188 



The following substances also exhibit circular polarization, and the 

 structure is, in all cases but one, negative. 



Hoof of the horse, vertical and trans- 

 verse sections. 



Hoof of an ass, transverse section. 



Transparent aperture in the wing of 

 a beetle. 



Hoof of rhinoceros. 



Horn of rhinoceros, transverse and 



vertical sections. 

 Horn of antelope. 

 Sections of hairs of animals. 



In conclusion, the author offered some observations on the forma- 

 tion and destruction of these discs. He regarded them as abnormal 

 crystallizations, in which the particles are in unstable equilibrium, and 

 have a constant tendency to arrange themselves according to their natu- 

 ral polarities. Hence, circular crystallizations are apt, after a longer or 

 shorter time, to disappear, the particles either dissolving, or assuming 

 the form of ordinary crystals, lying in all directions, or accumulated 

 in radial or circular lines. In oil of mace, the decomposition is 

 effected in a night ; in maunite, not for several years. 



The observations recorded in this paper, have occupied the author 

 during the last ten years, and must have an important bearing on 

 many unsettled questions in molecular philosophy. 



The following Donations to the Library were announced : — 



Oi-dnance Survey. Astronomical Observations made with Airy's 

 Zenith Sector, from 1842 to 1850, for the determination of 

 the Latitudes of various Trigonometrical Stations used in the 

 Ordnance Survey of the British Isles. By Captain W. Yol- 

 land. 4to. — From the Hon. Board of Ordnance, 



Archives du Museum d'Histoire Naturelle, publiees par les Pro- 

 fesseurs-Administrateurs de cet Etablissement. Tome VI., 

 Liv. 3 & 4. 4to. — From the Editors. 



The American Journal of Science and Arts. 2d Series. No. 43. 

 8vo. — From the Editors. 



