weeks' fasting, and is much increased by exercise. Lehmann and Frerichs, on the other hand, believe that the nature and quality of food influence the amount of urea. It is increased after meals; and Bechamp, by digesting albuminoids with permanganate of potash, has converted them into urea. His results have been questioned by Staedeler. Gelatin appears to form urea directly, not being appropriated by any tissue. Again, Draper found that the amount varied but slightly in a person kept at perfect rest while being treated for a fractured leg, and in another man who walked 13 miles daily at the rate of 4 miles per hour.

Much less is secreted during sleep, which is due either to diminished mental or muscular activity. Professor Haughton thinks mental exertion greatly increases the amount of urea. He thus assigns 533 grains as the daily average for the student, 400 for the physical labourer. Urea is not formed in the kidney, but is merely separated from the blood-so rapidly, however, that without taking great quantities of that fluid no trace can be obtained. As discovered by Prevost and Dumas, if the kidney be extirpated its amount increases to 1.4 per 1,000 of blood; while some of it, by the addition of water, is converted into carbonate of ammonia, and thus evolved from the mucous surfaces. In man disorganization of the kidneys is often so complete as to render impossible any separation of urea, which, being retained in the blood, often inflames the serous membranes, or if the retention be sudden, produces symptoms of narcotic poisoning, usually called "renal coma." Frerichs thinks such effects due to the conversion of the urea into carbonate of ammonia, which cannot, however, be usually detected in the breath. Urea is not a poison when taken into the digestive canal, for Segalas administered it as a diuretic in dropsy. Prof. Parkes states that liquor potassæ increases the amount of urea, as also does common salt. Alcohol, tea, &c., according to Böcker, check

its excretion. Disease sometimes increases the quantity excreted-a morbid condition, termed by Prout diabetes ureosus, or azoturia. Warncke (whose essay has been translated by that elegant scholar, Dr. W. D. Moore) found it increased during the first week of typhoid fever, after which it gradually decreased. This circumstance is diagnostic from gastric fever, in which it is at first decreased, and gradually rises in amount. 1,250 grains were daily excreted in pyemia, and 1,100 in fevers. Its amount is much decreased in cancerous cachexy, and lowered to its normal standard in Bright's disease, and Thudichum relates a case of ovarian tumour where but 75 grains passed daily. It occurred in the muscular substance of a cholera patient, in which tissue it cannot be found in health. The fluids of the eye and the liquor amnii normally contain urea.

Uric Acid (C10 H4 N4 O6) constitutes about Too of human urine, being combined with soda and ammonia, and, as some state, with lime. It is very insoluble. The almost solid urine of serpents and birds (guano) contains even 90 per cent. of urates. Carnivora excrete more of it than man, and the urine of herbivora contains none. It is a very weak acid, not reddening litmus, and displacing only a part of the carbonic acid combined with the base, thus converting a carbonate into a bicarbonate. Uric acid can be obtained by adding hydrochloric acid to urine, when in a few hours it subsides in beautiful crystalline grains, stained red by the colouring matter. Some of its crystalline forms are figured in the following page, that in the centre of the second figure having been observed by Mr. Richardson. Its presence is easily detected by forming murexid, or purpurate of ammonia of Prout (its discoverer), a splendid purple substance. To produce this reaction a little of the urine is heated on a slip of glass with a drop of nitric acid; ammonia is next added to the dried residue, when a crimson colour appears, changing to purple if liquor potassæ be poured on

it. The colour disappears on heating. Thein gives a similar reaction.

Uric acid appears to be a result of the oxidation of of nitrogenous tissues and food, further oxidation, according to Liebig, producing urea; so that Haughton regards the presence of uric acid as abnormal.

Diminished perspiration, respiration, or exercise, and increased animal food, augment the quantity excreted.

