Whilst still fluid and warm, add to it fifteen pounds of lime, previously slaked, and made into a milky mixture with two and a half gallons of water; then cover the vessel, and continue the steaming for several hours, or until the saponification shall be completed.  This may be known when a sample of the soap when cold gives a smooth and bright surface on being scraped with the finger-nail, and at the same time, breaks with a crackling noise.  By this process the fat or oil is decomposed, its acids uniting with the lime to form insoluble lime-soap, while the eliminated glycerine remains in solution in the water along with the excess of the lime.  After it has been sufficiently boiled, it is allowed to cool and to settle, and it is then to be strained.

The strained liquid contains only the glycerine and excess of lime, and requires to be carefully concentrated by heated steam.  During evaporation, a portion of the lime is deposited, on account of its lesser solubility in hot than in cold water.  The residue is removed by treating the evaporated liquid with a current of carbonic acid gas, boiling by heated steam to convert a soluble bicarbonate of lime that may have been formed into insoluble neutral carbonate, decanting or straining off the clear supernatant liquid from the precipitated carbonate of lime, and evaporating still further, as before, if necessary, so as to drive off any excess of water.  As nothing fixed or injurious is employed in this process, glycerine, prepared in this manner, may be depended upon for its almost absolute purity.

M. Jahn’s process is as follows:—­

Take of finely-powdered litharge five pounds, and olive oil nine pounds.  Boil them together over a gentle fire, constantly stirring, with the addition occasionally of a small quantity of warm water, until the compound has the consistence of plaster.  Jahn boils this plaster for half an hour with an equal weight of water, keeping it at the same time constantly stirred.  When cold, he pours off the supernatant fluid, and repeats the boiling three times at least with a fresh portion of water.  The sweet fluids which result are mixed, and evaporated to six pounds, and sulphuretted hydrogen conducted through them as long as sulphuret of lead is precipitated.  The liquid filtered from the sulphuret of lead is to be reduced to a thin syrupy consistence by evaporation.  To remove the brown coloring matter, it must be treated with purified animal charcoal.  However, this agent does not prevent the glycerine becoming slightly colored upon further evaporation.  It possesses also still a slight smell and taste of lead plaster, which may be removed by diluting it with water, and by digestion with animal charcoal, and some fresh burnt-wood charcoal.  After filtration, this liquid must be evaporated until it has acquired a specific gravity of 1.21, when it will be found to be free from smell, and of a pale yellow color.  For the preparation of glycerine, distilled water is necessary, to prevent it being contaminated with the impurities of common water.  Jahn obtained, by this method, from the above quantity of lead plaster, upwards of seven ounces of glycerine.—­Archives der Pharmacie.