=Cholesterol and phytosterol= are empirical names for certain closed-ring, monohydric alcohols which are found in relatively small amounts in all fats, the former term designating those found in animal fats and the latter those of plant origin. Their composition has not yet been definitely established. They are known to contain two, or three, closed rings, probably of the phenanthrene type; to form dichlor- and dibrom- addition products, showing that they contain one side-chain double linkage; and to yield ketones when oxidized, indicating that they are secondary alcohols. They form acetyl esters, or acetates, which can be separated from each other and identified by their crystal forms and melting points. Because of this fact and of the further fact that they are present in detectable quantities in practically all fats and oils, they afford a qualitative means of distinguishing between fats of animal and of plant origin. This possibility is the most interesting fact known concerning these complex alcohols; although their presence as esters in all plant and animal fats indicates that they must have some biological function. _Phytosterol_ is not a single alcohol, but a mixture of at least two, which have been separated and studied as _sitosterol_, C_{27}H_{43}OH, and _stigmasterol_, C_{30}H_{49}OH. As has been said, these are found in small proportions in all vegetable fats, being present in largest amounts in oily seeds, especially those of the legumes. The saponification of esters of cholesterol and phytosterol is a difficult and unsatisfactory process; but since this affords the only known means to distinguish between fats of plants and of animal origin, its technique has been fairly well worked out, and the process used in the study of the changes which take place in plant fats when they are used by animals as food. HYDROLYSIS AND SYNTHESIS OF FATS The reaction for the hydrolysis of fats has been discussed in connection with the process for the manufacture of glycerine. This reaction takes place very slowly with cold water alone, can be easily brought about by the action of superheated steam, and much more easily and rapidly in the presence of some catalyst (sulfuric acid is an especially effective catalyst for this purpose).