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 ina 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 | 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 Nicholl’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.” 
