124 PRACTICAL PHYSIOLOGY. [XIX. 



water to I litre. A few drops of HN0 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. 



12. Apparatus Required. Burette fixed in a stand, funnels, beakers, 

 filter-paper, glass rod, plate of glass, and three pipettes, 10, 15, and 20 cc. 



(a.) Collect the urine of the twenty-four hours, and measure the quantity. 



(b.) If albumin be present, separate it by acidification (acetic acid;, boiling, 

 and filtration. 



(c.) Mix 40 cc. of urine with 20 cc., i.e., half its volume, of a solution of 

 barium nitrate and barium hydrate (composed of one volume of solution of 

 barium nitrate and two volumes of barium hydrate, both saturated in the cold). 

 This precipitates the phosphates, sulphates, and carbonates. 



(d. } Filter through a dry filter to get rid of the above salts. While filtra- 

 tion is going on, fill the burette with the standard solution (SS.) of mer- 

 curic nitrate up to the mark on the burette. See that there are no air- 

 bubbles, and that the outflow tube is also filled. 



(e.) With a pipette take 15 cc. of the clear filtrate and place it in a beaker. 

 N.B. This corresponds to 10 cc. of urine. Place a few drops of the sodic 

 carbonate solution (the indicator) on a piece of glass resting on a black back- 

 ground. 



(/.) Note the height of the fluid in the burette. Run in the SS. of mer- 

 curic nitrate from the burette into the 15 cc. of the mixture, in small 

 quantities at a time, until the precipitate ceases. Stir and mix thoroughly 

 with a glas-s rod. After each addition, with the glass rod lift out a drop of 

 the mixture and place it on one of the drops of sodic carbonate until a pale 

 yellow colour is obtained. This indicates that all the urea has been precipi- 

 tated, and that there is an excess of mercuric nitrate. Read off the number 

 of cc. of the SS. used. 



(g. ) Repeat the experiment with a fresh 15 cc. of the filtrate, but run in the 

 greater part of the requisite SS. at once before testing with sodic carbonate. 



Read off the number of cc. of the SS. used, and deduct 2. cc. ; multiply by 

 .01, which gives the amount (in grams) of urea in 10 cc. of urine. 



Example. Suppose 27 cc. of theSS. were used, and the patient passed 1200 

 cc. of urine in twenty-four hours : then 25 x .01 = .25 gram urea in 10 cc. 



10 : 1200 : : .25 : x . i- = 30 grams of urea in twenty-four 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. 



Correction for Sodic Chloride. Two oc. were deducted in the above pro- 

 cess. Why? On adding mercuric nitrate to a solution 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 thft urea. Proofs of this : 



(a.) To a solution of sodic chloride (normal saline) add mercuric nitrate = 

 no precipitate. 



(b.) To a solution of sodi'; 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 = precipitate. 



