360 HAND-BOOK OF PHYSIOLOGY. 



diseases the quantity is reduced considerably below the healthy standard, 

 while in other affections it is above it. 



Estimation of Urea. A convenient apparatus for estimating the 

 quantity of urea in a given sample of urine is that devised by Russell and 

 West. 



Urea contains nearly half its weight of nitrogen; hence this gas may 

 be taken as a measure of the urea. A small quantity of urine is mixed 

 with a large excess of solution of sodium hypobromite, which completely 

 decomposes the urea, liberating all the nitrogen in a gaseous form: a 

 gentle heat promotes the reaction. The percentage of urea can of course 

 be readily calculated from the volume of nitrogen evolved from a measured 

 quantity of the urine, but this calculation is avoided by graduating the 

 tube in which the nitrogen is collected with numbers which indicate the 

 corresponding percentage of urea. CON 2 H 4 -j- 3NaBrO -f- 2NaHO = 

 3NaBr + 3H 2 + Na 2 C0 3 + N 9 . 



Uric Acid (C 5 H 4 N 4 3 ). This substance, which was formerly termed 

 lithic acid, on account of its existence in many forms of urinary calculi, 

 is rarely absent from the urine of man or animals, though in the feline 

 tribe it seems to be sometimes entirely replaced by urea. The pro- 

 portionate quantity of uric acid varies considerably in different animals. 

 In man, and Mammalia generally, especially the Herbivora, it is com- 

 paratively small. . In the whole tribe of birds, and of serpents, on the 

 other hand, the quantity is very large, greatly exceeding that of the urea. 

 In the urine of granivorous birds, indeed, urea is rarely if ever found, its 

 place being entirely supplied by uric acid. 



Variations in Quantity. The quantity of uric acid, like that of 

 urea, in human urine, is increased by the use of animal food, and de- 

 creased by the use of food free from nitrogen, or by an exclusively vege- 

 table diet. In most febrile diseases, and in plethora, it is formed in un- 

 naturally large quantities; and in gout it is deposited in, and around, 

 joints, in the form of urate of soda, of which the so-called chalk-stones 

 of this disease are principally composed. The average amount secreted 

 in twenty -four hours is 8*5 grains (rather more than half a gramme). 



Condition of Uric Acid in the Urine. The condition in which 

 uric acid exists in solution in the urine has formed ths subject of some 

 discussion, because of its difficult solubility in water. It is found chiefly 

 in the form of urate of sodium, produced by the uric acid as soon as it is 

 formed, combining with part of the base of the alkaline sodium phosphate 

 of the blood. Hippuric acid, which exists in human urine also, acts upon 

 the alkaline phosphate in the same way, and increases still more the quan- 

 tity of acid phosphate, on the presence of which it is probable that a part 

 of the natural acidity of the urine depends. It is scarcely possible to say 

 whether the union of uric acid with the base sodium and probably ammo- 

 nium, takes place in the blood, or in the act of secretion in the kidney: 



