THE CARBOHYDRATES. 15 



minutes, observing the spluttering that occurs, the liquid mean- 

 time becoming limpid. 



(a.) Test a portion of the liquid for glucose, taking care 

 that sufficient alkali is added to neutralise the surplus acid. 



(b.) Test it with iodine = a blue colour, showing that some 

 soluble starch (amidulin) still remains unconverted into a 

 reducing sugar. 



11. Circumpolarisation. Certain substances when dissolved 

 possess the power of rotating the plane of polarised light e.g., 

 the proteids, sugars, &c. The extent of the rotation depends on 

 the amount of the active substance in solution. The direction of 

 rotation i.e., to the right or the left is constant for each active 

 substance. Of course, light of the same wave-length must be 

 used. The light obtained from the volatilisation of common salt 

 is used. 



The term "specific rotatory power," or "specific rotation" of a 

 substance, is used to indicate the amount of rotation expressed 

 in degrees of the plane of polarised light, which is produced by 

 1 grm. of the substance dissolved in 1 cc. of liquid, when 

 examined in a layer 1 decimetre thick. 



Those substances which cause specific rotation are spoken of 

 as "optically active ;" those which do not, as "inactive." 



If n the observed rotation ; 



p = the weight in grammes of the active substances con- 



tained in 1 cc. of liquid ; 

 I = the length of the tube in decimetres ; 

 (a) D = the specific rotation for light corresponding to the 



light of a sodium flame ; 

 then 



The sign + or indicates that the substance is dextro- or 

 laevo-rotatory. Various instruments are employed. Use 



Laurent's Polarimeter. This instrument must be used in a 

 dark room. 



12. Determination of the Specific Rotatory Power of Dextrose. 



