Dictionary.net

A solution is made of 4.15 grams of copper sulphate, 10 grams of pure mannite, and 50 c.c. of glycerin in 250 c.c. of distilled water, and added to 250 c.c. of distilled water, containing 20.4 grams of caustic potash; when the mixture is cold 300 c.c. of strong ammonia are added, together with enough water to make a liter, and the solution is carefully filtered; 25 c.c. of this solution is heated with 50 c.c. of distilled water, and the urine is added drop by drop until the solution is colorless. The number of minims of urine necessary to decolorize the solution contain just a quarter of a grain of sugar.

See Purdy.