always mixed with the urate of soda, or uric acid, or urate of lime and magnesia. When burnt on platinum foil, there remains a white ash, containing soda and lime; the former demonstrated by imparting a yellow colour to the flame of the blowpipe, the latter by being soluble in acetic acid. and precipitated by oxalic acid.

The chemical characters of this substance are not less marked than the microscopic. It is rarely observed in urine until after it has cooled, a circumstance which makes it very difficult to explain how urate of ammonia can possibly form the nucleus of a concretion. It readily disappears when the urine is raised to the temperature at which it left the body, or when the urine is diluted with water. It is soluble in liquor ammonia and liquor potassæ, from which solutions uric acid is precipitated by the addition of acetic acid. When the sediment is brought into contact with acetic acid or hydrochloric acid, it is slowly transformed into uric acid crystals, ammonia entering into combination with the acid and forming a soluble salt. The dry deposit gives with nitric acid the characteristic reaction of murexide.

Urate of soda. C1H,NaNO-This salt may be obtained artificially, either by dissolving uric acid in caustic soda, and precipitating it by a current of carbonic acid, or by mixing the boiling caustic solution with bicarbonate of soda. In the latter case the urate is formed in delicate needle-shaped crystals. Occurrence: in the urine, dissolved and as sediment; in the blood during gout; in the gouty concretions near the joints.

It is soluble in 123 to 125 parts of boiling water, and requires 1100 to 1200 parts of water of 77° F. (25° C.) The solution is neutral, does not absorb carbonic acid; a precipitate is produced in it by the bicarbonates of the alkalies, and by the salts of baryta, lead, and silver.

Crystallization.-Urate of soda forms short hexagonal prisms, or thick six-sided plates, frequently uniting and forming star-like groups: the two opposite angles of these plates measure 74° 50', the four angles between these two 142° 35' (Gorup-Besanez, p. 185). In this form it is met with in urine undergoing the process of alkaline fermentation. The artificial salts, as obtained by the above process, or from the blood, form needles. If obtained by digesting pure uric acid in a hot solution of common phosphate of soda, it crys

fig. 4.

See also Dr. Golding Bird, loc. cit., p. 139, fig. 29. Funke, Atlas, taf. iv,

tallizes in needles, tufts, stars, spindle-like figures, and prisms, when it is allowed to separate from the solution on cooling. But if a solution of urate of soda be allowed to evaporate spontaneously, the salt is deposited in simple spherical masses and granules.

Dr. Golding Bird met with the urate of soda in the urine of persons labouring under fever, who were treated with carbonate of soda. It occurred in round, yellowish or white, opaque masses, furnished with projecting, generally curved processes.' I have observed the same deposit of an extremely striking character (Pl. II, fig. 3,) in the urine of a child suffering from petechial typhus, who was not treated with the carbonate of soda. The details of this case will be given under the paragraph, Deposits of Urates occurring in the urinary passages, p. 101. The same deposit occasionally occurs in the urine of children labouring under measles or scarlet fever.

Urate of lime. CH&CaNO+2Aq-This salt occurs occasionally in traces and small quantities in the urine, in sediments, and in concretions, particularly the gouty tumours near joints.

It may be obtained artificially, by mixing a solution of chloride of calcium with a boiling solution of urate of potash or soda. From this mixture it forms as an amorphous precipitate. If the urate employed have been a little alkaline, warty groups of needles are sometimes obtained. It is sparingly soluble in water, requiring 603 parts of cold and 276 parts of boiling water for solution. Its solubility becomes, however, increased by the presence of a small amount of chloride of potassium.

On combustion it leaves carbonate of lime, or caustic lime. On being digested in acetic acid, this salt is destroyed, uric acid being set free; from the acetate of lime, the base may be precipitated by oxalate of ammonia.

Differential diagnosis of urates.-As uric acid is readily set free by stronger acids, its presence is easily enough ascertained. The bases offer little difficulty to analysis, if they occur separately; but if they are mixed, as is mostly the case, the analysis becomes very complicated. It is for this reason that we do not know as yet whether the fixed or the volatile alkali is more commonly combined with uric acid. The question being moreover of no great practical importance, some time will probably elapse before it is finally settled.

