38 THE CHEMISTRY OF CATTLE FEEDING 



long. This angle is called the specific rotary power of the substance. 

 It is designated by the symbol [a] D when the rotation is referred to 

 the D line of the monochromatic light produced by holding a spoon 

 containing sodium chloride in the flame of an ordinary Bunsen 

 burner. The specific rotary power of each optically active 

 substance is a constant quantity (apart from the phenomena of 

 birotation, p. 22) and the direction may be indicated by the 

 ymbols 4- and according as it is dextro or laevo rotation. For 

 example, the specific rotatory power of cane sugar is given as 

 [a] D = + 66*5. The specific rotatory power of some substances 

 varies considerably at different temperatures. 



