ADDITIONS AND NOTES TO VOL. II. 547 



all the phosphoric acid has been precipitated, and there is a trace 

 of perchloride of iron in excess, we must moisten a slip of paper 

 saturated with ferrocyanide of potassium with a drop of the urine 

 to be tested ; if the excess is considerable it will be detected by 

 the formation of Prussian blue. Here, as in all cases in which 

 this method is employed, the quantity of the substance to be 

 determined in a previously determined volume of the urine, is 

 ascertained from the volume of the test-fluid which has been 

 expended in the experiment. 



We may proceed in a perfectly similar manner in the determi- 

 nation of the chlorine and sulphuric acid in the urine ; but here 

 we must acidify the urine with nitric or hydrochloric acid, and 

 bear in mind that, notwithstanding the free acid, organic matter, 

 combined with oxide of silver or baryta (as, for instance, uric acid, 

 &c.), is precipitated, although perhaps only in very small quanti- 

 ties, together with the chloride of silver and sulphate of baryta, 

 and hence rather more chlorine, and especially more sulphuric 

 acid, is always calculated than the urine actually contains. 



Liebig* has suggested a very ingenious method for deter- 

 mining volumetrically the amount of urea in the urine, which is 

 closely connected with a chemical fact that he has recently dis- 

 covered^ namely, that if bichloride of mercury in solution, and 



bicarbonate of potash in excess, be added to a solution of urea, we 



+ 

 obtain a compound of urea and mercury, U + 4 Hg O, which is 



perfectly insoluble in water. This method has, further, this ad- 

 vantage, that we simultaneously determine the amount of chlorine 

 in the urine. The following are the main steps in the process. 

 In order to remove the phosphates and sulphates of the urine, a 

 definite quantity of the fluid is mixed with half its volume of 

 a fluid, containing 1 volume of a saturated solution of nitrate of 

 baryta to 2 volumes of a saturated solution of caustic baryta. We 

 take about 15 c.c. of the filtered alkaline fluid, (which conse- 

 quently contains for every 3 volumes 2 volumes of urine), and 

 then, without neutralising it, we add from a burette a solution of 

 nitrate of mercury of known strength, as long as any precipitate is 

 formed. The mixture must be well stirred during this process. 



The precipitate is the above-mentioned compound of urea and 



+ 

 oxide of mercury, U + 4 HgO. When a few drops of the 



turbid fluid are poured into a watch-glass, and one drop of a 



* Ann. d. Ch. 11. Pharm. Bd. 85 ; S. 289-328. 

 t Ibid. Bd. 80, S. 12.3. 



