96 CHEMICAL PHYSIOLOGY. 



Example. Suppose 25 cc. of the SS. were used, and the patient 

 passed 1200 cc. of urine in 24 hours : then, 25 x -01 = -25 

 gramme urea in 10 cc. 



10 : 1200 : : -25 : a . - =30 grammes of urea in 24 



hours. 



This method yields approximately accurate results only when 

 the amount of urea is about 2 per cent. With a greater or less 

 percentage of urea, certain modifications have to be made. 



3. Correction for Sodic Chloride. Two cc. were deducted in the 

 above process. Why ? On adding mercuric nitrate to a solu- 

 tion containing sodic chloride, the mercuric nitrate is decomposed 

 and mercuric chloride formed, and as long as any sodic chloride 

 is present, there is no free mercuric nitrate to combine with the 

 urea. Proofs of this : 



(a.) To a solution of sodic chloride (normal saline), add 

 mercuric nitrate = precipitate. 



(b.) To a solution of sodic chloride (normal saline) add a 

 few crystals of urea, then add mercuric nitrate. At first 

 there is no precipitate, or, if there is, it is redissolved ; but 

 by-and-by a white precipitate is obtained. 



(c.) To a solution of urea (acid) add mercuric chloride = 

 no precipitate. 



4. Solutions Required. 



Baryta Mixture. Prepared as in Lesson XVI., 2, (c.) 



Mercuric Nitrate Solution (1 cc. = -01 grm. urea). Dis- 

 solve with the aid of gentle heat 77 -2 grammes of pure dry 

 oxide of mercury in as small a quantity as possible of HNO 3 , 

 evaporate to a syrup, and then dilute with water to 1 litre. 

 A few drops of HNO 3 will dissolve any of the basic salt 

 left undissolved. N.B. The exact strength of this solution 

 must be estimated by titrating it with a standard 2 per 

 cent, solution of urea. 



Sodic Carbonate Solution. 20 grains to the ounce of water. 



5. Apparatus Required. Burette fixed in a stand, funnels, 



