146 THE CARBOHYDRATES. [CH. V. 



obtained by heating sodium chloride or bromide in a plati- 

 num ring. The light emitted has a wave length correspond- 

 ing to the D line of the solar spectrum. A much more 

 brilliant illumination can be obtained by use of the green 

 rays emitted from a mercury lamp. The rotation being 

 greater with the shorter wave length, greater accuracy 

 can be obtained.* 



The rotation varies for different substances. It 

 is increased by increasing the concentration of the 

 solution or the length of the tube. It also varies with the 

 temperature, nature of the solvent, and the wave-length of 

 the light used. 



The specific rotatory power is the rotation observed 

 through a tube i decimetre in length of a solution calculated 

 to be 100 per cent. This is generally expressed as [a]. 

 If the sodium light is employed it is expressed as [a] D . 



- r x 100 



where r = the observed rotation. 



c = the concentration in grams, per 100 cc. 



/ = the length of the tube in decimetres. 

 If the temperature is defined, it is usually expressed by 



[a] D 20. 



If [a] D be known, the concentration in grammes per 

 100 cc. is given by 



P _ r x 100 



WD X /' 



The specific rotatory powers of the more common 

 sugars is shewn below. I am indebted to Dr. Lowry for 



* The apparatus in the author's laboratory consists of a triple field instru- 

 ment by Hilger, of London, fitted with a horizontal slit and a direct vision 

 spectroscope. A mercury lamp is used as the source of illumination. An 

 accuracy of 0-01 is easily obtained. 



