As this condition is met with only when the saccharine solution contains between 0.5 and 1 per cent. of sugar and when it is added very rapidly to the Fehling's solution, it is therefore necessary : (1) by means of a preliminary test with Fehling's solution or by polari zation, to estimate approximately the percentage of sugar, and then to dilute the urine for the definite test so that it contains only from 0.5 to 1 gm. of sugar in 100 c.c. ; (2) to add the Fehling's solution gradually to the diluted urine. If, for example, it were found that 20 c.c. of the diluted urine are needed to obtain the complete reaction, then in a second test 20 c.c. of urine are to be added quickly to the Fehling's solution. The end reaction does not take place, and some tenths of a cubic centimetre, say 0.5 c.c., must be added in order to obtain it. In this case, therefore, 20.5 c.c. of the diluted urine would exactly correspond to 20 c.c. of Fehling's solution, that is to say would contain just 0.1 gram of grape sugar. If the urine had previ ously been diluted ten times we should have the following computation: Application: 20 c.c. of Fehling's solution are diluted with SO c.c. of water and heated to boiling in a glass cucurbit having a capacity of about 200 c.c. The urine is now added from a burette to the boil ing fluid. After each addition of urine the urine is boiled for a min ute. The addition of urine and the boiling are continued until the fluid, which becomes turbid with the precipitated red oxide of cop per, ceases to be distinctly blue. When the color of the boiling fluid can no longer be clearly recognized, the cucurbit is removed from the fire after each addition of urine and subsequent boiling and held up between the eye and the window. There will then be seen, as
soon as the precipitate begins to sink, a light layer immediately below the surface of the fluid. As long as this ring appears blue or bluish-green the reduction is not completed, that is, a sufficient amount of urine has not been added; as soon as the ring is colorless the reduction is complete; when the ring assumes a yellow color too much urine has been added to the Fehling's solution.
This method gives, after some practice, very distinct results, and is in my opinion superior to all the other modifications of Fehling's titration method.
c. By Means of Aerometry and Fermentation.—The specific gravity of saccharine urine is reduced by fermentation, as the heavy sugar is taken away and light alcohol is formed in the fluid. Roberts and others have determined experimentally to what percentage of sugar a definite decrease in specific gravity following fermentation corre sponds. The method is easily carried out and gives approximately accurate results if (1) the specific gravity is obtained both times with the same temperature, (2) the aerometers are very exact, and (3) the yeast is absolutely pure.
Application: After the specific gravity has been ascertained, the yeast is added and the mixture is set aside in a temperature of from 25° to 30° C. (77° to 80° F.). The fermentation should be carried on until the urine gives no sugar reaction, and then the specific gravity is taken. The difference of specific gravity multiplied by 430 gives the percentage of sugar in the urine.