in the urine upon cooling. As a matter of fact, such a deposit may, and very often does, occur when the quantity of uric acid is both relatively and absolutely diminished. The separation of uric acid crystals from the urine is usually dependent upon one of three conditions:

1. A high degree of concentration of the urine, too little water being present to keep the uric acid in solution.

2. Marked acidity of the urine, the salts of uric acid being deprived of a part of the alkali; the larger the proportion of alkali combined with the uric acid, the more soluble it becomes.

3. A high percentage of uric acid. Any condition

that results in an increased formation of uric acid in the body causes its increase in the blood, and hence an increased amount in the urine, with, usually, a resulting deposition of the crystals.

When urine habitually contains a deposit of uric acid, an alkali or some substance with which the uric acid will combine should be administered, in order to prevent an irritation or inflammation of the urinary tract by the crystals.

In the consideration which follows the writer refers to the uric acid in solution (as urates) and not to a deposit of uric acid crystals.

Clinical Significance.-Uric acid is absolutely increased (1) by an abundant meat diet, especially when combined with a limited amount of outdoor exercise: in other words, increased metabolism together with limited oxidation. (2) In most of the acute diseases. (3) In acute diseases of the lung, as in pneumonia, or by any disease that interferes with respiration, as hydrothorax and pneumothorax, also by the upward pressure of abdominal tumors, marked ascites, etc. (4) In chronic heart disease or in any condition that interferes with the circulation. (5) In liver disease. (6) In disease of the spleen. (7) In various forms of anemia, especially splenic leukemia, in which case the proportion of uric acid to urea may be as high as I: 15, or even more. (8) In gout following the paroxysm. (9) In diabetes mellitus. 1 (10) Following the administration of nuclein. 2

1 The question as to whether uric acid is actually increased in diabetes mellitus has given rise to much controversy. It is probable that the increase,

if any, is not marked.

2" Monatsh. f. Chemie," Bd. XII, 234, 1891.

Uric acid is absolutely diminished (1) by low nitrogenous diet. (2) By the habitual ingestion of large quantities of water (long-continued use). (3) In most forms of advanced disease of the kidneys. (4) In gout during the paroxysm. (5) Following the administration of large doses of quinine. (6) In most of the general chronic dis

eases.

Detection.-1. The Murexide Test.-A small portion of urinary sediment is evaporated to dryness on a porcelain plate or capsule, and a drop or two of nitric acid is added to dissolve it. The mixture is then stirred thoroughly with a glass rod, and carefully evaporated to dryness over a spirit or a small Bunsen flame. When dry and cool, add one or two drops of ammonic hydrate, and if uric acid or urates are present, a beautiful purple-red color promptly appears, gradually diffusing itself over the bottom of the plate or capsule as the ammonia spreads.

2. Strongly acidulate the urine with concentrated hydrochloric acid, and allow the mixture to stand from eighteen to twenty-four hours. Usually, a deposit of uric acid crystals appears.

3. Uric acid may be detected in the urine and other fluids, when traces of the acid or of urates are present, by a method suggested by Garrod. A small portion of the suspected fluid is treated on a watch-glass with a few drops of glacial acetic acid. A few filaments of tow or very thin silk are placed in the mixture, and the whole is set aside under a bell-jar in a warm place for from twenty-four to forty-eight hours. Gradually, crystals of uric acid separate and are deposited upon the filaments. Their character may be readily recognized by microscopic examination.

4. When a solution of uric acid or of a urate is boiled with an alkaline solution of copper (Fehling's solution), a yellowish-red or reddish precipitate of suboxide of copper

occurs.

Quantitative Determination of Uric Acid.-Heintz's Method. Take 200 c.c. of filtered urine that is free from albumin, and add 10 c.c. of concentrated hydrochloric acid. Let this stand for twenty-four hours in a cool place, then collect the separated uric acid crystals on a previously dried and weighed filter-paper, and wash once or twice with cold distilled water. Dry the filter-paper holding the uric acid crystals at about 100° C.; cool and weigh. By subtracting

the weight of the filter-paper, the result will be the weight of the uric acid in 200 c.c. of urine.

This process can be considered only approximate, and should not be relied upon for accurate results. It frequently happens that urines containing uric acid do not give a precipitate by this method; it then becomes necessary to employ other longer and, probably, more accurate methods.

Fokker-Salkowski Method.1-Of the several gravimetric methods which have been suggested, this is perhaps the most reliable. It depends upon the precipitation of uric acid as ammonium urate, which, on account of its difficult solubility, can be easily handled, and with comparatively little loss.

