The recovery of glycerine is very closely allied with the soap-making industry, because glycerine is the very valuable by-product obtained in the saponification of oils and fats. No soap plant is, therefore, fully equipped unless it has some method whereby the glycerine is recovered and the importance of recovering this product cannot be too strongly emphasized. It has already been pointed out that neutral fats or the glycerides are a combination of fatty acid with glycerine. These are split apart in the process of saponification. While by the term _saponification_ as used in soap making it is inferred that this is the combination of caustic alkalis with the fatty acids to form soap, this term is by no means limited to this method of saponification, as there are various other methods of saponifying a fat. The chemical definition of saponification is the conversion of an ester, of which glycerides are merely a certain type, into an alcohol and an acid or a salt of this acid. Thus, if we use caustic alkali as our saponifying agent for a fat or oil, we obtain the sodium or potassium salt of the higher fatty acids or soap and the alcohol, glycerine. On the other hand, if we use a mineral acid as the saponifying agent, we obtain the fatty acids themselves in addition to glycerine. While the former is by far the most generally employed for making soap, other processes consist in saponifying the fats by some method other than caustic alkalis and then converting the fatty acids into soap by either neutralizing them with sodium or potassium carbonate or hydrate. It is important to again point out here that fats and oils develop free fatty acid of themselves and that the development of this acid represents a loss in glycerine. The selection of an oil or fat for soap making should therefore to a large extent be judged as to its adaptability by the free fatty acid content, as the higher this content is, the greater is the loss in the glycerine eventually obtained. Glycerine often represents the only profit to a soap manufacturer. It is indeed necessary to determine the percentage of free fatty acid before purchasing a lot of stock to be made into soap. In taking up the question of glycerine recovery we will consider the various methods thus: 1. Where the glycerine is obtained from spent lye by saponifying the fats or oils with caustic alkali. 2. Where the glycerine is obtained by saponifying the fats or oils by some other method than the above, of which there are the following: (a) Twitchell process. (b) Saponification by lime in autoclave. (c) Saponification by acid. (d) Saponification by water in autoclave. (e) Fermentative (Enzymes). (f) Krebitz process. RECOVERY OF GLYCERINE FROM SPENT LYE.