gas is read off, and the result multiplied by 100 in order to obtain the percentage of urea. Two forms of this apparatus are obtainable-one graduated to read fractions of a gram per cubic centimeter of urine, and the other graduated to read the number of grains of urea per fluidounce of urine.

The Doremus ureometer as modified by Professor J. D. Hinds (Fig. 6) has many advantages over the original form of apparatus. This instrument consists of a bulb with an upright graduated tube (a), the same as the original; near the lower portion of this tube is a horizontal glass connec

tion, which is provided with a ground glass stop-cock (b), and which supports another upright graduated tube (c) with a capacity of two cubic centimeters. The bulb and upright tube (a) are filled with the sodium hypobromite solution in precisely the same manner as previously described. The upright tube (c) is then filled to the zero mark with the urine to be tested. The stop-cock (b) is then turned, and exactly one cubic centimeter of the urine allowed to enter tube a with the reagent. As soon as the evolution of nitrogen gas is complete, the number of cubic centi

meters of the gas is read off, and the result multiplied by 100 in order to obtain the percentage of urea.

This form of apparatus1 gives more exact results than the original form, since the one cubic centimeter of urine required for the test is delivered with greater accuracy, and no nitrogen gas is lost by its escape from the bulb.

(c) Fowler's Hypochlorite Method (Differential Density). This method is based upon the fact that there is a difference in the specific gravity of urine before and after the decomposition of its urea by the hypochlorites; and that such difference bears a definite relation to the quantity of urea present. Dr. Fowler found that every degree of density lost corresponds to 0.77 of one per cent., or about 31⁄2 grains per fluidounce. The hypochlorite solution employed is Squibb's solution of chlorinated soda, or Labarraque's solution, of which seven parts will decompose the urea in one part of urine, unless the amount is very large, in which event the urine should be diluted by an equal bulk of water, and the result multiplied by 2. The presence of albumin and sugar does not interfere with this

test.

Process.-I. Add to I volume of the urine 7 volumes of the hypochlorite solution; effervescence due to the liberation of nitrogen will immediately take place. Shake the jar containing the mixture occasionally, and stand it aside for two hours, when the urea will have been decomposed. Now take the specific gravity of the quiescent fluid.

2. Ascertain the specific gravity of the mixed urine and hypochlorite solution before decomposition. To do this, multiply the specific gravity of the pure hypochlorite solution by 7, add this to the specific gravity of the pure urine, and divide by 8. The result is the specific gravity of the mixed fluid. From this subtract the specific gravity of the quiescent mixture after decomposition of the urea, multiply the difference by 0.77, and the result is the percentage of urea; or by 31⁄2, which gives the quantity of urea in grains per fluidounce.

1 This instrument, as well as the original Doremus ureometer, can be obtained at a moderate cost from Messrs. Eimer & Amend, 205 to 211 Third Ave., New York city.

2 Fowler, "Prize Essay to the Alumni Association of the College of Physicians and Surgeons," New York. Published in the "New York Medical Journal," July, 1887.

As changes of temperature affect the specific gravity and volume of liquids, the hypochlorite solution and urine should be mixed, and the jar set aside along with a bottle of the urine and the hypochlorite solution in the same place, subject to the same temperature. When decomposition is complete, the specific gravities can be taken, and the calculation made.

Example.-Suppose the specific gravity of the urine is 1010, and that of the hypochlorite solution is 1045, that of the mixed fluid will be

Now suppose the specific gravity of the decomposed fluid is 1038, then (1040 1038) X 0.77 = 1.54, the percent

age of urea.

URIC ACID.

C&H4N4O3.

Uric acid (H,U, expressed also U) is, in mammals, next to urea, the medium by which the largest quantity of nitrogen is excreted from the body. It is, however, in birds and reptiles the principal nitrogenous constituent of the urine.

Until recently, the theory of the formation of uric acid. was that it was a product of the metabolism of the nitrogenous material ingested, and that it represented an intermediate product between the nitrogenous substances and the final product, urea. The researches of Horbaczewski, 1 Hopkins and Hope, 2 Jerome, 3 and others tend to show that uric acid has an entirely different origin. It is now believed that uric acid is at least partly derived from the nucleins that form a constituent of all cell-nuclei, and which are taken into the body as food. The nucleins are capable of being split up into an albumin and nucleic acid, and it is thought that the uric acid is formed in the body from the nucleic acid through the oxidation of the xanthin or alloxur groups contained in a molecule of nucleic acid. It has been demonstrated that the ingestion of food that is rich in nucleins results in the formation and elimination of a much

1 "Monatsh. f. Chem.," S. 624, 1889.

