432 A MANUAL OF PHYSIOLOGY 



should be kept separately and mixed when required. Solution /. : 

 Dissolve 34*64 grammes pure cupric sulphate in distilled water, and 

 make up the volume to 500 c.c. Solution II. : Dissolve 173 grammes 

 Rochelle salt in 400 c.c. of water, add to this 51*6 grammes 

 sodium hydrate, and make up the volume with water to 500 c.c. 

 Keep in well-stoppered 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 sogar 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. 



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*05 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*05 x 2,000= 100 

 grammes sugar, in twenty-four hours. 



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

 means of estimating the quantity of sugar in pure and colourless 

 solutions, 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 rota- 

 tion, / 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 (/.<?., the amount of rotation expressed in degrees which is 

 produced by a layer of the substance i decimetre thick, when the 



Q 



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 5 2 '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. Among the best are those constructed 

 on what is called the ' half-shadow ' system. A half-shadow polari- 

 meter consists, like other polarimeters, of a fixed Nicol's prism (the 

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



