PRACTICAL EXERCISES 527 



bottles in the dark. For use, mix together equal volumes of the two 

 solutions. Ten c.c. of this mixture is reduced by 0-05 gramme dextrose. 

 To estimate the sugar in urine, put 10 c.c. of the mixture into a porcelain 

 capsule or glass flask, and dilute it four or five times with distilled water. 

 Dilute some of the urine, say ten or twenty times, according to the 

 quantity of sugar indicated by a rough determination. Run the 

 diluted urine from a burette into the Fehling's solution, bringing it to 

 the boil each time urine is added, until, on allowing the precipitate 

 to settle, the blue colour is seen to have entirely disappeared from the 

 supernatant liquid. The observation of the colour must be made while 

 the liquid is still hot. Benedict's modification of Fehling's solution* 

 may also be employed. 



Suppose that 10 c.c. of Fehling's solution is decolourized by 20 c.c. 

 of the ten-times diluted urine. Then 2 c.c. of the original urine contains 

 0-5 gramme dextrose. If the urine of the twenty-four hours (from 

 which this sample is assumed to have been taken) amounts to 4,000 c.c., 

 the patient will have passed 0-05x2,000=100 grammes sugar, in 

 twenty-four hours. 



(6) The polarimeter affords a rapid and, with practice, a delicate 

 means of estimating the quantity of sugar in pure and colourless solu- 

 tions, but diabetic urine must in general be first decolourized by adding 

 lead acetate and filtering off the precipitate. What is measured is the 

 amount by which the plane of polarization of a ray of polarized light of 

 given wave-length (say sodium light) is rotated when it passes through 

 a layer of the urine or other optically active solution of known thickness. 

 Let a be the observed angle of rotation, / the length in decimetres of 

 the tube containing the solution, w the number of grammes of the 

 optically active substance per c.c. of solution, and (a) D the specific 

 rotation of the substance for light of the wave-length of the part of the 

 spectrum corresponding to the D line (i.e., the amount of rotation 

 expressed in degrees which is produced by a layer of the substance 

 I decimetre thick, when the solution contains i gramme of it per c.c.). 



Then (a) D = -.. In this equation a and / are known from direct 



measurement; (a) D has been determined once for^all for most of the 

 important active substances, and therefore w is easily calculated. For 

 dextrose (a) D may be taken as 52-6. It varies somewhat with the 

 concentration, but for most investigations on the urine these variations 

 may be neglected. 



It is not possible to describe here the numerous forms of polarimeter 

 that are in use. Those constructed on what is called the ' half -shadow ' 

 system (Fig. 197) give sufficiently satisfactory results. A half -shadow 

 polarimeter consists, like other polarimeters, of a fixed Nicol's prism 

 (the polarizer), and a nicol capable of rotation (the analyzer). In 

 addition, there is an arrangement which rotates by a definite angle the 

 plane of polarization in one -half of the field, but not in the other 

 e.g., a small nicol occupying only half of the field. In the zero position 

 of the analyzer, both halves of the field are equally dark. The solution 

 to be investigated is placed in a tube of known length, the ends of which 



* It contains 17" 3 grammes of cupric sulphate, I73'o grammes of sodium 

 citrate, ioo'o grammes of anhydrous sodium carbonate made up with 

 distilled water exactly to one litre. In making the solution the citrate and 

 carbonate are dissolved with the aid of heat in about 600 c.c. ot water, and 

 then made up to about 800 c.c. The cupric sulphate is dissolved in about 

 too or 150 c.c. of water and added to the other solution, the whole being then 

 made up to a litre. 



