QUANTITATIVE ESTIMATION OF SUGAR. 221 



the thymol deep red. The subsequent addition of water causes a precipitate of similar colour, 

 which is insoluble in concentrated hydrochloric acid. Albumin, casein, and peptone give the 

 same reaction (Seegen), but the deposit on the addition of water is soluble in concentrated 

 hydrochloric acid. 



Other tests are described in 266. 



In all cases where albumin is present it must be removed in urine by acidulating with acetic 

 acid and boiling ; in blood, by adding four times its volume of alcohol and afterwards filtering, 

 while the alcohol is expelled by heat, 



150. QUANTITATIVE ESTIMATION OF SUGAR. I. By Fermentation. In the glass 

 vessel (fig. 161, a) a measured quantity (20 c.cm.) of the fluid (sugar) is placed along with some 

 yeast, while b contains concentrated sulphuric acid. The__ 

 whole apparatus is then weighed. When exposed to a suffi- 

 cient temperature (10 to 40 C), the sugar splits into 2 

 molecules of alcohol and 2 of carbon dioxide, 



C 6 H 12 6 - 2(G 2 H 6 0) + 2(C0 2 ), 

 Grape-sugar =; 2 alcohol + 2 carbon dioxide ; 



and in addition there are formed traces of glycerine and suc- 

 cinic acid. The C0. 2 escapes from b, and as it passes through 

 the H 2 S0 4 , the C0 2 yields to the latter its water. The 

 apparatus is weighed after two days, when the reaction is Fig. 16L 



ended, and the amount of sugar is calculated from the loss of Apparatus for the quantitative 

 weight in the 20 c.cm. of fluid. 100 parts of water-free sugar estimation of sugar by fer- 

 = 48*89 parts C0 2 , or 100 parts C0 2 correspond to 204*54 mentation, 

 parts of sugar. 



II. Titration. By means of Fehling's solution, which is made of such a strength that all the 

 copper in 10 cubic centimetres of the solution is reduced bv - 05 grammes of grape-sugar 

 (267). 



III. Circumpolarisation. The saccharimeter of Soleil-Ventzke may be used to determine the 

 amount of sugar present. It may also be used for the quantitative estimation of albumin. 

 Sugar rotates the ray of polarised light to the right and albumin to the left. The amount of 

 rotation, or "specific rotatory power," is directly proportional to the amount of the rotating 

 substance present in the solution, so that the amount of rotation of the ray indicates the amount 

 of the substance present, By the term "specific rotatory power" is meant the degree of 

 rotation which is produced by 1 grm, of the substance dissolved in 1 c.cm. of water, when 

 examined in a layer 1 decimeter thick. For yellow light the specific rotation of grape-sugar is 

 + 56. 



In fig. 162 the light from the lamp falls upon a crystal of calc-spar. Two Nicol's prisms are 

 placed at v and s, v is movable round the axis of vision, while s is fixed. In m Soleil's double 

 plate of quartz is placed, so that one-half of it rotates the ray of polarised light as much to the 

 right as the other rotates it to the left. In n the field of vision is covered by a plate of left- 

 rotatory quartz. At b c is the compensator, composed of two right-rotatory prisms of quartz, 

 which can be displaced laterally by the milled head, g, so that the polarisied light passing 

 through the apparatus can be made to pass through a thicker or thinner layer of quartz. When 

 these right-rotatory prisms are placed in a certain position, the rotation of the left-rotatory 

 quartz at n is exactl}' neutralised. In this position the scale on the compensator has its nonius 

 exactly at o, and both halves of the double plate at m appear to have the same colour to the 

 observer, who from v looks through the telescope placed at e. Rotate the Nicol's prism at v 

 until a bright rose-coloured field is obtained. In this position the telescope must be so adjusted 

 that the vertical line bounding the two halves shall be distinctly visible. The apparatus is now 

 ready for use. 



Fill a tube, 1 decimetre in length, with urine containing sugar or albumin, the urine being 

 perfectly clear. The tube is placed between m and n. By rotating the Nicol's prisms, v, the 

 rose-colour is again obtained. The compensator at g is then rotated until both halves of the 

 field of vision have exactly the same colour. When this is obtained, read off on the scale the 

 number of degrees the nonius is displaced to the right (sugar) or to the left (albumin) from zero. 

 The number of degrees indicates directly the number of grammes of the rotating substance 

 present in 100 c.c. of the fluid. If the fluid is very dark coloured, it must be decolourised by 

 filtering it through animal charcoal (Seegen), [or the colouring matter may be preciptated by the 

 addition of lead acetate. ] If the sugary urine contains albumin, the latter must be removed by 

 boiling and filtration. A turbidity not removed by .filtration may be got rid of by adding a drop 

 of acetic acid or several drops of sodic carbonate or milk of lime, and afterwards filtering. [One 

 may also employ the apparatus of Mitscherlich, or the " half-shadow apparatus" of Laurent.] 



151. MECHANISM OF THE DIGESTIVE APPARATUS. This embraces 

 the following acts : 



