In a porcelain dish are placed fifty cubic centimeters of the alkaline copper solution and an equal quantity of water and the mixture maintained in ebullition for two or three minutes. The dish is then placed on a boiling water-bath and twenty-five cubic centimeters of a reducing sugar of approximately one per cent strength added at once. The reduction is thus secured at a temperature below 100°, which is an important consideration in securing the minimum decomposing effect of the alkali upon the sugar. The dish is kept upon the water-bath for about ten minutes when the reduction is complete and the supernatant liquor should still be intensely blue. The precipitate is washed by decantation with boiling water, taking care to avoid the loss of any of the cuprous oxid. The washing is continued until the wash waters are neutral to phenolphthalein. The cuprous oxid is then washed into a pyknometer of from twenty to twenty-five cubic centimeters capacity, the exact content of which has been previously determined at zero. It is filled with boiling water, the stopper inserted, and after cooling the flask is weighed. Let _P_ be the weight of the pyknometer plus the liquid and the precipitate, the total volume of which is equal to the capacity of the flask at the temperature at which it was filled, that is _V_ₜ = _V_₀ [1 + 3β(_t_-_t_₀)]. This formula is essentially that given in paragraph =51=, for calculating the volume of a pyknometer at any temperature, substituting for 3β, γ the cubical expansion of glass, _viz_., 0.000025. The specific gravity of the dry cuprous oxid is Δ = 5.881 and let the specific gravity of water at the temperature of filling, which can be taken from any of the tables of the density of water, be _d_. The total weight _p_ of the precipitated suboxid may then be calculated by the following formula: _P_-_V_ₜ_d_ _P_ = -----------. 1 - _d_ ---- Δ The density of water at 99°, which is about the mean temperature of boiling water for laboratories in general, is 0.95934, and this may be taken as the weight of one cubic centimeter for purposes of calculation in the formula above. In order to obtain exact results, it is important that the weight _P_ be reduced to a vacuum. The weight of cuprous oxid not varying proportionally to the weight of reducing sugar, it is necessary to prepare a table showing the principal numerical values of the two, in order to be able to calculate easily all the possible values, either directly from the table or by appropriate interpolations. Following are the chief values which are necessary for the calculation: Milligrams Milligrams Milligrams Milligrams cuprous oxid. dextrose. cuprous oxid. dextrose.