§ 2. Specific Gravity.—By means of the urinometer we ascer tain how far urine differs in specific gravity from pure water; and it is of great importance to bear in mind that the instrument does no more. It shows how much soluble matter heavier than water is contained in a given quantity of fluid, but it does not teach what that solid matter is. It may consist of salts, of urea, or of sugar, and we can only determine its nature by chemical analysis. A generally high or low specific gravity, as ascertained by testing a portion of the mixed secretion of the whole day, and especially when found persistently so by repeated examina tion from day to day, is of more importance than any one eva cuation of the bladder being above or below the average. In estimating the importance of specific gravity as an indication of disease, we have to take into account the quantity passed during the twenty-four hours. Any deviation from the normal standard is of value when the average quantity of urine is secreted, but it may be taken as certain evidence of disease if a high specific gravity accompany excessive secretion, or a low specific gravity be noted when the urine is scanty. The observation that at any one period of the day the specific gravity is much above the standard may lead to the detection of some disorder in the assimi lating processes which would otherwise escape notice: its being casually below the standard is of little moment ; it is not uncom mon in hysteria ; it may happen in consequence of the person having imbibed an unnecessary quantity of liquid, or having taken some aliment or stimulant which has accidentally acted as a diuretic. The circumstances under which the more important variations occur must be again adverted to; it need only be stated here, that when persistent, the minimum is observed in albuminuria, the maximum in diabetes.
§ 3. General Appearance.—The urine, after standing some time in the vessel, may be perfectly transparent throughout, or a sedi ment may rest at the bottom, leaving the supernatant fluid quite clear ; in other cases the whole is more or less opaque ; and this opacity may increase towards its lower part, or in addition there may be a distinct deposit.
a. Transparent urine varies in color from a paleyellow, hardly perceptible, to a deep amber, in conditions of health ; and within certain limits these variations are proportional to the amount of animal matters present. When the color is deep, the relative amount of water is usually small, and the specific gravity high, and very generally the salts as well as the extractive matters are in excess, and are deposited when the urine is cold, unless they be held in solution by some unusual circumstance. In all cases of diuresis the urine is pale and limpid from an excess of water, and perhaps the absence of color is most striking after an hys terical paroxysm.
Deep-colored transparent urine may be taken generally as in dicating excessive metamorphosis of tissue; it has perhaps a more intimate relation to the secretion of coloring matter by the liver than to any other circumstance. When the blood becomes satu
rated with bile in jaundice, the urine acquires the color of porter; and it is only by pouring a small quantity into a white porringer or into a test-tube that we can ourselves that it is not opaque. This condition is essentially ifferent from the secretion of dark-colored urine of high specific gravity, although the shade of color in slight jaundice may be exactly the same. It may be added here, as we shall not have again occasion to refer to it, that the addition of nitric acid, converting the color into green, is the readiest test of the actual presence of biliary coloring matter in the urine.
b. When the whole of the urine is opaque, it presents either an appearance of unusual whiteness, in consequence of the minute opaque particles being colorless ; or it is unnaturally dark from the adventitious presence of coloring matter, and this is most commonly derived from an admixture of blood.
The white varieties are chiefly of two kinds—an admixture of mucus or pus, and turbidity as caused by chemical decomposition; the two being very often found together. Healthy mucus floats as a cloud, which may leave the edges nearly transparent as it accumulates towards the bottom of the vessel ; pus renders the whole of the urine more or less opaque, but forms a distinct sedi ment when allowed to stand, very often carrying down with it some portion of earthy salts ; altered pus, or ropy mucus as it used to be called, collects into a stringy mass at the lower part of the vessel, the whole urine being turbid from decomposition : urine mixed with leucorrhceal discharge, and that which is altered by decomposition, are both wholly opaque; if there be any sedi ment it is quite independent of the opacity, which has no tendency to form a deposit.
Opaque urine of deep color may be produced by a combination of one of the white varieties with coloring matter of bile, which is of itself really transparent: in such cases the existence of jaun dice would explain its meaning : it is much more commonly produced by a certain admixture of blood, when the color varies from a pinkish hue to a deep brown. These varieties depend more upon the condition of. the urine itself than upon the causes which give rise to the effusion of blood ; alkalescence or acidity is especially prone to produe such changes ; but as a general rule the passive hemorrhage occurring in organic disease of the kidney is far more frequently brownish than pink. It sometimes gives merely a slight smokiness to the urine, which is then rather hazy than opaque. When more severe and more active hemor rhage occurs, coagula are often found of such a size as to be readily recognized by the naked eye : microscopic examination affords some assistance in determining to which of these causes the blood is to be attributed.