1 See his fig. 28.

If a sediment simply filtered from the urine be taken for analysis, an ash is always obtained on combustion. For the epithelia from the kidneys and urinary passages, and the mucus from the bladder contain alkali and lime; and as they are mixed with the deposit, and burnt with it, they are placed to the account of the uric acid. In all cases, therefore, it is necessary to redissolve the deposit in the urine by the application of heat, and to filter, when all organized particles, with the mucus and uric acid crystals, will remain on the filter. On cooling, the deposit will form again, and may now by filtration be obtained free from admixtures. But colouring matter, containing iron, still adheres to the urate; and however small the amount may be, it would leave a residue on combustion. This colouring matter must therefore be removed by washing the deposit on the filter with boiling alcohol.

If there is more deposit in urine than it will dissolve on the application of heat, which is sometimes the case where the deposit has come ready formed from the bladder, it becomes necessary either to dilute the urine until the deposit is dissolved, to filter it while hot, and to evaporate again to the former bulk, or to filter the urine cold, and then dissolve the urates on the filter by the assistance of hot water. This latter solution may then be kept separate, and on evaporation will leave the urates in a pure state.

Under all circumstances, the deposits of urates may be collected on a filter, washed with alcohol, dissolved in boiling water and filtered; the filtered liquid after evaporation leaves the pure urates.

To determine the urates dissolved in urine, the latter must be evaporated to dryness, and extracted with alcohol; the residue consists of uric acid, urates, and inorganic salts. The urates, which form part of the scum of urine while it is

evaporating, appear as spherical masses, with many spicula protruding from their surfaces. These bodies frequently lie in a film of phosphate of lime and magnesia. When urine is evaporating spontaneously, similar forms are frequently seen. In a concentrated urine from fever, urates formed on spontaneous evaporation which exactly resembled the fig. 281 of Dr. G. Bird's work. Hot water dissolves the urates and inorganic salts from the residue obtained after extraction. If the solution be now concentrated, and cooled down, a part of the urates will precipitate, after which they may be collected on a filter, washed, and analysed with regard to their proportions of bases. From the urates in solution with the inorganic salts the uric acid is obtained by the admixture of another acid, and from the composition of the first portion of the urates the amount of the bases which were combined with this uric acid may be calculated.

Amount of Uric Acid discharged during twenty-four

hours.

Uric acid being a normal, i. e., a constant ingredient of urine, there is reason to expect that its amount may stand in a certain proportion to the ingesta and the organic changes. But mere reflection on the nature of uric acid, on its relations to urea, uric oxyde, and hypoxanthine, or its occurrence or absence in certain animals and classes of animals, on the mode in which its occurrence and amount in animals is influenced by various conditions, natural or artificial-cannot supply the want of direct observation on this point, however valuable may be the suggestions derived therefrom. The number of direct observations is very small, as compared with the number of attempts to supply this want hypothetically by observations on lower animals. And of the existing observations a great number are entirely useless, because they refer only to 1000 parts of urine, with no regard to the time in which it was discharged. Some otherwise valuable analyses have been executed upon abnormal subjects, as those of Lehmann upon himself. The only really useful series of analyses we possess are those of Becquerel2 and Neubauers. According to the former, the medium amount of uric acid discharged during twenty-four hours is 0.49 to 0.56 grammes. Neubauer obtained the following results in a series of observations on two healthy individuals—

1 Vide Beale, loc. cit., p. 213.

Séméïotique des Urines,' Paris, 1841, p. 44.

3 Vogel, loc. cit., p. 249.

Lehmann found that he discharged on an average 1.18 grammes of uric acid during twenty-four hours. As he himself supposes this to be an abnormal quantity, we must leave the observation out of consideration at present, since we are engaged in determining the normal quantity.

Considering the very limited number of observations, the following quantities of uric acid for twenty-four hours may be said to be within the range of perfect health:

Minimum.

0.02 grammes.

Medium.1 0.5 grammes.

Maximum. 10 grammes.

The fluctuations of uric acid in health are therefore more considerable than those of any other constituent of urine, not only in the same individual during so short a period as five days, as shown by Neubauer's first series of observations on the first individual, but the averages of different individuals show considerable variations, for which an ample allowance must be made, before in any given case the amount discharged can be said to be abnormal; viz., below or above the average. The 118 grammes of Lehmann is clearly abnormal, as is also his average amount of urea.2 The idiosyncrasy of this experimenter is most probably a large appetite.

The relations of urea to uric acid are intimate; their quantities are in certain proportions. For 350 grammes of

1 Dr. G. Bird's medium is 8.1 grains of uric acid during twenty-four hours, which is very nearly the above medium, which is 7.72 grains.

2 Vide Dr. G. Bird, loc. cit., p. 7, and Simon's 'Beiträge zur Physiol. und Pathol. Chemie und Microscopie,' Berlin, 1843, B. i, p. 190.