PRACTICAL EXERCISES 5*9 



Salts il will float on the top. The difference is due to an alteration in 

 the surface tension of the urine produced by the bile-salts. We must 

 exclude the presence of acetic acid, alcohol, ether, chloroform, turpen- 

 tine, benzine and its derivatives, phenol and its derivatives, anilin and 

 soaps, all of which also cause such an alteration in the surface tension 

 of urine that the sulphur sinks to the bottom. The urine should be 

 fresh, and if it has to be kept it should be preserved from decomposition 

 by cyanide of mercury, which does not alter the surface tension. The 

 reaction has the great advantage over other tests of being easily carried 

 out at the bedside. 



Acetone (i) Legal' s Test (Rothera's modification). To 5 to 10 c.c. 

 of the acetone-containing urine add enough ammonium sulphate crystals 

 to form a layer at the bottom of the test-tube, then 2 or 3 drops cf 

 a fresh 5 per cent, solution of sodium nitro-prusside and i to 2 c.c. of 

 strong ammonia. The development of a colour like that of perman- 

 ganate of potassium, often in the form of a ring a little above the 

 undissolved salt, indicates the presence of acetone. The reaction must 

 not be declared negative till half an hour has elapsed. The colour 

 slowly fades. 



(2) Where there is doubt as to the presence of acetone, it is best first 

 to distil it over. Put 250 to 500 c.c. of the urine suspected to contain 

 acetone into a litre flask. Add a few c.c. of phosphoric acid; connect 

 the flask with a worm, and distil over the urine into a small flask. 

 For qualitative tests it is best to collect only the first 20 to 30 c.c., 

 as most of the acetone is contained in this. Test the distillate for 

 acetone by (i) or by 



Lieben's Test. To a few c.c. of the distillate in a test-tube add a few 

 drops of solution of iodine in potassium iodide, and then sodium or 

 potassium hydroxide. A precipitate of yellow iodoform crystals (six- 

 sided tables) is thrown down if acetone be present. Examine them 

 under the microscope. On heating, the odour of iodoform may be 

 recognized. If the precipitate is amorphous it may be dissolved in 

 ether (free from alcohol), which is allowed to evaporate on a slide, 

 when crystals may be obtained. 



Determination of the Freezing-Point of Urine.* Study Beckmann's 

 apparatus shown in Fig. 171, p. 427. Note the large thermometer D 

 graduated in hundredths of a degree centigrade. It is inserted through 

 a rubber cork into the inner test-tube A. A platinum wire, F, bent 

 at the lower end into a circle or a spiral, which passes easily up and 

 down between the bulb of the thermometer and the tube, serves to stir 

 the urine. The thermometer must be so supported by the rubber 

 cork that the bulb is in the axis of the tube and a centimetre or two from 

 the bottom of it. The side-piece E on the tube A is not absolutely 

 necessary, but it is convenient for ' inoculating ' the urine with a crystal 

 of ice at the proper time. A passes through a rubber cork into a shorter 

 and wider outer glass tube B. The space between A and B serves as a 

 badly conducting mantle, which prevents too rapid cooling of the 

 contents of A. B passes through a hole in the metal or wooden cover 

 of a strong glass jar, C, which contains the freezing mixture. B should 

 fit the hole so tightly that it does not bob up out of the mixture when 

 A is removed. In C is a stirrear, G, of strong copper wire, the end of 

 which passes through the lid. This serves to stir up the freezing 

 mixture from time to time. 



Pulverize some ice by pounding it in a strong wooden box with a 

 heavy piece of wood. Take the inner tube with the thermometer out 

 of the apparatus. It is convenient to take the thermometer out of the 



* This is not often of clinical value, but it affords an opportunity for the 

 student to practise a method of great importance in physiology. 



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