INGREDIENTS OF THE URINE. 379 



Urea. This is the most important constituent of the urine, both in 

 regard to character and amount, forming more than one-half the entire 

 quantity of its solid ingredients, and over 80 per cent, of all those of an 

 organic nature. The most important fact known with regard to the 

 origin of urea is, that it is not formed in the kidneys, but pre-exists in 

 the blood in small proportion, and is drained away from the circulat- 

 ing fluid during its passage through the renal vessels. This was first 

 shown by the experiments of Prevost and Dumas, 1 who, after extir- 

 pating the kidneys, or tying the renal arteries in the living animal, 

 found the quantity of urea in the blood increased in marked proportion, 

 owing to the arrest of its elimination by the kidneys. It has also been 

 found in the blood of the human subject in cases of renal disease, 

 sometimes in so large a proportion as 1.5 parts per thousand, 2 or nearly 

 ten times its normal quantity. It has not been found, however, in suf- 

 ficient quantity in any of the solid tissues to indicate the immediate 

 source of its production. It is either formed in the blood itself, by 

 transformation of some previous nitrogenous combination, or it is ab- 

 sorbed by the blood too rapidly to be detected as an ingredient of the 

 solid tissues. 



Urea is obtained most readily from the urine by first converting it 

 into the form of a nitrate. For this purpose the fresh urine is evapo- 

 rated over the water-bath until it is reduced to one-quarter of its original 

 volume. It is then filtered, and the filtered fluid mixed with an equal 

 quantity of nitric acid, which produces nitrate of urea. This salt, being 

 less soluble than urea, rapidly separates in the form of abundant crys- 

 talline scales. The c^stalline deposit is separated from the mother 

 liquor, mixed with water, and decomposed by the addition of barium 

 carbonate, which sets free the urea, with the formation of barium nitrate. 

 This process is continued so long as carbonic acid is given off;- after 

 which the whole is evaporated to dryness, and the dry residue extracted 

 with absolute alcohol, which dissolves out the urea. The alcoholic solu- 

 tion is then filtered and evaporated until the urea separates in a crys- 

 talline form. 3 



The quantity of urea in a given volume of urine is readily ascertained 

 by decomposing it, according to Davy's method, with a solution of so- 

 dium hypochlorite. A long and narrow graduated glass tube, open at 

 one extremity, and capable of holding about 50 cubic centimetres of 

 fluid, is filled to a little more than one-third its capacity with mercury, 

 upon which are poured 3 or 4 cubic centimetres of the urine to be ex- 



1 Prevost and Dumas, Annales de Chimie et de Physique, 1823, tome xxiii. 

 p. 90 ; Segalas, Journal de Physiologie, tome ii. p. 354 ; Mitscherlich, Tiede- 

 mann. and Gmelin, Poggendorf's Annalen, band xxxi. p. 303 ; Cl. Bernard, 

 Liquides de 1'Organisme. Paris, 1859, tome ii., Deuxieme Legon. 



2 In Milne Edwards, Legons sur la Physiologie. Paris, 1857, tome i. p. 298. 



3 Hoppe-Seyler, Handbuch der Physiologisch- und Pathologisch-Chemischen 

 Analyse. Berlin, 1870, p. 120. 



