For qualitative testing Johnson directs: To a drachm of urine in a test-tube add a few drops, enough to give a distinct yellow color, of a saturated solution of picric acid. Add about 10 drops of liquor potassæ and boil. If sugar is present, the mixture becomes promptly red in hue. [Illustration: FIG. 5. _Johnson's Picro-Saccharimeter_. The shading of the side tube indicates the ferric-acetate standard. The darker shading at the bottom of the graduated tube shows the saccharine fluid, darkened by boiling with picric acid and potash, and occupying ten divisions between dilution.] The quantitative estimation is based upon an accurate approximation, by dilution, of the color of the tested fluid with that of the standard solution. Johnson recommends the picro-saccharimeter figured in the text. This is a stoppered tube twelve inches long and three-quarters of an inch in diameter, graduated into ten, and each of these again into ten other equal divisions. By the side of this tube, and held in position by an S-shaped band of metal, is a stoppered tube of equal diameter and about six inches long, containing the standard solution corresponding to the reaction of the one grain of grape-sugar with picric acid and potash diluted four times. It has been found that ten minims of a cold saturated solution of picric acid are rather more than sufficient for decomposition by one drachm of a solution of grape-sugar in the proportion of one grain to the ounce. A drachm of the solution will therefore contain one-eighth of a grain of sugar, which is the strength of the solution used in making the standard-color liquid. In making the analysis, while the quantity of liquor potassæ used is always the same and the dilution is always to four drachms, the picric acid must be added in proportion to the amount of sugar present, so that if the urine contains as much as six grains to the fluidounce, sixty drops or a fluidrachm of the picric-acid solution would have to be used; and when the proportion of sugar is higher than this, the urine should be diluted with distilled water five or ten times before commencing the analysis, and the degree of dilution remembered in the computation. If, now, a drachm of a solution of grape-sugar, containing two grains to the ounce, be mixed with the same quantity of liquor potassæ and picric acid and increased by the addition of distilled water to four drachms in the boiling tube, and boiled as before for sixty seconds, the result will be a mixture of much darker color than will be produced by the one-grain solution; but if the dark liquid be diluted with its own volume of water, the color will be the same as that of the one-grain solution or the standard.