid (C18H36O2) is found in most fats, occurring more abundantly in the solid fats and especially in those having high melting points.

Butyric acid is a mobile liquid, mixing in all proportions with water, alcohol, and ether, boiling without decomposition, and readily volatile with steam. With increasing molecular weight the acids of this series regularly show increasing boiling or melting points, decreasing solubility, and loss of volatility with steam. For ordinary temperatures the dividing line between liquids and solids falls at about capric acid. Stearic acid is a crystalline solid, insoluble in water, and only moderately soluble in alcohol and ether.

Acids Of The Series CNH2N-2O2

These are unsaturated compounds. Each molecule contains one ethylene linkage or "double bond," and can take up by addition two atoms of halogen to form a saturated compound.* These unsaturated acids have, as a rule, much lower melting points than the saturated acids containing the same number of carbon atoms. The glycerides show correspondingly lower melting points than those of the saturated fatty acids and are therefore found more largely in the soft fats and the fatty oils. Such soft fats or fatty oils can be hardened to any desired consistency (up to that of stearin) by hydrogenation, which changes the unsaturated fatty acid radicles into the corresponding members of the saturated series. In recent years this process has been exploited commercially and large quantities of refined cottonseed oil are now hydrogenated to the consistency of lard and sold under trade names as lard substitutes. Other oils are also hardened by hydrogenation.

Phycetoleic acid (C16H30O2) is obtained from seal oil and sperm oil; an isomeric acid, hypogaic, occurs in peanut oil.

Oleic acid (C18H34O2) occurs as glyceride in nearly all fats and fatty oils and is much the most important member of the series. Many of the typical oils of both animal and vegetable origin, such as lard oil and olive oil, consist mainly of olein.

Erucic acid (C22H42O2) is obtained from rape seed and mustard seed oils, and is not found in animal fats except when oils which contain this acid have been fed to the animal.

The gradual change in physical properties with increasing molecular weight which is noticeable in the stearic acid series is not apparent in this series, probably because the known acids of the series differ as regards the position of the double bond and are therefore not strictly homologous.

* The relative number of double bonds is measured analytically by determining the percentage of iodine which the fat or fatty acid will absorb. Thus pure oleic acid (mol. wt. 282) absorbs 2 atoms of iodine, giving an "iodine number" of 90; pure linoleic acid would absorb 4 atoms of iodine to the molecule, giving an "iodine number" about twice as great.

Other Unsaturated Fatty Acids

Adds of the series CnH2n-4O2, CnH2n-6O2, and CnH2n-8O2 have been found to occur as glycerides in some of the fats. Linoleic acid, C18H32O2, and linolenic acid, C18H30O2, are the best known of these acids. They are found abundantly in linseed oil and in others of the so-called "drying oils," which on account of the affinity for oxygen of their highly unsaturated glycerides are gradually oxidized to solids on exposure to the air. Fatty acids having the same number of double bonds, but not the same property of oxidizing to hard, solid films are found in fatty oils of animal origin, especially those obtained from marine animals and from fishes. Since the acids of this series have still lower melting points than the corresponding acids of the oleic series, and since the physical properties of the glycerides follow those of the fatty acids which they contain, a fat containing an acid isomeric with linoleic or linolenic acid will be more fluid at any given temperature than one containing oleic acid in the same proportion. Hence, it is apparent that glycerides of the highly unsaturated and more fluid acids are physiologically adapted to the cold-blooded animals, and it is found that they are especially abundant in fish fat; the acids of the series CnH2n-8O2 have been obtained as yet only from fish oils.