BY DR. LAUDER BRUNTON. 549 



, required to decompose the chlorides, and does not, therefore, 

 take part in the urea reaction. 



If this correction be constantly introduced in a series of ob- 

 servations, and, as has been already pointed out, the urine be 

 not of very exceptional composition, results are obtained which 

 are ve r.y n ea rly ,co r rec t , and which are comparable the one with 

 the other. If, however, the urine in cases of pneumonia or of 

 fevers were under investigation, the error introduced by the 

 application of this arbitrary correction would generally be very 

 great. 



In such cases we must adopt a more scientific method of 

 avoiding the error introduced by the presence of chlorides. We 

 must in the first place determine, by the standard xofation of 

 mercuric nitrate for chlorine, the amount ot chlorine, calcu- 

 lated as JSTa'Or present in 10 cubic centimetres of the urine, i. e., 

 in 15 cubic centimetres of the filtrate obtained on mixing two 

 volumes of urine with one volume of baryta mixture, and we 

 must then remove the whole of the Cl from a fresh quantity 

 of filtrate by a standard solution of nitrate of silver. To do 

 this we require a solution of nitrate of silver exactly equivalent 

 to the solution of nitrate of mercury which has been used. If 

 11.601 grammes of fused silver nitrate be dissolved in distilled 

 water, and diluted to the volume of 1 litre, the solution will be 

 of the required strength, i. e., 1 cubic centimetre will exactly 

 precipitate 0.010 gramme of chloride of sodium. 



Take 30 cubic centimetres of the filtrate from the mixture of 

 baryta mixture and urine, and, having added a drop of nitric 

 acid, pour in from a burette, or from a finely divided pipette, 

 twice as many cubic centimetres of the nitrate of silver solu- 

 tion as the number of cubic centimetres of nitrate of mercury 

 solution required in the chlorine determination. A precipitate 

 of chloride of silver will fall, and the filtrate may now be sub- 

 jected to analysis for urea. 



An example will help to make the course of these operations 

 clear. 



Fortycubic^ centimetres of the urine of a boy suffering from 

 typhus lever were mixed with 20 cubic centimetres of baryta 

 mixture, and the fluid was filtered. 15 cubic centimetres of the 

 filtrate was placed in a beaker, and the standard solution of 

 tncrcnrif for chlorine was added, until a permanent and dense 

 cloud had formed. The number of cubic centimetres added 

 was 4.5. As each cubic centimetre of the standard solution 

 corresponds to 0.010 gramme of Cl calculated as NaCl, the 

 quantity in 10 cubic centimetres amounted to 0.045 gramme. 

 30 cubic centimetres of the filtrate from the baryta mixture 

 and urine were now taken and treated with 4.5 X 2, i. e., 9 cubic 

 centimetres of nitrate of silver solution. The fluid was filtered. 

 Now 39 cubic centimetres of the mixture of urine, baryta so- 



