(_c_) _Decomposition of exudates_ anywhere in the body, as in empyema, bronchiectasis, and large tuberculous cavities. {64} _Detection of indican_ depends upon its decomposition and oxidation of the indoxyl set free into indigo-blue. * * * * * Obermayer's Method.--In a test-tube take equal parts of the urine and Obermayer's reagent and add a small quantity of chloroform. Mix by inverting a few times; avoid shaking violently. If indican be present in excess, the chloroform, which sinks to the bottom, will assume an indigo-blue color. The depth of color indicates the comparative amount of indican if the same proportions of urine and reagents are always used. The indican in normal urine may give a faint blue by this method. Urine of patients taking iodids gives a reddish-violet color, which disappears upon addition of a few drops of strong sodium hyposulphite solution. Bile-pigments, which interfere with the test, must be removed (p. 48). _Obermayer's reagent_ consists of strong hydrochloric acid (sp. gr., 1.19), 1000 parts, and ferric chlorid, 2 parts. This makes a yellow, fuming liquid which keeps well. * * * * * 4. Urea.--From the standpoint of physiology urea is the most important constituent of the urine. It is the principal waste-product of metabolism, and constitutes about one-half of all the solids excreted--about 30 gm. in twenty-four hours. It represents 85 to 90 per cent. of the total nitrogen of the urine, and its quantitative estimation is a simple, though not very accurate, method of ascertaining the state of nitrogenous excretion. Normally, the amount is greatly influenced by exercise and diet. Pathologically, urea is increased in fevers, in diabetes, and especially during resolution of pneumonia and absorption of large exudates. Other factors being equal, the amount of urea indicates the activity of metabolism. In {65} this connection the relation between the amounts of urea and the chlorids is important. The amount of urea is normally about twice that of the chlorids. If the proportion is much increased above this, increased tissue destruction may be inferred, since other conditions which increase urea also increase chlorids. [Illustration: FIG. 19.--Crystals of nitrate of urea (upper half) and oxalate of urea (lower half) (after Funke).]