Since the molecular weight of caustic potash (56) is greater than that of caustic soda (40) more potash is required to saponify a pound of fat. The resulting potash soap is correspondingly heavier than a soda soap. When salt is added to a potassium soap double decomposition occurs, the potassium soap being transformed to a sodium soap and the potassium uniting with the chlorine to form potassium chloride. This was one of the earliest methods of making a hard soap, especially in Germany, where potash was derived from leeching ashes of burned wood and plants. SODIUM CARBONATE (SODA ASH). While carbonate of soda is widely distributed in nature the source of supply is entirely dependent upon the manufactured product. Its uses are many, but it is especially important to the soap industry in the so called carbonate saponification of free fatty acids, as a constituent of soap powders, in the neutralization of glycerine lyes and as a filler for laundry soaps. The old French Le Blanc soda process, which consists in treating common salt with sulphuric acid and reducing the sodium sulphate (salt cake) thus formed with carbon in the form of charcoal or coke to sodium sulphide, which when treated with calcium carbonate yields a mixture of calcium sulphide and sodium carbonate (black ash) from which the carbonate is dissolved by water, has been replaced by the more recent Solvay ammonia soda process. Even though there is a considerable loss of salt and the by-product calcium chloride produced by this process is only partially used up as a drying agent, and for refrigerating purposes, the Le Blanc process cannot compete with the Solvay process, so that the time is not far distant when the former will be considered a chemical curiosity. In the Solvay method of manufacture sodium chloride (common salt) and ammonium bicarbonate are mixed in solution. Double decomposition occurs with the formation of ammonium chloride and sodium bicarbonate. The latter salt is comparatively difficultly soluble in water and crystallizes out, the ammonium chloride remaining in solution. When the sodium bicarbonate is heated it yields sodium carbonate, carbon dioxide and water; the carbon dioxide is passed into ammonia which is set free from the ammonium chloride obtained as above by treatment with lime (calcium oxide) calcium chloride being the by-product. Sal soda or washing soda is obtained by recrystallizing a solution of soda ash in water. Large crystals of sal soda containing but 37% sodium carbonate are formed. POTASSIUM CARBONATE. Potassium carbonate is not extensively used in the manufacture of soap. It may be used in the forming of soft soaps by uniting it with free fatty acids. The methods of manufacture are the same as for sodium carbonate, although a much larger quantity of potassium carbonate than carbonate of soda is obtained from burned plant ashes. Purified potassium carbonate is known as _pearl ash_. ADDITIONAL MATERIAL USED IN SOAP MAKING.