24 LOUIS PASTEUR 



many chemical compounds assume. Besides, simi- 

 lar compounds such as the salts of a given acid 

 often have crystals of the same general shape. 



As a rule, crystals are regular in form and may 

 be divided by a plane into two symmetrical halves, 

 but the crystals of some substances cannot be so 

 divided; they appear to exhibit a certain degree of 

 distortion, but they are distorted in a very definite 

 way. Some are spoken of as right-handed and 

 others as left-handed. Quartz has the peculiar 

 property of crystallizing in both forms. Its right- 

 handed and left-handed crystals are exactly alike 

 except that the one is like the mirrored image of 

 the other. They cannot be superposed; they are 

 related much as our right and left hands. 



The asymmetry of crystals is evinced not only 

 by their form, but by their peculiar action on light. 

 Light passing in a certain direction through Iceland 

 spar or quartz, for instance, becomes polarized. 

 Physicists have shown that light probably consists 

 of very minute undulations or waves which are sup- 

 posed to occur at right angles to the line of trans- 

 mission although these waves may occur 

 at various angles to each other, thus: — 7\~~ 

 When a beam of light is passed through a 

 polarizing prism it is found to be incapable of pass- 



