bi/ transmission through Fluids. 417 



veil of mystery and obscurity which has hitherto perplexed 

 and thwarted many who, in common with myself, have assayed 

 to follow out the experiments detailed by Biot and others, I 

 may not be considered anxious to throw discredit on their la- 

 bours ; but candidly detailing the result of my own experi- 

 ments, I can only say to them, as well as to others, " Si 

 quid novisti rectius istis, candidus imperti ; si non, his utere 

 mecum." 



The point in which the results of my own experiments 

 disagree with those of other observers, is in regard to the al- 

 leged possession of opposite rotations by certain fluids, which 

 have the property of deviating the plane of polarization of a 

 beam of polarized light transmitted through them, some of 

 those fluids being said to produce a right-handed rotation, 

 others a /^-handed rotation. On examining a great variety 

 of samples of the particular fluids said to possess opposite 

 rotations, I found that such fluids deviated the plane of pola- 

 rization in only one direction, namely, towards the right- 

 hand. The only fluids in which I have hitherto been able to 

 discover a decided /^-handed rotation is in the essential oil 

 of lavender, and in that of cubebs*. The interesting fact, if 

 true, that solutions of sugar, when obtained from different 

 sources and in different conditions, possess in some cases a 

 right-handed and in others a left-handed rotation, has not 

 been corroborated by my own observations. When such so- 

 lutions did exhibit any decided rotating influence, it was uni- 

 formly right-handed. It is, however, curious that solutions 

 of cane-sugar differ from those of grape, potato and diabetic 

 sugar in this respect; that whilst the former possessed a 

 powerful rotating influence, the latter exhibited little or no 

 rotating energyf, although their solutions were in some cases 

 sufficiently concentrated to furnish crystals after a few days' 

 repose. 



It might perhaps be supposed that such result proceeded 

 only from my own want of information on the subject, or a 

 deficiency of my powers of observation ; indeed, I long so 

 considered it myself, so much so, that, not satisfied with obser- 

 vations made with apparatus procured in this country, I com- 



* The amount of left-handed rotation in oil of cubebs is much greater 

 than that of oil of lavender. This oil (which was colourlessl, and some others 

 contained in the table accompanying this communication, have been exa- 

 mined since my paper was read, the samples having been kindly furnished 

 by Mr. Warington of Apothecaries' Hall. Samples of oil of cubebs obtained 

 elsewhere were coloured, and possessed much less rotating energy, being 

 probably adulterated with oil of peppermint. 



t Although possessed of no rotating energy they have a depolarizing in- 

 fluence, as subsequently described. 



Phil. Mag. S. 3. Vol. 25. No. 168. Dec. 184.4. 2 E 



