32 PHYSIOLOGICAL CHEMISTRY. 



pulverized dextrose in a test-tube. After the sugar has melted and 

 turned brown, allow the tube to cool, add water and warm. The 

 coloring 1 matter produced is known as caramel. 



13. Demonstration of Optical Activity. A demonstration of 

 the use of the polariscope, by the instructor, each student being 

 required to take readings and compute the " specific rotation." 



USE OF THE POLARISCOPE. 



For a detailed description of the different forms of polariscopes, 

 the method of manipulation and the principles involved the student 

 is referred to any standard text-book of physics. A brief descrip- 

 tion follows: -An ordinary ray of light vibrates in every direction. 

 If such a 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 sub- 

 stances (sugars, proteins, etc.) have the power of twisting or rotat- 

 ing this plane of polarized light, the extent to which the plane is 

 rotated depending upon the number of molecules which the polar- 

 ized 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 one gram of 

 substance dissolved in I c.c. of water in a tube one decimeter in 

 length. The specific rotation, (a)/>, 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 

 ascertain the per cent of the substance in solution, this may be 

 obtained by the use of the following formula, 



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

 substance in solution. 



