THE KIDNEYS AND URIXE. 359 



U.ea is colorless when pure; when impure, yellow or brown: without 

 smell, and of a cooling nitre-like taste; has neither an acid nor an alka- 

 line reaction, and deliquesces in a moist and warm atmosphere. At 59 F. 

 (15 C.) it requires for its solution less than its weight of water; it is 

 dissolved in all proportions by boiling water; but it requires five times its 

 weight of cold alcohol for its solution. It is insoluble in ether. At 248 F. 

 (120 C.) it melts without undergoing decomposition; at a still higher 

 temperature ebullition takes place, and carbonate of ammonium sublimes; 

 the melting mass gradually acquires a pulpy consistence; and if the heat 

 is carefully regulated, leaves a grey-white powder, cyanic acid. 



Chemical Nature of Urea. The chemical nature of urea is ex- 

 plained elsewhere, 1 but it will be as well to mention here that urea is 

 isomeric with ammonium cyanate, and that it was first artificially pro- 

 duced from this substance. Thus: Ammonium cyanate (NH 4 .CNO) 

 = urea (CH 4 ~N" 2 0). The action of heat upon urea in evolving ammonium 

 carbonate and leaving cyanic acid, is thus explained. A similar de- 

 composition of the urea with development of ammonium carbonate 

 ensues spontaneously when urine is kept for some days after being voided, 

 and explains the ammoniacal odor then evolved (p. 356). The urea is some- 

 times decomposed before it leaves the bladder, when the mucous mem- 

 brane is diseased, and the mucus secreted by it is both more abundant, 

 and, probably, more prone to act as a ferment; although the decomposi- 

 tion does not often occur unless atmospheric germs have had access to 

 the urine. 



Variations in the Quantity of Urea. The quantity of urea ex- 

 creted is, like that of the urine itself, subject to considerable variation. 

 For a healthy adult 500 grains (about 32 -5 grms.) per diem maybe taken 

 as rather a high average. Its percentage in healthy urine is 1 *5 to 2 '5. 

 It is materially influenced by diet, being greater when animal food is ex- 

 clusively used, less when the diet is mixed, and least of all with a vegeta- 

 ble diet. As a rule, men excrete a larger quantity than women, and per- 

 sons in the middle periods of life a larger quantity than infants or old 

 people. The quantity of urea excreted by children, relatively to their 

 body-weight, is much greater than in adults. Thus the quantity of urea 

 excreted per kilogram of weight was, in a child, 0'8 grm. : in an adult 

 only 0*4 grm. Regarded in this way, the excretion of carbonic acid 

 gives similar result, the proportion in the child and adult being as 82 : 34. 



The quantity of urea does not necessarily increase and decrease with 

 that of the urine, though on the whole it would seem that whenever the 

 amount of urine is much augmented, the quantity of urea also is usually 

 increased; and it appears that the quantity of urea, as of urine, may be 

 especially increased by drinking large quantities of water. In various 



1 Appendix. 



