523 
light, got from a spirit-lamp with salted wick, and with the 
same results, 
*¢ 3. Upon substituting for the light of the spirit that 
transmitted through a window-blind of a yellowish colour, the 
phenomena of the last experiment were very distinctly repro- 
duced. 
** 4, In operating, by aid of the apparatus used in the 
preceding experiments, with heterogeneous or solar light, and 
making the analyser revolve, a series of tints were produced, 
much feebler than those exhibited by quartz, but following 
apparently the same law. 
** From these facts I think I am justified in concluding, 
that this hyalite exercises the power denominated rotatory 
polarization. This property, too, it possesses, no matter in 
what direction it is traversed by the plane polarized ray, a cir- 
cumstance which would seem to identify it with that exerted 
by liquids, and distinguish it from the analogous influence of 
quartz, which is manifested only in the direction of the optic 
axis. 
‘* Before concluding this paper I may be permitted to 
give expression to an opinion, which is, I believe, shared by 
most mineralogists, namely, that hyalite, opal, and calcedony, 
have all had a similar origin, or were originally silex in the 
gelatinous condition which it assumes when certain alkaline 
silicates are dissolved in dilute acid, and their solutions are 
slowly evaporated. All three include water, the hyalite in 
greatest, and the calcedony in smallest quantity ; and in some 
specimens that I have seen, these minerals may be observed 
to pass by insensible gradations into each other. This is well 
illustrated by some lumps of the Mexican hyalite which have 
nodules of calcedony attached to them, the latter mineral dif- 
_ fering in no respect from the former, save in being less trans- 
_ parent, and containing a smaller quantity of water,—its amount 
being about 0.53 per cent., according to a single experiment. 
_ The water, too, in all, is, I believe, present, not in a state 
VOL, II. 22 
