LE NOBEL'S TEST. — Le Nobel and Fehr hold that Legal's test is only re liable when much acetone is present; and that, when there is only a small quantity of it in the urine, the test may be fallacious, since other substances con tained in the urine can produce a red color with the nitroevanide of soda. The most characteristic point of the test is, according to Fehr, the appearance of the violet hue, which causes the red color to become crimson or purple, and not pure red.
Le Nobel proposes to substitute a solu tion of ammonia for the solution of soda, when the test is, in other respect, made according to the indications of Legal; the fluid containing acetone is not im mediately colored, but after some time, when the liquid is shaken with air or some drops of a strong acid added (the alkaline reaction being maintained), the fluid takes a rose-red color, increasing gradually and changing after some time to violet vine-red. By heating the fluid
the color disappears, but returns on cooling down; when boiled with acids it changes into greenish violet. Le Nobel's test is more delicate than Legal's, and will reveal 0.00025 gramme of acetone.
FEHR'S TEST.—Fehr also employs the test after the method of Legal, but pro poses, when the color of the urine after the addition of solution of soda is pass ing from dark red to yellow, to float sonic drops of acetic acid on the urine. When the test-tube is slightly rotated so that only a small quantity of the acid mingles with the urine, a beautiful violet color will appear when acetone is pres ent, the intensity of the color being pro portionate to the quantity of acetone contained in the urine.