344: EXCRETION. 



tion of 1.4 per thousand. 1 It is not yet known from what source 

 the urea is originally derived ; whether it be produced in the blood 

 itself, or whether it is formed in some of the solid tissues, and thence 

 taken up by the blood. It has not yet been found, however, in any 

 of the solid tissues, in a state of health. 



Urea is obtained most readily from the urine. For this purpose 

 the fresh urine is evaporated in the water bath until it has a syrupy 

 consistency. It is then mixed with an equal volume of nitric acid, 

 which forms nitrate of urea. This salt, being less soluble than pure 

 urea, rapidly crystallizes, after which it is separated by filtration 

 from the other ingredients. It is then dissolved in water and decom- 

 posed by carbonate of lead, forming nitrate of lead which remains 

 in solution, and carbonic acid which escapes. The solution is then 

 evaporated, the urea dissolved out by alcohol, and finally crystal- 

 lized in a pure state. 



Urea has no tendency to spontaneous decomposition, and may 

 be kept, when perfectly pure, in a dry state or dissolved in water, 

 for an indefinite length of time. If the watery solution be boiled, 

 however, the urea is converted, during the process of ebullition, 

 into carbonate of ammonia. One equivalent of urea unites with 

 two equivalents of water, and becomes transformed into two equiva- 

 lents of carbonate of ammonia, as follows : 



C 2 H 4 N 2 2 =Urea. 

 0,=Water. 



Various impurities, also, by acting as catalytic bodies, will in- 

 duce the same change, if water be present. Animal substances in 

 a state of commencing decomposition are particularly liable to act 

 in this way. In order that the conversion of the urea be thus pro- 

 duced, it is necessary that the temperature of the mixture be not 

 far from 70 to 100 F. 



The quantity of urea produced and discharged daily by a healthy 

 adult is, according to the experiments of Lehmann, about 500 

 grains. It varies to some extent, like all the other secreted and 

 excreted products, with the size and development of the body. 

 Lehmann, in experiments on his own person, found the average 

 daily quantity to be 487 grains. Prof. William A. Hammond, 2 

 who is a very large man, by similar experiments found it to be 



1 Robin and Verdeil, vol. ii. p. 502. 



2 American Journal Med. Sci , Jan., 1855, and April, 1856. 



