MONOSACCHARIDES. 195 



We designate the optical activity of the carbohydrates with the 

 letter I- for levogyrate, d- for dextrogyrate, and r- for the racemic. 

 These are only partly indicative. Thus dextrorotatory glucose is 

 designated d-glucose, levorotatory Z-glucose, but EMIL FISCHER has 

 used these signs in another sense. He designates by these signs the 

 mutual relationship of the various kinds of sugars instead of their 

 optical activity. For example, he does not designate the levorotatory 

 levulose Z-levulose, but d-levulose, showing its close relation to dextro- 

 rotatory d-glucose. This designation is generally accepted, and the 

 above-mentioned signs only show the optical properties in certain cases. 



Specific rotation means the rotation in degrees produced by 1 gm. substance- 

 dissolved in 1 cc. liquid placed in a tube 1 dcm. long. The reading is ordinarily 

 made at 20 C. and with the monochromatic sodium light. The specific rotation 

 with this light is represented by ()D, and is expressed by the following formulae 



()D = 7, in which a represents the reading of degrees, 1 the length of the 



tube in decimeters, and p the weight of substance in 1 cc. of the liquid. In- 

 versely the per cent P of substance can be calculated, when the specific rotation 



is known, by the formula P = i , in which s represents the known specific- 



S.L 



rotation. 



In the determination of the change in specific rotation with various concen- 

 trations we must know also the amount of substance in grams in 1 gram of the 

 solution (p) and the specific gravity of the solution (d) at 20. The rotation 



is calculated according to the formula ()D = T-J-. 



A freshly prepared solution of a substance often shows a different rotation 

 from one that has been allowed to stand for some time (multirotation) . The 

 correct values which are found on allowing the solution to stand for a sufficiently 



