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 suppos- 
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 
~~. 
