UREA. 161 



urine do not modify the result of the experiment ; the essential 

 point in this method, which is somewhat more complicated, but 

 doubtless more accurate than that by nitric acid, consists in our 

 determining, by means of bichloride of platinum, the amount of 

 potash and ammonia (if the latter be present) in a specimen of 

 urine, and in our then treating a second specimen with sul- 

 phuric acid, and gradually heating it to 180 or 200, or as long 

 as any effervescence continues ; the fluid is then filtered, and the 

 amount of ammonia determined by bichloride of platinum ; deduct- 

 ing from the precipitate thus obtained that which was yielded by the 

 other specimen (corresponding to the potassio-chloride of platinum,) 

 we can easily calculate the amount of urea from the ammonio- 

 chloride of platinum, or from the platinum itself left on the inci- 

 neration of the residue. 



A still better method, by which urea may be determined quan- 

 titatively, although not perfectly free from error, has been given by 

 Millon.* It is based on the fact that urea is decomposed by nitrous 

 acid into nitrogen and carbonic acid; to effect this object a solu- 

 tion of nitrite of suboxide of mercury is dissolved in nitric acid, 

 and added to a weighed portion of urine ; on warming this mixture 

 there is a development of nitrogen and carbonic acid, which latter 

 gas is caught in a potash-apparatus and weighed. Some of the 

 extractive matters might yield carbonic acid, even if none of the 

 other constituents of the urine did so ; this, however, is denied by 

 Millon. It must also be recollected that the urine always contains 

 free carbonic acid in solution. 



Finally, a method has been proposed by R. Bunsenf for the 

 quantitative determination of urea, founded on the property that its 

 solutions undergo decomposition in closed vessels at a temperature of 

 from 120 to 240; the carbonic acid which is thus formed is com- 

 bined with baryta, and the amount of urea is calculated from that 

 of carbonate of baryta. 



Physiological Relations. 



Occurrence. Urea is one of the principal products of excretion 

 of the kidneys : hence it chiefly occurs in the urine. Although it 

 constitutes the greatest part of the solid constituents of the urine, 

 it is contained in the liquid urine in very variable quantities in con- 

 sequence of the physiological relations, in accordance with which 

 the amount of water in the urinary secretion varies in so extraor- 



* Compt. rend. T. 26, pp. 119-121. 

 t Ann. d. Ch. u, Pharm. Bd. 65, S. 375-387. 



M 



