222 GLUCOSES, SACCHAROSES, ETC. 



constant of the carbohydrate in question must he knoAA'n. The 

 results are generally calculated from the equation 



where C is the weight of the substance (expressed in grams) con- 

 tained in a litre of solution, A the rotation-constant of the 

 substance to be estimated, a the observed rotation, and L the 

 length of the column of solution in millimetres. If the concen- 

 tration of the solution has an influence on the optical activity, 

 then^j, the weight of substance in grams in 100 cc. of the solution, 

 must also be taken into account.^ The same must be done for 

 the temperature when that exerts any influence on the rotatory 

 power (invert-sugar) (cf. § 199). 



Suppose for instance a solution of grape-sugar in a tube 0*1 

 metre long to have shown a rotatory power -f 3°, then (using 

 Tollens and Grote's constant), 



C = 1883-2 x^-^^ 



For a solution of levulose with rotatory power - 3°, 



C = 943-4 X ^§^ 



For a solution of cane-sugar with rotatory j^ower +3° (using 



Wild's constant), 



C= 1505-6 X y^^j. 



In the first instance the solution would contain 56-496 grams 

 of dextrose per litre ; in the second, 28-302 grams of levulose in 

 the third, 45-168 grams of cane-sugar. See also § 210. 



§ 209. Edimation of two Sugars. — Tavo sugars present in solution 

 together may be accurately estimated, provided that the rotatory 

 power of each is known, and the other conditions in § 208 are 

 fulfilled. This is frequently the case Avith solutions of dextrose 

 and levulose. Neubauer^ then recommends observing the rotatory 

 power, and estimating the sugar with Fehling's solution. If the 

 latter indicates 15 per cent, of sugar, then if levulose alone were 

 present, the rotatory power (by sodium-light, in a tube 0-1 metre 



^ See Hesse, Annal. d. Chem. und Thann. clxxvi. 95, 1875, and Tollens, 

 Ber. d. d. chem. Ges. xi. 1800. 



2 Ber. d. d. chem. Ges. x. 827, 1877. 



