THE ORGANIC CONSTITUENTS OF THE URINE. 233 



compounds of mercury, which latter would not give up their entire 

 amount of nitrogen if sodium hydrate alone were present. The talcum 

 or zinc merely prevents an unduly violent bumping when boiling. 

 The retort is then immediately connected with a condenser through 

 the intervention of a Kjeldahl distilling tube. The condensing tube 

 dips into a nitrogen bulb, which contains a carefully measured 

 amount of a one-fourth normal solution of sulphuric acid ; 30 c.c. 

 are usually sufficient. The mixture is now distilled until about 

 two-thirds have passed over. The condenser is rinsed with a 

 little distilled water, which is added to the distillate. After the 

 addition of a few drops of tincture of cochineal the excess of 

 acid is retitrated with a one-fourth normal solution of sodium 

 hydrate. The difference indicates the amount of acid which was 

 consumed in uniting with the liberated ammonia. As 1 c.c. of the 

 one-fourth normal solution represents 0.0035 gramme of nitrogen, 

 the amount contained in the 5 c.c. of urine is ascertained by multi- 

 plying the number of cubic centimeters employed by this figure, 

 from which the total amount of twenty-four hours is then readily 

 calculated. The corresponding amount of albumin is obtained by 

 multiplying this figure by 6.25. The method, as just described, 

 appears simple enough, but in reality requires a considerable amount 

 of experience to obtain figures that are reliable. With experience, 

 however, the method is exceedingly accurate. In every case, of 

 course, chemically pure reagents are necessary, and it is well to test 

 -these with care before proceeding. 



Uric Acid. 



Whereas in mammals, the amphibia, and fishes urea is the most 

 important end-product of nitrogenous metabolism, the greater por- 

 tion of the urinary nitrogen in birds and reptiles is eliminated as 

 uric acid. 



Origin. The formation of uric acid in birds and reptiles is analo- 

 gous to the formation of urea in the mammal. It is derived in 

 the last instance from the albumins of the tissues and from the 

 ingested food, and, like urea, is formed synthetically in the liver. 

 This is true at least of the greater portion ; while a variable frac- 

 tion originates from the nucleins, viz., the xanthin bases. Organic 

 ammonium salts, amido-acids, urea, and ammonium carbonate, when 

 given to birds in their food, appear in the urine as uric acid, and 

 it is now thought that here also the greater portion of the nitrogen 

 is carried to the liver as ammonium lactate. We accordingly find 

 that after extirpation of the liver almost all the urinary nitrogen 

 appears in this form, and that ammonium carbonate when jjiven by 

 the mouth is eliminated as such. Of the manner in which the 

 synthesis of uric acid is effected in the liver, however, we know 

 but little, Urea or ammonium carbonate cannot, of course, give 

 rise to its formation alone, as the available amount of carbon is 



