26 



PhACTICAL PHYSIOLOGY. 



[III. 



If a = the observed rotation ; 



"p = the weight in grams of the active substance contained in i cc of 



liquid ; 

 Z = 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 is a so-called 

 polarimeter," and must be used in a dark room (fig. 12). 



half-shadow 



Fig 12.— Laurent's Half- Shadow Polarimeter. 



18. Determination of the Specific Rotatory Power of Dextrose. 



(a.) Fill one of the decimetre tubes with distilled water, taking care that 



\ 



