SECT. XXII. DISCOVERIES OF M. BIOT. 191 



do not possess circular polarization themselves, the angle of 

 deviation remaining exactly the same as before the mixture. 

 Whence M. Biot infers that the action exercised by the liquids 

 in question does not depend upon their mass, but that it is a 

 molecular action exercised by the ultimate particles of matter, 

 which depends solely upon the individual constitution, and is 

 entirely independent of the positions and mutual distances of 

 the particles with regard to each other. These important dis- 

 coveries show that circular polarization surpasses the power of 

 chemical analysis in giving certain and direct evidence of the 

 similarity or difference existing in the molecular constitution of 

 bodies, as well as of the permanency of that constitution, or of 

 the fluctuations to which it may be liable. For example, no 

 chemical difference has been discovered between syrup from the 

 sugar-cane and syrup from grapes. Yet the first causes the 

 plane of polarization to revolve to the right, and the other to 

 the left ; therefore some essential difference must exist in the 

 nature of their ultimate molecules. The same difference is to 

 be traced between the juices of such plants as give sugar similar 

 to that from the cane, and those which give sugar like that 

 obtained from grapes. 



If chlorate of soda be dissolved in water, the liquid has no 

 circular polarization ; but if the solution be allowed to crystallize, 

 some of the crystals turn the light to the right and others to the 

 left. Now, if all those of one kind be gathered together and 

 dissolved a second time, the liquid will have no circular polariza- 

 tion ; but if crystals be allowed to form, some will turn the light 

 to the right and others to the left, although only one kind was 

 dissolved.* 



It is a fact established by M. Biot, that in circular polariza- 

 tion the laws of rotation followed by the different simple rays of 

 light are dissimilar in different substances. Whence he infers 

 that the deviation of the simple rays from one another ought 

 not to result from a special property of the luminous principle 

 only, but that the proper action of the molecules must also concur 

 in modifying the deviations of the simple rays differently in 

 different substances. 



One of the many brilliant discoveries of M. Fresnel is the pro- 



* M. Mar bach of Breslau. 



