CARBOHYDRATES 39 



of the same sugar, dissolved in the same volume of water, and 

 placed in a tube of the same length, will always cause the same 

 angular deviation, or rotation, of the plane in which the polarized 

 light which passes through it is vibrated. In other words, the 

 same number of molecules of the optically active substance in 

 solution will always produce the same rotatory effect. This is 

 called the specific rotatory power of the substance in question. 

 It is expressed as the number of degrees of angular deviation of the 

 plane of polarized light caused by a column of the solution exactly 

 200 mm. in length, the concentration of the solution being 100 

 grams of substance in 100 cc. at a temperature of 20 C. Actual 

 determinations of specific rotatory power are usually made with 

 solutions more dilute than this standard, and the observed devia- 

 tion multiplied by the proper factor to determine the effect which 

 would be produced by the solution of standard concentration. If 

 the direction of the deviation is to the right (i.e., in the direction in 

 which the hands of the clock move) it is spoken of as " dextro " 

 rotation and is indicated by the sign +, or the letter d; while 

 if in the opposite direction, it is called " levo " rotation and indi- 

 cated by the sign , or the letter I. For example, the specific 

 rotation of ordinary glucose is +52.7; of fructose, 92; of 

 sucrose, +66.5. 



Reducing Action. All of the hexose sugars are active reducing 

 agents. This is because of the aldehyde group which they con- 

 tain. Many of the common heavy metals, when in alkaline solu- 

 tions, are strongly reduced when boiled with solutions of the hexose 

 sugars. Alkaline copper solutions yield a precipitate of red 

 cuprous oxide; ammoniacal silver solutions give silver mirrors; 

 alkaline solutions of mercury salts are reduced to metallic mer- 

 cury, etc. Any sugar which contains a potentially active alde- 

 hyde group will exhibit this reducing effect and is known as a 

 " reducing sugar." In some of the di- and tri-saccharides, the 

 linkage of the hexose components together is through the aldehyde 

 group, in such a way that it loses its reducing effect; such sugars 

 are known as " non-reducing." Advantage is taken of this prop- 

 erty for both the detection and quantitative determination of the 

 " reducing sugars." A standard alkaline copper solution of definite 

 strength, known as " Fehling's solution," is added to the solution 

 of the sugar to be tested and the mixture boiled, when the char- 

 acteristic brick-red precipitate appears. If certain standard 



