490 A MANUAL OF PHYSIOLOGY 



(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. Various modifications of Fehling's 

 solution, which have for their object the prevention of the precipitate 

 of cuprous oxide, with its disturbing effect upon the reading of the 

 end-point, have been devised. Thus, in Pavy's solution ammonia 

 holds the cuprous oxide in solution, and the end-point is the dis- 

 appearance of the blue colour. Ten c.c. of Pavy's solution = i c.c. 

 of Fehling's solution = 0*005 gramme of dextrose. 



(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 (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) n 

 has been determined once for all for most of the important active 

 substances, and therefore w is easily calculated. For dextrose () 

 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 polari- 

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

 shadow ' system (Fig. 182) 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 are closed by glass discs secured by brass 

 screw caps. The glass discs must be slid on, so as to exclude all air. 

 The tube having been introduced between the polarizer and analyzer, 

 the sharp vertical line which indicates the division between the two 

 half-fields is focussed with the eye-piece, and then the analyzer is 

 rotated till the two halves are again equally shadowed. The angle 

 of rotation, a, is read off on the graduated arc, which is provided 

 with a vernier. 



Pentoses reduce Fehling's solution, but do not give the yeast test. 

 They give the following characteristic tests, which may be performed 

 with gum arabic, which contains arabinose, one of the pentoses : 



(i) Phloroglucin Reaction. Warm in a test-tube some pure con- 

 centrated hydrochloric acid to which an equal volume of distilled 

 water has been added. Add" phloroglucin until a little remains un- 

 dissolved. Add a small quantity of gum arabic, and keep the test- 



