Fats and oils are identified by determinations of their physical properties, such as specific gravity, melting point, refractive index, etc., and by certain special color reactions for particular oils; or by measurements of certain chemical constants, such as the percentage of free fatty acids which they contain, the saponification value (i.e., the number of milligrams of KOH required to completely saponify one gram of the fat), the iodine number (percentage by weight of iodine which is absorbed by the unsaturated fatty acids present in the fat), percentage of water-insoluble fatty acids obtained after saponification and acidifying the resultant soap, etc., etc. Most of these tests must be carried out under carefully controlled conditions in order to insure reliable identifications, and need not be discussed in detail here. Full directions for making such tests, together with tables of standard values for all common fats and oils, may be found in any reference book on oil analysis. PHYSIOLOGICAL USE OF FATS AND OILS In animal organisms, fats are the one important form of energy storage. They also form one of the most important supplies of energy reserve material in plants. Carbohydrates commonly serve this purpose in those plants whose storage reservoirs are in the stems, tubers, etc.; but in most small seeds the reserve supply of energy is largely in the form of oil, and even in those seeds which have large endosperm storage of starch, the embryo is always supplied with oil which seems to furnish the energy necessary for the first germinative processes. Fats are the most concentrated form of potential energy of all the different types of organic compounds which are elaborated by plants. This is because they contain more carbon and hydrogen and less oxygen in the molecule than any other group of substances of vegetable (or animal) origin. It has been pointed out that a quantity of fat capable of yielding 100 large calories of heat will occupy only about 12 cc. of space, whereas from 125 to 225 cc. of space in the same tissue would be required for the amount of starch of glycogen necessary to yield the same amount of heat, or energy, when oxidized. The fats undoubtedly catabolize first by hydrolysis into glycerol and fatty acids, and then by oxidation possibly first into carbohydrates and then finally into the end-products of oxidation, namely, carbon dioxide and water. The following hypothetical equation to represent the oxidation of oleic acid into starch, suggested by Detmer, is interesting as a suggestion of how much oxygen is required and how much heat would be liberated by such a transformation: C_{18}H_{34}O_{2} + 27O = 2(C_{6}H_{10}O_{5}) + 6CO_{2} + 7H_{2}O Complete oxidation of oleic acid to the final end-products, carbon dioxide and water, would require much more oxygen, thus: C_{18}H_{34}O_{2} + 51O = 18CO_{2} + 17H_{2}O.