In the new method the ordinary fehling solution is employed and it is prepared for use in the following way: Ten cubic centimeters of the fehling solution, or half that quantity of each of the component parts kept in separate bottles, are placed in a porcelain dish with forty cubic centimeters of water and brought to the boiling-point. To the boiling liquid is added, from a pipette, a five per cent solution of potassium cyanid until the blue color just disappears, or only a very faint tint of blue remains, avoiding any excess of the cyanid. A second portion of the fehling solution equal to that first employed is added, and to the boiling mixture the solution of sugar is added, from a burette, until the blue color disappears. The contents of the dish should be kept boiling during the addition of the sugar solution. The volume used will contain fifty milligrams of dextrose. The sugar solution should be of such a strength as to contain no more than half a per cent of reducing sugar. The principle of the preparation of the solution may be stated as follows: If to a solution of copper sulfate, potassium be added until the blue color disappears, a double cyanid of copper and potassium cyanid is formed according to the following reaction: CuSO₄ + 4KCN = Cu(CN)₂.2KCN + K₂SO₄. This double cyanid is a salt of considerable stability. It is not decomposed by alkalies, hydrogen or ammonium, sulfid. With mineral acids it gives a whitish, curdy precipitate. With fehling solution the same double cyanid is formed as that described above. If, however, fehling solution be present in excess of the amount necessary to form the double cyanid of copper, this excess can be used in the oxidation of reducing sugar and the colorless condition of the solution will be restored as soon as the excess of the fehling is destroyed. The double cyanid holds in solution the cuprous oxid formed and thus complete decoloration is secured.