Acute inflammatory diseases, gout and rheumatism, increase the quantity of uric acid generated in the system; but more of it will be passed with the urine when they are subsiding. Parkes procured 17.28 grains daily from a fever patient. Dr. Garrod has detected urate of soda in the blood of gouty patients, from whose urine it disappears before an attack of gout, which is then due to its accumulation in the blood. The readiest way to obtain uric acid from the blood of a gouty patient, is to apply a blister and add acetic acid to the blister-serum, through which a fine thread is passed; it soon crystallizes upon the thread. Tophi or chalk stones are composed of urate of soda; hence physicians prefer potash or lithia as remedy for gout, the salts they form being

soluble. These deposits are formed about the smaller joints, and, as Dr. Benson has observed, under the skin of the ear. I had lately the advantage of observing the action of lithia employed by Dr. Garrod in the University College Hospital. In many debilitating diseases, chlorosis, hysteria, ramollissement, spermatorrhoea, &c., the amount of uric acid may be decreased one-half. Liebig's theory that excess of uric acid occurs in diseases where the oxygenation of the blood is prevented, is not supported by clinical observation. The dependence of lithuria on obstructed action of the skin, and the eliminative and curative power of that surface are well established; and uric acid calculi rarely occur in warm climates, where cutaneous excretion is abundant. Bird found crystals of urate of soda in eczematous discharges from gouty patients. Among remedial agents whose actions the physiological source and chemical properties of uric acid explain, may be mentioned exercise, diminished animal food, diaphoretics, and such solvents as potash, and its salts with organic acids, lithia used by Garrod, phosphate of soda suggested by Liebig, and benzoic acid recommended by Ure.

Hippuric Acid (C18 H8 NO5 HO), discovered by Liebig, is found in the urine of horses-hence its nameof cows, which contain 1.3 per cent., and some other vegetable feeders. Uric acid replaces it in the urine of all animals while suckling. A minute trace can be discovered in human urine, and a large quantity was daily excreted by a patient of Mr. Tuffnell, in the Military Prison. Hydrochloric acid throws it down, as it does uric acid, than which it is much more soluble, and distinctly acid, reddening litmus at once. Ure found that benzoic acid, cinnamic acid, or oil of bitter almonds, if taken, were converted into hippuric acid, and so carried off in urine. A converse change is said to occur if cow's urine be allowed to become stale, hippuric changing into benzoic acid, an occurrence which also takes place in

horses if overworked. After bile and purpurine, hippuric acid contains more carbon than any other substance obtainable from the human body. With regard to its physiological and pathological relations we know little, save that it has been found in the blood of oxen, that it appears analogous to bile-pigment, and is absent from the urine in jaundice.

Creatin and Creatinin are two other nitrogenized products discovered in the urine by Pettenkofer, and proved to exist in muscle-juice by Liebig, a pound of flesh giving about 5 grains of the former. Their chemical composition only differs in creatin containing two more equivalents of water. Creatin has been discovered in the blood. Four grains of creatin and 7 of creatinin are daily excreted, the amount being proportional to muscular waste. The heart yields most creatin.

Kiestin is a term applied to a substance which rises, as a cotton-like cloud, to the surface of the urine of pregnant women. It is said by some to be merely a mixture of ammoniaco-magnesian phosphate and fungoid growths developed by putrescence, and by others to be composed of a fatty and a casein-like substance, thus excreted while the blood contains casein and no milk is produced. Its peculiar appearance, and cheese-like odour when putrifying, afford reliable signs of pregnancy or mole-pregnancy.

We shall here describe two nitrogenized bodies which can be hardly assigned as normal constituents, though there is some reason to suppose they exist in, or are formed from the extractives.

Xanthin, or uric oxide, is a very rare constituent of calculi discovered by Dr. Marcet, and never has occurred except in children. Its chemical characters are very similar to those of guanin and hypo-xanthin, which may be found abundantly in the spleen and cirrhosed liver. Bence Jones is the only chemist who has found it crystalline. It forms the greater part of the urine