PRELIMINARY OPERATIONS.
It is the practice of some physicians to direct the patient to preserve all the urine passed in twenty-four hours, and to forward this in one bottle for analysis. Others, again, merely send a small sample of "morning" and "evening" urine in separate phials, desiring only a comparative report. In the former case the volume should be accurately measured, and the quantity noted either in fluid ounces or cubic centimeters before commencing the analysis. This need not be done if small samples only are received. The color should be noted. It varies greatly, through every shade of yellow and amber to dark brown, with a tinge of green or red, if the coloring matter of bile or blood is present. Also note relative transparency or cloudiness, specific gravity, and reaction, as all these observations are useful in diagnosis. Odor is not quite so important. The specific gravity should be taken at about 60° F. in an ordinary specific gravity bottle, or more conveniently by means of a good urinometer. In the latter case it is very important to have an instrument of known accuracy, many of those in the market being valueless. Urinometers of glass, though fragile, are decidedly more cleanly and less liable to get out of order than the gilded brass instruments carried in the pocket by many physicians. Mr. J.J. Hicks, of 8 Hatton Garden, E.C., manufactures a very creditable "patent urinometer" at an extremely low cost. Healthy urine has a density of from 1.015 to 1.025; but variations from this range are common.
A fair quantity of the urine, after shaking, should be placed in a tall conical glass vessel, to allow easy collection of the precipitate for subsequent, microscopical examination. If an abundant amorphous deposit of a fawn or pink—from uroerythrin—color slowly settles and is readily diffused, urates in excess can be anticipated. Their presence is proved by the readiness with which they dissolve on warming with the supernatant urine to about the temperature of the blood. No difficulty is experienced if small quantities of albumen are present, as that body is not coagulated until the temperature rises much higher. A sandy precipitate of free uric acid will not dissolve on warming the urine, and its identity can further be determined by means of the microscope, or by applying a well-known color-reaction. A grain or so is oxidized into reddish alloxan and alloxantin by carefuly evaporating with a few drops of strong nitric acid on a piece of porcelain. A little ammonia is then added, when the fine purple murexide stain will be produced.
It is always advisable to mention the reaction to test papers of all samples received. Urine is normally acid, but there are certain diseases which render fluid neutral or alkaline. The urea of acid urine on standing is changed by a putrefactive ferment into ammonic carbonate, but this decomposition in a state of health should not take place for at least twenty-four hours. Alkalies, or organic salts of alkaline metals, when taken as medicine render the urine alkaline, and the indication is then not of much moment; but if none of these causes exist, the condition is of serious diagnostic import. Where it is desired to determine the degree of acidity of the urine voided, say, by a gouty patient, a dilute volumetric solution of caustic soda should be employed, using a few drops of an alcoholic solution of phenolphthalein as an indicator, and reporting in terms of oxalic acid. The soda solution may conveniently contain the equivalent of one milligramme of recrystallized oxalic acid (H2C2O4.2H2O) in each cubic centimeter.
UREA.
Carbamide, as it is called by systematic chemists, or urea, is next to water the largest constituent of urine, and forms about one-third of its total solids. Derived from ammonic carbonate by abstracting two molecules of the elements of water, it is readily converted by putrefaction into that salt, and the urine under these circumstances becomes strongly alkaline in reaction. Earthy phosphates then fall naturally out of solution, so that the putrid fluid is always well furnished with sediment. Nitrogen that has served its purpose as muscle or other proteid leaves the animal economy chiefly in the form of urea, and its proportion in the urine, therefore, is a fair index of the activity of wasting influences.
For its determination Knop's sodic hypobromite method, on account of its convenience, is now generally preferred. The volumetric process of Liebig, which depends on the formation of an insoluble compound of urea with mercuric nitrate, possesses no advantages and is troublesome to work. The principle of the hypobromite process is simple. In a strongly alkaline solution urea is broken up by sodic hypobromite, its nitrogen being evolved in the gaseous state, and its carbon and hydrogen oxidized to carbonic anhydride and water respectively. The volume of free nitrogen obtained bears a direct ratio to the amount of urea decomposed.
Among the number of instruments which have been introduced for the purpose of conveniently measuring the evolved gas, that of Gerrard, an illustration of which we give, is one of the simplest, cheapest, and best. The ureometer tube, b, is connected at the base with a movable reservoir, c, and by means of a rubber tube passing through a cork at the top to the generating bottle, a. To use the apparatus, fill b to zero with water and have the reservoir placed so high that it contains only an inch or so of the liquid. Replace the cork with attached tube tightly in b. Now pour into the generating bottle 25 c.c. of a solution prepared by dissolving 1 part of caustic soda in 2½ parts of distilled water, and dexterously break in the liquid a tube containing 2.2 c.c. of bromine. The tubes will be found very convenient, obviating entirely the suffocating fumes diffused in the act of measuring bromine. Allow to stand in the solution of sodic hypobromite thus prepared a test tube containing exactly 5 c.c. of the urine under examination. Cork the bottle as shown in the illustration, see that the water is at zero, and that the liquid in the reservoir is at the same level, and then allow the urine to gradually mix with the hypobromite solution. Cool the evolved gas by placing the bottle in cold water, adjust the levels of the water in the tube and reservoir (to obviate a correction for pressure), and read off the percentage of urea in terms of which the tube is graduated. Stale urine, the urea of which has largely been converted into ammonic carbonate, still yields a very fair result, that salt being also completely split up by the powerful oxidant employed. Should the urine contain albumen, it is advisable to remove it by boiling and filtering, as, although only slowly decomposed by the hypobromite solution, it communicates to the liquid such a tendency to froth that the disengagement of the nitrogen is seriously impeded. Most of those alkaloids which might possibly be present do not yield the gas when treated in this manner, and therefore may be disregarded.