=Recovery and Purification of Grease.=--The degreasing operation, which is applied usually to bones and to skin glue scutch, was once brought about by steaming only, but is now accomplished with the assistance of fat solvents, though in the latter case steaming together with mechanical centrifugal force has proved sufficiently successful. On the Continent carbon disulphide was once largely used as solvent, and in this country benzene has been employed, but their low volatility and high inflammability, as well as their expense, make both these substances somewhat unsuitable, and it is now usual to make use of petroleum oils, whether Scotch, American or Russian. A fraction which boils about the same temperature as water is usually employed, and all of it must be volatile under 280° F. Before the actual grease extraction the bones should be sorted over and unsuitable substances (horns, gravel, iron, etc.) removed. They are also usually put through a mill and roughly crushed or broken. The actual grease extraction plant consists of large copper vessels which will each take 5 tons of bones. These extractors are arranged in sets so that the degreasing is proceeding in some whilst the others are being emptied and recharged. The doors for charging and emptying must be securely fastened. When the extractor is charged the solvent is run in and heated by a steam coil which eventually causes it to distil. After some hours the remainder, which has dissolved much grease, is run off, and a fresh lot of solvent is added and heated up. After four such extractions only about 1/4 per cent. of grease remains in the bones. To remove the remainder of the solvent high-pressure steam (80 lbs.) is blown through the bones. The extractor is then opened and the degreased and somewhat dried bones are mechanically conveyed to the cleansing mill. The grease solutions obtained are subjected again to steam with a view to removing the solvent and obtaining it for repeated use in this sense. The efficient distillation and recovery of the solvent is indeed an essential element in the success of the process. The greases obtained, whether by the use of fat solvents or by skimming off during extraction, or in any other way, are mixed together as is appropriate to their origin and purity, and subjected to further purification, the object of which is to remove gelatinous and albuminous matters, and to decompose lime or soda soaps. The precise methods of purification are, of course, dependent mainly upon the impurities known to be present, but the readiest method is to give the grease further steaming or boiling with water, and so effect by washing and by solvent action the elimination of non-fatty matters. In many cases it is found advantageous to employ mineral acids or oxidizing agents to assist the process. The process may be repeated as often as is desired.