548 THE SECRETIONS. 



aqueous solution of pure urea, containing 2 grammes of per- 

 fectly pure urea in 100 cubic centimetres. The quantity of so- 

 lution in the beaker will then contain 0.200 gramme of urea. 



The solution of mercuric nitrate is then added and the fluid 

 stirred ; an abundant snow-white precipitate falls. When the 

 precipitation appears to be ncnrly completed, a drop of the fluid 

 holding the precipitate in suspension is added to a drop of 

 ^solution of sodium carbonate on a porcelain slab. If the urea 

 be not completely precipitated, TTO cnange of color will be ob r 

 served when the two fluids are mixed. The mercuric nitrate 

 solution is then added drop by drop, and the process of testing 

 with the solution of Na. 2 C0 3 on the slab repeated from time to 

 time. At last a yellow color will appear. This will indicate 

 that the solution of mercury has been added in excess. The 

 number of cubic centimetres of solution added indicates the 

 number of c. c. which are equivalent to 0.200 gramme of urea. 

 As it is convenient to have a solution of mercuric nitrate, of 

 which 10 cubic centimetres shall precipitate 100 milligrammes 

 of urea (0.100), or 1 cubic centimetre 10 milligrammes, it is 

 essential to dilute the solution which has been prepared, in the 

 same manner as was indicated in the case of the solution for 

 the determination of chlorine. 



Haying prepared the solution of mercuric nitrate for urea, 

 and the baryta mixture, the analysis of urine can be rapidly 

 effected. 40 cubic centimetres of urine are mixed with 20 cubic 

 centimetres of baryta mixture; 15 cubic centimetres of the fil- 

 trate are precipitated with the mercury solution, until a yellow 

 reaction with solution of Na 2 CO.< is obtained. 



The number of cubic centimetres of the mercury solution 

 u-ed, minus 2 and multiplied by 0.010 gramme, indicates very 

 closely the amount of urea, expressed in fractions of a gramme, 

 contained in 10 cubic centimetres of urine, provided that the 

 urine be (rf'Tiverage composition, i. e., that it contains no ab- 

 normal substances, that the amount of chlorine in it be about 

 the average, and that it be neither very concentrated nor very 

 dilute. 



The statements made in the preceding paragraph indicate 

 many circumstances which have to be taken into account, and 

 many corrections which have to be introduced in order to give 

 to Lirbig\ method the accuracy of which it is capable. 



In pointing out these corrections, an explanation must be 

 given of the empirical statement, "that the number of cubic 

 "'fri'8 of mercury solution used, minus 2, and multiplied 

 by 0.01 grm., indicates very closely the amount of urea, ex- 

 d in fractions of a gramme, contained in 10 cubic centi- 

 f urine." The reason for subtracting 2 cubic centi- 

 metres is, that in average urines this volume of the solution is 



