522 



the water was in one 6.35, and in the other 8.68 parts in 100. 

 The Mexican hyalite, therefore, is quite peculiar as respects 

 the amount of its constituent water, this being not more than 

 two-fifths of what has been found in the least hydrated variety 

 of the mineral whose analysis has been published. 



" The most remarkable circumstance connected with this 

 mineral remains to be mentioned. The hyalites are, in all 

 treatises on mineralogy, described as single refractors ; but, 

 upon transmitting through the Mexican hyalite a ray of plane- 

 polarized light, and examining it in the usual manner, with an 

 analyzing eye-piece, it was found to possess in a very marked 

 degree the property of depolarizing the ray. The action, 

 however, exerted by it, appeared of an anomalous kind ; for, 

 while in all doubly-refracting minerals, whether uniaxal or 

 biaxal, there are two positions of the crystal, in which, if inter- 

 posed (for example) between two tourmalines whose axes are 

 crossed, the light continues excluded, there is no position into 

 which a lamina of this hyalite can be brought, by revolving it 

 in its own plane, in which the light will not be restored. 



" The action, therefore, of this mineral on polarized light 

 is not of the ordinary kind, and can, as far as my knowledge 

 of this difficult subject extends, only be explained by sujjpos- 

 ing that, like rock crystal in one particular direction, and cer- 

 tain essential oils and aqueous solutions of sugar, dextrine, &c., 

 it possesses the power of changing the position of the plane 

 of polarization of a plane polarized ray, or of causing it to 

 revolve through a greater or less angle. This view would 

 seem to be corroborated by the following experiments. 



"1. A ray of homogeneous red light was made to pass 

 through a lamina of the hyalite, on which nearly parallel 

 planes were cut and polished, this lamina being interposed 

 between a pair of crossed tourmalines. The light was thus 

 restored, and, by rotating the tourmaline next the eye to the 

 right, it was again very nearly extinguished. 



" 2. The preceding experiment was repeated with yellow 



