524 



of chemical, but of mechanical union, and gives to these mine- 

 rals the different degrees of transparency which they exhibit^ 

 just as water confers the same property upon specimens of 

 hydrophane or tabasheer, into whose pores it has been intro- 

 duced by capillary absorption. 



" From the analogy as to chemical constitution between 

 hyalite and calcedony, one might be inclined to conclude that 

 both would modify light transmitted through them in a similar 

 manner. This idea, however, would appear to be inconsistent 

 with experiment, for a plate of agate is said to polarize light 

 in the ordinary manner, or in a single plane ; and Sir David 

 Brewster is of opinion that the action which it exerts is of the 

 same nature with that which belongs to tourmaline. These 

 statements are certainly not true of the mineral which 1 have 

 this evening brought under the notice of the Academy. 



" P. S Since the preceding remarks were put together, 



I have found that a ray of light polarized by reflexion, and 

 made to disappear by the interposition of a NichoU's prism, was 

 restored when made to pass through a particle of hyalite from 

 Frankfort on the Maine. All varieties, therefore, of this mi- 

 neral may be concluded to possess the same optical proper- 

 ties. 



" In respect to the experiments above detailed, I have found 

 that the results are somewhat different, according to the part 

 of the lamina of hyalite traversed by the light, the change in 

 the position of the plane of polarization not having always the 

 same direction or value, and being sometimes null. This fact 

 would seem to point to a different explanation of the pheno- 

 mena from that already suggested, and to identify the optical 

 characters of hyalite with those of rock crystal, the differences 

 being explicable upon the hypothesis of the former mineral 

 being composed of a multitude of minute crystals of the latter, 

 thrown together without any regular or symmetric arrange- 

 ment." 



