black. It will also be necessary to introduce shades of red, for it will be found that the pigments of the blood are under certain conditions capable of imparting a red color to the urine. The color of the urine is subject to several changes not found in the ordinary scales when certain substances have been taken internally. Salol and many compounds allied to the phenols color the urine a smoky black. Rhubarb, senna, and chrysarobin color the urine a deep red, if the fluid be alkaline, and so simulate the appearance of a hemoglobinuria. This will be detected on the addition of an acid. The color disappears. Santonin produces a bright-yellow urine. Methylene-blue when taken internally produces a urine of a marked bluish-green tint.

The Specific Gravity. The specific gravity of the urine varies in health with the amount of fluid taken internally and the amount excreted by the skin. Thus in summer, when perspiration is likely to be profuse, the urine may be of high specific gravity; while in winter, when the skin does not act so freely, the specific gravity is likely to be lower.

In the various forms of nephritis, except the acute cases, the specific gravity is diminished; while in glycosuria, even when large amounts of urine are being passed, the specific gravity rises above normal.

The specific gravity of the urine is determined with a special form of areometer called a urinometer. The most useful form is that of Squibb. This has a limited range, being used only for fluids. whose specific gravity varies between 1000 and 1060.

This instrument (Fig. 15) differs from the ordinary form of areometer in having a spindle-shaped bulb, and in having the vessel in which it is immersed provided with flutings. In this way the friction. between the sides of the vessel and the bulb of the spindle is reduced to a minimum. It is designed to

give its readings at room-temperature (25° C.), and is therefore more easily used than those which require the instrument to be used at 15° C., a temperature which is only to be obtained by cooling the urine. All urinometers as they are obtained from the dealer should be checked by immersion in fluids

of known specific gravity. The 1000 mark may be checked by immersing in distilled water. A fluid of a specific gravity of 1020 may be obtained by dissolving 5 grams of crystallized sodium sulfate in 100 c. c. of water; 10 grams of this salt in 100 c.c. of water will yield a fluid of a specific gravity of 1040. By interpolation a table can thus be constructed, giving the error of the instrument at any part of the scale. method will yield sufficiently accurate results for ordinary work.

This

The urine is placed in the vessel and the urinometer floated in it. Care must be taken to have sufficient urine so that the bottom of the spindle is above the bottom of the urinometer vessel. It is better to have sufficient urine so that the surface of the fluid is level with the top of the cylinder. In this way it is easier to make the readings. In reading the urinometer, the upper part of the urine, where it is drawn up on the scale of the urinometer, is taken as the indication of the reading. This is shown in Fig. 17. The urinom

FIG. 17.