At the outbreak of war, the German supplies of potassium compounds ceased as far as the allied nations were concerned, and an older method of making potassium chloride from _orthoclase_ or potash-felspar was revived. This involves the heating of the powdered mineral to a high temperature after mixing it with calcium chloride, lime, and a little fluorspar. The potassium chloride is then extracted from the fused mass with water. This method has been worked with great success in America, and it is claimed that potassium chloride can be made in that country at a cost which is lower than that formerly paid for the imported article. Mild potash and soda are so very similar in chemical properties that in most cases it is immaterial which compound is used. In all cases in which there is this choice, soda is employed, both because it is cheaper and because it is more economical, for 106 parts of soda ash are equivalent to 138 parts of potash. There are, however, some occasions when soda cannot be substituted, notably for the manufacture of hard glass and soft soap, and for the preparation of caustic potash, potassium dichromate, and other potassium salts. Potassium Bicarbonate. This resembles the corresponding sodium salt in nearly every respect. It is, however, much more readily soluble in water, so much so, that it is not possible to obtain this substance by the Solvay method. It is made from potassium carbonate by saturating a strong aqueous solution of that substance with carbon dioxide. CHAPTER IX CAUSTIC ALKALIS The Alkali Metals. The discovery of current electricity in 1790 furnished the chemist with a very powerful agency for bringing about the decomposition of compounds. Hydrogen and oxygen were soon obtained by passing an electric current through acidulated water; and in 1807, Sir Humphry Davy, who is perhaps better remembered for his invention of the miners’ lamp, isolated the metals sodium and potassium by subjecting caustic soda and caustic potash respectively to the action of the current.