544 THE SECRETIONS. 



or in the water oven (fig. 339), until a nearly dry residue is 

 obtained. The capsule with its contents is then heated in an 

 air oven whose temperature is maintained at 120 C. The 

 capsule is, after some time, allowed to cool in an exsiccator 

 (fig. 340) and rapidly weighed. The drying and weighing 

 should be repeated until the weight of the capsule and residue 

 is constant. In order to secure accuracy, the capsule in which 

 the evaporation is carried on should be fitted with a ground 

 til:i>- plate, which should be placed over it, when it is trans- 

 ferred from the air oven to the exsiccator, and from the ex- 

 siccator to the balance. 



It is absolutely essential that the weighing should be con- 

 ducted with the greatest possible rapidity* as the dried urinary 

 solids are highly hygroscopic. 



Instead of measuring the urine used in the analysis, a 

 weighed quantity may lie taken. 



** 193. Determination of the Amount of Chlorine 

 contained in Urine. 



Hi/ L /<'!>/<< '.< M.'thod It has been already mentioned that 

 when a solution of mercuric nitride is added to a solution of 

 ureri. :i dense white precipitate, is formed, which consists of 

 compounds of urea with mercuric oxide. 



If the solution of mercuric nitrate be sufficiently diluted, 

 and be added iii sutlicient quantity, the compound formed con- 

 tains four molecules of mercuric oxide for each molecule of 

 urea. 



If, however, a solution of mercuric nitrate be added to a 

 solution of urea and chloride of sodium, no precipitate will at 

 first be formed, the reaction between the urea and oxide of 

 mercury not occurring until the double decomposition between 

 the mercuric nitrate and sodium chloride has been completed, 

 thus: 



Hg 2N0 3 + 2NaCl=Hg 01,+ 2NaN0 8 . 



As soon, however, as this has occurred, a white precipitate 

 of the mercuric oxide and urea compound falls. 



Liebir's method of determining the amount of chlorine in 

 urine is based upon the reactions which have been referred to. 



In order to enable the student to determine the amount of 

 chlorine by Liebig's method, we shall describe, in the first 

 place, the method of preparing the standard solution of nitrate 

 of ineivury, and, in the second place, the method to be fol- 

 lowed in determining by its aid the quantity of chlorine in 

 urine. 



/'reparation of standard solution of mercuric nitrate for 

 the ffilhnafian of clilnrinc in Urine. 



The following solutions are required: 



1st. A solution of mercuric nitrate of such a strength that 



