The specific gravity of the fermented urine should be compared with that of the urine soon after it is passed, because saccharine urine under suitable circumstances undergoes fermentation without the addition of yeast; and, the specific gravity being thus lowered spontaneously, the reduction in the urine fermented by yeast would appear less than it actually is. At the same time, care should be taken that the urine is of the same temperature when the specific gravity is taken before and after fermentation. The Picric Acid and Potash Test.--Although attention was called in 1865 by C. D. Braun,[40] a German chemist, to a reaction between grape-sugar and picric acid, as the result of which the latter is converted into picramic acid, very little attention seems to have been paid to this announcement. Quite ignorant of it, George Johnson rediscovered this reaction in 1882, and published it in 1883.[41] It is applicable to both qualitative and quantitative purposes. In order to make use of it, a standard comparison-solution is made as follows: Take 1 fluidrachm of a solution of grape-sugar, 1 grain to the fluidounce; mix it in a long test-tube with half a drachm of liquor potassæ (U. S. P. or B. P.) and ten minims of a saturated solution of picric acid; dilute the mixture to 4 fluidrachms with distilled water, to facilitate which a tube used for the purpose may be marked at 4 fluidrachms. Raise the mixture to the boiling-point, and continue the boiling for sixty seconds, to ensure complete reaction between the sugar and picric acid. During the boiling the pale-yellow color of the liquid is changed to a vivid claret-red. Cool the liquid by cautiously immersing the tube in cold water, and if it is not then at the level of the 4-drachm mark, raise it to this by adding distilled water. The standard color thus obtained is that which results from the decomposition of picric acid by a grain of sugar to the ounce, four times diluted, or by a solution of sugar containing one-quarter of a grain per ounce. But the picramic solution rapidly becomes pale on exposure, so it becomes necessary to make a more permanent solution to use as a standard. This may be accomplished by combining liquor ferri perchloridi drachm j, liquor ammonii acetatis drachms iv, acidum aceticum (glacial) drachms iv, and water enough to make ounces iiss. The color of this is identical with that of the picric acid reduced by a one-grain solution diluted four times, and, {214} according to Johnson, it will retain its color unchanged for at least six months. At the same time, whenever a new solution is made it should be compared with that of the one-quarter grain per ounce solution of sugar, boiled with picric acid and potash. [Footnote 40: "Ueber die Umwandlung der Pikrinsaüre in Pikramminsaüre, und Ueber die Nachweisung der Traubenzucker," _Zeitschrift für Chemie_, 1865.] [Footnote 41: _British Medical Journal_, March, 1883.]