In Chapter V. processes of soap-making are described in which (1) the glycerine is retained in the finished soap, and (2) the glycerine is contained in the lyes, in very dilute solution, contaminated with salt and other impurities. These lyes, though now constituting the chief source of profit in the manufacture of cheap soaps, were till early in last century simply run down the drains as waste liquor.
Much attention has been devoted to the purification and concentration of glycerine lyes; and elaborate plant of various forms has been devised for the purpose.