32 PHYSIOLOGICAL CHEMISTRY 



ray is caused to pass through a " polarizing" Nicol prism it is resolved 

 into two rays, one of which vibrates in every direction as before and a 

 second ray which vibrates in one plane only. This latter ray is said to 

 be polarized. Many organic substances (sugar, proteins, etc.) have the 

 power of twisting or rotating this plane of polarized light, the extent to 

 which the plane is rotated depending upon the number of molecules 

 which the polarized light passes. Substances which possess this power 

 are said to be " optically active/' The specific rotation of a substance is 

 the rotation expressed in degrees which is afforded by i gram of sub- 



FIG. 5. ONE FORM OF LAURENT POLARISCOPE. 



B, Microscope for reading the scale; C, a vernier; E, position of the analyzing Nicol prism; 

 H, polarizing Nicol prism in the tube below this point. 



stance dissolved in i c.c. of water in a tube one decimeter in length. 

 The specific rotation, () D , may be calculated by means of the following 

 formula: 



in which 



D = sodium light. 

 a = observed rotation in degrees. 

 p = grams of substance dissolved in i c.c. of liquid. 

 / = length of the tube in decimeters. 



If the specific rotation has been determined and it is desired to ascer- 

 tain the per cent of the substance in solution, this may be obtained by 

 the use of the following formula, 



P = 



(a) D l 



The value of p multiplied by 100 will be the percentage of the sub- 

 stance in solution. 



