CHAPTER X FATS AND OILS, WAXES, AND LIPOIDS Included in this group are several different kinds of compounds which have similar physical properties, and which, in general, belong to the type of organic compounds known as esters, i.e., alcoholic salts of organic acids. The terms "oil," "fat," and "wax," are generally applied more or less indiscriminately to any substance which has a greasy feeling to the touch and which does not mix with, but floats on, water. There are many oils which are of mineral origin which are entirely different in composition from natural fats. These have no relation to plant life and will not be considered here. The natural fats, vegetable oils, and plant waxes are all esters. There is no essential difference between a fat and an oil, the latter term being usually applied to a fat which is liquid at ordinary temperatures. The waxes, however, are different in chemical composition from the fats and oils, being esters of monohydric alcohols of high molecular weight, such as cetyl alcohol, C_{16}H_{33}OH, myristic alcohol, C_{30}H_{61}OH, and cholesterol, C_{27}H_{45}OH; whereas the fats and oils are all esters of the trihydric alcohol glycerol, C_{3}H_{5}(OH)_{3}. Lipoids are much more complex esters, having some nitrogenous, or phosphorus-containing, group and sometimes a sugar in combination with the fatty acids and glycerol which make up the characteristic part of their structure. In general, waxes and lipoids have a harder consistency than fats: but this is not always the case, since "wool-fat" and spermaceti, both of which are true waxes in composition, are so nearly liquid in form as to be commonly called fats; while certain true fats, like "Japan wax," are so hard as to be commonly designated as waxes. It is plain that physical properties alone cannot be relied upon in the classification of these bodies. In fact, there is no single definite property by which members of this group can be accurately identified. There are many other types of substances belonging to entirely different chemical groups, which have oily, or fat-like, properties. A. FATS AND OILS OCCURRENCE