THE URINE. 327 



quantity of water in the urine ; its total amount of solid matter re- 

 maining about the same. If, however, both its quantity and mean 

 specific gravity be increased or diminished at the same time, or if either 

 one be increased or diminished while the other remains stationary, 

 this would show an actual change in the amount of solid ingredients, 

 and consequently an abnormal condition. 



Ingredients of the Urine. 



The chemical composition of the urine, as derived from numerous 

 analyses, is as follows : 



COMPOSITION OF THE URIXE. 



Water 950.00 



( Urea . . 26.20 

 Nitrogenous I Creatinine 0>87 



1 Sodium and potassium urates . . 1.45 



;es> [ Sodium and potassium hippurates . . 0.70 



Sodium biphosphate .... 0.40 



Sodium and potassium phosphates . . 3.35 



Mineral salts. Lime and magnesium phosphates . . 0.83 



Sodium and potassium chlorides . . IL 



Sodium and potassium sulphates . . 3.30 



Mucus and coloring matter . . . 0.35 



1000.00 



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

 character and amount, forming more than one-half 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 and is drained 

 away from the circulating fluid during its passage through the renal 

 vessels. It has been found in the blood of the human subject in cases 

 of renal disease, in so large a proportion as 1.5 parts per thousand,* 

 or nearly ten times its normal quantity. 



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

 a nitrate. For this purpose the fresh urine is evaporated over the 

 water-bath to one-quarter of its original volume. It is then filtered, 

 and the filtered fluid mixed with an equal quantity of nitric acid. The 

 nitrate of urea thus produced, being less soluble than urea, is deposited 

 in abundant crystalline scales. The deposit is separated by filtration 

 .from the mother liquor, mixed with water, and decomposed by the 

 addition of barium carbonate, which sets free the urea, with the forma- 

 tion 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 the 



* In Milne Edwards, Le9ons sur la Physiologic. Paris, 1857, tome L, p. 298. 



