If the urine contains very minute quantities of sugar, the reaction is less satisfactory. The copper is reduced, but the suboxide is so small in quantity that it is obscured by the excess of copper solution, and a mixture results which is greenish or greenish-yellow or yellow or milky, and on standing a small yellow sediment falls to the bottom. Now, it dare not be said that it is sugar which produces such reaction. It may be {212} sugar, but it may also be uric acid. Uric acid is really more frequently a source of error than is commonly supposed. I have myself seen the reaction due to it so vivid that I did not suspect it could be due to any reducing agent excepting sugar; but, noting the next day a copious sediment of uric acid which had fallen during the night, a testing of the supernatant fluid then revealed no reaction whatever. Such a urine, after being treated by the lead process to get rid of the uric acid, fails also to respond. But this process is very tedious,[39] and cannot be conveniently carried out by the busy practitioner. The same thing is, however, accomplished by treating the urine with hydrochloric acid, which in twenty-four hours precipitates all of the uric acid. Simple precipitation by lead acetate solution and filtration does not answer, because all of the uric acid is not thus removed. Other substances, as hippuric acid, urates, hypoxanthin, etc., are said to act similarly, but they produce no practical interference with the test. On the other hand, a small amount of sugar may be present and yet fail to show the reaction, because the cuprous oxide is held in solution by certain substances. Such are ammonia and nitrogenous matters, including albumen, creatinin, pepsin, peptones, urinary coloring matters, etc. The latter probably produce their effect through the ammonia which is given off while heating them in the presence of an alkali. Hence all albumen should be precipitated and filtered out of urines suspected to contain sugar, and the heat applied should not be too great. Finally, excess of glucose will also hold in solution cuprous oxide, so that the suspected urine should not be added in too large a quantity at a time, but rather drop by drop. [Footnote 39: The details of this process will be found in the writer's work on the _Practical Examination of Urine_, 5th ed., 1883, p. 63.]