CRYSTALLIZABLE NITROGENOUS MATTERS. 115 



proportion in the urine. This is shown by the analyses of Picard, 

 who found, in the dog, the proportion of urea in the blood of the 

 renal arteries 0.36 per thousand, in the renal veins 0.18 per thousand. 



After extirpation of the kidneys, in the dog,* the urea in the blood 

 of the general circulation increases in twenty-four hours from 2^ to 

 nearly 8 times its former proportion. The same effect is produced by 

 tying the renal arteries, or by ligature of both ureters, which arrests 

 the functional activity of the kidneys. Grehant corroborated the 

 observations of Picard in regard to a diminished proportion of urea in 

 the blood of the renal vein, as compared with that of the renal artery, 

 in the healthy animal ; but after ligature of the ureter, the proportion 

 of urea was no longer diminished while passing through the kidney. 

 It is plain from these experiments that the immediate source of urea 

 is not in the kidneys, but in some other part or parts of the general 

 system. It has been found in the lymph, the aqueous and vitreous 

 humors of the eye, the crystalline lens, the liver and the spleen, and in 

 minute quantity in the perspiration. 



Though urea is evidently derived from the nitrogenous organic sub- 

 stances, the exact manner and place of its formation in the body have 

 not been determined. It has been artificially produced by Bechampf 

 from albuminous matter, placed in contact with potassium permanganate 

 in watery solution, and subjected to a heat of GO or 80 C. This reac- 

 tion has been confirmed by Ritter,J in whose experiments 30 grammes 

 of albumen furnished 0.09 gramme of urea, and the same quantity of 

 fibrine, 0.07 gramme; while from 30 grammes of gluten, in an average 

 of three experiments, there was obtained 0.27 gramme of urea. This 

 process, however, is not one of simple oxidation, but an oxidation with 

 decomposition, in which various other substances are produced at the 

 same time. 



Urea is a colorless, neutral substance, very soluble in water and in 

 boiling alcohol, less so in cold alcohol, nearly insoluble in ether. It 

 crystallizes in four-sided prisms, which are decomposed on being heated 

 above 120 C. Its pure watery solution may be kept without change 

 at ordinary temperatures ; but by continued boiling, or by a short 

 boiling in the presence of alkalies, it is decomposed with the production 

 of ammonium carbonate. If heated with water in an hermetically sealed 

 tube to 180 C. it undergoes the same alteration. This change takes 

 place with the assumption of the elements of water, as follows : 



Urea. Ammonium carbonate. 



CH 4 X S O 4- 2H 2 O = (XH 4 ) 2 CO 3 . 



Daily quantity of Urea and its variations. The quantity of urea 



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

 p. 90 ; Segalas, Journal de Physiologic, tome ii., p. 354 ; Mitecherlicb, Tiedemann and 

 Gmelin, Poggendorf's Annalen, Land xxxi., p. 303; Cl. Bernard, Liquides de 

 1'Organisme. Paris, 1859, tome ii., Deuxieme Le9on. Grehant, Centralblatt fiirdie 

 Medicinischen "Wissenschaften. Berlin, 1870, p. 249. 



f Coraptes Eendus de 1'Academie des Sciences. Paris, 1870, tome Ixx., p. 866. 



J Comptes Rendus, 1871, Ixxiii., p. 1219. 



