70 ORGANIC AGRICULTURAL CHEMISTRY 



let it suffice to state that when light passes through certain 

 crystalline substances, e.g., Iceland spar (crystallized calcium 

 carbonate), it undergoes double refraction, that is, the vibra- 

 tions are broken into two sets, the planes of which are at right 

 angles to each other. One set of vibrations is called the ordi- 

 nary ray and the other the extraordinary ray. When another 

 crystal or, in practice, another half of a crystal reversed, is 

 next interposed, the extraordinary ray only passes through the 

 second half of the double crystal. On emerging the light is 

 found to consist of vibrations all in one plane or in parallel 

 planes and is said to be polarized. When such a double crys- 

 tal has been mounted in a particular form of optical apparatus 

 containing a second double crystal like the first, it is possible 

 to observe this polarized light and note how it is affected by 

 different substances. Such an instrument is known as a polar- 

 iscope. 



Now when certain substances, some in crystalline form and 

 others in solution, are examined in this instrument, they exhibit 

 certain definite characteristic effects upon the polarized light. 

 When a solution of ordinary sour milk lactic acid is so examined, 

 it is found to have no effect upon the polarized light, i.e. it 

 is, as we say, inactive toward polarized light. When, however, 

 the structurally identical substance obtained from flesh is simi- 

 larly examined, it is found that it turns the plane of the polarized 

 light to the right. It is termed, therefore, dextro (right) rotatory. 

 In the same way when the third structurally identical lactic 

 acid, obtained from cane sugar by the special form of bacterium, 

 is examined, it is found to ttirn the plane of the polarized light to 

 the left, and is termed levo (left) rotatory. And when sour milk 

 lactic acid, the inactive form, is split by means of its strychnine 

 salt, there are obtained both the dextro-rotatory and the levo- 

 rotatory compounds. We shall find that other compounds, 

 especially tartaric acid and the sugars, possess these same prop- 

 erties. It should be emphasized that these differences in physi- 

 cal properties as exhibited toward polarized light indicate no 

 difference whatever in the chemical structure of the compounds, 



