60*2 M. BIOT ON THE APPLICATION OF CIKCULAR POLARIZATION 



light transmitted is white ; whence results the facility of making obser- 

 vations with a light of this nature as rigorously as with simple light, 

 and in a manner infinitely more convenient in practice, and more deli- 

 cate with respect to the appreciations which can be made by its means. 



The single fact that the rotations are proportional to the degree of 

 thickness in liquid mediums under normal incidences, proves to every 

 natural philosopher familiar with the general laws of mechanics that 

 the action thus exerted is molecular ; that is, that the total deviation 

 observed through a limited thickness is the sum of the infinitely small 

 angular deviations successively produced by the groups of atoms which 

 compose each infinitely thin layer of the simple or compound substance 

 exercising a power of this nature. This molecular power is of such 

 great importance, and is so evidently the principle of all the chemical 

 applications of the phsenomenon which are possible, that I have em- 

 ployed the most minute attention and the most diversified tests in 

 order indubitably to establish its truth. 



I endeavoured in the first instance to remove all idea that the effect 

 could arise from a certain actual relation of position existing among 

 the molecular groups of the active medium. For this purpose I agi- 

 tated the particles by moving the medium during the transmission of 

 the ray, and modified their intervals by the application of heat, without 

 however carrying it so far as chemically to alter the atomic groups ; 

 the total deviation remained the same, as the mechanical laws had pre- 

 viously indicated. I again removed these groups much further, and, so 

 to speak, indefinitely, by mixing active with inactive liquids, or with 

 those having an action upon each other either in a similar or in a con- 

 trary direction : the total deviation produced by the mixed system was 

 always rigorously the sum of the partial deviations that the luminous 

 ray would have undergone in traversing the same sum of active and 

 inactive groups placed in succession in separate tubes. These expe- 

 riments, which M. Chevreul says require now to be made*, I performed, 

 and published in the memoirs of the Academy sixteen years ago; and I 

 almost lost my life at that time in performing an experiment by which 

 I proved that the essence of turpentine preserves its rotatory property 

 in the state of vapour in motion. 



I established all these laws in 1818f , and no addition had been made 

 to them until a more profound examination of these laws, and an ap- 

 paratus of much greater sensibility, furnished me with indications of the 

 rotatory property infinitely more delicate than those which I had pre- 

 viously employed ; and I succeeded by these means, two years ago, in 

 discovering this property in a great number of substances of organic 



• Rapport silt I'Jmidon, cliap. v., § 72. Annales, p. 266. 



f Vide the Memoires de I' Academic des Sciences fw tiie vear 1817. 



