4 PHYSIOLOGICAL CHEMISTRY. 



prism is usually some form of a Nicol prism and is so constructed that 

 only one of the polarized rays produced at the start is allowed to emerge 

 and pass through the instrument. The plane in which this ray vibrates 

 is called the plane of polarization. Such plane polarized light passes 

 through air, water, alcohol, ether, glass and many other transparent sub- 

 stances without change; that is the direction in which the light vibrates 

 remains unaltered. But many organic substances, liquids or solids dis- 

 solved, have the remarkable property of causing this plane of polarization 

 to change direction ; in other words the plane of vibration of the light suf- 

 fers a twist or rotation in passing through a column of the liquids. Sub- 

 stances which have the power of changing the direction of the. plane of 

 vibration of polarized light passing through them are called " active " sub- 

 stances and the extent of the rotation is dependent on the number of mole- 

 cules which the light passes. In the case of homogeneous liquids like oil 

 of turpentine the rotation varies with the length of the column through 

 which the light must pass, while in the case of dissolved solids, sugar 

 solutions for example, the amount of the twist or rotation varies with 

 the length of the column of solution, and also with its concentration or 

 number of molecules in a given volume. An instrument which has some 

 device which enables the observer to read off this rotation in degrees is 

 called a polarimeter, and the number of degrees read constitutes a measure 

 of the strength or concentration of the substance. 



In order to compare the rotation of substances the term " specific rota- 

 tion" has been introduced. This, as applied to liquids, may be defined as 

 the rotation which a substance would exhibit if examined in a column 100 

 millimeters in length having a concentration of I gram of active substance 

 to each cubic centimeter. This rotation must therefore be a calculated one, 

 and is found as illustrated by this concrete case. Consider a solution 

 made by dissolving 25 gm. of pure cane sugar in distilled water and diluted 

 to make exactly 100 cc. This is then examined in a polarization tube, 

 which is a long tube of glass or metal having ends of plane polished glass 

 perfectly parallel to each other. The sugar solution forms then a clear 

 transparent column of definite length, which, assume in this case, is 200 

 millimeters. By examination in the polarimeter it is found now that this 

 solution rotates the plane of polarized sodium light through 33.25. For a 

 solution with 100 grams to 100 cc. the rotation by calculation should be 

 four times this, or 133, in the 200 mm. tube or 66.5 in the 100 mm. or 

 standard tube. This is then the specific rotation, and we express it by 

 the formula: 



[a] D = 66.5, 



in which [a] is the usual symbol for the specific rotation, and the D the 

 indication that the observation is made with sodium light, a without the 

 brackets is the angle of rotation as read off. Remembering the definition 

 of specific rotation we have this general formula as applied to solutions: 



r , _ IOQ X 

 H - IX c 