Process.-Take 200 c.c. of urine, render alkaline with 20 c.c. of sodic hydrate, allow it to stand an hour, and to the filtered mixture add 10 c.c. of a solution of ammonium

chloride (15). Let this stand for forty-eight hours. Transfer the precipitate to a previously dried and weighed filter-paper, and wash two or three times with distilled water. Then treat the precipitate several times with dilute hydrochloric acid (1 : 10) until all of the ammonium urate has been decomposed. This leaves on the filter all of the uric acid except what has been dissolved and carried. through into the filtrate. This filtrate is, therefore, saved and allowed to stand six hours, after which any separated uric acid crystals are thrown on to the weighed filter-paper. The precipitate, which represents the total uric acid in 200 c.c. of urine, is washed twice with water (30 c.c.), then with alcohol until the acid reaction has disappeared, and finally the filter-paper is dried and weighed. The difference in weight plus 0.03 gram (correction for solubility) gives the quantity of uric acid in 200 c.c. of urine. From this calculate the amount for twenty-four hours.

If the urine is dilute, it should be concentrated so as to have a specific gravity of from 1017 to 1020 before the foregoing test is applied. The presence of albumin in the urine does not interfere with the test.

Hopkin's Method. In this process the uric acid and all of the urates are precipitated by saturating the urine with ammonium chloride, which converts all into ammonium urate. This is then filtered out, and the uric acid separated

1 "Pflüger's Archiv," Bd. x, 157, 1875; "Virchow's Archiv," Bd. LXVIII, 401, 1876.

by the action of hydrochloric acid. The final estimation is then made by titrating with a standard solution of potassium permanganate, which Hopkins has found to be more accurate than weighing. Exceedingly accurate results are claimed for this process, and it has the advantage of being conducted with ease and comparative rapidity.

The following solution is required: A twentieth-normal solution of potassium permanganate. This solution is prepared by dissolving 1.577 grams of pure crystals of potassium permanganate in about 900 c.c. of distilled water. A portion of this solution is then placed in a burette and titrated against a decinormal solution of oxalic acid as follows: Take 10 c.c. of the decinormal oxalic acid solution in a beaker, add some dilute sulphuric acid, and heat this mixture to 60° C. To this hot mixture add the solution of potassium permanganate until a faint but permanent pink color is produced. Note the number of cubic centimeters of the permanganate solution used, and then dilute the remainder so that 20 c.c. of it will exactly correspond to 10 c.c. of the decinormal solution of oxalic acid.

Each cubic centimeter of the twentieth-normal solution of potassium permanganate corresponds to 0.00375 gram of uric acid.

The permanganate solution will usually retain its strength for several weeks, but it should always be restandardized by titration with oxalic acid before it is used.

The process, as applied to all urines, normal and pathologic, is as follows:

1. In Normal Urine without Deposit.-(a) To 100 c.c. of the urine is added ammonium chloride until practically saturated; about 35 grams are necessary. When a small quantity of the chloride remains undissolved, even after brisk stirring at intervals of a few minutes, saturation is nearly complete. As the temperature of the urine again. rises, from the depression due to the process of solution, any residual crystals will, for the most part, dissolve; but there is no necessity for adding more.

(b) After having stood for two hours or longer,-better with occasional agitation to promote subsidence,—the precipitate produced is filtered through a thin filter-paper, and washed three or four times with a saturated solution of ammonium chloride. The filtrate should remain perfectly clear.

(c) With a jet of hot distilled water the urate, which will be somewhat pigmented, is now washed off the filter into a small beaker, and heated just to boiling with an excess of hydrochloric acid. It is then allowed to stand, in order that the uric acid may separate completely. Two hours are sufficient if the liquid be cooled. The acid is then filtered off and washed with cold distilled water. The filtrate should be measured before the washing is begun, and one milligram added to the final result for each 15 c.c. of liquid present. This need never be more than from 20 to 30 c.c.

(d) The acid is now again washed off the filter with hot water, sodium carbonate is added, it is warmed until dissolved, and the solution then made up to 100 c.c. Being transferred to a flask of sufficient capacity, it is mixed with 20 c.c. of concentrated pure sulphuric acid, and immediately titrated with the twentieth-normal potassium permanganate solution. The latter should be added slowly toward the end of the reaction, the close of which is marked by the first approach of a pink color, which is permanent for an appreciable interval. The flask should be agitated throughout the titration.

Since each cubic centimeter of the potassium permanganate solution is equal to 0.00375 gram of uric acid, the number of cubic centimeters of permanganate solution multiplied by 0.00375, plus the correction of one milligram for each 15 c.c. of liquid present, will give the amount of uric acid in the 100 c.c. of urine used. From this the quantity of uric acid in the twenty-four-hour urine can be readily calculated.

2. Acid Urines Containing Cystin.-The author recommends the addition of a small amount of ammonic hydrate; heat and filter. The ammonium chloride may be added while the urine is still warm.

3. Alkaline Urines with an Abundant Deposit of Phosphates.-Filter off the phosphates after complete precipitation by heat. The ammonium urate separates more rapidly in alkaline than in acid urine. The only objection to adding ammonic hydrate in all cases is its tendency to precipitate the phosphates.

4. Albuminous Urines.-Albumin does not interfere with the accurate determination of uric acid by this method, but requires a little longer digestion with an excess of hydrochloric acid, in order to form the soluble acid-albumin.