STRUCTURE AND FUNCTION OF THE KIDNEYS. 365 



combines with some acids, like a weak base; and may thus be conveni- 

 ently procured in the form of crystals of nitrate or oxalate of urea 

 (Figs. 252 and 253). 



Urea is colorless when pure; when impure it may be yellow or brown: 

 'it is without smell, and of a cooling nitre-like taste; it has neither an 

 acid nor an alkaline reaction, and deliquesces in a moist and warm at- 

 mosphere. At 59 F. (15 C.) it requires for its solution less than its 

 own 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 gray- 

 white powder, cyanic acid. 



Chemical Nature. Urea is isomeric with ammonium cyanate, NH 4 , 

 CNO. It was first of all artificially prepared from that substance. It is usu- 



FIG. 252. Crystals of Urea nitrate. FIG. 253. Crystals of Urea oxalate. 



ally considered to be a diamide of carbonic acid, in other words, carbonic 

 acid, CO (OH)' a , with two of hydroxyl, (OH)' 2 replaced by two of amido- 

 gen (NH 2 )' 2 . It may also be written as if it were a monamide of carbamic 

 acid (COOHNHJ, thusCONH,. NH 2 ; one of amidogen NH a in the latter 

 replacing one of hydroxyl in the former. On heating, urea is converted 

 into ammonium carbonate and cyanic acid. A similar decomposition of 

 the urea with development of ammonium carbonate ensues spontaneously 

 when urine is kept for some days after being voided, and explains the am- 

 moniacal odor then evolved. The urea is sometimes decomposed before 

 it leaves the bladder, when the mucous membrane is diseased, and the 

 mucus secreted by it is both more abundant, and, probably, more prone 

 to act as a ferment; although the decomposition does not often occur 

 unless atmospheric germs have had access to the urine. 



Variations in Quantity excreted. The quantity of urea excreted is, 

 like that of the urine itself, subject to considerable variation. For a 

 healthy adult 500 grains (about 32.5 grms.) per diem may be taken as 

 rather a high average. Its percentage in healthy urine is 1.5 to 2.5. Its 

 amount is materially influenced by diet, being greater when animal food 



