BIOT AND J. HERSCHEL 11 



and left-handed plagihedrons oblige us to admit as 

 present in the quartz, we can place the parallel, but 

 inverse, actions which it has on polarized light. 



I have just spoken of structure. It is necessary 

 to make here an important remark: this action on 

 polarized light manifests itself only in crystallized quartz. 

 With the amorphous quartz, or silica in solution 

 in any liquid whatsoever, we no longer find a trace of 

 it. Furthermore, the action takes place only on a ray 

 of polarized light traversing the crystal in the direction of 

 its longer axis and parallel to that axis, or at least in a 

 direction very little inclined away from it. It dimin- 

 ishes rapidly in proportion to the augmentation of the 

 inclination, and there is no longer a trace of it when 

 the ray traverses the crystal obliquely and in the direc- 

 tion of its shorter diameter. 



This circumstance, which connected the rotary power 

 with the molecular files of Delafosse, was so much 

 the more curious as it did not occur at all in the other 

 substances in which Biot had also discovered the rotary 

 power. Almost all of these substances were products of 

 animal or vegetable life: sugar, tartaric acid, different 

 essences, albumen, etc. But those which could crystal- 

 lize, the sugar and the tartaric acid, had no polarizing 

 action in the crystalline state. All, on the contrary, 

 when dissolved in water or any liquid whatsoever, rotated 

 the plane of polarization, some to the right, some to the 

 left. This rotation is always the same for the same solu- 

 tion when the density is the same, regardless of the direc- 

 tion in which the light ray is made to traverse the liquid 

 which is being examined, and we can agitate this liquid 

 during the observation without changing in any way the 

 quantity and direction of the rotation, a fact which well 

 demonstrates that it does not depend on the internal 

 arrangement of the active molecules in the solvent. 