66

2 Journ. of Physiol.," XXIII, p. 271. 3 Journ. of Physiol.," XXII, p. 146.

larger quantity of uric acid than the ingestion of an equal amount of food that is poor in nucleins. The chief evidence, however, in favor of the view that nucleins play a rôle as precursors of uric acid is based upon the results of thymus feeding. The experiments of Hopkins and Hope, however, show that extracts of the thymus gland may be prepared which contain only traces of nucleins and nucleic acid, but which, when ingested, produce the characteristically large excretion of uric acid. It, therefore, appears that some more soluble constituent of the diet acts either as a direct precursor, or as a factor in the formation of uric acid.

Our knowledge of this subject is yet too meager to warrant the conclusion that this new theory fully explains the formation of uric acid, but there can be no doubt that the nucleins (nucleic acid) play an important part in its formation.

When uric acid is referred to as a constituent of normal urine, it is never to its free state that allusion is made, but to its combinations chiefly with potassium, sodium, and ammonium, and also with calcium and magnesium; such combinations being usually known as mixed urates.

Under ordinary conditions uric acid exists in the urine in the form of urates. Since uric acid is dibasic,—that is, has two replaceable atoms of hydrogen,-two forms of salts exist―i. e., acid urates of potassium, sodium, and ammonium, in which only one atom of the hydrogen is replaced by the positive elements or radicles; and normal (neutral) salts of the same substances, in which both atoms of hydrogen are replaced. According to Neubauer and Vogel, there are two forms of acid urates-monacid urates (biurate), and triacid urates (quadriurate or tetraurate). The normal salts are readily soluble in water at 70° F., but the acid urates are only feebly soluble, while uric acid itself is almost insoluble in water. Hence, the precipitation of the acid urates or uric acid often occurs when the urine cools, or is allowed to stand in a cold place. A urine containing a deposit of acid urates (amorphous urates) is usually more or less concentrated, and always contains a relative excess of the acid urates. If a strong acid be added to a urine that contains a relative excess of urates, they are precipitated on account of the feeble solubility of the acid urates and the almost insoluble uric acid. Also, if the urine con

tains an excess of normal urates, they are partially decomposed by the acid, which chemically unites with the excess of the base to form acid urates, hence their precipitation. Thus, in the nitric-acid test for albumin (performed according to instructions given on p. 122) a white zone of acid urates is frequently seen above the zone of albumin (Fig. 15), or above where the zone of albumin would be if present. It should be borne in mind that a zone of urates may be present when albumin is absent.

Pure uric acid is soluble in 16,000 parts of cold water and in 1600 parts of boiling water; impure uric acid is more readily soluble in water than the pure. Its cold solutions do not show an acid reaction with litmus paper. Uric acid is insoluble in alcohol and ether, but dissolves in warm glycerin, from which, on cooling, it separates in crystalline form. It is insoluble in strong mineral acids, but is soluble in alkaline hydrates as well as in alkaline carbonates, phosphates, lactates, and acetates. It is more soluble in solu

tions of urea than in water (Rüdel).

On boiling, uric acid reduces alkaline solutions of copper; before reduction occurs, however, a white precipitate, consisting of cuprous urate, is formed.

When uric acid is artificially decomposed, an interesting series of products results, the most important of which is Whether similar changes take place in the body is still a matter of doubt.

urea.

The following is a list of the principal changes which may be brought about by various reagents:

1. When uric acid is reduced with weak sodium amalgam, two substances-xanthin (C,H,NO,) and hypoxanthin or sarkin (C,H,N,O)—may be obtained. Their formulas differ from that of uric acid in containing one or two atoms less oxygen respectively than that substance.

2. When uric acid is heated in a closed tube with hydrochloric acid, it is decomposed into glycocoll, carbolic acid, and ammonia:

CHNO2+5H2O = C2H¿ÑO2 + 3CO2 + 3NH3.

3. By the action of cold, concentrated nitric acid, uric acid takes up water and oxygen, forming alloxan and urea :

2C,H,NO, + 2H2O + O2 = 2C,H ̧Ñ‚О ̧ + 2CON,H ̧.

Uric acid.

2